The structure and functions of blood vessels

The cardiovascular system transports blood around the body, supplying oxygen and nutrients while removing waste products. Blood vessels form a network that carries blood, with each type adapted to its specific role. These vessels have a hollow centre, known as the lumen, through which blood flows.

Main types of blood vessels

• Arteries - These carry blood away from the heart. They have thick, muscular walls to carry blood flowing at high pressure. Most arteries transport oxygenated blood, except for the pulmonary arteries.
• Veins - These return blood to the heart. Their walls are thinner and contain less muscle than arteries. Most veins carry deoxygenated blood, except for the pulmonary veins.
• Capillaries - These are tiny vessels that carry blood through the body to exchange oxygen, carbon dioxide, and nutrients with body tissues. Their walls are very thin, so substances can easily pass through.

Smaller blood vessels and blood flow pathway

Blood flow involves smaller vessels that branch from the main types.

Types of smaller blood vessels:

• Arterioles - Small branches that branch off arteries.
• Venules - Small vessels that meet to form veins.

The pathway starts with oxygenated blood flowing through arteries into arterioles, then into capillaries. After gas exchange between capillaries and body tissues, blood travels from capillaries into venules, then into veins.

Blood pressure

Blood pressure is how strongly the blood presses against the walls of blood vessels.

The components of blood

Blood is a fluid tissue that carries essential substances around the body. It consists of cells, platelets, and plasma.

Key components and their roles

• Red blood cells - These cells carry oxygen around the body for energy release needed by muscles during physical activity.
• White blood cells - These fight against disease to maintain health and performance.
• Platelets - Small cell fragments that help blood to clot at wounds to prevent infection.
• Plasma - The liquid part of blood that carries everything in the bloodstream, including blood cells, digested food (e.g., glucose) and waste (e.g., carbon dioxide).

Changes in blood vessels during exercise

When you exercise, your body adapts to meet the higher demand for oxygen and nutrients in muscles. During exercise, blood is redistributed around the body to increase oxygen supply to muscles.

Vasodilation and vasoconstriction

• Vasodilation - Blood vessels in muscles widen to allow more blood flow.
• Vasoconstriction - Blood vessels in inactive organs narrow to restrict blood flow.

Managing blood pressure

During intense activity, arteries widen to prevent blood pressure from getting too high.

Vascular shunting and temperature regulation

Vascular shunting is the redistribution of blood during exercise to prioritise muscles over less essential areas. This occurs because muscles need blood more than some organs.

How vascular shunting works

1. Blood vessels in muscles undergo vasodilation to increase flow.
2. Vessels in inactive organs experience vasoconstriction to limit flow.
3. As a result, blood normally supplied to organs like the gut and liver is shunted to the muscles.

This process requires both vasodilation and vasoconstriction.

Temperature regulation through blood flow

Working muscles generate heat which warms the blood. To prevent overheating:

Body's cooling responses:

• Blood is shunted closer to the skin so heat can escape through radiation, which causes redness.
• Sweating begins to help maintain body temperature.