Physiology 6 - Microcirculation

Question 1

Why is the consistency of interstitial fluid more like a gel than a free flowing liquid?

Answer 1

Collagen fibres and proteoglycan filaments (interstitium) are dispersed in the interstitial fluid, increasing viscosity

Question 2

Why do colloids (plasma proteins) not move out of the capillaries?

Answer 2

In a nutshell, they are too large - barred from crossing by capillary walls. A few however, may escape

Question 3

Explain the concept behind oncotic pressure.

Answer 3

Large amount of plasma protein (colloid), predominantly albumin, in the capillary
Smaller quantity of albumin in the interstitial space
Oncotic pressure in capillary is therefore high (roughly 28mmHg)
Oncotic pressure in interstitial space is lower (5-8mmHg)
Water is drawn across toward the majority of solute, according to laws of osmosis, so moves into capillary

Question 4

What is the average hydrostatic pressure at the arterial end of circulation?

Answer 4

30-40mmHg

Question 5

Negative hydrostatic pressure in the interstitial space leads to…?

Answer 5

Water drawn out of capillaries

Question 6

Positive hydrostatic pressure in the interstitial space leads to…?

Answer 6

Water pulled into capillaries

Question 7

What is the average hydrostatic pressure at the venous end of circulation?

Answer 7

10-15mmHg

Question 8

What would be the result of having purely hydrostatic pressure pushing water out?

Answer 8

Oedema

Question 9

Give 4 methods which increase venous pressure and hence increase venous return.

Answer 9

Increase blood volume
Increase inspiration movements
Increase skeletal muscle movement (as a pump)
Increase activity of sympathetic nerves to veins

Question 10

List the chain of events starting with: Venous pressure ^ —> Venous return ^ —>

Answer 10

Atrial pressure ^
End diastolic ventricular volume ^
Stroke volume ^
Cardiac output ^

Question 11

What is the interstitium (ECM) composed of?

Answer 11

Collagen
Proteoglycan filaments
Interstitial fluid

Question 12

What are low molecular weight solutes (e.g. Cl-, K+ or Na+) known as?

Answer 12

Crystalloids

Question 13

What are large molecular weight plasma proteins such as albumin known as?

Answer 13

Colloids

Question 14

What is bulk flow?

Answer 14

Distribution of the extracellular fluid

Question 15

What is normal oncotic pressure in the capillary?

Answer 15

28mmHg

Question 16

What is normal oncotic interstitial pressure?

Answer 16

5-8mmHg

Question 17

Why is capillary oncotic pressure higher than interstitial oncotic pressure?

Answer 17

Larger amount of albumin present in the blood than in the interstitium

Question 18

What are normal arteriole and venule hydrostatic pressures?

Answer 18

Arteriole: 30-40mmHg
Venule: 10-15mmHg

Question 19

What does the hydrostatic pressure do?

Answer 19

Forces fluid out of the capillaries and into the interstitium

Question 20

What direction does fluid move as interstitial pressure alternates between positive and negative?

Answer 20

Forces fluid in to the capillary when positive

Draws fluid in to the interstitium when negative

Question 21

What is normal interstitial hydrostatic pressure?

Answer 21

Negligible - 0mmHg

Question 22

How is fluid built up in the interstitium removed?

Answer 22

Via the lymphatic system

Question 23

What is the overall movement of fluid at the arteriole end of the capillary?

Answer 23

Hydrostatic pressure > Oncotic pressure – fluid moves out of capillary

Question 24

What is the overall movement of fluid at the venule end of the capillary?

Answer 24

Hydrostatic pressure < Oncotic pressure – fluid moves into capillary

Question 25

Overall, do capillaries lose or gain water as blood moves from arteriole to venous end?

Answer 25

Lose more than they gain

Question 26

What four factors is the lymphatic system responsible for regulating?

Answer 26

Concentration of proteins in interstitial fluids (lymphatics allow movement of large molecules)
Volume of interstitial fluid
Interstitial fluid pressure
Immune response

Question 27

What is the pressure range of the systemic venous circulation?

Answer 27

3-18mmHg

Question 28

What results from increasing venous return?

Answer 28

Increased EDV and therefore increased CO

Question 29

Which nervous system controls the capacity of veins?

Answer 29

Sympathetic n.s.

Question 30

What four factors increase venous return to the heart?

Answer 30

Sympathetic innervation (smooth muscle contraction)
Muscle pumps
Inspiratory movements (due to descending diaphragm and decreased thoracic pressure)
Blood volume

Question 31

What is orthostatic (postural) hypotension, when does it occur and what is the effect it has on the heart?

Answer 31

An immediate effect when going from supine to upright. Around 500 ml of blood moves from the upper body to the legs. There is a ↓ venous return, therefore ↓ cardiac output, therefore ↓ BP. Reflex vasoconstriction in legs and lower abdomen after a few seconds delay