Summary

Summary

The vascular system of the human body is made up of the systemic and pulmonary circuits. The five general classes of blood vessels include the arteries, arterioles, capillaries, venules, and veins. The walls of arteries con-sist of three distinct layers and a blood-containing space known as the lumen. In arteries and arterioles, vasomo-tor fibers receive impulses to contract and reduce blood vessel diameter, which is called vasoconstriction. When inhibited the muscle fibers relax, and the vessel’s diam-eter increases, which is called vasodilation. The arterial system consists of the aorta and pulmonary arteries.

The smallest diameter blood vessels are capillar-ies, which connect the smallest arterioles to the small-est venules. Capillaries form interwoven networks called capillary beds or plexuses. The formation of new blood vessels, either in the embryo or when the body cells are oxygen-starved, is called angiogenesis. Venules are microscopic vessels that link capillaries to veins, which carry blood back to the atria. Vein walls are similar but not identical to arteries but have poorly developed middle layers. Many veins have flap-like valves projecting inward from their linings, prevent-ing blood from flowing backward. Veins often act as blood reservoirs. Vascular­ anastomoses are intercon-nections formed by blood vessels.

The blood must continue to circulate to sustain life, with the heart acting as the circulation pump. Blood pressure is calculated by multiplying cardiac output by peripheral resistance. It is the pressure exerted by the blood’s circulating volume on the walls of the arteries, veins, and heart chambers. The maxi-mum pressure during ventricular contraction is called the systolic pressure. The lowest pressure that remains in the arteries before the next ventricular contraction is called the diastolic pressure. The venous blood pres-sure is steady and regular and does not pulsate with ventricular contractions like the arterial blood pres-sure. Blood volume is the sum of formed elements and plasma volumes in the vascular system.

Most neural controls of blood vessels operate because of reflex arcs, which involve baroreceptors and related afferent fibers. Baroreceptors are activated by increased arterial pressure. Chemoreceptors in the aortic arch and large neck arteries send impulses to the cardioacceleratory­ center to increase cardiac out-put. The brain stem’s medulla oblongata integrates reflexes that maintain blood pressure. Short-term ­hormonal controls involve ADH, angiotensin II, ANP, and the hormones of the adrenal medulla. Long-term control of blood pressure involves the kidneys. Homeostatic imbalances in blood pressure involve hypertension and ­hypotension.

Arteries are deep, whereas veins are either deep or superficial. Veins have more interconnections than arteries. The aorta is the body’s largest artery and emerges from the left ventricle of the heart. It consists of four portions: the ascending aorta, aortic arch, descend-ing aorta, and abdominal aorta. The head and neck are supplied by four paired arteries: the common carotid arteries plus three branches from each subclavian artery. The subclavian arteries branch to supply all portions of the upper limbs. The abdominal aorta branches to form the abdominal arteries. The common iliac arteries are divided into the internal and external iliac arteries.

Three major veins return blood from the body to the right atrium: the coronary sinus, superior vena cava, and inferior vena cava. From the head and neck three pairs of veins collect most of the draining blood: the external jugular veins, internal jugular veins, and ver-tebral veins. The deep upper limb veins and most of the pelvic and lower limb veins follow their related arteries’ paths and therefore have the same names. The path-way of blood flow from the gastrointestinal tract and spleen to the liver via the portal vein and its tributaries is called the hepatic portal circulation. Aging causes atherosclerosis in the blood vessels. Venous valves weaken and purple varicose veins appear. Hyperten-sion is common, along with heart attacks, renal failure, strokes, and vascular disease. Normal blood pressure is signified by a reading of 120/80 or less, and hyperten-sion is signified by a reading of 140/90 or more.