# Biomedical Engineering Theory And Practice/Physiological System
## Cardiovascular Structure and Function

As all the cell in the human body could not exchange with nutrients, oxygen, carbon dioxide, and the waste products of metabolism, energy and momentum, the high way network in the physiological system transport the mass between the cell in order to hold all the body. This high way network, called cardiovascular system, includes a pumping station, the heart; a working fluid, blood; a complex branching configuration of distributing and collecting pipes and channels, blood vessels; and a complicated means for intrinsic (inherent) and extrinsic (autonomic and endocrine) control.

### The blood

The blood supplies oxygen and nutrients including constitutional elements to tissues and remove waste products. Blood also transport hormones and other substances to tissues and organs.
Blood consists of plasma(55% of blood volume) and blood cell or hematocytes (approximately, %8±1 of body weight). Hematocytes are suspended in continus plasma fluid and could divided into red blood cells (erythrocytes, totalling nearly 95% of the formed elements), white blood cells(leukocytes, averaging <0.15% of all hematocytes), and platelets (thrombocytes, on the order of 5% of all blood cells). Hematocytes are derived in the active (“red”) bone marrow from undifferentiated stem cells (called hemocytoblasts) and matured through hematocytopoiesis.

### The Heart

The human heart occupies about 0.47% of the body weight and it rests on the diaphragm, between the lower part of the two lungs. This small important organ protected by the third and sixth ribs in the central portion of the thoracic cavity of the body. The heart is divided by a tough muscular wall- the interatrial-interventricular septum.The left side of the heart drives oxygen-rich blood through the aortic semilunar outlet valve into the systemic circulation, which carries the fluid to the whole body. The right side of the heart drives this oxygen-poor blood through the pulmonary semilunar (pulmonic) outlet valve into the pulmonary circulation, which carries the fluid to the lungs. Through breathing, the oxygen is supplied and the carbon dioxide is purged.After that, the blood goes to the heart and the cycles begin all over again.

### The blood vessel

Blood vessels are the ‘road’ of the blood that is distributed inside the body through human circulatory system. Through these vessels, the blood is sent to the whole body and help optimizing the organ’s function. There are three main types of blood vessels: the arteries, which transport the blood away from the heart; the capillaries, which can the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart.
The arteries and veins have three layers
* Tunica intima (the innermost and thinnest layer): It is made up of single layer of simple squamous endothelial cells and is supported by an internal elastic lamina.
* Tunica media (the thickest layer in arteries)made up of smooth muscle cells and elastic tissue. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the level of the vessel.
* Tunica adventitia(the thickest layer in veins) entirely made of connective tissue. It contains nerves that supply the vessel as well as nutrient capillaries.

## Endocrine System

'See also Wikipedia,endocrine system and Human Physiology/The endocrine system
The endocrine system means the collection of glands of an organism that secrete hormones(in other words, produce messengers like small molecules) directly into the circulatory system to be carried toward a distant target organ. In order to grow, maintain a constant temperature, produce offspring, or perform the basic actions and functions, essentially, hormones like small chemicals(in other words, messengers) should enter the blood stream. So, the endocrine system could provides an electrochemical connection from the hypothalamus of the brain to all the organs for controlling the body metabolism, growth and development, and reproduction.
The endocrinology is a relatively long history. But In the late 1960s as sensitive and relatively specific analytical methods introduced, the measurement of low concentrations of circulating hormones is easier and cheaper. Since then, it is easier to understand endocrine physiology and mechanisms of regulation and control. Competitive protein binding and radioimmunoassays brought progress of the study about the physiology of individual endocrine glands and of the neural control of the pituitary gland and the overall feedback control of the endocrine system. Cellular and molecular biology and recombinant DNA technology helped the endocrine system research, too.
At the same time, the interactive researches between mathematical modeling and experimental studies make it possible to understand endocrine dynamics.

### Hormones and signal, interaction between the tissue and the cell

Hormones can be classified into four groups according to the molecular structure and characteristics: (1) steroid hormones, (2) peptide and protein hormones, (3) amino acids derivatives, principally the aromatic amino acid tyrosine, and (4) the eicosanoids (fatty acid derivatives).
1. Steroids are lipids, more specifically, derivatives of cholesterol produced by chemical modification.
2. Peptide and protein hormones are synthesized in the cellular endoplasmic reticulum and then transferred to the Golgi apparatus where they are packaged into secretory vesicles for export.
3. Amino acid derivatives: There are two groups of hormones derived from the amino acid tyrosine;thyroid hormones and catecholamines. Thyroid hormones are basically a “double” tyrosine ring incorporating three or four iodine atoms. Catecholamines include epinephrine and norepinephrine that have the capability of functioning as both hormones and neurotransmitters.
4. Eicosanoids are large groups of polyunsaturated fatty acids derivatives like the prostaglandins, prostacyclins, leukotrienes, and thromboxanes.

