Session 8 - Special Circulations

Question 1

What are the mean arterial, capillary and venous pressures of the lung

Answer 1

12-15mmHg
9-12mmHg
5mmHg

Question 2

What adaptations are there in the lungs to promote efficient gas exchange

Answer 2

High density of capillaries, therefore large surface area

Short diffusion distance

Question 3

What is the optimal ventilation-perfusion ratio

Answer 3

0.8

Question 4

How is optimal ventilation-perfusion ratio maintained

Answer 4

Via hypoxic pulmonary vasoconstriction

Alveolar hypoxia results in vasoconstriction of pulmonary vessels therefore perfusion is matched to ventilation

Question 5

What are the effects of chronic hypoxic vasoconstriction

What might be a cause?

Answer 5

Can cause right heart failure as there is a chronic increase in vascular resistance which leads to chronic pulmonary hypertension and therefore a high after load on the right ventricle
Can occur at altitude or as consequence to disease such as emphysema

Question 6

Which vessels of the lungs are most affected by gravity

Answer 6

The lower vessels as there is greater hydrostatic pressure

Question 7

How does exercise affect pulmonary blood flow

Answer 7

Increased cardiac output therefore an increase in pulmonary arterial pressure which opens the apical capillaries therefore an increased uptake of O2 in the lungs
As the blood flow increases the capillary transit time is reduced (1s at rest - 0.3s)

Question 8

How is tissue fluid formed

Answer 8

Determined by starling forces

Question 9

How is tissue fluid formed in the lungs

Answer 9

Determined by starling forces
Hydrostatic pressure of blood within the capillary pushes the fluid out
Oncotic pressure exerted by large molecules draws fluid into the capillaries
Low pressure in the lungs minimises the amount of lymph formed

Question 10

Explain how an increase in capillary pressure can cause oedema

Answer 10

An increase in capillary pressure means that there is an increase in the hydrostatic pressure and therefore more fluid is pushed out of the capillary
This can be caused by an increase in left atrial pressure e.g. due to mitral valve stenosis or left ventricular failure

Question 11

How does the cerebral circulation meet the high demand for O2

Answer 11

High capillary density
High basal flow rate - X10 average for whole body
High O2 extraction

Question 12

What occurs in the brain if the blood flow is impaired

Answer 12

Neurones are very sensitive to hypoxia

Irreversible damage occurs in roughly 4 mins

Question 13

How is secure blood supply ensured

Answer 13

Structurally - Anastomoses Circle of Wilis

Functionally - Myogenic auto regulation and metabolic factors

Question 14

What is Myogenic regulation

Answer 14

Changes in transmural pressure lead to a response in the vessels
Increase in blood pressure –> vasoconstriction
Decrease in blood pressure –> vasodilation
Fails below 50mmHg

Question 15

How does metabolic regulation maintain cerebral blood flow

Answer 15

Cerebral vessels are very sensitive to change in pCO2
Increase in pCO2 –> vasodilation
Decrease in pCO2 –> vasoconstriction

Question 16

Which metabolites produce an increase in local blood flow

Answer 16

Increase pCO2

Increase in [K+]

Question 17

Which metabolites produce an increase in local blood flow

Answer 17

Increase pCO2
Increase in [K+]
Increase in Adenosine - very active in cerebral arterioles
Decrease in pO2

Question 18

Explain Cushing’s Reflex

Answer 18

An increase in intracranial pressure impairs blood flow therefore Cushing’s reflex prevents this
Impaired flow to the vasomotor control regions of the brainstem increases the sympathetic activity –> increase BP –> maintained cerebral flow

Question 19

Where do the right and left coronary arteries arise from?

Answer 19

The aortic sinuses

Question 20

By how much can the work rate of the heart increase

Answer 20

5X

Question 21

What is the diffusion distance

Question 22

What is produced by the coronary endothelium that maintains high basal flow and explain its mechanism of action

Answer 22

Nitric Oxide
Activates guanylate cyclase which increases cGMP therefore decreases conc. intracellular Ca2+ –> relaxation of vascular smooth muscle

Question 23

What % of coronary arteries are perfused at any one time in skeletal muscle at rest

Answer 23

50%

Question 24

What induces increased flow in skeletal muscle

Answer 24

Metabolic hyperaemia

Production of agents that are vasodilators K+, osmolarity, inorganic phosphates, adenosine, H+

Question 25

Explain the action of adrenaline in skeletal muscle

Answer 25

As well as having alpha 1 receptors that bind NA and cause vasoconstriction arterioles in skeletal muscle also have beta 2 receptors that have a high affinity for adrenaline and cause vasodilation —> facilitates the fight or flight response

Question 26

Explain the role of the cutaneous circulation in temperature regulation

Answer 26

Apical skin has arterovenous anastomoses which regulate heat loss from apical skin. Apical skin has a high surface area to volume ratio
AVA under neural control therefore when there is a decrease in core temp. this increases sympathetic tone in AVAs so there is decreased flow to the AVAs –> less heat loss (vice versa with increase in core temp)