The Pulmonary and Systemic Circulations Are Arranged in Series, but the Various OrgansWithin the Systemic Circulation Are Arranged in Parallel
As shown in Figure 18-3, blood is pumped from the left ventricle into the aorta. The aorta divides and subdivides to form many arteries, which deliver fresh, oxygenated blood to
Brain, head, and neck
FIGURE 18-3 General layout of the cardiovascular system, showing that the systemic and pulmonary circulations are arranged in series and that the organs within the systemic circulation are arranged in parallel.
LA, Left atrium; LV, left ventricle; PA, pulmonary artery; PV, pulmonary vein; RA, right atrium; RV, right ventricle. Oxygenated blood has a bright-red color; deoxygenated blood is darker and bluish red. (Modified from MiInorWR: Cardiovascular physiology, NewYork, 1990, Oxford University Press.)each organ of the body» except the lungs. The pattern of arterial branching that delivers blood of the same composition to each organ is called parallel. After blood passes through the capillaries within individual organs» it enters veins. Small veins combine to form progressively larger veins, until the entire blood flow is delivered to the right atrium by way of the venae cavae. The blood vessels between the aorta and the venae cavae (including the blood vessels in all organs of the body except the lungs) are collectively called the systemic circulation. From the right atrium, blood passes into the right ventricle, which pumps it into the pulmonary artery. The pulmonary artery branches into progressively smaller arteries, which deliver blood to each lung capillary. Blood from lung capillaries is collected in pulmonary veins and brought to the left atrium. Blood then passes into the left ventricle, completing the circuit. The blood vessels of the lungs, including the pulmonary arteries and veins, constitute the pulmonary circulation. The pulmonary circulation and the heart are collectively termed the central circulation.
The pulmonary circulation and the systemic circulation are arranged in series; that is, blood must pass through the pulmonary vessels between each passage through the systemic circuit.In one pass through the systemic circulation, blood generally encounters only one capillary bed before being collected in veins and returned to the heart, although a few exceptions to this rule exist. One exception occurs in the splanchnic circulation (blood supply of the digestive organs). As shown in Figure 18-3, blood that leaves the gastric, splenic, or mesenteric capillaries enters the portal vein. The portal vein carries splanchnic venous blood to the liver, where the blood passes through another set of capillaries before it returns to the heart. This arrangement of two systemic capillary beds in series is called a portal system. The splanchnic portal system allows nutrients that have been absorbed from the gastrointestinal tract to be delivered directly to the liver. There the nutrients are transformed for storage or allowed to pass into the general circulation. The liver also receives some blood directly from the aorta through the hepatic artery.
The kidneys also contain a portal system. As shown in Figure 18-3, blood enters a kidney by way of a renal artery and passes through two sets of capillaries (called glomerular and tubular) before returning to the venous side of the systemic circulation. Large amounts of water, electrolytes, and other solutes are filtered out of the blood as it passes through the glomerular capillaries. Most of this filtered material is subsequently reabsorbed into the bloodstream as it flows through the peritubular capillaries. The remainder becomes urine. The kidneys use the renal portal system to adjust the amounts of water, electrolytes, and other critical solutes in the blood.
A third portal system is found in the brain and is important in the control of hormone secretion by the pituitary gland. After traversing capillaries in the hypothalamus, blood enters portal vessels that carry it to the anterior pituitary gland (adenohypophysis) and to another set of capillaries (see Figures 33-16 and 33-17). As blood traverses the hypothalamic capillaries, it picks up substances that control the release of pituitary hormones. When this blood reaches capillaries in the anterior pituitary gland, these substances diffuse out of the bloodstream and into the pituitary interstitial fluid, where they act on pituitary cells to increase or decrease their secretion of specific hormones. This system is called the hypothalamic-hypophyseal portal system.
To summarize, except for a few specialized portal systems, blood encounters only one capillary bed in a single pass through the systemic circulation.