### Hormone actions at cell level
## Nervous System
### Nervous System Organization

The nervous system can be defined as the network of nerve cells and fibers that sends messages for controlling movement and feeling between the brain and the other parts of the body. This nervous system is divided into two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The three basic functions of the nervous system:
* Motor output: Respond via muscle or glandular action
* Sensory input: Receive sensations from inside and outside the body
* Integration: Process and interpret sensations and make decisions

#### The Central Nervous System

The CNS consists of the brain and spinal cord. It is integrative and control centers.

#### The Peripheral Nervous System

The peripheral nervous system is the part of the nervous system that is composed of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to make communication lines between CNS and the rest of body. The peripheral nervous system is divided into sensory (afferent) division and motor (efferent) division. The main function of the sensory (afferent) division is to conduct impulses from receptors to the CNS. The sensory division consist of somatic (skin, muscle, joints) and visceral (organs) sensory neurons. The motor (efferent) division is composed of motor neurons. It conducts impulses from the CNS to effectors (muscles and gland). The motor division could be divided into the somatic nervous system and the autonomic nervous system(ANS) whether it is voluntary or not. The somatic nervous system provide voluntary control and conducts impulses from CNS to skeletal muscles. The autonomic nervous systems are involuntary but they can often work in conjunction with the somatic nervous system as ,within both systems, there are inhibitory and excitatory synapses between neurons. The main function of ANS is to conduct impulses from CNS to cardiac muscles, smooth muscles, and glands. The ANS is divided into three main sub-systems: the parasympathetic nervous system (PSNS), sympathetic nervous system (SNS), and the enteric nervous system (ENS). Depending on the circumstances, these sub-systems may work independently or co-operatively. ENS is composed of a mesh-like system of neurons that controls the function of the gastrointestinal system. The parasympathetic system is responsible for stimulation of "rest-and-digest" or "feed and breed". The sympathetic nervous system is related to stimulate activities associated with the fight-or-flight response(also called the fight, flight, freeze, or fawn response [in PTSD], hyperarousal, or the acute stress response).

### Nervous Tissue

The nervous tissue is the main component of the central nervous system and the branching peripheral nerves of the peripheral nervous system. It is densely packed and intertwined and composed of neurons and neuroglial(supporting) cells.

#### Neuroglial cells

Neuroglia cells(also called glial cells) are non-neuronal cells that maintain homeostasis, form myelin, and support and protect neurons in the brain and peripheral nervous system.
In PNS, neuroglial cells consist of Schwann cells,satellite cells and enteric glial cells. The Schwann cells form myelin sheath around large nerve fibers in PNS and is also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons. The Satellite cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia. It may aid in controlling chemical environment of neurons. The enteric glial cells could be found in the intrinsic ganglia of the digestive system. They may have many roles in the enteric system, some related to homeostasis and muscular digestive processes.
In CNS, there are Astrocytes, Microglia, Ependymal Cells and Oligodendrocytes as supporting cells. Astrocytes occupy half of neural volume and project with bulbous ends that cling to neurons and capillaries (therefore connecting neurons to blood/nutrient supply). It controls chemical environment around neurons (buffer K+ in extracellular space and/or recapture neurotransmitters released). Microglia are the resident macrophages of the brain and spinal cord and act as the first and main form of active immune defense in the CNS. Oligodendrocytes provide support and insulation to axons in the central nervous system of some vertebrates, equivalent to the function of Schwann cells in the PNS. Oligodendrocytes do this by creating the myelin sheath, which is 80% lipid and 20% protein. Ependymal cells is the thin epithelium-like lining the spinal cord and the ventricular system of the brain. They creates a barrier between CNS cavities and tissues surrounding cavities. Their cilia circulates the cerebrospinal fluid and protect the brain.

#### Neurons

The neurons is an electrically excitable cell that send messages through electrochemical processes.The human brain has approximately 100 billion neurons. These neurons are amitotic and have a high metabolic rate. Neurons are structurally composed of cell body(soma) and one or more process.
* The soma is the body of the neuron. As it contains the nucleus, most protein synthesis occurs here. The nucleus can range from 3 to 18 micrometers in diameter.Most neuron cell bodies located within CNS.Clusters of cell bodies in CNS are called nuclei. Few/clusters of cell bodies in PNS are called ganglia
* Processes are called either tracts (in CNS) or nerves (in PNS). Neurons have specialize cell parts called dendrites and axons.
** Dendrites are cellular extensions with many branches. Dendrites receive chemical signals as well as conduct electrical signals (graded potentials).
** Axons transmit graded potential away from cell body to axonal terminal. Many neurons have only one axon, but this axon may keep extensive branching, helping communication with many target cells. The part of the axon from the soma is called the axon hillock. The axon hillock is the part of the neuron that has the greatest density of voltage-dependent sodium channels. This makes it the most easily excited part of the neuron and the spike initiation zone for the axon: in electrophysiological terms it has the most negative action potential threshold.
* Some neurons can be repaired through the processes of fragmentation, proliferation. elongation.
* Electrochemical signals transmitted with the help of myelin sheath (protein-lipoid) which insulates nerve fibers (long axons) and enhance the transmission. Nodes of Ranvier also help the transmission of nerve impulses. Myelinated processes form the white matter of nervous tissue and unmyelinated processes form the gray matter of nervous tissue.
* The axon terminal includes synapses where neurotransmitter chemicals are released to communicate with target neurons.
* Fully differentiated neurons are permanently postmitotic;

