Functions in the rapid internal transport of substances. 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 the tissue fluid surrounding them – the interstitial fluid. Circulating connective tissue (blood) interacts with tissue fluid, making continual deliveries and pickups that help keep conditions tolerable for enzymes and other molecules. The heart generates pressure that keeps blood flowing.
Like many animals, you have a closed circulatory system. Blood is confined within the continuously connected walls of the heart and blood vessels, including capillary beds. At these beds, the flow fans out through vast numbers of small-diameter blood vessels. The heart’s right half pumps oxygen-poor blood to the lungs, where the blood picks up oxygen and gives up carbon 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 left half of the heart pumps oxygenated blood to all tissues where oxygen is needed. An elaborate network of drainage vessels picks up excess interstitial fluid and reclaimable solutes, then returns them to the circulatory system. Blood, a connective tissue, has multiple functions. It transports oxygen, nutrients, and other solutes to cells. The volume of blood depends on body size; the average-size adult human has about 6 to 8 percent of their body weight in blood, which amounts to about four or five quarts. Red blood cells, white blood cells, and platelets are its components, accounting for 50 to 60 percent of the total blood volume.
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 substances. Erythrocytes, or red blood cells, are biconcave disks. The red blood cell count is the number of cells of a given type in a microliter of blood. The average number of red blood cells is 5.4 million in males and 4.8 million in females.
Platelets arise from stem cells in the bone marrow. Each platelet only lasts five to nine days, but hundreds of thousands are always circulating in the blood. If the volume of blood were to decrease by more than 30 percent, then circulatory shock would follow. A defensive response called agglutination occurs when proteins called antibodies that are circulating in plasma act against 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.
In the circulatory system, the heart pumps blood into large-diameter arteries. From there, blood flows into small, muscular arterioles, which branch into even smaller diameter capillaries. Blood flows continuously from capillaries into small 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 carrying 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. It must be a truly durable pump, having beaten 2.5 billion times during a seventy-year lifespan.
Each half of the heart has two chambers – an atrium. 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 about seventy-five times before reaching another cell body cluster, the AV node. This is the only electrical bridge between the atria and ventricles (which connective tissue insulates everywhere else). After the AV node, conducting cells are arranged as a bundle in 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 forceful 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. 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 circuit, and is lowest in the relaxed state. The average resting value stays fairly constant over a few weeks, even months, at about 120/80 mm Hg. An increase in blood.
