MAVO Year 4 · Transport and immune defence · Section 13.2

Circulation and blood vessels

Double circulation

The heart and blood vessels form the cardiovascular system. Humans have double circulation: during one complete journey through lungs and body, blood passes through the heart twice.

The pulmonary circulation carries blood from the right side of the heart to the lungs. There it takes up oxygen and releases carbon dioxide, then returns to the left side of the heart.

The systemic circulation carries oxygen-rich blood from the left side to body tissues. It supplies oxygen and nutrients, collects carbon dioxide and other wastes, and returns blood towards the right side.

Arteries

Arteries carry blood away from the heart. Many experience high, pulsating pressure and have strong muscular, elastic walls. They often lie relatively deep, although some are close to the surface. Arteries are defined by flow direction, not oxygen content: pulmonary arteries carry relatively oxygen-poor blood.

Capillaries

Arteries branch into smaller vessels and ultimately capillary networks. Capillary walls consist mainly of one layer of endothelial cells, allowing exchange between blood and tissues. Oxygen and nutrients move towards cells, while carbon dioxide and other products move towards blood according to local conditions.

Veins

Veins carry blood towards the heart. Their pressure is generally lower and their walls thinner than those of corresponding arteries. Many veins, especially in the limbs, have valves limiting backward flow. Contractions of surrounding muscles help move blood along.

Heat transport and vessel names

Blood distributes heat released by metabolism. Changing flow through skin vessels helps regulate heat loss: dilation generally increases it and constriction reduces it. Many vessels are named after their location or the organ they serve.

Blood supply to the liver

The hepatic portal vein carries blood from the gut and associated organs to the liver, often rich in recently absorbed nutrients. The hepatic artery supplies more highly oxygenated blood. The liver regulates substances such as glucose, storing some as glycogen and releasing glucose when needed. Blood leaves through hepatic veins towards the vena cava.