#### Classification of Neurons
##### Structural Classification

* Anaxonic:Axons can be indistinguishable from dendrite. They can be found in brain. Their functions are poorly understood.
* Multipolar neurons:They possesses a single (usually long) axon and many dendrite. Three or more processes (usually with a single axons) are needed. They are common type, major neurons in CNS.
** Golgi I: neurons with a long axon that begins in the grey matter of CNS and may extend from there . Examples are pyramidal cells, Purkinje cells, and anterior horn cells.
** Golgi II:Neurons have either no axon or else a short axon that does not send branches out of the gray matter of CNS. The best example is the granule cell.
* Bipolar neurons :Axon and single dendrite on opposite ends of the soma.They are rare in adult but may be found in retina and olfactory mucosa.
* Unipolar neurons or pseudounipolar:Only one protoplasmic process (neurite) extends from the cell body and forms CNS and PNS. Most neurons are multipolar. But,many types of primary sensory neurons are pseudounipolar.

##### Functional Classification

* Sensory (afferent) neurons:They carry nerve impulses from receptors or sense organs toward CNS.
* Motor (efferent) neurons: receive signals from the brain and spinal cord to cause muscle contractions and affect glandular outputs/
* Association neurons (interneurons):Neurons that forms a connection between other neurons.

#### Nerve Fibre and Connective Tissue

A nerve contains two types of tissue: nerve fiber and connective tissue. Nerve fibre is the organ that make up peripheral connective tissue(PNS). It consists of an axon or long dendrite, myelin sheath (if existent) and their Schwann cells. They serve as an information pipelines that let the brain and the spinal cord communicate with the other tissues and organs.

## Vision System

'See also Wikipedia,Visual System
The visual system is composed of three things: the central nervous system, eyes and light. It detects and interprets the information about the visual objects from visible light and guide body movements in relation to visual objects.

### Eye

The light reflects off the visual image and comes back to your eye. Light then enters through the outer part of the eye, called the cornea. The cornea is clear like a window. The cornea helps the eye to focus. “To focus” means to make things look sharp and clear as the film and the electronic sensor in the camera does. After then, the light rays go through an opening called the pupil which is the dark round circle in the middle of the colored part of your eye . The colored part is called the iris. When the light is bright, the iris loses the pupil until the right amount of light gets in. When the light is dim, the iris is reverse to let in more light. The function of iris in eyes is the same to the iris in camera. The eye has a lens to focus the rays of light. The lens of the eye is behind the iris. Light passes through the lens on its way to the back of the eye.

### Retina

In the back of eyes, lining the inside of the eye is the retina.The retina includes 130 million tiny photoreceptor cells which contain particular protein molecules called opsins. An opsin absorbs a photon (a particle of light) and transmits a signal to the cell through a signal transduction pathway. In human opsin, two types of opsins participate in conscious vision: rod opsins and cone opsins. Rod opsins (rhodopsins, usually denoted Rh),employed in night vision, are thermally stable, and are found in the rod photoreceptor cells. Cone opsins, used in color vision, are less-stable opsins in the cone photoreceptor cells. Cone opsins could be subdivided according to their absorption maxima (λmax), the wavelength at which the highest light absorption is observed. So, humans have four opsins as follows:
* Rhodopsin (Rh1, OPN2, RHO) – expressed in rod cells, used in night vision
* Three cone opsins (also known as photopsins) – expressed in cone cells, used in color vision
** Long Wavelength Sensitive (OPN1LW) Opsin – λ$_{max}$ in the red region of the electromagnetic spectrum. Despite its name, this receptor has a secondary response in the violet high frequencies
** Middle Wavelength Sensitive (OPN1MW) Opsin – λ$_{max}$ in the green region of the electromagnetic spectrum
** Short Wavelength Sensitive (OPN1SW) Opsin – λ$_{max}$ in the blue region of the electromagnetic spectrum
In the retina, the photo-receptors synapse directly onto bipolar cells, which in turn synapse onto ganglion cells of the outermost layer, which will then conduct action potentials to the brain. Based on their projections and functions, there are five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain:
* Midget cells with small recepter field(Parvocellular, or P pathway; P cells):About 80% of all retinal ganglion cells are midget cells in the parvocellular pathway. They receive inputs from relatively few rods and cones. In many cases, they are connected to midget bipolars, which are linked to one cone each. They have slow conduction velocity.They can respond to the changes in color but less sensitive to contrast
* Parasol cell (Magnocellular, or M pathway; M cells):About 10% of all retinal ganglion cells are parasol cells, and these cells are part of the magnocellular pathway. They receive inputs from relatively many rods and cones. They have fast conduction velocity, and can respond to low-contrast stimuli, but are not very sensitive to changes in color
* K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth;
* Photosensitive ganglion cells
* Other ganglion cells projecting to the superior colliculus for eye movements (saccades)

### Brain
## Auditory System
### The Peripheral Auditory System
#### Outer ear

The sounds are collected by the pinna, the visible part of the external ear and guided to the middle ear by the external auditory canal, a deceptively simple tube. The ear canal amplifies sounds between 3 and 12 kHz. At the far end of the ear canal is the tympanic membrane, which marks the beginning of the middle ear.

