12.3 Circulation

12.3 Circulation

Vertebrates move oxygen, nutrients, hormones, and wastes in a closed circulation. The blood stays inside a heart and a connected set of vessels. It does not slosh through a body cavity the way the blood of many invertebrates does. The heart raises the pressure. Arteries carry blood away from the heart. Capillaries exchange materials with the fluid around cells. Veins return blood to the heart. How many chambers the heart has, and whether oxygen-rich blood can mix with oxygen-poor blood, is the comparison across the vertebrate classes. The human circuit is the one to trace one chamber at a time.

Chambers and circuits

A fish heart has two chambers, one atrium and one ventricle, and a single circuit. The ventricle pumps blood to the gills. After the gill capillaries, the same blood continues on to the body and only then returns to the atrium. Pressure falls as the blood crosses the gills, so the body tissues receive blood at a lower pressure. There is no second pump waiting after the respiratory surface.

Amphibians and most reptiles have three chambers: two atria and one ventricle. One atrium receives blood returning from the body. The other receives blood returning from the lungs, or from the lungs and the skin in a typical frog. Those streams meet in the single ventricle. Ridges in the ventricle and in the outgoing artery reduce mixing, but some oxygen-rich blood still blends with oxygen-poor blood.

Birds, mammals, and crocodilians have four chambers and a full double circuit. The right side of the heart handles the pulmonary stream. The left side handles the systemic stream. The two streams do not share a ventricle. The left side can raise a high pressure for the body while the lungs receive a gentler pressure from the right side. A human heart is this four-chamber plan.

The human double path

Blood from the body enters the right atrium through the superior and inferior venae cavae. It crosses the tricuspid valve into the right ventricle. The right ventricle pumps it through the pulmonary semilunar valve into the pulmonary arteries, which lead to the lungs. Those arteries are full of oxygen-poor blood. They are still arteries, because they leave the heart. In the pulmonary capillaries the blood gains oxygen and gives up carbon dioxide, the exchange described for the alveoli.

Pulmonary veins return the oxygen-rich blood to the left atrium. Blood crosses the mitral valve, also called the bicuspid valve, into the left ventricle. The left ventricle has the thickest muscular wall, because it must drive blood through the whole systemic circuit, from brain to feet. It pumps through the aortic semilunar valve into the aorta. Arteries and arterioles branch into capillary beds. Venules and veins collect the blood, and the venae cavae complete the loop at the right atrium.

Valves keep the stream pointed the right way. The atrioventricular valves close when the ventricles contract, so blood cannot fall back into the atria. The semilunar valves close when the ventricles relax, so blood cannot drop back from the aorta or the pulmonary trunk. Veins in the limbs have valves as well. Skeletal muscles around those veins squeeze blood toward the heart, and the valves stop it from slipping backward between squeezes.

Walls, exchange, and the beat

Artery walls are thick, muscular, and elastic, which suits a vessel that receives blood straight from a contracting ventricle. Vein walls are thinner, and many veins have a wider lumen than the artery serving the same region. Capillary walls are a single layer of endothelium. Water, gases, nutrients, and wastes cross that layer. Not every drop of fluid that leaves a capillary at its arterial end returns at its venous end. Lymph vessels collect the extra interstitial fluid and eventually empty it back into the blood. The circulation stays closed because that fluid is returned, not because every drop stays inside one capillary.

The sinoatrial node, in the wall of the right atrium, sets the pace. It is the pacemaker. Each impulse spreads across the atria, pauses briefly at the atrioventricular node, and then travels through conducting fibers into the ventricular muscle. The pause gives the atria time to finish filling the ventricles before the ventricles contract.

Systole and diastole

Systole is contraction. Diastole is relaxation. In ventricular systole the two ventricles contract together. The right ventricle ejects blood into the pulmonary arteries. The left ventricle ejects blood into the aorta. In diastole the muscle relaxes, the semilunar valves are shut, and both sides of the heart fill. A commonly cited resting arterial pressure for a healthy young adult is about 120 mm Hg during ventricular systole and about 80 mm Hg during diastole. The first number is systolic pressure. The second is diastolic pressure. The figures describe the systemic arteries, the high-pressure side, not the gentler pulmonary arteries.

Picture one red blood cell in the right ventricle. It enters a pulmonary artery, crosses a pulmonary capillary, and rides a pulmonary vein into the left atrium and then the left ventricle. Only then does the aorta carry it to a systemic capillary, where it gives up oxygen. A vena cava brings it home to the right atrium. Calling every artery oxygen-rich breaks this loop at the first step, because the pulmonary arteries are the oxygen-poor exceptions.

The right side of the heart is the pulmonary pump. The left side is the systemic pump. The sinoatrial node sets the rhythm, and systole is the contraction that ejects the blood.