Mass Transport in Animals - Blood Vessels

Question 1

What are the types of blood vessels?

Answer 1

Arteries - carry blood away from the heart into arterioles.
Arterioles - smaller arteries that control blood flow from arteries to capillaries.
Capillaries - tiny vessels that link arterioles to veins.
Veins - carry blood from capillaries back to the heart.

Question 2

What do arteries do?

Answer 2

Carry blood away from the heart into arterioles.

Question 3

What do arterioles do?

Answer 3

Smaller arteries

- Control blood flow from arteries to capillaries under low blood pressures.

Question 4

What is the capillary?

Answer 4

Tiny vessels that link arterioles to veins.

• Function is to to exchange metabolic materials such as oxygen, CO2 and glucose between the blood and the cells of the body.
• Flow of blood is slow so that there is more time for the exchange of materials.

Question 5

What do veins do?

Answer 5

• Carry blood from capillaries in the tissues back to the heart.
• Slowly, under low pressure

Question 6

Which vessels have the same layered structure?

Answer 6

Arteries, Arterioles and Veins

Question 7

What are the layers of the arteries, arterioles and veins?

Answer 7

Tough fibrous outer layer - Resists pressure changes from both inside an outside.
Muscle layer - Contract and control the flow of blood.
Elastic layer - Helps maintain blood pressure through recoil (stretching and springing back).
Endothelium (Thin inner layer lining) - Smooth to reduce friction and thin to allow diffusion.
Lumen - (Not actually a layer) The central cavity of the blood vessel where blood flows through.

Question 8

What differs between each blood type?

Answer 8

The relative proportions of each layer.

Question 9

What are the similarities and differences between arteries and arterioles?

Answer 9

• Same relative proportions of each layer.

- Arterioles have a smaller diameter but have a larger muscle layer and lumen.

Question 10

How does the structure of an artery relate to it’s function?

Answer 10

Thicker muscle layer than veins - Smaller arteries can be constricted and dilated to control the volume of blood passing through them.
Thicker elastic layer than veins - Helps to maintain the high pressure of blood flow.
Overall thick wall - Resist the vessel bursting under pressure.
No valves (except in arteries leaving the heart) - As blood is under constant high pressure due to the heart pumping blood into the arteries, blood therefore tends to not flow back.

Question 11

How does the structure of an arteriole relate to its function?

Answer 11

Muscle layer is relatively thicker than in arteries - Contraction of this muscle layer allows constriction of the lumen of the arteriole. This restricts the flow of blood and controls its movement into the capillaries that supply the tissues with blood.
Elastic layer is relatively thinner than in arteries - Due to low blood pressure.

Question 12

How does the structure of the vein relate to its function?

Answer 12

Muscle layer is thin compared to arteries - Veins carry blood away from the tissues so their constriction and dilation cannot control the flow of blood to the tissues.
Elastic layer is relatively thin compared to arteries - Low pressure, will not burst and will not create recoil action.
Overall thickness of the wall is small - No need for thick wall as there is a low risk of bursting due to a low blood pressure. Allows them to be flattened easily when muscles contract/
Valves throughout - Ensure that blood does not flow back which it might do otherwise due to the low pressure. Also, when muscles contract veins are compressed which pressurises blood within them. Valves prevent this from backflow occurring.

Question 13

How is the structure of a capillary related to it’s function?

Answer 13

Walls consist mostly of the lining layer - making them extremely thin so the distance over which diffusion takes place is short. This allows for rapid diffusion of materials between the blood and their cells.
Numerous and highly branched - Provides a large surface area for gas exchange
Narrow diameter - No cell is far from a capillary and there is a short diffusion pathway
Very narrow lumen - Brings the RBC closer to the cells to supply oxygen, reducing the diffusion distance.
Spaces between the endothelial cells - Allow WBC to escape to deal with infections in the tissues.

Question 14

What is tissue fluid?

Answer 14

• A watery liquid containing glucose, amino acids, fatty acids, ions in solution and oxygen.
• Tissue fluid supplies all these substances to the tissues.
• In return it receives CO2 and other waste materials from the tissues.

Question 15

How is tissue fluid formed?

Answer 15

• Blood pumped by the heart passes along arteries, then to the narrower arterioles and then to the even narrower capillaries.
• Pumping by the heart creates hydrostatic pressure at the arterial end of the capillaries.
• This hydrostatic pressure causes tissue fluid to move out of the blood plasma.
• This pressure is opposed by two other forces:
  Hydrostatic pressure of the tissue fluid outside the capillaries which resist outward movement of liquid.
  The lower water potential of the blood, due to the plasma proteins that causes water to move back into the blood within the capillaries.
• The combined effect of all these forces is to create an overall pressure that pushes tissue fluid out of the capillaries at the arterial end.
• This pressure is only enough to push small molecules out of the capillaries, leaving all cells proteins in the blood because these are too large to cross the membranes.
• This is called ultrfailtration.

Question 16

What happens to the tissue fluid after it has exchanged materials with the cells it bathes in?

Answer 16

• It returns to the circulatory system.
• Most returns via the capillaries.
• This happens through as the loss of tissue fluid from the capillaries reduces the hydrostatic pressure inside them.
  By the time the blood has reached the venous end of the capillary network, its hydrostatic pressure is usually lower than that of the tissue fluid outside it.
  Therefore tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them.
  As well as this the plasma has lost water and still contains proteins so it has a lower water potential than the tissue fluid.
  Water then leaves the tissue by osmosis down a water potential gradient.

Question 17

What happens to the tissue fluid that does not return to the capillaries?

Answer 17

Remainder is carried back to the lymphatic system.

Question 18

What happens to the tissue fluid after it has exchanged materials with the cells it bathes in?

Answer 18

Most return back to the circulatory system through the capillaries but some go back to the lymphatic system.

Question 19

How do the contents of the lymphatic system move?

Answer 19

• By the hydrostatic pressure of the tissue fluid that has left the capillaries.
• Contraction of body muscles that squeeze the lymph vessels - valves in the lymph vessels ensure that the fluid inside moves away from the tissues in the direction of the heart.