#### Middle ear

The middle ear is composed of the ear drum (tympanic membrane), attached to the inner ear through a delicate bone structure (malleus, incus and stapes). The middle ear bones (ossicles) and the muscles which keeps them in place are the smallest in the human body. One of the main functions of the middle ear is to transfer the sound from the air to the fluids in the inner ear efficiently. If the sound were to have an impact directly on the inner ear, most of it would simply be reflected back because acoustical impedance of the air is different from that of the fluids.
The middle ear behaves as an impedance-matching device that improves sound transmission, reduces the amount of reflect sound and protects the inner ear from excessive sound pressure. This protection is controlled by the brain through the middle ear’s muscles to tense and untense the bone structure with a reaction speed as fast as 10 milliseconds. The middle ear’s connection to the inner ear is the smallest bone in the human body: the stapes (or stirrup bone). It is about 3 mm long and weighs about 3 mg.

#### Inner ear

The inner ear consists of the cochlea and the auditory nerve for hearing and vestibular system for balance. The cochlea, snail-shaped, bony structure, converts sound pressure patterns from the outer ear into electrochemical impulses which are passed on to the brain via the auditory nerve. The vestibular system consists of a series of fluid-filled compartments (three semi-circular canals and two larger divisions) that contain the sense organs for balance and movement. The vestibular sensors detect angular movements, direction and velocity of the head. This information about equilibrium is sent to the brain by the vestibular nerves.

### The Central Auditory System

The encoded sound information enter the vestibulocochlear nerve, through intermediate stations such as the cochlear nuclei and superior olivary complex of the brainstem and the inferior colliculus of the midbrain and it is further processed at each waypoint. Finally, the information reaches the thalamus, and from there it is relayed to the cortex. In the human brain, the primary auditory cortex is placed in the temporal lobe.

## Gastrointestinal System

The gastrointestinal tract (GIT) is composed of a hollow muscular tube from the oral cavity, where food enters the mouth, continuing through the pharynx, oesophagus, stomach and intestines to the rectum and anus, where food is expelled. There are a variety of accessory organs that help the tract by secreting enzymes to enhance breaking down food into its component nutrients. Thus the salivary glands, liver, pancreas and gall bladder have functions in the digestive system. Food is propelled along the length of the GIT by peristaltic movements of the muscular walls.
The whole digestive tract is about nine metres long. The tract is divided into upper and lower tracts, and the intestines parts.

### Oral cavity

Oral cavity is known as mouth buccal cavityor in Latin cavum oris.The oral cavity is the first part of the alimentary canal that take food and saliva. It is lined by a stratified squamous oral mucosa with keratin covering those areas subject to significant abrasion, such as the tongue, hard palate and roof of the mouth. Mastication indicates the mechanical breakdown of food by chewing and chopping actions of the teeth. The tongue, a strong muscular organ, let the food bolus to come in contact with the teeth. It is also the sensing organ of the mouth for touch, temperature and taste using its specialised sensors known as papillae.
Human saliva is 99.5% water, while the other 0.5% consists of electrolytes, mucus, glycoproteins, enzymes, and antiseptic compounds such as secretory IgA and lysozyme.
Insalivation means the mixing of the oral cavity contents with salivary gland secretions. The mucin (a glycoprotein) in saliva acts as a lubricant. The oral cavity plays a limited role in the digestion of carbohydrates. The enzyme serum amylase, a component of saliva, starts the process of digestion of complex carbohydrates. The final function of the oral cavity is to absorb small molecules such as glucose and water, across the mucosa. From the mouth, food passes through the pharynx and oesophagus via the action of swallowing.

### Salivary glands

Three pairs of salivary glands work with the oral cavity. Each is a complex gland with a lot of acini lined by secretory epithelium. The acini secrete their contents into specialized ducts. Each gland is divided into smaller lobes. Salivation occurs because of the taste, smell or even watching food. This occurs in response to nerve signals that indicate the salivary glands to secrete saliva to prepare and moisten the mouth. Each pair of salivary glands secretes saliva with slightly different compositions. They also secrete amylase, an enzyme that degrades starch into maltose.

#### Parotids

The parotid gland is a major salivary gland in humans. The parotid glands are largest(in saliva gland), bilateral and irregular shaped glands located inferior and anterior to the external acoustic meatus draining their secretions into the vestibule of oral cavity through Stensen duct or parotid duct. They provides 25% of the total salivary volume. They are situated below the zygomatic arch (cheekbone) and cover part of the mandible (lower jaw bone). An enlarged parotid gland can be easier felt when one clenches their teeth. The parotids secretes salivary alpha-amylase (sAA). Immunoglobins are secreted help to fight microorganisms and a-amylase proteins start to break down complex carbohydrates

#### Submandibular

The paired submandibular glands or submaxillary glands are major salivary glands located beneath the lower jaws, superior to the digastric muscles. They secrete around 60–67% of the total volume of saliva although they are much smaller than the parotid glands. These glands produce a more viscostic(thick) secretion, rich in mucin and with a little bit protein. Mucin is a glycoprotein which act as the lubrication of the food bolus as it travels through the esophagus..

