Blood Vessels

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Arteries, Arterioles and Veins

The circulatory system is made up of different blood vessels. Each blood vessel is specialised for its function. These blood vessels include:

Arteries

Arteries transport blood away from the heart to the organs.
Artery walls have thick layers of muscle.
- The walls maintain a high pressure so blood can be pumped around the body.
Elastic fibres in the artery wall allow the arteries to stretch.
The endothelium is folded which also allows the arteries to stretch.

Arterioles

When arteries reach an organ, they split into many smaller vessels called arterioles.
The direction of blood flow can be controlled by contracting the arterioles to restrict blood flow and relaxing the arterioles to allow blood to flow.

Veins

Veins transport blood back to the heart.
The lumen of veins is wider than the arteries which allows the blood to flow at low pressure.
There is a thin muscle wall and elastic tissue in the vein walls.
Valves are located throughout the veins to ensure blood flows towards the heart.

Capillaries

Arterioles branch into very small vessels, called capillaries. Capillaries have thin walls and pass very close to the body cells to allow exchange between blood and the organs.

Walls

Capillary walls (endothelium) are only one cell thick.
This means that the diffusion distance of substances (e.g. oxygen) from the bloodstream to the tissues is very short.
A short diffusion distance allows for efficient exchange.

Location

Capillaries pass very close to the body cells.
There is a very short diffusion distance between the capillaries and the body cells.
Exchange between the capillaries and the body cells is rapid.

Surface area

Arterioles split into millions of capillaries that can form networks around the body cells (e.g. the network around the alveoli).
The networks create a large surface area for exchange of substances between the bloodstream and the tissues.
Networks of capillaries are called capillary beds.

Tissue Fluid

Substances carried in the capillaries are transported to the cells via the tissue fluid. Tissue fluid is a combination of oxygen, water and nutrients. The fluid surrounds the cells in the body.

Pressure filtration

In order for the substances carried in the bloodstream (e.g. oxygen) to diffuse into the cells, they must first move out from the capillaries.
Substances move into tissue fluid from the capillaries in a process called pressure filtration.

Pressure gradient

There is high pressure in the capillaries at the entrance to a capillary bed.
The high pressure creates a pressure gradient between the inside of the capillaries and the outside.

Movement of fluid

The fluid in the capillaries flows down the pressure gradient into the surrounding space.
- This is pressure filtration.
The fluid that has moved out of the capillaries creates the tissue fluid.
Substances in the tissue fluid can now diffuse or be transported into the cells.

Decreasing pressure

The movement of fluid out of the capillaries causes the pressure inside the capillaries to decrease.
This means that the pressure in the capillaries at the exit of the capillary bed is lower than the pressure at the entrance to the capillary bed.

Water potential gradient

When fluid moves out of the capillaries, plasma proteins remain inside the capillaries.
The plasma protein concentration inside the capillaries increases and the water potential decreases.
A water potential gradient is established and water diffuses via osmosis back into the capillaries from the tissue fluid.