14 Capillary exchange

Question 1

What effect does increased metabolism have on blood pressure?

Answer 1

• causes localized bp increase

- increased metbolism –> changes in co2 o2 and pH K+ etc –> dilation of arterioles –> increased blood flow

Question 2

What substances increase in concentration in active tissues?

Answer 2

CO2 ADP and organic acids, adenosine and prostaglandin

Question 3

What happens when there is an increase in CO2 in tissues?

Answer 3

autoregulation: Arteries dilate, increase in blood flow.
if auto regulation is ineffective nural mechanisms (short term) or endocrine mechanisms (long term) cause changes in systemic circulation (blood volume and heart rate)
except in pulmonary circulatio where an increase in CO2 can cause vasoconstriction of the arteries to protect the lung tissue

Question 4

How does the endocrine system effect blood pressure?

Answer 4

• changes the extracellular fluid volume via the kidney/hormones
• ADH and aldosterone increase NA content and so water content and total volume of the blood increasing pressure (increasing blood volume increases co2 in the blood)
• arterial natriuretic hormone causes decreased plasma NA content so less water and and pressure in theblood

Question 5

Name some structural adaptation of the capillaries (3)

Answer 5

• Slow blood flow
• Thin walls
• 8 micrometers in diameter

Question 6

What opposes hydrostatic pressure in capillaries?

Answer 6

osmotic forces (lots of proteins in capillaries)

Question 7

What is the net filtration pressure?

Answer 7

Net hydrostatic pressure - net colloid osmotic pressure

Question 8

What are the ranges of hydrostatic pressure in the capillaries?

Answer 8

18 (V) - 35 (A) mmHg

Question 9

What is the net hydrostatic pressure?

Answer 9

Capillary hydrostatic pressure (CHP) - hydrostatic pressure of ISF (taken as 0)

Question 10

What is colloid osmotic pressure?

Answer 10

The attraction of proteins for water

Question 11

What is the net colloid osmotic pressure?

Answer 11

Blood colloid osmotic pressure (BCOP approx 25 mmHg) - colloid osmotic pressure of ISF (ICOP taken as 0 as ISF contains no proteins)

Question 12

Describe capillary exchange in terms of pressure

Answer 12

• Arteriole end: net hydrostatic pressure exceeds net colloid osmotic pressure (25mmHg) so fluid etc moves out
• reaches midpoint where forces are balanced (acc slightly toward venous end)
• Venous end: hydrostatic pressure is less than 25mmHg so fluid moves back into capillary

Question 13

Where is the transition point of filtration vs absorption in the capillary?

Answer 13

Towards the venous end since the max filtration pressure is higher than the max reabsorption pressure…this mean more fluid is filtered out than reabsorbed so lymph has to get involved

Question 14

Defects in capillary exchange:

Answer 14

Oedema
systemic -fluid collects in extremities
pulmonary - fluid builds up in alveoli

Question 15

Special consideration of the brain in circulation:

Answer 15

• blood flow is maintained at a constant level, even in emergencies (even when the rest of the body constricts, the brain vessels may vasodilate)
• four arteries and various anastamoses used to supply the brain so temporary loss of arterial supply in one isn’t a big deal
• High blood flow rate for mass of tissue (neurones have a poor reserve capacity for metabolites)

Question 16

What is the blood flow rate in the brain?

Answer 16

higher than av. - 750ml/min

Question 17

Adaptations of coronary circulation..

Answer 17

due to the compression of the vessels during systole there is a high capillary density leading to a higher capacity (70%) for O2 extraction

Question 18

What does vasoconstriction in another part of the body mean for coronary flow?

Answer 18

It increases it

Question 19

Name some local metabolites that act as vasodilators in the coronary vessels:

Answer 19

decreased O2
Adenosine
lactic acid

Question 20

Adaptations of pulmonary circulation:

Answer 20

• lower blood pressure than systemic circulation (10mmHg, not 35)
• arteries and arterioles have thinner walls with less muscle/elastic fibres so are wider and shorter
• more distensible so can accommodate changes in CO with little change in pressure

Question 21

Why is there lower BP in the lungs?

Answer 21

BCOP exceeds CHP so fluid can be absorbed along the entire length of the capillary so there is no interference with gas exchange
In cases of high BP the opposite dynamic takes over and can cause fluid build up in lungs, interfering with gas exchange

Question 22

Local metabolite effects on pulmonary circulation:

Answer 22

• opposite to rest of the body

- high o2 levels causes vasodilation to increase BF and O2 pick up