Human Heart – Diagram and Anatomy of the Heart Heart, Posterior Full Heart Description [Continued from above] . . . pulmonary arteries and veins, and the vena cava. The inferior tip of the heart, known as the apex, rests just superior to the diaphragm . The base of the heart is located along the body’s midline with the apex pointing toward the left side. Because the heart points to the left, about 2/3 of the heart’s mass is found on the left side of the body and the other 1/3 is on the right. Anatomy of the Heart Pericardium The heart sits within a fluid-filled cavity called the pericardial cavity. The walls and lining of the pericardial cavity are a special membrane known as the pericardium. Pericardium is a type of serous membrane that produces serous fluid to lubricate the heart and prevent friction between the ever beating heart and its surrounding organs. Besides lubrication, the pericardium serves to hold the heart in position and maintain a hollow space for the heart to expand into when it is full.

The pericardium has 2 layers—a visceral layer that covers the outside of the heart and a parietal layer that forms a sac around the outside of the pericardial cavity. Structure of the Heart Wall The heart wall is made of 3 layers: epicardium, myocardium and endocardium. Epicardium. The epicardium is the outermost layer of the heart wall and is just another name for the visceral layer of the pericardium. Thus, the epicardium is a thin layer of serous membrane that helps to lubricate and protect the outside of the heart. Below the epicardium is the second, thicker layer of the heart wall: the myocardium. Myocardium. The myocardium is the muscular middle layer of the heart wall that contains the cardiac muscle tissue . Myocardium makes up the majority of the thickness and mass of the heart wall and is the part of the heart responsible for pumping blood. Below the myocardium is the thin endocardium layer. Endocardium. Endocardium is the simple squamous endothelium layer that lines the inside of the heart.

The endocardium is very smooth and is responsible for keeping blood from sticking to the inside of the heart and forming potentially deadly blood clots. The thickness of the heart wall varies in different parts of the heart. The atria of the heart have a very thin myocardium because they do not need to pump blood very far—only to the nearby ventricles. The ventricles, on the other hand, have a very thick myocardium to pump blood to the lungs or throughout the entire body. The right side of the heart has less myocardium in its walls than the left side because the left side has to pump blood through the entire body while the right side only has to pump to the lungs. Chambers of the Heart The heart contains 4 chambers: the right atrium , left atrium , right ventricle , and left ventricle . The atria are smaller than the ventricles and have thinner, less muscular walls than the ventricles. The atria act as receiving chambers for blood, so they are connected to the veins that carry blood to the heart. The ventricles are the larger, stronger pumping chambers that send blood out of the heart.

The ventricles are connected to the arteries that carry blood away from the heart. The chambers on the right side of the heart are smaller and have less myocardium in their heart wall when compared to the left side of the heart. This difference in size between the sides of the heart is related to their functions and the size of the 2 circulatory loops. The right side of the heart maintains pulmonary circulation to the nearby lungs while the left side of the heart pumps blood all the way to the extremities of the body in the systemic circulatory loop. Valves of the Heart The heart functions by pumping blood both to the lungs and to the systems of the body. To prevent blood from flowing backwards or “regurgitating” back into the heart, a system of one-way valves are present in the heart. The heart valves can be broken down into two types: atrioventricular and semilunar valves. Atrioventricular valves. The atrioventricular (AV) valves are located in the middle of the heart between the atria and ventricles and only allow blood to flow from the atria into the ventricles.

The AV valve on the right side of the heart is called the tricuspid valve because it is made of three cusps (flaps) that separate to allow blood to pass through and connect to block regurgitation of blood. The AV valve on the left side of the heart is called the mitral valve or the bicuspid valve because it has two cusps. The AV valves are attached on the ventricular side to tough strings called chordae tendineae . The chordae tendineae pull on the AV valves to keep them from folding backwards and allowing blood to regurgitate past them. During the contraction of the ventricles, the AV valves look like domed parachutes with the chordae tendineae acting as the ropes holding the parachutes taut. Semilunar valves. The semilunar valves, so named for the crescent moon shape of their cusps, are located between the ventricles and the arteries that carry blood away from the heart. The semilunar valve on the right side of the heart is the pulmonary valve , so named because it prevents the backflow of blood from the pulmonary trunk into the right ventricle.

The semilunar valve on the left side of the heart is the aortic valve , named for the fact that it prevents the aorta from regurgitating blood back into the left ventricle. The semilunar valves are smaller than the AV valves and do not have chordae tendineae to hold them in place. Instead, the cusps of the semilunar valves are cup shaped to catch regurgitating blood and use the blood’s pressure to snap shut. Conduction System of the Heart The heart is able to both set its own rhythm and to conduct the signals necessary to maintain and coordinate this rhythm throughout its structures. About 1% of the cardiac muscle cells in the heart are responsible for forming the conduction system that sets the pace for the rest of the cardiac muscle cells. The conduction system starts with the pacemaker of the heart—a small bundle of cells known as the sinoatrial (SA) node. The SA node is located in the wall of the right atrium inferior to the superior vena cava . The SA node is responsible for setting the pace of the heart as a whole and directly signals the atria to contract.

