THE LIVING WORLD
Unit Six. Animal Life
The vertebrate circulatory system, also called the cardiovascular system, is made up of three elements: (1) the heart, a muscular pump that pushes blood through the body; (2) the blood vessels, a network of tubes through which the blood moves; and (3) the blood, which circulates within these vessels.
Blood moves through the body in a cycle, from the heart, through a system of vessels: From the arteries and arterioles, into the capillaries, and then back to the heart through the venules and veins as shown here:
Blood leaves the heart through vessels known as arteries. From the arteries, the blood passes into a network of smaller arteries called arterioles, shown in figure 23.2a 1. From these, it is eventually forced through a capillary bed 2, a fine 1 atticework of very narrow tubes called capillaries (from the Latin, capillus, “a hair”). While passing through the capillaries, the blood exchanges gases and metabolites (glucose, vitamins, hormones) with the cells of the body. After traversing the capillaries, the blood passes into a fourth kind of vessel, the venules, or small veins, 3. A network of venules empties into larger veins that collect the circulating blood and carry it back to the heart.
Capillary beds can be opened or closed, based on the physiological needs of the tissues. Smooth muscles (discussed later) can control blood flow to the arterioles, but, in addition, flow through the capillaries can be controlled by the relaxation or contraction of small circular muscles called precapillary sphincters. The closing of the precapillary sphincters by contraction is shown in figure 23.2b with the blood being diverted from the capillary bed.
Figure 23.2. The capillary network connects arteries with veins.
Through-flow channels connect arterioles directly to venules. Branching from these through-flow channels is a network of finer channels, the capillaries. Most of the exchange between the body tissues and the red blood cells occurs while they are in this capillary network. The flow of blood into the capillaries is controlled by bands of muscle called precapillary sphincters located at the entrance to each capillary. (a) When a sphincter is open, blood flows through that capillary. (b) When a sphincter contracts, it closes off the capillary.
The capillaries have a much smaller diameter than the other blood vessels of the body. Blood leaves the mammalian heart through a large artery, the aorta, a tube that in your body has a diameter of about 2 centimeters (about the same as your thumb). But when blood reaches the capillaries, it passes through vessels with an average diameter of only 8 micrometers, a reduction in radius of some 1,250 times!
This decrease in size of blood vessels has a very important consequence. Although each capillary is very narrow, there are so many of them that the capillaries have the greatest total cross-sectional area of any other type of vessel. Consequently, this allows more time for blood to exchange materials with the surrounding extracellular fluid. By the time the blood reaches the end of a capillary, it has released some of its oxygen and nutrients and picked up carbon dioxide and other waste products. Blood loses most of its pressure and velocity in passing through the vast capillary networks, and so is under very low pressure when it enters the veins. The blood flow through the capillaries is like water flowing out of the sprinkler head of a watering can—the stream of blood spreads out to many small streams. These smaller streams don’t flow with as much force, nor as quickly, as the larger stream that entered the capillary bed.
Arteries: Highways from the Heart
The arterial system, composed of arteries and arterioles, carries blood away from the heart. An artery is more than simply a pipe. Blood comes from the heart in pulses rather than in a smooth flow, slamming into the artery in great big slugs as the heart forcefully ejects its contents with each contraction. The artery has to be able to expand to withstand the pressure caused by each contraction of the heart. An artery, then, is designed as an expandable tube, with its walls made up of four layers of tissue. Figure 23.3a shows these layers pulled out from the artery, like a telescope so they are more easily seen. The innermost thin layer is composed of endothelial cells. Surrounding them is a layer of elastic fibers and then a thick layer of smooth muscle, which in turn is encased within an envelope of protective connective tissue. Because this sheath and envelope are elastic, the artery is able to expand its volume considerably when the heart contracts, shoving a new volume of blood into the artery—just as a tubular balloon expands when you blow more air into it. The steady contraction of the smooth muscle layer strengthens the wall of the vessel against overexpansion.
Figure 23.3. The structure of blood vessels.
(a) Arteries, which carry blood away from the heart, are expandable and are composed of layers of tissue. (b) Capillaries are simple tubes whose thin walls facilitate the exchange of materials between the blood and the cells of the body. (c) Veins, which transport blood back to the heart, do not need to be as sturdy as arteries. The walls of veins have thinner muscle layers than arteries, and they collapse when empty. Note, this drawing is not to scale; as stated in the text, arteries can be up to 2 centimeters in diameter, the capillaries are only about 8 micrometers in diameter, and the largest veins can be up to 3 centimeters in diameter.
