The circulatory system, which functions in the rapid internal transport of substances to and from cells. Your body's differentiated cells, which perform specialized tasks, cannot fend for themselves. Different types must interact in coordinated ways to maintain the composition, volume, and temperature of a tissue fluid surrounding them, the interstitial fluid. A circulating connective tissue-blood-interacts with tissue fluid, making continual deliveries and pickups that help keep conditions tolerable for enzymes and other molecules that carry out cell activities. A muscular puni p, the heart, generates pressure that keeps blood flowing. Like many anirnals, you have a closed circulatory system- blood is confined within the continuously connected walls of the heart and blood vessels.

The required slowdown proceeds a capillary beds. At such beds, the flow fans out through vast numbers of sen all-diameter blood vessels called capillaries. In birds and mamntals, the heart's right half pumps oxygen-poor blood to the lungs, where blood picks up oxygen and gives up car- bon dioxide. The freshly oxygenated blood then flows to the heart's left half.

This is the pulmonary circuit. By contrast, in a systemic circuit, the I tft half of the heart pumps oxygenated blood to all tissues where oxygen is used and carbon dioxide forms. an elaborate network of drainage vessels picks up excess interstitial fluid and reclaimable solutes, then returns them to the circulatory system. This network is part of the lymphatic system. 27.2 Blood, a connective tissue, has multiple functions.

It transports oxygen, nutrients, and other solutes to cells. The volume of blood depends on body size average-siz adult humans is about 6 to 8 percent of the body weight. That amounts to about four or five quarts. Plasma, red blood cells, venite blood cells, and platelets are its components Plasma normally accounts for 50 to 60 percent of the total blood volume. Pla sena Plasma, which is mostly water, functions as a transport medium for blood cells and platelets. It also serves as a solvent for ions and molecules, including hundreds of different kinds of plasma proteins.

Some of the plasma proteins transport lipids and fat-soluble vitamins through the body. Erythrocytes, or red blood cells, are biconcave disks, like doughnixts with a squashed-in center instead of a hole. A cell count is the number of cells of a given type in a micro- I liter of blood. The average number of red blood cells is 5.4 million in males and 4.8 million in females. Leukocytes, or white brood cells, (arise from stem cells in bone marrow. Each platelet only lasts five to nine days, but hundreds of thousands are always circulating in blood.

27.4 if the volume of blood were to decrease by more than 30 percent, then circulatory shock would follow and could lead to death. In a defensive response called agglutination, proteins called antibodies that are circulating in plasma act against the foreign cells and cause them to clump. Molecular variations in one kind of self marker on red blood cells are analyzed in ABO blood typing. 27.5 In the human cardiovasc- c ular system, a heart pumps blood into large-diameter arteries.

From there, blood flows into small, muscular arterioles, which branch into the even smaller diameter capillaries introduced earlier. Blood flows contintrously from capillaries into sma!! venules, then into large- diameter veins that return blood to the heart. The pulmonary circuit, a short loop, rapidly oxygenates blood. It leads from the heart's right half to capillary beds in both lungs, then returns to the heart's left half.

The systemic circuit is a longer loop. It starts at the heart's left half and the aorta (the main artery carry- ing oxygenated blood away from the heart), branches to all organs and tissues with metabolically active cells, then converges into major veins that deliver oxygen- poor blood to the heart's right half. 27.6 the human heart beats about 2.5 billion times during a seventy-year life span, and you know it must be a truly durable pump. Each half of the heart has two chambers-an atrium (plural, atria) and a ventricle. The sequence of contraction and relaxation is a cardiac cycle... About 1 percent of the cardiac muscle cells don't contract.

Instead, they function as a cardiac conduction system. These specialized cells initiate and Propagate waves of excitation abo ttl seventy-five times a minute. The wave passes through the wall to another cell body cluster, the AV node. This is the only electrical bridge between atria and ventricles (which connective tissue insulates every. where else). After the AV node, conducting cells are arranged as a bundle i = the partition between the heart's two halves. The cells then branch, and the branches deliver the excitatory wave up the ventricle walls.

The ventricles contract in response with a forvisting movement, upward from the heart's apex, that ejects blood into the great arteries. The SA node fires action potentials faster than the rest of the system and serves as the cardiac pacemaker. 27.7 Blood pressure, the fluid pressure generated by heart contractions, is highest in contracting ventricles. During the time it takes for a given volume of blood to leave and reenter the heart, pressure is still high in arteries, then drops along the cirettit, and is lowest tn the relaxed atria the average restin n value stars fairly constant o- few weeks, even months about 120/80 mm Hg. ' is vasodilation, an increase in blood vessel diameter. If the resting value falls, the brain makes the heart beat faster and contract more forcefully. And it makes rings of arteriole muscles contract.

The result is vasoconstriction, a decrease in blood vessel diameter. Capillaries thread through nearly every tissue, and at least one of then is as close as 0.001 centimeter to every living cell. 27.8 Cardiovascular disorders are the leading cause of death in the United States. They affect at least 40 million people and kill about a million each year.

The most common disorders are called hypertension (sustained high blood pressure) and atherosclerosis (progressive thickening of the arterial wall and narrowing of the arterial lumen). Extensive research indicates the following risk factors have roles in cardiovascular disorders: 1. Smoking 2. Genetic predisposition to heart failure 3.

High level of cholesterol in the blood 4. High blood pressure 5. Obesity 6. Lack of regular exercise 7. Diabetes mellitus 8.

Age (the order you get, the greater the risk) 9- Gender (until age fifty, males are at much greater risk than females) Hypertension is called the "silent killer" ' because affected persons may have no outavard symptoms. ' With arteriosclerosis, arteries thicken and lose elasticity. 1 When circulating in blood, cholesterol is bound to proteins as low-density lipoproteins (L DLs). These bind to receptors on cells throughout the body. 338 holt.