box 12.1. Clinical Correlations: Hypertension 336

box 12.2. Clinical Correlations: Atherosclerosis 337

box 12.3. Clinical Correlations: Ischemic Heart Disease 346

v overview of the cardiovascular system

The cardiovascular system includes the heart, blood vessels, and lymphatic vessels

The cardiovascular system is a transport system that car- Blood vessels provide the route by which blood circuities blood and lymph to and from the tissues of the body. lates to and from all parts of the body. The heart pumps The constitutive elements of these fluids include cells, nu- the blood through the arterial system under significant trients, waste products, hormones, and antibodies. pressure; blood is returned to the heart under low pressure with the assistance of negative pressure in the thoracic cavity during inspiration and compression of the veins by skeletal muscle. The blood vessels are arranged so that blood delivered from the heart quickly reaches a network of narrow, thin-walled vessels, the blood capillaries, within or in proximity to the tissues in every part of the body.

In the capillaries, a two-directional exchange of fluid occurs between the blood and tissues. The fluid, called blood filtrate, carrying oxygen and metabolites, passes through the capillary wall. In the tissues, these molecules are exchanged for carbon dioxide and waste products. Most of the fluid reenters the distal or venous end of the blood capillaries. The remaining fluid enters lymphatic capillaries as lymph and is ultimately returned to the bloodstream through a system of lymphatic vessels that join the blood system at the junction of the internal jugular veins with the subclavian veins. Normally, many of the white blood cells conveyed in the blood leave the blood vessels to enter the tissues. This occurs at the level of the postcapillary venules. When pathologic changes occur in the body, as in the inflammatory reaction, large numbers of white blood cells emigrate from these venules.

Arteries are the vessels that deliver blood to the capillaries. The smallest arteries, called arterioles, are functionally associated with networks of capillaries into which they deliver blood. The arterioles regulate the amount of blood that enters these capillary networks. Together, the arterioles, associated capillary network, and postcapillary venules form a functional unit called the microcirculatory or microvascular bed of that tissue (Fig. 12.1). Veins, beginning with the postcapillary venule, collect blood from the microvascular bed and carry it away.

Two circuits distribute blood in the body: the systemic and the pulmonary circulation

Two pathways of circulation are formed by the blood vessels and the heart:

• Pulmonary circulation conveys blood from the heart to the lungs and from the lungs to the heart (Fig. 12.2). ® Systemic circulation conveys blood from the heart to other tissues of the body and from other tissues of the body to the heart.

Although the general arrangement of blood vessels in both circulations is from arteries to capillaries to veins, in some parts of the systemic circulation it is modified so that a vein or an arteriole is interposed between two capillary networks; these vessels constitute portal systems. Venous portal systems occur in vessels carrying blood to the liver, namely, the hepatic portal system (portal vein) and, in vessels leading to the pituitary, the hypothalamic-hypophyseal portal system.

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