Arterioles differ from arteries in two ways. They are smaller in diameter, and the muscle layer that surrounds an arteriole can be relaxed under the influence of hormones to enlarge the diameter. When the diameter increases, the blood flow also increases, an advantage during times of high body activity. Most arterioles are also in contact with nerve fibers. When stimulated by these nerves, the muscle lining of the arteriole contracts, constricting the diameter of the vessel. Such contraction limits the flow of blood to the extremities during periods of low temperature or stress. You turn pale when you are scared or cold because the arterioles in your skin are constricting. You blush for just the opposite reason. When you overheat or are embarrassed, the nerve fibers connected to muscles surrounding the arterioles are inhibited, which relaxes the smooth muscle and causes the arterioles in the skin to expand, bringing heat to the surface for escape.
Capillaries: Where Exchange Takes Place
Capillaries are where oxygen and food molecules are transferred from the blood to the body’s cells and where waste carbon dioxide is picked up. To facilitate this back-and-forth traffic, capillaries are narrow (figure 23.4) and have thin walls across which gases and metabolites pass easily. Capillaries have the simplest structure of any element in the cardiovascular system. They are built like a soft-drink straw, simple tubes with walls only one cell thick (see figure 23.3b). The average capillary is about 1 millimeter long and connects an arteriole with a venule. All capillaries are very narrow, with an internal diameter of about 8 micrometers, just bigger than the diameter of a red blood cell (5 to 7 micrometers). This design is critical to the function of capillaries. By bumping against the sides of the vessel as they pass through (like the cells in figure 23.4), the red blood cells are forced into close contact with the capillary walls, making exchange easier.
Figure 23.4. Red blood cells within a capillary.
The red blood cells in this capillary pass along in single file. Red blood cells can pass through capillaries even narrower than their own diameter, pushed along by the pressure of the pumping heart.
Almost all cells of the vertebrate body are no more than 100 micrometers from a capillary. At any one moment, about 5% of the circulating blood is in capillaries, a network that amounts to several thousand miles in overall length. If all the capillaries in your body were laid end to end, they would extend across the United States! Individual capillaries have high resistance to flow because of their small diameters. However, the total cross-sectional area of the extensive capillary network (that is, the sum of all the diameters of all the capillaries, expressed as area) is greater than that of the arteries leading to it. As a result, the blood pressure is actually far lower in the capillaries than in the arteries. This is important, because the walls of capillaries are not strong, and they would burst if exposed to the pressures that arteries routinely withstand.
Veins: Returning Blood to the Heart
Veins are vessels that return blood to the heart. Veins do not have to accommodate the pulsing pressures that arteries do because much of the force of the heartbeat is weakened by the high resistance and great cross-sectional area of the capillary network. For this reason, the walls of veins have much thinner layers of muscle and elastic fiber, as seen in figure 23.3c. An empty artery will stay open, like a pipe, but when a vein is empty, its walls collapse like an empty balloon. In figure 23.5 you can see a vein and an artery side-by-side. The vein on the left is partially collapsed, while the artery on the right still holds its shape.
Figure 23.5. Veins and arteries.
The vein (left) has the same general structure as the artery (right), but an artery retains its shape when empty, while a vein collapses.
Because the pressure of the blood flowing within veins is low, it becomes important to avoid any further resistance to flow, lest there not be enough pressure to get the blood back to the heart. Because a wide tube presents much less resistance to flow than a narrow one, the internal passageway of veins is often quite large, requiring only a small pressure difference to return blood to the heart. The diameters of the largest veins in the human body, the venae cavae, which lead into the heart, are fully 3 centimeters; this is wider than your thumb! Pressure alone cannot force the blood in the veins back to the heart but several features provide help. Most significantly, when skeletal muscles surrounding the veins contract, they move blood by squeezing the veins. Veins also have unidirectional valves (the small flaps within the vein in figure 23.6) that ensure the return of this blood by preventing it from flowing backward. These structural features keep the blood flowing in a cycle through the circulatory system.
Figure 23.6. Flow of blood through veins.
Venous valves ensure that blood moves through the veins in only one direction back to the heart. This movement of blood is aided by the contraction of skeletal muscles surrounding the veins.
Key Learning Outcome 23.2. The vertebrate circulatory system is composed of arteries, which carry blood away from the heart; capillaries, a network of narrow tubes across whose thin walls the exchange of gases and food molecules takes place; and veins, which return blood from the capillaries to the heart.