Session 8 - Special Circulations Flashcards
What are the mean arterial, capillary and venous pressures of the lung
12-15mmHg
9-12mmHg
5mmHg
What adaptations are there in the lungs to promote efficient gas exchange
High density of capillaries, therefore large surface area
Short diffusion distance
What is the optimal ventilation-perfusion ratio
0.8
How is optimal ventilation-perfusion ratio maintained
Via hypoxic pulmonary vasoconstriction
Alveolar hypoxia results in vasoconstriction of pulmonary vessels therefore perfusion is matched to ventilation
What are the effects of chronic hypoxic vasoconstriction
What might be a cause?
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
Which vessels of the lungs are most affected by gravity
The lower vessels as there is greater hydrostatic pressure
How does exercise affect pulmonary blood flow
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)
How is tissue fluid formed
Determined by starling forces
How is tissue fluid formed in the lungs
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
Explain how an increase in capillary pressure can cause oedema
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
How does the cerebral circulation meet the high demand for O2
High capillary density
High basal flow rate - X10 average for whole body
High O2 extraction
What occurs in the brain if the blood flow is impaired
Neurones are very sensitive to hypoxia
Irreversible damage occurs in roughly 4 mins
How is secure blood supply ensured
Structurally - Anastomoses Circle of Wilis
Functionally - Myogenic auto regulation and metabolic factors
What is Myogenic regulation
Changes in transmural pressure lead to a response in the vessels
Increase in blood pressure –> vasoconstriction
Decrease in blood pressure –> vasodilation
Fails below 50mmHg
How does metabolic regulation maintain cerebral blood flow
Cerebral vessels are very sensitive to change in pCO2
Increase in pCO2 –> vasodilation
Decrease in pCO2 –> vasoconstriction
Which metabolites produce an increase in local blood flow
Increase pCO2
Increase in [K+]
Which metabolites produce an increase in local blood flow
Increase pCO2
Increase in [K+]
Increase in Adenosine - very active in cerebral arterioles
Decrease in pO2
Explain Cushing’s Reflex
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
Where do the right and left coronary arteries arise from?
The aortic sinuses
By how much can the work rate of the heart increase
5X
What is the diffusion distance
What is produced by the coronary endothelium that maintains high basal flow and explain its mechanism of action
Nitric Oxide
Activates guanylate cyclase which increases cGMP therefore decreases conc. intracellular Ca2+ –> relaxation of vascular smooth muscle
What % of coronary arteries are perfused at any one time in skeletal muscle at rest
50%
What induces increased flow in skeletal muscle
Metabolic hyperaemia
Production of agents that are vasodilators K+, osmolarity, inorganic phosphates, adenosine, H+
Explain the action of adrenaline in skeletal muscle
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
Explain the role of the cutaneous circulation in temperature regulation
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)