The signal from the SA node is picked up by another mass of conductive tissue known as the atrioventricular (AV) node. The AV node is located in the right atrium in the inferior portion of the interatrial septum. The AV node picks up the signal sent by the SA node and transmits it through the atrioventricular (AV) bundle. The AV bundle is a strand of conductive tissue that runs through the interatrial septum and into the interventricular septum. The AV bundle splits into left and right branches in the interventricular septum and continues running through the septum until they reach the apex of the heart. Branching off from the left and right bundle branches are many Purkinje fibers that carry the signal to the walls of the ventricles, stimulating the cardiac muscle cells to contract in a coordinated manner to efficiently pump blood out of the heart. Physiology of the Heart Coronary Systole and Diastole At any given time the chambers of the heart may found in one of two states: Systole. During systole, cardiac muscle tissue is contracting to push blood out of the chamber. Diastole.

During diastole, the cardiac muscle cells relax to allow the chamber to fill with blood. Blood pressure increases in the major arteries during ventricular systole and decreases during ventricular diastole. This leads to the 2 numbers associated with blood pressure—systolic blood pressure is the higher number and diastolic blood pressure is the lower number. For example, a blood pressure of 120/80 describes the systolic pressure (120) and the diastolic pressure (80). The Cardiac Cycle The cardiac cycle includes all of the events that take place during one heartbeat. There are 3 phases to the cardiac cycle: atrial systole, ventricular systole, and relaxation. Atrial systole: During the atrial systole phase of the cardiac cycle, the atria contract and push blood into the ventricles. To facilitate this filling, the AV valves stay open and the semilunar valves stay closed to keep arterial blood from re-entering the heart. The atria are much smaller than the ventricles, so they only fill about 25% of the ventricles during this phase. The ventricles remain in diastole during this phase.

Ventricular systole: During ventricular systole, the ventricles contract to push blood into the aorta and pulmonary trunk. The pressure of the ventricles forces the semilunar valves to open and the AV valves to close. This arrangement of valves allows for blood flow from the ventricles into the arteries. The cardiac muscles of the atria repolarize and enter the state of diastole during this phase. Relaxation phase: During the relaxation phase, all 4 chambers of the heart are in diastole as blood pours into the heart from the veins. The ventricles fill to about 75% capacity during this phase and will be completely filled only after the atria enter systole. The cardiac muscle cells of the ventricles repolarize during this phase to prepare for the next round of depolarization and contraction. During this phase, the AV valves open to allow blood to flow freely into the ventricles while the semilunar valves close to prevent the regurgitation of blood from the great arteries into the ventricles. Blood Flow through the Heart Deoxygenated blood returning from the body first enters the heart from the superior and inferior vena cava .

The blood enters the right atrium and is pumped through the tricuspid valve into the right ventricle. From the right ventricle, the blood is pumped through the pulmonary semilunar valve into the pulmonary trunk . The pulmonary trunk carries blood to the lungs where it releases carbon dioxide and absorbs oxygen. The blood in the lungs returns to the heart through the pulmonary veins . From the pulmonary veins, blood enters the heart again in the left atrium. The left atrium contracts to pump blood through the bicuspid (mitral) valve into the left ventricle. The left ventricle pumps blood through the aortic semilunar valve into the aorta. From the aorta, blood enters into systemic circulation throughout the body tissues until it returns to the heart via the vena cava and the cycle repeats. The Electrocardiogram The electrocardiogram (also known as an EKG or ECG) is a non-invasive device that measures and monitors the electrical activity of the heart through the skin. The EKG produces a distinctive waveform in response to the electrical changes taking place within the heart.

The first part of the wave, called the P wave, is a small increase in voltage of about 0.1 mV that corresponds to the depolarization of the atria during atrial systole. The next part of the EKG wave is the QRS complex which features a small drop in voltage (Q) a large voltage peak (R) and another small drop in voltage (S). The QRS complex corresponds to the depolarization of the ventricles during ventricular systole. The atria also repolarize during the QRS complex, but have almost no effect on the EKG because they are so much smaller than the ventricles. The final part of the EKG wave is the T wave, a small peak that follows the QRS complex. The T wave represents the ventricular repolarization during the relaxation phase of the cardiac cycle. Variations in the waveform and distance between the waves of the EKG can be used clinically to diagnose the effects of heart attacks, congenital heart problems, and electrolyte imbalances. Heart Sounds The sounds of a normal heartbeat are known as “lubb” and “dupp” and are caused by blood pushing on the valves of the heart.

The “lubb” sound comes first in the heartbeat and is the longer of the two heart sounds. The “lubb” sound is produced by the closing of the AV valves at the beginning of ventricular systole. The shorter, sharper “dupp” sound is similarly caused by the closing of the semilunar valves at the end of ventricular systole. During a normal heartbeat, these sounds repeat in a regular pattern of lubb-dupp-pause. Any additional sounds such as liquid rushing or gurgling indicate a structure problem in the heart. The most likely causes of these extraneous sounds are defects in the atrial or ventricular septum or leakage in the valves. Cardiac Output Cardiac output (CO) is the volume of blood being pumped by the heart in one minute. The equation used to find cardiac output is: CO = Stroke Volume x Heart Rate Stroke volume is the amount of blood pumped into the aorta during each ventricular systole, usually measured in milliliters. Heart rate is the number of heartbeats per minute. The average heart can push around 5 to 5.5 liters per minute at rest.

Prepared by Tim Taylor, Anatomy and Physiology Instructor Muscular System - Muscles of the Human Body Muscular System Home > Muscular System Muscular System The muscular system is responsible for the movement of the human body. Attached to the bones of the skeletal system are about 700 named muscles that make up roughly half of a person’s body weight. Each of these muscles is a discrete organ constructed of skeletal muscle tissue, blood vessels, tendons, and nerves. Muscle tissue is also found inside of the heart, digestive organs, and blood vessels. In these organs, muscles serve to move substances throughout the body.... Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image Muscle Types There are three types of muscle tissue: Visceral, cardiac, and skeletal. Visceral Muscle. Visceral muscle is found inside of organs like the stomach , intestines, and blood vessels. The weakest of all muscle tissues, visceral muscle makes organs contract to move substances through the organ. Because visceral muscle is controlled by the unconscious part of the brain, it is known as involuntary muscle—it cannot be directly controlled by the conscious mind.

