8.2 Animal Transport

Circulation transports respiratory gases, nutrients, wastes, hormones, heat, immune cells and antibodies. In an open system, hemolymph leaves vessels and bathes organs in body spaces; pressure and flow are relatively low. In a closed system, blood remains within vessels, allowing higher pressure, faster delivery and precise redistribution. Vertebrate circulation is closed. Fish have a single circuit through heart, gills and body, whereas mammals have pulmonary and systemic circuits arranged as a double circulation.

The mammalian heart is a double pump. The right atrium receives deoxygenated blood through the venae cavae and the right ventricle sends it through the pulmonary artery. Oxygenated blood returns in pulmonary veins to the left atrium, and the left ventricle ejects it through the aorta. Vessel names indicate direction relative to the heart, not oxygen content. The pulmonary artery and vein are therefore deliberate exceptions to the careless rule that arteries always carry oxygenated blood.

Atrioventricular valves prevent ventricular pressure from driving blood back into atria; semilunar valves prevent arterial blood returning to ventricles. Tendinous cords and papillary muscles stop atrioventricular valve flaps inverting. The left ventricular wall is thicker because systemic circulation has greater resistance and requires a larger pressure than the short pulmonary circuit. The septum prevents mixing and lets the two sides operate at different pressures.

During diastole the chambers relax and venous return fills the atria and then the ventricles through open atrioventricular valves. Atrial systole completes ventricular filling. Ventricular systole raises ventricular pressure, closes the atrioventricular valves and, once ventricular pressure exceeds arterial pressure, opens the semilunar valves. During early ventricular relaxation both valve sets are briefly closed until ventricular pressure falls below atrial pressure and filling begins again.

The sinoatrial node initiates each heartbeat and spreads excitation across the atria. The atrioventricular node delays transmission, allowing atrial contraction to finish, then conducting tissue carries excitation through the septum and ventricular walls. Ventricular contraction begins near the apex so blood is driven toward the arteries above. Autonomic nerves and hormones change the node's rate, but the basic rhythm is myogenic: it originates within heart tissue.

Cardiac output is the volume pumped by one ventricle per minute. It equals heart rate multiplied by stroke volume. Exercise increases venous return, sympathetic stimulation and usually stroke volume as well as heart rate. A numerical answer must keep units consistent: multiplying beats per minute by cubic centimeters per beat gives cubic centimeters per minute, which can then be converted to cubic decimeters per minute.

At the arterial end of a capillary bed, hydrostatic pressure tends to force water and small solutes through gaps into tissue fluid, while cells and most plasma proteins remain in blood. Plasma proteins create colloid osmotic attraction that favors water return. As hydrostatic pressure falls along the capillary, reabsorption becomes more important. Not all fluid returns directly; lymphatic capillaries collect the excess and eventually return it to veins.

Organ perfusion is dynamic. During exercise, arterioles serving skeletal and cardiac muscle dilate while flow to some digestive organs falls. Local carbon dioxide, acidity, low oxygen and temperature changes can influence vessels, while sympathetic signals and adrenal hormones coordinate a body-wide response. The body does not simply increase flow everywhere; finite cardiac output is allocated according to immediate need.

The lymphatic system begins as blind-ended vessels among tissues. It returns leaked fluid and plasma proteins, transports absorbed lipids from intestinal lacteals and passes lymph through nodes rich in immune cells. Lymph has no central pump; valves, skeletal-muscle movement and pressure changes maintain one-way flow. Failure of drainage produces edema because tissue fluid accumulates faster than it is returned.