3.5 The Circulatory System in Animals

Question 1

Why is a cirulatory system needed?

Answer 1

Diffusion distances too long

Question 2

What is the advantage of a double circulatory system?

Answer 2

No mixing of oxygenated and deoxygenated blood.

Question 3

Describe cardiac diastole.

Answer 3

• The ventricles and atria are both relaxed
• The pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close
• The atria continue to fill with blood
• Pressure in the atria rises above that in the ventricles, forcing the AV valves open
• Blood flows passively into the ventricles without need of atrial systole

Question 4

Describe atrial systole.

Answer 4

• The walls of the atria contract
• Atrial volume decreases / pressure increases
• The pressure in the atria rises above that in the ventricles, forcing the atrioventricular valves open
• Blood is forced into the ventricles
• There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria
• Ventricular DIAstole coincides with atrial systole

Question 5

Describe ventricular systole.

Answer 5

• The walls of the ventricles contract
• Ventricular volume decreases / pressure increases
• The pressure in the ventricles rises above that in the atria
• This forces the AV valves to close, preventing back flow of blood
• The pressure in the ventricles rises above that in the aorta and pulmonary artery
• This forces the semilunar valves open so blood is forced into the arteries
• Atrial DIAstole coincides with ventricular systole
• Relaxed atria begin to fill with blood again

Question 6

What are arteries?

Answer 6

Vessels that transport blood away from the heart (usually at high pressure).
- thick walls
- elastic fibres

Question 7

What are veins?

Answer 7

Vessels that transport blood to the heart (usually at low pressure).
- thin walls
- valves
- larger lumen

Question 8

What are arterioles?

Answer 8

Blood vessels that transport blood into capillaries.
- muscle cells

Question 9

What are capillaries?

Answer 9

Smallest blood vessel responsible for the exchange of substances (e.g, O2)
- thin walls
- leaky

Question 10

Describe the structure and function of capillaries.

Answer 10

• Small diameter (lumen):
  This forces the blood to travel slowly which provides more opportunity for diffusion to occur
• Capillaries branch between cells:
  Substances can diffuse between the blood and cells quickly as there is a short diffusion distance
• The wall of the capillary is made solely from a single layer of endothelial cells:
  The wall is only one cell thick – this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
• The cells of the wall have pores which allow blood plasma to leak out and form tissue fluid

Question 11

What is tissue fluid?

Answer 11

As blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body.

Question 12

How is tissue fluid formed?

Answer 12

1. When blood is at the arteriole end of a capillary, the hydrostatic pressure is great enough to push molecules out of the capillary
2. Proteins remain in the blood, creating a water potential gradient between the capillary and the tissue fluid
3. However, overall movement of water is out from the capillaries into the tissue fluid by osmosis via the lymphatic system
4. At the venule end of the capillary, less fluid is pushed out of the capillary as pressure within the capillary is reduced
5. The water potential gradient between the capillary and the tissue fluid remains the same as at the arteriole end, so water begins to flow back into the capillary from the tissue fluid
6. Overall, more fluid leaves the capillary than returns, leaving tissue fluid behind to bathe cells
7. If blood pressure is high then the pressure at the arteriole end is even greater
8. This pushes more fluid out of the capillary and fluid begins to accumulate around the tissues.