#### Sublingual

The sublinguals are the smallest salivary glands, covered by a thin layer of tissue at the floor of the mouth. They produce only about 5% of the saliva volume. Their secretions produced are very sticky due to the large amount of mucin. They aid in buffering and lubrication.

### Upper gastrointestinal tract

The upper gastrointestinal tract is composed of the esophagus, stomach, and duodenum.

#### Esophagus

It is known as the foodpipe or gullet. The Esophagus is a fibromuscular tube of about 25cm in length and 2cm in diameter. It extends generally from around the level of the sixth cervical vertebra (C6) behind the cricoid cartilage, enters the diaphragm at about the level of the tenth thoracic vertebra (T10), and ends at the cardia of the stomach, at the level of the eleventh thoracic vertebra (T11). The wall of the oesophagus from the lumen outwards is made up of mucosa, sub-mucosa (connective tissue), layers of muscle fibers between layers of fibrous tissue, and an outer layer of connective tissue.This muscle are supplied by the oesophageal nerve plexus. This nerve plexus surrounds the lower portion of the oesophagus. The oesophagus functions primarily as a transport medium between compartments.The esophagus has a rich blood supply and vascular drainage. It is clinically investigated through X-rays using barium, endoscopy, and CT scans.

#### Stomach

The stomach is a J shaped expanded bag, located just left of the midline between the oesophagus and the duodenum (the first part of the small intestine). It secretes protein-digesting enzymes called proteases and strong acids to help food digestion through segmentation before sending partially digested food (chyme) to the small intestines.
It is divided into four main regions and has two borders called the greater and lesser curvatures.
* The first section is the cardia which surrounds the cardial orifice where the oesophagus enters the stomach,the point at which the epithelium changes from stratified squamous epithelia to columnar.
* The fundus is formed by the upper curvature of the organ and the superior, dilated portion of the stomach that has contact with the left dome of the diaphragm.
* The body is the largest center section between the fundus and the curved portion of the J. This is where most gastric glands are located and most mixing of the food occurs.
* The pylorus is the curved base of the stomach. Gastric contents are expelled into the proximal duodenum via the pyloric sphincter. The inner surface of the stomach is contracted into numerous longitudinal folds called rugae. These allow the stomach to stretch and expand when food enters. In adult humans, the stomach has a relaxed, near empty volume of about 45 to 75 milliliters. When it expands, it normally does to hold about one litre of food. The stomach of a newborn human baby could retain about 30 milliliters.
The functions of the stomach is as follows:
* The short-term storage of ingested food.
* Breakdown of food by churning and mixing.
* Chemical digestion of proteins by acids and enzymes.
* Stomach acid kills bugs and germs.
* Some absorption of food such as alcohol.

#### Liver

The liver is a large, reddish-brown organ located in the right upper quadrant of the abdomen. It is covered by a strong capsule and divided into four lobes namely the right, left, caudate and quadrate lobes.The liver function is various by the liver cells or hepatocytes. Currently, there is no artificial organ or device conducting all the functions of the liver. Some functions can be carried out by liver dialysis, an experimental treatment for liver failure.
But the liver is the only human internal organ could naturally regenerate from lost tissue; as little as 25% of a liver can regenerate into a whole liver. Regeneration is very speedy. The liver would return to a normal size within two weeks although the removal is greater than 50% of the liver by mass.This is due to the hepatocytes which go from the quiescent G0 phase to the G1 phase and undergo mitosis. This process is activated by the p75 receptors.
The liver is thought to have over 500 separate functions, usually working with other systems and organs.

#### Gall bladder

The gallbladder is a small organ where bile is stored, before it is released into the small intestine. It is a hollow, pear shaped organ that sits in a depression on the posterior surface of the liver’s right lobe.In adults, the gallbladder is 8cm(3.1 in) in length and 4cm(1.6 in) in diameter when fully extended. The gallbladder can store about 100mL.
It is divided into a fundus, body and neck . It empties via the cystic duct into the biliary duct system. The main functions of the gall bladder are storage and concentration of bile produced by liver. Bile is released from the gall bladder by contraction of its muscular walls in response to hormone signals from the duodenum because of food.
The Gallbladder bile is composed of 92% water, 6% bile salts, 0.3% bilirubin, 0.9-2.4% fats (Cholesterol, fatty acids and lecithin), and 200 mEq/L inorganic salts.

