Transport in the bodyIB MYP Sciences: Revision notes

The circulatory system is made of the heart, blood vessels and blood. It transports oxygen, glucose, carbon dioxide, urea, hormones and heat around the body.

The heart is a muscular pump with four chambers:

• Right atrium receives deoxygenated blood from the body (through the vena cava)
• Right ventricle pumps blood to the lungs (through the pulmonary artery)
• Left atrium receives oxygenated blood from the lungs (through the pulmonary vein)
• Left ventricle pumps blood to the body (through the aorta)

The atria receive blood and the ventricles pump it out. Valves stop the blood flowing backwards. The wall of the left ventricle is thicker because it pumps blood further, at higher pressure.

In single circulation (for example in a fish) blood passes through the heart once on each circuit of the body. Blood loses pressure when it passes through the gills, so it travels around the body slowly.

In double circulation (for example in humans) blood passes through the heart twice on each circuit:

1. Right side of the heart: deoxygenated blood goes to the lungs and picks up oxygen (the pulmonary circulation)
2. Left side of the heart: oxygenated blood goes to the body (the systemic circulation)

The blood returns to the heart after the lungs to be pumped again at high pressure, so oxygen is delivered to the cells quickly. This suits active animals with a high demand for oxygen.

• Arteries carry blood away from the heart at high pressure. They have thick, elastic, muscular walls and a narrow lumen, so they can stretch and recoil with each beat.
• Veins carry blood towards the heart at low pressure. They have thinner walls, a wide lumen and valves that stop backflow.
• Capillaries are the smallest vessels. They have walls one cell thick, which gives a short diffusion distance so substances pass easily between the blood and the body cells.

Capillaries link arteries to veins.

Blood is made of cells in a liquid called plasma:

• Red blood cells: contain haemoglobin, which combines with oxygen to carry it from the lungs to the cells. They have no nucleus and a biconcave shape.
• White blood cells: defend the body against pathogens by engulfing microbes or making antibodies.
• Platelets: small cell fragments that help the blood to clot at a wound, sealing it and preventing microbes entering.
• Plasma: the pale yellow liquid that transports dissolved glucose, carbon dioxide, urea, hormones and heat.

Each time the heart pumps, blood is forced into the arteries and their elastic walls stretch and recoil. This can be felt as the pulse, for example at the wrist or the neck. The pulse rate equals the heart rate.

Heart rate = beats per minute (bpm). A resting heart rate is typically 60 to 100 bpm.

Worked example: 20 beats are counted in 15 seconds. Heart rate = 20 × 4 = 80 bpm.

During exercise the heart rate increases because the muscles respire faster and need more oxygen and glucose, and carbon dioxide has to be removed more quickly. Veins have no pulse because blood is at low pressure.