The term “smooth muscle” is often used to describe visceral muscle because it has a very smooth, uniform appearance when viewed under a microscope. This smooth appearance starkly contrasts with the banded appearance of cardiac and skeletal muscles. Cardiac Muscle. Found only in the heart , cardiac muscle is responsible for pumping blood throughout the body. Cardiac muscle tissue cannot be controlled consciously, so it is an involuntary muscle. While hormones and signals from the brain adjust the rate of contraction, cardiac muscle stimulates itself to contract. The natural pacemaker of the heart is made of cardiac muscle tissue that stimulates other cardiac muscle cells to contract. Because of its self-stimulation, cardiac muscle is considered to be autorhythmic or intrinsically controlled. The cells of cardiac muscle tissue are striated—that is, they appear to have light and dark stripes when viewed under a light microscope. The arrangement of protein fibers inside of the cells causes these light and dark bands. Striations indicate that a muscle cell is very strong, unlike visceral muscles. The cells of cardiac muscle are branched X or Y shaped cells tightly connected together by special junctions called intercalated disks.

Intercalated disks are made up of fingerlike projections from two neighboring cells that interlock and provide a strong bond between the cells. The branched structure and intercalated disks allow the muscle cells to resist high blood pressures and the strain of pumping blood throughout a lifetime. These features also help to spread electrochemical signals quickly from cell to cell so that the heart can beat as a unit. Skeletal Muscle. Skeletal muscle is the only voluntary muscle tissue in the human body—it is controlled consciously. Every physical action that a person consciously performs (e.g. speaking, walking, or writing) requires skeletal muscle. The function of skeletal muscle is to contract to move parts of the body closer to the bone that the muscle is attached to. Most skeletal muscles are attached to two bones across a joint, so the muscle serves to move parts of those bones closer to each other. Skeletal muscle cells form when many smaller progenitor cells lump themselves together to form long, straight, multinucleated fibers. Striated just like cardiac muscle, these skeletal muscle fibers are very strong. Skeletal muscle derives its name from the fact that these muscles always connect to the skeleton in at least one place.

Gross Anatomy of a Skeletal Muscle Most skeletal muscles are attached to two bones through tendons. Tendons are tough bands of dense regular connective tissue whose strong collagen fibers firmly attach muscles to bones. Tendons are under extreme stress when muscles pull on them, so they are very strong and are woven into the coverings of both muscles and bones. Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. One of the bones is pulled towards the other bone, which remains stationary. The place on the stationary bone that is connected via tendons to the muscle is called the origin. The place on the moving bone that is connected to the muscle via tendons is called the insertion. The belly of the muscle is the fleshy part of the muscle in between the tendons that does the actual contraction. Names of Skeletal Muscles Skeletal muscles are named based on many different factors, including their location, origin and insertion, number of origins, shape, size, direction, and function. Location. Many muscles derive their names from their anatomical region. The rectus abdominis and transverse abdominis, for example, are found in the abdominal region .

Some muscles, like the tibialis anterior , are named after the part of the bone (the anterior portion of the tibia ) that they are attached to. Other muscles use a hybrid of these two, like the brachioradialis, which is named after a region (brachial) and a bone ( radius ). Origin and Insertion. Some muscles are named based upon their connection to a stationary bone (origin) and a moving bone (insertion). These muscles become very easy to identify once you know the names of the bones that they are attached to. Examples of this type of muscle include the sternocleidomastoid (connecting the sternum and clavicle to the mastoid process of the skull) and the occipitofrontalis (connecting the occipital bone to the frontal bone ). Number of Origins. Some muscles connect to more than one bone or to more than one place on a bone, and therefore have more than one origin. A muscle with two origins is called a biceps. A muscle with three origins is a triceps muscle. Finally, a muscle with four origins is a quadriceps muscle. Shape, Size, and Direction. We also classify muscles by their shapes.

For example, the deltoids have a delta or triangular shape. The serratus muscles feature a serrated or saw-like shape. The rhomboid major is a rhombus or diamond shape. The size of the muscle can be used to distinguish between two muscles found in the same region. The gluteal region contains three muscles differentiated by size—the gluteus maximus (large), gluteus medius (medium), and gluteus minimus (smallest). Finally, the direction in which the muscle fibers run can be used to identify a muscle. In the abdominal region, there are several sets of wide, flat muscles. The muscles whose fibers run straight up and down are the rectus abdominis , the ones running transversely (left to right) are the transverse abdominis, and the ones running at an angle are the obliques. Function. Muscles are sometimes classified by the type of function that they perform. Most of the muscles of the forearms are named based on their function because they are located in the same region and have similar shapes and sizes. For example, the flexor group of the forearm flexes the wrist and the fingers.

