Transport in Animals

Circulatory Systems

Large animals cannot rely on diffusion alone to supply their cells. They have a circulatory system: the heart pumps blood around the body in blood vessels.

ExtendedSingle circulation of a fish

In a fish the blood passes through the heart once on each complete circuit. The heart pumps deoxygenated blood to the gills, where it picks up oxygen. The oxygenated blood then flows on to the body and returns to the heart.

ExtendedDouble circulation of a mammal

In a mammal the blood passes through the heart twice on each complete circuit. The right side pumps deoxygenated blood to the lungs; it returns oxygenated to the left side, which pumps it to the body. Blood returns from the body deoxygenated, to the right side.

Advantage: blood loses pressure in the lung capillaries, so returning it to the heart lets it be pumped again. It reaches the body at higher pressure and flows faster.

The Heart

The mammalian heart is a muscular organ divided by a septum into a right side and a left side. Each side has an upper atrium and a lower ventricle.

The atria receive blood and the ventricles pump it out. One-way valves in the heart prevent backflow.

ExtendedValves, walls and the septum

The valves between atria and ventricles are the atrioventricular valves; those at the base of the aorta and pulmonary artery are the semilunar valves. The left ventricle has a thicker muscle wall than the right, because it pumps blood at high pressure to the whole body, while the right pumps blood a short distance to the lungs at lower pressure. The ventricles have thicker walls than the atria, because they pump blood out of the heart at higher pressure, while the atria only push blood into the ventricles. The septum keeps oxygenated and deoxygenated blood apart, so the blood pumped to the body is fully oxygenated.

The Heart in Action

Blood is pumped away from the heart in arteries and returns to the heart in veins. The activity of the heart may be monitored in three ways.

Investigating exercise and heart rate

Count the pulse for a set time at rest. Take exercise, such as stepping or jogging, for a fixed time. Count the pulse again straight afterwards, then at regular intervals until it returns to the resting value. Heart rate increases during exercise and takes time to recover.

ExtendedHow the heart works and why exercise raises heart rate

Blood flows into the atria, which contract and push it into the ventricles. The ventricles then contract and force the blood out into the arteries. The valves open to let blood through and close to stop it flowing back.

During exercise the muscles respire faster, so the heart beats faster to deliver more oxygen and glucose and to remove carbon dioxide more quickly.

Coronary Heart Disease

In coronary heart disease the coronary arteries become blocked by fatty deposits. Less blood reaches the heart muscle, so it receives less oxygen.

Risk factors

Diet and exercise

A diet low in saturated fat and salt, and regular exercise, reduce the risk. Exercise strengthens the heart muscle and helps to control body mass. Neither removes the risk completely, since age, sex and genes cannot be changed.

Blood Vessels

Three types of vessel carry the blood: arteries, veins and capillaries.

Capillaries allow the exchange of materials between the blood and the cells of the tissues.

ExtendedStructure and function

Arteries carry blood at high pressure, so they have thick, strong walls and a narrow lumen. Veins carry blood at low pressure, so their walls are thinner, their lumen is wide and valves stop backflow. Capillaries have walls one cell thick, giving a short diffusion distance. They are very narrow and permeable, so they pass close to every cell.

The Main Blood Vessels

You need to identify the main vessels to and from the heart, the lungs and the kidneys.

ExtendedVessels of the liver

Three vessels serve the liver. The hepatic artery brings oxygenated blood to the liver. The hepatic portal vein brings blood from the small intestine, carrying absorbed nutrients. The hepatic veins carry blood away from the liver.

Worked example (naming the vessel): Name the vessel that carries blood from the left ventricle to the body. Step 1. The left ventricle pumps blood away from the heart. Step 2. A vessel carrying blood away from the heart is an artery. Step 3. It leads to the whole body, so it is the aorta. Answer: Aorta.

Components of Blood

Blood has four components: red blood cells, white blood cells, platelets and plasma.

White Cells and Clotting

White blood cells defend the body against pathogens in two ways: by phagocytosis and by antibody production.

Blood clotting

When a vessel is cut, the blood forms a clot. The platelets are involved in clotting. Clotting has two roles: it prevents blood loss and it prevents the entry of pathogens.

ExtendedLymphocytes and phagocytes

A phagocyte has a lobed or irregular nucleus and engulfs pathogens by phagocytosis. A lymphocyte has a large round nucleus that fills most of the cell, and produces antibodies.

ExtendedThe clotting process

Clotting is the conversion of soluble fibrinogen to insoluble fibrin. The fibrin forms a mesh of fibres across the wound, which traps blood cells and forms the clot.