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The main function of the cardiovascular system is to transport materials such as hormones and nutrients to the cells of the body and to remove secretions and waste products. The energy of the heart’s contractions propels the blood through the blood vessels to all of the tissues of the body. The arteries carry blood away from the heart and branch into progressively smaller vessels. The smallest vessels are the capillaries, with walls consisting of only one cell layer of simple squamous endothelium. The systemic capillaries are the site of exchange of materials between the blood and body tissues. The pulmonary capillaries are the site of exchange between the blood and the alveolar gas. The capillaries are so numerous and well distributed that almost no cell in the body is greater than 70 μ away from a capillary.

Certain substances (such as oxygen, nutrients, and hormones) contained in the blood pass either through the capillary endothelium or through the spaces where endothelial cells meet. These substances diffuse through the interstitial fluid in the area between the capillary and the cell (Claude Bernard’s Internal Milieu) according to the concentration gradient. These materials then interact with receptors on the cell membrane or pass through the cell membrane into the cell if the substances are lipid soluble. Carbon dioxide; metabolic waste products; and secretory products such as hormones, autacoids (biologically active substances such as prostaglandins), and stored nutrients released for use elsewhere in the body follow the same route but in the opposite direction. Concentration gradients determine the direction of movement.

The systemic arterial blood vessels play an important role in maintaining the blood pressure and in distributing the blood flow to the different vascular beds by changing the tone or level of contraction of the vascular smooth muscle (VSM) in their walls. Many factors influence the tone of the VSM such as endothelial and cellular metabolism as well as autonomic input. Elasticity of the vessel wall changes the pulsatile input from the heart into a steady flow through the capillaries.


Although the heart is a single organ anatomically, physiologically it acts as two pairs of pumps arranged in series. The right side of the heart consists of a thin-walled, weakly pumping right atrium and a thicker-walled, stronger right ventricle. The right atrium receives venous blood from the tissues of the body via the superior and inferior vena cava, as well as from the heart itself through the coronary sinus which drains most of the blood from the coronary arteries. This blood, called venous return, flows through the right atrium to the right ventricle when the tricuspid or right atrioventricular (AV) valve is open and at the end of the filling phase, right atrial contraction pushes more blood into the right ventricle. The right ventricle pumps the blood through the pulmonic ...

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