The supinator is a muscle that supinates the wrist by rolling it over to face palm up. In the leg, there are muscles called adductors whose role is to adduct (pull together) the legs. Groups Action in Skeletal Muscle Skeletal muscles rarely work by themselves to achieve movements in the body. More often they work in groups to produce precise movements. The muscle that produces any particular movement of the body is known as an agonist or prime mover. The agonist always pairs with an antagonist muscle that produces the opposite effect on the same bones. For example, the biceps brachii muscle flexes the arm at the elbow . As the antagonist for this motion, the triceps brachii muscle extends the arm at the elbow. When the triceps is extending the arm, the biceps would be considered the antagonist. In addition to the agonist/antagonist pairing, other muscles work to support the movements of the agonist. Synergists are muscles that help to stabilize a movement and reduce extraneous movements. They are usually found in regions near the agonist and often connect to the same bones. Because skeletal muscles move the insertion closer to the immobile origin, fixator muscles assist in movement by holding the origin stable.

If you lift something heavy with your arms, fixators in the trunk region hold your body upright and immobile so that you maintain your balance while lifting. Skeletal Muscle Histology Skeletal muscle fibers differ dramatically from other tissues of the body due to their highly specialized functions. Many of the organelles that make up muscle fibers are unique to this type of cell. The sarcolemma is the cell membrane of muscle fibers. The sarcolemma acts as a conductor for electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules (T-tubules) that help carry these electrochemical signals into the middle of the muscle fiber. The sarcoplasmic reticulum serves as a storage facility for calcium ions (Ca2+) that are vital to muscle contraction. Mitochondria, the “power houses” of the cell, are abundant in muscle cells to break down sugars and provide energy in the form of ATP to active muscles. Most of the muscle fiber’s structure is made up of myofibrils, which are the contractile structures of the cell. Myofibrils are made up of many proteins fibers arranged into repeating subunits called sarcomeres. The sarcomere is the functional unit of muscle fibers.

(See Macronutrients for more information about the roles of sugars and proteins.) Sarcomere Structure Sarcomeres are made of two types of protein fibers: thick filaments and thin filaments. Thick filaments. Thick filaments are made of many bonded units of the protein myosin. Myosin is the protein that causes muscles to contract. Thin filaments. Thin filaments are made of three proteins: Actin. Actin forms a helical structure that makes up the bulk of the thin filament mass. Actin contains myosin-binding sites that allow myosin to connect to and move actin during muscle contraction. Tropomyosin. Tropomyosin is a long protein fiber that wraps around actin and covers the myosin binding sites on actin. Troponin. Bound very tightly to tropomyosin, troponin moves tropomyosin away from myosin binding sites during muscle contraction. Muscular System Physiology Function of Muscle Tissue The main function of the muscular system is movement. Muscles are the only tissue in the body that has the ability to contract and therefore move the other parts of the body.

Related to the function of movement is the muscular system’s second function: the maintenance of posture and body position. Muscles often contract to hold the body still or in a particular position rather than to cause movement. The muscles responsible for the body’s posture have the greatest endurance of all muscles in the body—they hold up the body throughout the day without becoming tired. Another function related to movement is the movement of substances inside the body. The cardiac and visceral muscles are primarily responsible for transporting substances like blood or food from one part of the body to another. The final function of muscle tissue is the generation of body heat. As a result of the high metabolic rate of contracting muscle, our muscular system produces a great deal of waste heat. Many small muscle contractions within the body produce our natural body heat. When we exert ourselves more than normal, the extra muscle contractions lead to a rise in body temperature and eventually to sweating. Skeletal Muscles as Levers Skeletal muscles work together with bones and joints to form lever systems. The muscle acts as the effort force; the joint acts as the fulcrum; the bone that the muscle moves acts as the lever; and the object being moved acts as the load.

There are three classes of levers, but the vast majority of the levers in the body are third class levers. A third class lever is a system in which the fulcrum is at the end of the lever and the effort is between the fulcrum and the load at the other end of the lever. The third class levers in the body serve to increase the distance moved by the load compared to the distance that the muscle contracts. The tradeoff for this increase in distance is that the force required to move the load must be greater than the mass of the load. For example, the biceps brachia of the arm pulls on the radius of the forearm, causing flexion at the elbow joint in a third class lever system. A very slight change in the length of the biceps causes a much larger movement of the forearm and hand, but the force applied by the biceps must be higher than the load moved by the muscle. Motor Units Nerve cells called motor neurons control the skeletal muscles. Each motor neuron controls several muscle cells in a group known as a motor unit. When a motor neuron receives a signal from the brain, it stimulates all of the muscles cells in its motor unit at the same time.

The size of motor units varies throughout the body, depending on the function of a muscle. Muscles that perform fine movements—like those of the eyes or fingers—have very few muscle fibers in each motor unit to improve the precision of the brain’s control over these structures. Muscles that need a lot of strength to perform their function—like leg or arm muscles—have many muscle cells in each motor unit. One of the ways that the body can control the strength of each muscle is by determining how many motor units to activate for a given function. This explains why the same muscles that are used to pick up a pencil are also used to pick up a bowling ball. Contraction Cycle Muscles contract when stimulated by signals from their motor neurons. Motor neurons contact muscle cells at a point called the Neuromuscular Junction (NMJ). Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma known as the motor end plate. The motor end plate contains many ion channels that open in response to neurotransmitters and allow positive ions to enter the muscle fiber.

The positive ions form an electrochemical gradient to form inside of the cell, which spreads throughout the sarcolemma and the T-tubules by opening even more ion channels. When the positive ions reach the sarcoplasmic reticulum, Ca2+ ions are released and allowed to flow into the myofibrils. Ca2+ ions bind to troponin, which causes the troponin molecule to change shape and move nearby molecules of tropomyosin. Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together. ATP molecules power myosin proteins in the thick filaments to bend and pull on actin molecules in the thin filaments. Myosin proteins act like oars on a boat, pulling the thin filaments closer to the center of a sarcomere. As the thin filaments are pulled together, the sarcomere shortens and contracts. Myofibrils of muscle fibers are made of many sarcomeres in a row, so that when all of the sarcomeres contract, the muscle cells shortens with a great force relative to its size. Muscles continue contraction as long as they are stimulated by a neurotransmitter.

