Transport

The Human Circulatory System

The blood system consists of a network of tubes called blood vessels throughout which the heart pumps blood. The major blood vessels are the arteries and veins.

The circulatory system functions in the delivery of oxygen, nutrient molecules, and hormones and the removal of carbon dioxide, ammonia and other metabolic wastes.

Blood Vessels

Arteries

Arteries are pressure reservoirs that can ‘smooth out’ pulsations in blood pressure. The artery walls are elastic and stretch to take the blood. Then they contract and bounce back to force the blood along. This bouncing back can be felt as a ‘pulse’ as the blood flows through.

Arteries branch into arterioles as they get smaller. Arterioles eventually become capillaries, which are very thin and branching.

Veins

Veins carry blood from every tissue in the body to the heart. The blood has lost almost all its pressure in the capillaries, so it is at low pressure inside veins and moving slowly. Veins therefore don’t need thick walls and they have a larger lumen that arteries, to reduce the resistance to flow. They also have semi-lunar valves to stop the blood flowing backwards. The blood is squeezed along by the muscles.

Capillaries

The arteries branch many times until the smallest branches form capillaries. Capillaries are very narrow, and allow red blood cells to just squeeze through.

Capillaries are very thin and provide a large surface of exchange between them and the surrounding cells.

The Heart

Our heart is made up nearly completely of muscle called cardiac muscle which contracts and relaxes rhythmically for a lifetime. It is divided into four chambers: the two atria are the receiving chambers and the two ventricles are the pumping chambers. Our average heart rate is about 70 beats per minute.

Like other tissues, heart muscle needs oxygen and other nutrients so it has a constant blood supply which is provided by the coronary arteries.

The structure of the heart ensures that blood flows in a circuit. The right side of the heart receives blood from the body which is deoxygenated and high in carbon dioxide. It then pumps the blood to the lungs to become oxygenated. The left side of the heart receives the freshly oxygenated blood from the lungs, then pumps it out to the rest of the body.

The mammalian heart is responsible for pumping blood through the pulmonary and the systemic circulation simultaneously. This means that when the heart is relaxed (diastole), blood flows into both the right and left atria of the heart at the same time. When the heart contracts (systole), blood is pumped out of both ventricles at the same time.

Important points to remember

1. The RIGHT ventricle pumps blood to the LUNGS in the PULMONARY artery
2. The LEFT ventricle pumps blood to the rest of the body in the AORTA
3. Deoxygenated blood returns to the RIGHT ATRIUM in the VENA CAVA
4. Oxygenated blood returns to the LEFT ATRIUM in the pulmonary vein