The Circulatory System

The circulatory system has the major function of circulating blood that transports oxygen and nutrients throughout the body. Major organs are the lungs, heart, and blood vessels. The functional unit are the red blood cells.

Red Blood Cell

Mammals have a closed circulatory system called the cardiovascular system. The heart has one or two atria, chambers that take in blood that returns to the heart and one or two ventricles, chambers that pump blood out of the heart. Blood vessels contain arteries, veins, and capillaries. Arteries carry blood away from the heart and towards organs, and they branch into arterioles into capillaries in organs.

Capillaries are microscopic and porous vessels. Capillary beds are the networks that infiltrate tissues. Chemicals are diffused through the capillary walls. Capillaries converge into venules that converge into veins which carry blood back to the heart.

Heart Diagram

The cardiac cycle is the complete sequence of heart pumping and filling. The contraction phase is called the systole and the relaxation is called diastole. Cardiac output is the amount of blood/minute that left the ventricle pumps into the systemic circuit. This depends on the heart rate (#beats per minute) and stroke volume (amount of blood pumped by the left ventricle in each contraction). The heart has four valves with flaps of connective tissue to prevent back-flow. Between each atrium and ventricle is an atrioventricular valve which are held by fibers and the pressure by ventricle contraction closes the valves. Semi-lunar valves are at the 2 exits of the heart, the aorta leaves the left ventricle and the pulmonary artery leaves the right ventricle. These open by pressure by ventricle contraction.

Blood flow will begin at the pulmonary lung circuit, where the right ventricle pumps blood to the lungs through the pulmonary arteries. Blood flows through capillary beds in the lungs gets oxygen and trashes CO2. The oxygen rich blood uses the pulmonary veins to return from the lungs to the left atrium. The blood flows into the left ventricle as it opens and then contracts, pumping the blood out to body tissues through the systematic circuit.

Blood leaves the left ventricle through the aorta, which then supplies blood to arteries throughout the body. Coronary arteries branch from the aorta supplying blood to the heart itself. Branches lead to capillary beds in forelimbs and the head. The aorta then goes to the posterior arteries to arterioles and capillary beds in the abdomen, organs, and the legs. Oxygen and carbon dioxide diffuse in the capillary walls, oxygen entering cells and carbon dioxide entering into a large vein in the anterior vena cava. The large posterior vena cava vein drains blood from the hind limbs and trunk. The two veins empty the blood into the right atrium, where it flows into the right ventricle.

Written By: Neil 7/23/2019


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