When a motor neuron stops the release of the neurotransmitter, the process of contraction reverses itself. Calcium returns to the sarcoplasmic reticulum; troponin and tropomyosin return to their resting positions; and actin and myosin are prevented from binding. Sarcomeres return to their elongated resting state once the force of myosin pulling on actin has stopped. Types of Muscle Contraction The strength of a muscle’s contraction can be controlled by two factors: the number of motor units involved in contraction and the amount of stimulus from the nervous system. A single nerve impulse of a motor neuron will cause a motor unit to contract briefly before relaxing. This small contraction is known as a twitch contraction. If the motor neuron provides several signals within a short period of time, the strength and duration of the muscle contraction increases. This phenomenon is known as temporal summation. If the motor neuron provides many nerve impulses in rapid succession, the muscle may enter the state of tetanus, or complete and lasting contraction. A muscle will remain in tetanus until the nerve signal rate slows or until the muscle becomes too fatigued to maintain the tetanus. Not all muscle contractions produce movement.

Isometric contractions are light contractions that increase the tension in the muscle without exerting enough force to move a body part. When people tense their bodies due to stress, they are performing an isometric contraction. Holding an object still and maintaining posture are also the result of isometric contractions. A contraction that does produce movement is an isotonic contraction. Isotonic contractions are required to develop muscle mass through weight lifting. Muscle tone is a natural condition in which a skeletal muscle stays partially contracted at all times. Muscle tone provides a slight tension on the muscle to prevent damage to the muscle and joints from sudden movements, and also helps to maintain the body’s posture. All muscles maintain some amount of muscle tone at all times, unless the muscle has been disconnected from the central nervous system due to nerve damage. Functional Types of Skeletal Muscle Fibers Skeletal muscle fibers can be divided into two types based on how they produce and use energy: Type I and Type II. Type I fibers are very slow and deliberate in their contractions. They are very resistant to fatigue because they use aerobic respiration to produce energy from sugar. We find Type I fibers in muscles throughout the body for stamina and posture.

Near the spine and neck regions, very high concentrations of Type I fibers hold the body up throughout the day. Type II fibers are broken down into two subgroups: Type II A and Type II B. Type II A fibers are faster and stronger than Type I fibers, but do not have as much endurance. Type II A fibers are found throughout the body, but especially in the legs where they work to support your body throughout a long day of walking and standing. Type II B fibers are even faster and stronger than Type II A, but have even less endurance. Type II B fibers are also much lighter in color than Type I and Type II A due to their lack of myoglobin, an oxygen-storing pigment. We find Type II B fibers throughout the body, but particularly in the upper body where they give speed and strength to the arms and chest at the expense of stamina. Muscle Metabolism and Fatigue Muscles get their energy from different sources depending on the situation that the muscle is working in. Muscles use aerobic respiration when we call on them to produce a low to moderate level of force. Aerobic respiration requires oxygen to produce about 36-38 ATP molecules from a molecule of glucose.

Aerobic respiration is very efficient, and can continue as long as a muscle receives adequate amounts of oxygen and glucose to keep contracting. When we use muscles to produce a high level of force, they become so tightly contracted that oxygen carrying blood cannot enter the muscle. This condition causes the muscle to create energy using lactic acid fermentation, a form of anaerobic respiration. Anaerobic respiration is much less efficient than aerobic respiration—only 2 ATP are produced for each molecule of glucose. Muscles quickly tire as they burn through their energy reserves under anaerobic respiration. To keep muscles working for a longer period of time, muscle fibers contain several important energy molecules. Myoglobin, a red pigment found in muscles, contains iron and stores oxygen in a manner similar to hemoglobin in the blood. The oxygen from myoglobin allows muscles to continue aerobic respiration in the absence of oxygen. Another chemical that helps to keep muscles working is creatine phosphate. Muscles use energy in the form of ATP, converting ATP to ADP to release its energy. Creatine phosphate donates its phosphate group to ADP to turn it back into ATP in order to provide extra energy to the muscle.

Finally, muscle fibers contain energy-storing glycogen, a large macromolecule made of many linked glucoses. Active muscles break glucoses off of glycogen molecules to provide an internal fuel supply. When muscles run out of energy during either aerobic or anaerobic respiration, the muscle quickly tires and loses its ability to contract. This condition is known as muscle fatigue. A fatigued muscle contains very little or no oxygen, glucose or ATP, but instead has many waste products from respiration, like lactic acid and ADP. The body must take in extra oxygen after exertion to replace the oxygen that was stored in myoglobin in the muscle fiber as well as to power the aerobic respiration that will rebuild the energy supplies inside of the cell. Oxygen debt (or recovery oxygen uptake) is the name for the extra oxygen that the body must take in to restore the muscle cells to their resting state. This explains why you feel out of breath for a few minutes after a strenuous activity—your body is trying to restore itself to its normal state.