#### Pancreas

The pancreas is a lobular, pinkish-grey organ located in the abdominal cavity behind the stomach. Its head communicates with the duodenum and its tail extends to the spleen. The organ is about 5.75-9.5 cm in length with a long, slender body connecting the head and tail segments. The pancreas has both exocrine and endocrine functions.
Endocrine indicates production of hormones which occurs in about a million cell clusters called islets of Langerhans. Four main cell types exist in the islets and can be classified by their secretion: α cells secrete glucagon (increase glucose in blood), β cells secrete insulin (decrease glucose in blood), Δ cells secrete somatostatin (regulates/stops α and β cells) and PP cells, or γ (gamma) cells, secrete pancreatic polypeptide. . The exocrine (secretrory) portion occupies 80-85% of the pancreas.
It is composed of numerous acini (small glands) that secrete fluid into ducts which eventually lead to the duodenum. The fluid contains digestive enzymes that pass to the small intestine. These enzymes enhance breaking down the carbohydrates, proteins and lipids (fats) in the chyme.

### Lower gastrointestinal tract

The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. The small intestine consists of the duodenum, jejunum and ileum while the large intestine consists of the cecum, colon, rectum, and anal canal.

#### Small Intestine

The small intestine starts from the duodenum, which receives food from the stomach. It averages 5.5~6m in length, extending from the pyloric sphincter of the stomach to the ileo-caecal valve separating the ileum from the caecum. The small intestine is compressed into a lot of folds and occupies a large portion of the abdominal cavity.
* Duodenum: The duodenum is the proximal C-shaped hollow jointed tube(about 25–38 cm long) that curves around the head of the pancreas . It receives gastric chyme from the stomach, together with digestive liquids from the pancreas (digestive enzymes) and the gall bladder (bile). The digestive enzymes break down proteins and bile and emulsify fats into micelles.
* Jejunum:The jejunum is about 2.5 m long and located between the distal end of duodenum and the proximal part of ileum. It contains the plicae circulares (also called circular folds or valves of Kerckring), and villi that increase the surface area of the GI Tract. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream. It is in the jejunum where the majority of digestion and absorption occurs.
* Ileum:The ileum is the longest segment and contains villi similar to the jejunum. Its function is to absorb vitamin B12 and bile salts.For fat-soluble vitamin(Vitamin A, D, E and K) absorption, bile acids are necessary.

#### Large Intestine

The large intestine is horse-shoe shaped and extends around the small intestine like a frame. Its function is to absorb body water from the matter, and then to pass useless waste material from the body. It is subdivided into the appendix, caecum, ascending, transverse, descending and sigmoid colon, and the rectum. It has a length of about 1.5m and a width of 7.5cm.

### Layers of Gastrointestinal Tract(GI)

The gastrointestinal tract is a muscular tube lined by epithelium, a special layer of cells. The contents of the tube are considered external to the body and are continuous with the outside at the mouth and the anus. The GI tract can be divided into four concentric layers as follows :
* Mucosa
* Submucosa
* Muscularis externa (the external muscular layer)
* Adventitia or serosa

#### Mucosa

The innermost layer of the gastrointestinal tract is surrounding the lumen, or open space within the tube. This layer contacts with digested food (chyme). The mucosa cosists of:
* Epithelium: It is supported by an underlying connective tissue layer called the lamina propria. Areas like the mouth and oesophagus are covered by a stratified squamous (flat) epithelium so they can survive the wear and tear of passing food. Simple columnar (tall) or glandular epithelium lines the stomach and intestines to help secretion and absorption.
* Lamina propria: A layer of connective tissue. It contains blood vessels, nerves, lymphoid tissue and glands that support the mucosa.
* Muscularis mucosae: Beneath the lamina propria is a thin layer of smooth muscle called the muscularis mucosa which helps passing material and aids the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis.

#### Submucosa

The submucosa surrounds the muscularis mucosa and consists of fat,dense, irregular fibrous connective tissue and larger vessels , lymphatics, and nerves branching into the mucosa. At its outer margin there is a specialized nerve plexus(enteric nervous plexus) called the submucosal plexus or Meissner plexus. This supplies the mucosa and submucosa.

#### Muscularis externa

This smooth muscle layer consists of inner circular and outer longitudinal layers of muscle fibres separated by the myenteric plexus or Auerbach plexus. Neural innervations control the contraction of these muscles. The coordinated contractions of these layers is called peristalsis and moves the food down through the tract. From the mouth down to the stomach, Food is called a bolus(ball of food). After the stomach, the food is partially digested and semi-liquid, and is called as chyme. In the large intestine the remaining semi-solid substance is referred to as faeces.

#### Adventitia or serosa

Adventitia is the outermost connective tissue covering of an organ, vessel, or other structure. It is also called the tunica adventitia or the tunica externa. In GI tract, the muscularis externa is covered mostly by serosa. But, at the oral cavity, thoracic esophagus, ascending colon, descending colon and the rectum, the muscularis externa is bounded by adventitia. (The muscularis externa of the duodenum is covered by both tissue types.) Generally, if it belong to the digestive tract that is free to move, it is bounded by serosa, but if it is relatively rigidly fixed, it is covered by adventitia.
See also Wikipedia,Electrogastrogram

## Respiratory System

The Respiratory System is indispensable to every human. Through the respiratory system, oxygen enters our bodies and carbon dioxide leaves our bodies. The respiratory system is composed of the air pathways,the lungs, alveoli, pulmonary vasculature, respiratory muscles, and surrounding tissues and structures.

