Circulatory System

The circulatory system is a major organ system of the body. The circulatory system transports oxygen and nutrients in the blood to all of the cells in the body. In addition to transporting nutrients, this system also picks up wastes produced by metabolic processes and delivers them to other organs for disposal. The circulatory system, sometimes called the cardiovascular system, consists of the heart, blood vessels, and blood. The heart provides the "muscle" needed to pump blood throughout the body. Blood vessels are the conduits through which blood is transported and blood contains the valuable nutrients and oxygen that are needed to sustain tissues and organs. The circulatory system circulates blood in two circuits: the pulmonary circuit and systemic circuit.

## Circulatory System Function

The circulatory system provides several necessary functions in the body. This system works in conjunction with other systems to keep the body functioning normally. The circulatory system makes respiration possible by transporting carbon dioxide to the lungs and delivering oxygen to cells. The circulatory system works with the digestive system to carry nutrients processed in digestion (carbohydrates, proteins, fats, etc.) to cells. The circulatory system also makes cell to cell communication possible and regulates internal body conditions by transporting hormones, produced by the endocrine system, to and from targeted organs. The circulatory system helps to remove wastes by transporting blood to organs such as the liver and kidneys. These organs filter waste products, such as ammonia and urea, which are removed from the body via the excretory system. The circulatory system is also a major means of transportation throughout the body for germ-fighting white blood cells of the immune system.

## Pulmonary and Systemic Circuits

### Pulmonary Circuit

The pulmonary circuit is the path of circulation between the heart and the lungs. Blood is pumped to the various places of the body by a process known as the cardiac cycle. Oxygen depleted blood returns from the body to the right atrium of the heart by two large veins called vena cavae. Electrical impulses produced by cardiac conduction cause the heart to contract. As a result, blood in the right atrium is pumped to the right ventricle. On the next heart beat, the contraction of the right ventricle sends the oxygen-depleted blood to the lungs via the pulmonary artery. This artery branches into left and right pulmonary arteries. In the lungs, carbon dioxide in the blood is exchanged for oxygen at lung alveoli. Alveoli are small air sacs that are coated with a moist film that dissolves air. As a result, gasses can diffuse across the thin endothelium of the alveoli sacs. The now oxygen-rich blood is transported back to the heart by the pulmonary veins. The pulmonary veins return blood to the left atrium of the heart. When the heart contracts again, this blood is pumped from the left atrium to the left ventricle.

### Systemic Circuit

The systemic circuit is the path of circulation between the heart and the rest of the body (excluding the lungs). Oxygen-rich blood in the left ventricle leaves the heart via the aorta. This blood is circulated to the rest of the body by various major and minor arteries.

* **Coronary Arteries:** blood vessels that branch off from the ascending aorta. They supply blood to the heart.
* **Brachiocephalic Artery:** supplies blood to the head, neck and arms.
* **Celiac Artery:** supplies blood to the abdominal area.
* **Splenic Artery:** supplies blood to the spleen, stomach, and pancreas.
* **Renal Arteries:** supply blood to the kidneys.
* **Common Iliac Artery:** supplies blood to the legs and feet.

Gas, nutrients, and waste exchange between blood and body tissues takes place in the capillaries. Blood flows from arteries to smaller arterioles and on to the capillaries. In organs such as the spleen, liver, and bone marrow that do not have capillaries, this exchange occurs in vessels called sinusoids. After passing through the capillaries or sinusoids, the blood is transported to venules, to veins, to the superior or inferior vena cavae, and back to the heart.

### Lymphatic System and Circulation

The lymphatic system contributes significantly to the functioning of the circulatory system by returning fluid to the blood. During circulation, fluid gets lost from blood vessels at capillary beds and seeps into the surrounding tissues. Lymphatic vessels collect this fluid and direct it toward lymph nodes. Lymph nodes filter the fluid of germs and the fluid is eventually returned to blood circulation through veins located near the heart.