Prepared by Tim Taylor, Anatomy and Physiology Instructor Human Heart: Anatomy, Function & Facts Human Heart: Anatomy, Function & Facts By Tanya Lewis, Staff Writer | March 22, 2016 01:07pm ET MORE Credit: Dreamstime The human heart is an organ that pumps blood throughout the body via the circulatory system, supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes. "The tissues of the body need a constant supply of nutrition in order to be active," said Dr. Lawrence Phillips, a cardiologist at NYU Langone Medical Center in New York. "If [the heart] is not able to supply blood to the organs and tissues, they'll die." Human heart anatomy In humans, the heart is roughly the size of a large fist and weighs between about 10 to 12 ounces (280 to 340 grams) in men and 8 to 10 ounces (230 to 280 grams) in women, according to Henry Gray's " Anatomy of the Human Body ." The physiology of the heart basically comes down to "structure, electricity and plumbing," Phillips told Live Science. The human heart is about the size of a fist.

Credit: tlorna Shutterstock The human heart has four chambers: two upper chambers (the atria) and two lower ones (the ventricles), according to the National Institutes of Health . The right atrium and right ventricle together make up the "right heart," and the left atrium and left ventricle make up the "left heart." A wall of muscle called the septum separates the two sides of the heart. A double-walled sac called the pericardium encases the heart, which serves to protect the heart and anchor it inside the chest. Between the outer layer, the parietal pericardium, and the inner layer, the serous pericardium, runs pericardial fluid, which lubricates the heart during contractions and movements of the lungs and diaphragm. The heart's outer wall consists of three layers. The outermost wall layer, or epicardium, is the inner wall of the pericardium. The middle layer, or myocardium, contains the muscle that contracts. The inner layer, or endocardium, is the lining that contacts the blood.

The tricuspid valve and the mitral valve make up the atrioventricular (AV) valves, which connect the atria and the ventricles. The pulmonary semi-lunar valve separates the right ventricle from the pulmonary artery, and the aortic valve separates the left ventricle from the aorta. The heartstrings, or chordae tendinae, anchor the valves to heart muscles. The sinoatrial node produces the electrical pulses that drive heart contractions. Human heart function The heart circulates blood through two pathways: the pulmonary circuit and the systemic circuit. In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs, then returns as oxygenated blood to the left atrium of the heart via the pulmonary vein. In the systemic circuit, oxygenated blood leaves the body via the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen. Deoxygenated blood returns via veins to the venae cavae, re-entering the heart's right atrium.

Of course, the heart is also a muscle, so it needs a fresh supply of oxygen and nutrients, too, Phillips said. The cardiovascular system circulates blood from the heart to the lungs and around the body via blood vessels. Credit: The BioDigital HumanTM developed by NYU School of Medicine and BioDigital Systems LLC "After the blood leaves the heart through the aortic valve, two sets of arteries bring oxygenated blood to feed the heart muscle," he said. The left main coronary artery, on one side of the aorta, branches into the left anterior descending artery and the left circumflex artery. The right coronary artery branches out on the right side of the aorta. Blockage of any of these arteries can cause a heart attack , or damage to the muscle of the heart, Phillips said. A heart attack is distinct from cardiac arrest, which is a sudden loss of heart function that usually occurs as a result of electrical disturbances of the heart rhythm. A heart attack can lead to cardiac arrest, but the latter can also be caused by other problems, he said. The heart contains electrical "pacemaker" cells, which cause it to contract — producing a heartbeat.

"Each cell has the ability to be the 'band leader' and [to] have everyone follow," Phillips said. In people with an irregular heartbeat, or atrial fibrillation, every cell tries to be the band leader, he said, which causes them to beat out of sync with one another. A healthy heart contraction happens in five stages. In the first stage (early diastole), the heart is relaxed. Then the atrium contracts (atrial systole) to push blood into the ventricle. Next, the ventricles start contracting without changing volume. Then the ventricles continue contracting while empty. Finally, the ventricles stop contracting and relax. Then the cycle repeats. Valves prevent backflow, keeping the blood flowing in one direction through the heart. Facts about the human heart A human heart is roughly the size of a large fist. The heart weighs between about 10 to 12 ounces (280 to 340 grams) in men and 8 to 10 ounces (230 to 280 grams) in women. The heart beats about 100,000 times per day (about 3 billion beats in a lifetime).

An adult heart beats about 60 to 80 times per minute. Newborns' hearts beat faster than adult hearts, about 70 to 190 beats per minute. The heart pumps about 6 quarts (5.7 liters) of blood throughout the body. The heart is located in the center of the chest, usually pointing slightly left. Editor’s Note: If you’d like more information on this topic, we recommend the following book: Layers of the Heart Walls About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Layers of the Heart Walls The heart wall is comprised of three layers: the outer epicardium, the middle myocardium, and the inner endocardium.

Learning Objective Distinguish between the epicardium, myocardium, and endocardium layers of the heart wall Key Points The epicardium is a thin layer of connective tissue and fat serving as an additional layer of protection for the heart . It is considered a continuation of the serous pericardium . The myocardium is the muscle tissue of the heart, composed of cardiac muscle cells called cardiomyocytes that receive nervous stimulation from the sinoatrial (SA) and atrioventricular (AV nodes via the Purkinje fibers . Cardiomyocytes are shorter than skeletal myocytes, and contain fewer nuclei. Cardiac muscle is striated. The endocardium is composed of endothelial cells that provide a smooth, non-adherent surface for blood collection and pumping and may help regulate contractility. An infection or inflammation of the endocardium is called infective endocarditis. Terms Register for FREE to remove ads and unlock more features! Learn more Full Text The heart wall is comprised of three layers, the epicardium (outer), myocardium (middle), and endocardium (inner). These tissue layers are highly specialized and perform different functions.