### Respiratory System

* Lungs:There are two lungs in the human chest; the right lung is made up of three incomplete divisions called lobes, and the left lung has two, the space for the heart.
* Conducting Air pathway: Before gas exchange and respiration, Air should be moved, filtered, warmed and humidified.
* Alveoli:The gas change occurs.For efficiency, the alveolar walls is very thin.
* Pulmonary circulation: the part of the cardiovascular system which carries oxygen-poor blood away from the heart, to the lungs, and returns oxygen-rich blood back to the heart.
* Respiratory muscles: the various muscles of respiration participate in both inspiration and expiration. The main muscles are the diaphragm, the external intercostal and the interchondral part of the internal intercostal muscles. Both the external intercostal muscles and the intercondral elevate the ribs, so increasing the width of the thoracic cavity, while the diaphragm contracts to increase the vertical dimensions of the thoracic cavity, and also helps in the elevation of the lower ribs. Accessory muscles are usually used when the body needs to process energy quickly.

## Practise
### Fill in the gap

* Blood consists of liquid portion (plasma) and __a___different kinds of cells. ((a) two (b) three)
* The clotting of blood is done by___b_. ((a)thrombocyctes (b) leucocytes)

## Reference

* ↑ Template:DorlandsDict
* ↑ Allostatic load notebook: Parasympathetic Function - 1999, MacArthur research network, UCSF
* ↑ John Barton Furness (15 April 2008). The Enteric Nervous System. John Wiley & Sons. ISBN 978-1-4051-7344-5. Retrieved 16 January 2013.
* ↑ Jessen, Kristjan R. & Mirsky, Rhona Glial cells in the enteric nervous system contain glial fibrillary acidic protein Nature 286, 736–737 (14 August 1980); doi:10.1038/286736a0
* ↑ Jessen, K. R. & Mirsky, R. (2005), "The origin and development of glial cells in peripheral nerves", Nature Reviews Neuroscience 6 (9): 671–682
* ↑ Hanani, M. Satellite glial cells in sensory ganglia: from form to function. Brain Res. Rev. 48:457–476, 2005
* ↑ Bassotti, G. et al, Laboratory Investigation (2007) 87, 628–632
* ↑ Lawson L J, Perry V H, Gordon S (1992). "Turnover of resident microglia in the normal adult mouse brain". Neuroscience. 48: 405–415. doi:10.1016/0306-4522(92)90500-2.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Gehrmann J, Matsumoto Y, Kreutzberg GW (1995). "Microglia: intrinsic immuneffector cell of the brain". Brain Research Reviews. 20 (3): 269–287. doi:10.1016/0165-0173(94)00015-H. PMID 7550361.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Carlson, Physiology of Behavior, 2010
* ↑ Johansson CB, Momma S, Clarke DL, Risling M, Lendahl U, Frisén J (January 1999). "Identification of a neural stem cell in the adult mammalian central nervous system". Cell. 96 (1): 25–34. doi:10.1016/S0092-8674(00)80956-3. PMID 9989494.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ Chudler, Eric H. "Brain Facts and Figures". Neuroscience for Kids. Retrieved 2009-06-20.
* ↑ Herrup K, Yang Y (2007). "Cell cycle regulation in the postmitotic neuron: oxymoron or new biology? (Ma3371545712287 Content-Dispositin2124)". Nat. Rev. Neurosci. 8 (5): 368–78. doi:10.1038/nrn2124. PMID 17453017.
* ↑ "Golgi type I neuron definition". Dictionary.com. 2008. Retrieved 2008-12-25.
* ↑ "Golgi type II neuron definition". Dictionary.com. 2008. Retrieved 2008-12-25.
* ↑ Martin, John Harry (2003). Neuroanatomy. McGraw-Hill Professional. ISBN 0-07-138183-X.
* ↑ Mathpages http://www.mathpages.com/home/kmath579/kmath579.htm
* 1 2 University of California excerpts from "Theory of Color"
* ↑ Mathpages http://www.mathpages.com/home/kmath579/kmath579.htm
* ↑ "eye, human."Encyclopædia Britannica. 2008.
* 1 2 3 Principles of Neural Science 4th Ed. Kandel et al.
* ↑ Kong F, Singh RP (June 2008). "Disintegration of solid foods in human stomach". J. Food Sci. 73 (5): R67–80. doi:10.1111/j.1750-3841.2008.00766.x. PMID 18577009.
* ↑ "Length of a Human Intestine". Retrieved 2 September 2069. {{cite web}}: Check date values in: |accessdate= (help)
* ↑ Gray, Henry (1918). "2a. The Mouth". Gray's Anatomy. {{cite book}}: Cite has empty unknown parameter: |1= (help)
* ↑ Maton, Anthea; Jean Hopkins; Charles William McLaughlin; Susan Johnson; Maryanna Quon Warner; David LaHart; Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1.
* ↑ Physiology at Medical College of George Section 6 Chapter4
* ↑ Edgar, M.; Dawes, C.; O'Mullane, D. (2004). Saliva and Oral Health (3rd ed.). British Dental Association. ISBN 0-904588-87-4.
* ↑ Human Anatomy, Jacobs, Elsevier, 2008, page 193
* ↑ Arhakis A, et al., Salivary Alpha-Amylase Activity and Salivary Flow Rate in Young Adults, The Open Dentistry Journal, 2013, 7, 7-15 at http://benthamscience.com/open/todentj/articles/V007/7TODENTJ.pdf
* ↑ Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, p. 155
* ↑ Illustrated Dental Embryology, Histology, and Anatomy, Bath-Balogh and Fehrenbach, Elsevier, 2011, page 137
* ↑ Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, p. 156
* ↑ Upper Gastrointestinal Tract at the US National Library of Medicine Medical Subject Headings (MeSH)
* ↑ Davidson's Principles and Practice of Medicine (21st ed. ed.). Edinburgh: Churchill Livingstone/Elsevier. 2010. p. 838. ISBN 978-0-7020-3084-0. {{cite book}}: |edition= has extra text (help); |first= has generic name (help); |first= missing |last= (help)
* 1 2 3 Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W.M. Mitchell ; illustrations by Richard; Richardson, Paul (2005). Gray's anatomy for students. Philadelphia: Elsevier/Churchill Livingstone. pp. 192–194. ISBN 978-0-8089-2306-0.{{cite book}}: CS1 maint: multiple names: authors list (link)
* ↑ BBC news article
* ↑ Sherwood, Lauralee (1997). Human physiology: from cells to systems. Belmont, CA: Wadsworth Pub. Co. ISBN 0-314-09245-5. OCLC 35270048.
* ↑ Dieter Häussinger, ed. (2011). Liver Regeneration. Berlin: De Gruyter. p. 1. ISBN 9783110250794.
* ↑ Suzuki K, Tanaka M, Watanabe N, Saito S, Nonaka H, Miyajima A (2008). "p75 Neurotrophin receptor is a marker for precursors of stellate cells and portal fibroblasts in mouse fetal liver". Gastroenterology. 135 (1): 270–281.e3. doi:10.1053/j.gastro.2008.03.075. PMID 18515089.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ "Where is the Gallbladder Located in the Body". Buzzle.com. 2013-02-28. Retrieved 2013-08-18.
* ↑ Jon W. Meilstrup (1994). Imaging Atlas of the Normal Gallbladder and Its Variants. Boca Raton: CRC Press. p. 4. ISBN 0-8493-4788-2.
* ↑ Deakin, Barbara Young ... ; drawings by Philip J.; et al. (2006). Wheater's functional histology : a text and colour atlas (5th ed. ed.). [Edinburgh?]: Churchill Livingstone/Elsevier. p. 298. ISBN 978-0-443-06850-8. {{cite book}}: |edition= has extra text (help); Explicit use of et al. in: |first= (help)
* ↑ Guyton and Hall (2011). Textbook of Medical Physiology. U.S.: Saunders Elsevier. p. 784. ISBN 978-1-4160-4574-8. Bile acts to some extent as a surfactant, helping to emulsify the fats in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side
* ↑ Khan, Ali Nawaz. "Chronic Pancreatitis Imaging". Medscape. Retrieved 5 January 2014.
* ↑ Hellman B, Gylfe E, Grapengiesser E, Dansk H, Salehi A (2007). "[Insulin oscillations--clinically important rhythm. Anti-diabetics should increase the pulsative component of the insulin release]". Lakartidningen (in Swedish). 104 (32–33): 2236–9. PMID 17822201.{{cite journal}}: CS1 maint: multiple names: authors list (link)
* ↑ BRS physiology 4th edition ,page 255-256, Linda S. Constanzo, Lippincott publishing
* ↑ National Library of Medicine,2011 MeSH,Lower Gastrointestinal Tract
* ↑ Kapoor, Vinay Kumar (13 Jul 2011). Gest, Thomas R. (ed.). "Large Intestine Anatomy". Medscape. WebMD LLC. Retrieved 2013-08-20.
* ↑ Gray, Henry (1918). Gray's Anatomy. Philadelphia: Lea & Febiger.
* ↑ http://www.britannica.com/EBchecked/topic/275485/human-body
* ↑ van Gijn J; Gijselhart JP (2011). "Treitz and his ligament". Ned Tijdschr Geneeskd. 155 (8). PMID 21557825. {{cite journal}}: Invalid |display-authors=2 (help); Unknown parameter |author-separator= ignored (help)
* ↑ The Gale Group, Inc. (2008). Gale Encyclopedia of Medicine
* ↑ "large intestine". NCI Dictionary of Cancer Terms. National Cancer Institute, National Institutes of Health. Retrieved 2014-03-04.
* ↑ Virtual Medical Center, Gastrointestinal System
* ↑ Abraham L. Kierszenbaum (2002). Histology and cell biology: an introduction to pathology. St. Louis: Mosby. ISBN 0-323-01639-1.
* ↑ eMedicineDictionary, additional text.