During ventricular contraction, the wave of depolarization from the SA and AV nodes moves from within the endocardial wall through the myocardial layer to the epicardial surface of the heart. The wall of the heart is composed of three layers, the thin outer epicardium, the thick middle myocardium, and the very thin inner endocardium. The dark area on the heart wall is scarring from a previous myocardial infarction (heart attack). Epicardium The outer layer of the heart wall is the epicardium. The epicardium refers to both the outer layer of the heart and the inner layer of the serous visceral pericardium, which is attached to the outer wall of the heart. The epicardium is a thin layer of elastic connective tissue and fat that serves as an additional layer of protection from trauma or friction for the heart under the pericardium. This layer contains the coronary blood vessels, which oxygenate the tissues of the heart with a blood supply from the coronary arteries . Myocardium The middle layer of the heart wall is the myocardium—the muscle tissue of the heart and the thickest layer of the heart wall. It is composed of cardiac muscle cells, or cardiomyocytes.

Cardiomyocytes are specialized muscle cells that contract like other muscle cells, but differ in shape. Compared to skeletal muscle cells, cardiac muscle cells are shorter and have fewer nuclei. Cardiac muscle tissue is also striated (forming protein bands) and contains tubules and gap junctions , unlike skeletal muscle tissue. Due to their continuous rhythmic contraction, cardiomyocytes require a dedicated blood supply to deliver oxygen and nutrients and remove waste products such as carbon dioxide from the cardiac muscle tissue. This blood supply is provided by the coronary arteries. Endocardium The inner layer of the heart wall is the endocardium, composed of endothelial cells that provide a smooth, elastic, non-adherent surface for blood collection and pumping. The endocardium may regulate metabolic waste removal from heart tissues and act as a barrier between the blood and the heart muscle, thus controlling the composition of the extracellular fluid in which the cardiomyocytes bathe. This in turn can affect the contractility of the heart. This tissue also covers the valves of the heart and is histologically continuous with the vascular endothelium of the major blood vessels entering and leaving the heart.

The Purkinje fibers are located just beneath the endocardium and send nervous impulses from the SA and AV nodes outside of the heart into the myocardial tissues. The endocardium can become infected, a serious inflammatory condition called infective endocarditis. This and other potential problems with the endocardium may damage the valves and impair the normal flow of blood through the heart. Myocardium - RightDiagnosis.com Myocardium Myocardium: the middle muscular layer of the heart wall Source: WordNet 2.1 Myocardium : muscle tissue of the heart composed of striated, involuntary muscle known as cardiac muscle. Source: CRISP Myocardium : The muscle tissue of the HEART composed of striated, involuntary muscle known as cardiac muscle. Source: MESH OBO (Open Biomedical Ontologies) Summary Information: Myocardium Myocardium: The myocardium is the muscular tissue that makes up the heart, a vital internal organ located in the pericardium in the thorax of the chest. The myocardium is supplied with blood by the coronary arteries. The myocardium is involuntary muscle that is activated by an electrical conductions system.

The myocardium pumps oxygenated blood from the lungs into and throughout the body and pump deoxygenated blood from the body to the lungs, where it is exchanged for freshly oxygenated blood. The myocardium includes four pumping chambers: the left ventricle, right ventricle, left atrium, and right atrium. In a healthy person, these chambers work together to pump and move blood in a regular, strong rhythm. The myocardium receives deoxygenated blood from the body through the superior and inferior vena cavas. Blood then flows through the right atrium and is pumped through the right ventricle into the pulmonary artery to the lungs, where gas exchange occurs and the blood is re-oxygenated. The myocardium receives oxygenated blood from the lungs from the pulmonary vein. It enters the left atrium and is pumped through the left ventricle and the aorta to the rest of the body. Conditions that afflict the myocardium include myocarditis, myocardial infarction, infection, heart disease, cardiac arrhythmias, angina, trauma, heart failure, birth defects, cardiac arrest, and atherosclerosis.

Closely Related Organs: Myocardium Myocarditis - Texas Heart Institute Heart Information Center Myocarditis Related terms: inflammation of the wall of the heart, myocardium The myocardium is the muscular wall of the heart, or the heart muscle. It contracts to pump blood out of the heart and then relaxes as the heart refills with returning blood. The myocardium's smooth outer membrane is called the epicardium. Its inner lining is called the endocardium. Myocarditis is an inflammation of the myocardium. When the heart becomes inflamed, it cannot pump as well because of damage to its cells and swelling (edema). The heart muscle may be damaged even more if your body's immune system sends antibodies to try to fight whatever started the inflammation. Sometimes, these antibodies attack the tissues of your heart instead. If too many heart muscle cells are damaged, the heart muscle becomes weakened. In some cases, this process happens very quickly and results in heart failure or even sudden death. More often, the heart tries to heal itself by changing the damaged or dead heart muscle cells into scar tissue. Scar tissue is not like heart muscle tissue because it does not contract and it cannot help the heart to pump.

If enough scar tissue forms in the heart, it can lead to congestive heart failure or dilated cardiomyopathy . What causes myocarditis? Myocarditis is a rare condition. The inflammation of the heart muscle may be caused by A viral, bacterial, or fungal infection. Rheumatic fever , which can occur if the antibodies that your body sends to fight a strep infection attack the tissues of your joints or heart instead. Drug or chemical poisoning. Connective tissue diseases, such as lupus or rheumatoid arthritis. What are the symptoms? With a mild case of myocarditis, you may not feel any symptoms at all. You may have a fever, an achy feeling in your chest, and severe fatigue, as if you have a bad cold or flu. Some people have an irregular heartbeat ( arrhythmia ) or trouble breathing. Usually, a mild case of myocarditis will go away without any lasting damage. Severe cases may not be diagnosed until you have the symptoms of heart failure. Even severe cases may go away without notice, but these severe cases usually cause ongoing and irreversible damage to the heart muscle. How is myocarditis diagnosed? Myocarditis is hard to diagnose because it can resemble many other diseases.

Your doctor may suspect that you have myocarditis if your symptoms have appeared within 6 months of having an infection. A number of tests may be used to confirm the diagnosis: With a stethoscope, doctors can listen for a rapid heartbeat (called tachycardia). What is the Myocardium? (with pictures) What is the Myocardium? Last Modified Date: 17 January 2017 Copyright Protected: You won't believe these 10 facts about people The myocardium is the muscle layer of the heart, responsible for the heart's pumping action, which supplies the entire body with blood. The myocardium consists of cardiac muscle, a type of muscle unlike any other muscle in the body. Cardiac muscle combines features of skeletal muscle, which controls voluntary body movement, and smooth muscle, which controls the movement of all body organs other than the heart. The myocardium is the middle layer of the cardiac wall; the outermost layer is the epicardium, while the innermost is the endocardium . The epicardium consists mostly of connective tissue and serves to protect the inner structures of the heart. The endocardium is a thin layer of epithelial cells , similar to that which lines the inside of blood vessels .

The cardiac muscle that makes up this structure is involuntary, like the smooth muscle in the body's other organs. Involuntary muscle is not under conscious control, and contrasts with voluntary skeletal muscle, which is attached to the skeleton and used for skeletal movement like walking and standing. Cardiac muscle is more similar in structure, however, to skeletal muscle than to smooth muscle. Both cardiac muscle and skeletal muscle are striated, meaning the muscle fibers are arranged into parallel bundles, and have alternating thick and thin protein filaments. Striated muscle is better suited to brief, intense contractions than smooth muscle. While skeletal muscle fibers are arranged into regular, non-branching bundles, the muscle fibers of the myocardium branch at irregular angles, and connect to other muscle cells at junctions called intercalated discs. The cells that make up cardiac muscle are called cardiac myocytes, or cardiomyocytes. They also differ from skeletal muscle cells in that they require extracellular calcium for contraction to take place. The contractions of the myocardium are responsible for pumping oxygenated blood throughout the body, providing the body with the oxygen and other nutrients it needs to function properly. The heart muscle also pumps deoxygenated blood into the lungs so that it can be oxygenated again.

After the blood has delivered oxygen throughout the body, deoxygenated blood returns to the heart, which in turn pumps the blood into the lungs. After the blood is reoxygenated in the lungs, it returns to the heart to be pumped throughout the body once again. Like all body tissues, the myocardium itself requires a blood supply in order to function; the coronary arteries supply the heart muscle with blood. Ad Heart Anatomy: size, location, coverings and layers : Anatomy & Physiology anchors it to surrounding structures prevents overfilling of the heart with blood Deep to the fibrous pericardium is the serous pericardium, a thin, two-layer serous membrane that forms a closed sac around the heart. It’s parietal layer lines the internal surface of the fibrous pericardium. At the superior margin of the heart, the parietal layer attaches to the large arteries exiting the heart, then it turns inferiorly and continues over the external heart surface as the visceral layer, also called the epicardium, which is an integral part of the heart wall. Between the parietal and visceral layers is the pericardial cavity containing a film of serous fluid.

The serous membranes are lubricated by the serous fluid glide past each other, allowing the heart to work in a friction-free environment. Imbalances Peracarditis– inflamation of the pericardium, roughens the serous membrane surfaces. As the beating heart rubs against its pericardial sac, it creates a creaking sound that can be heard with a stethoscope. Peracarditis is characterized by deep pain to the sternum and may lead to impeded heart activity. Cardiac tamponade– when large amounts of inflammatory fluid seep into the pericardial cavity, the excess fluid compresses the heart and limits its ability to pump blood. Physicians treat cardiac tamponade by inserting a syringe into the pericardial cavity and draining excess fluid. Layers of the Heart Wall The heart wall is composed of three layers: the epicardium, myocardium, and endocardium. Location of the heart in the mediastinum. The superficial epicardium is the visceral layer of the serous pericardium. The middle layer is the myocardium and is composed mainly of cardiac muscle and forms the bulk of the heart.

The myocardium is the layer that contracts and is composed of cardiac muscle cells tethered to one another by crisscrossing tissue fibers arranged in circular bundles. These bundles link all parts of the heart together. Connective tissue fibers form a dense network called the fibrous cardiac skeleton that reinforces the myocardium and anchors cardiac muscle fibers. Without this additional support, the vessels and valves might eventually become stretched because of the continuous stress of blood pumping through them. The cardiac skeleton also limits the spread of action potentials because it is not electrically excitable. The pericardial layers and layers of the heart wall The third layer of the heart wall is called the endocardium. It is a white sheet of endothelium (squamous epithelium) resting on a connective tissue layer. It is located on the inner myocardial surface and lines the heart chambers. The endocardium is continuous with the endothelial linings of the blood vessels leaving and entering the heart. In the next section we will discuss chambers, vessels and valves of the heart. Related Posts Anatomy Human Anatomy & Biology Anatomy Information and resources on the anatomy of the human body. Learn about the brain, heart, blood vessels and more.

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Question: Myocardium is the muscular tissue of which part of the human body?
Answer: Heart