L14 - Special Circulations Flashcards
Why is there a minimal difference between the pressure in the right ventricle and the left atrium?
Because the pulmonary circulation is a low pressure system
What are the normal pressures in the pulmonary arteries, pulmonary capillaries and pulmonary veins ?
Pulmonary arteries - 12-15mmHg
Pulmonary capillaries - 9-12mmHg
Pulmonary veins - 5mmHg
What features of the pulmonary circulation allow it to have low resistance?
Short, wide vessels
Lots of capillaries
Arterioles have relatively low amount of smooth muscle
Give two adaptations of the alveoli that promotes efficient gas exchange
High density of capillaries in alveolar wall - means a big SA for exchange
Very thin endothelium and epithelium of alveoli means a short diffusion distance
Explain the difference between ventilation and perfusion of the alveoli
Ventilation - the amount of air that reaches the alveoli
Perfusion - the amount of blood that reaches the alveoli
Optimal gas exchange is dependent on the ratio between ventilation and perfusion - the ‘V:Q ratio’. What is the optimal ratio?
0.8
Hypoxic pulmonary vasoconstriction is the most important mechanism in ensuring that perfusion matches ventilation of the alveoli. Explain this mechanism
Alveolar hypoxia results in the smooth muscle of pulmonary vessels to said alveoli vasoconstriction, this means blood is diverted away from poorly ventilated alveoli
Explain how chronic hypoxic vasoconstriction which can occur as a consequence of emphysema or at high altitudes can cause right ventricular failure
Chronic increase in vascular resistance (due to everywhere being poorly perfused in emphysema) leads to pulmonary hypertension. This means the vascular resistance is very high -> high afterload on right ventricle -> hypertrophy then failure
Describe briefly ways in which exercise affects pulmonary blood flow to maximise oxygen uptake
Apical vessels which are normally closed at rest due to gravity are opened
Cardiac output increases which slightly increases the pulmonary arterial pressure
Both these things together cause an increase in blood flow and as this happens capillary transit time by the RBC’s is increased without compromising gas exchange
Describe how the forces in the lung capillaries ensure a low formation of extracellular fluid in the lung normally
Hydrostatic pressure and the interstitial oncotic pressure together equal the plasma oncotic pressure at the capillary level
When do we see pulmonary oedema? Give an example of a condition which would cause this
Pulmonary oedema is occurs when the normally low hydrostatic pressure of the pulmonary capillaries and increases resulting in oedema
Mitral valve stenosis - marrow means blood backs up into the left atrium, backflow of pressure into the lung -> pulmonary hypertension
Left ventricular failure - same mechanism as above. Can’t cope with the amount of blood -> backflow of pressure
Why are symptoms of pulmonary oedema worse when lying down?
What is prescribed for pulmonary oedema?
Because less of the pulmonary capillaries are distended and under more pressure, gravity isn’t assisting
Diuretics prescribed
The brain has a high oxygen demand especially when compared to its mass. How does the cerebral circulation meet the brains high demand for oxygen?
1) high capillary density - increases SA for gas exchange
2) high basal flow rate
3) high oxygen extraction
Describe how a ‘secure’ blood flow to the brain is supplied both structurally and functionally
Anastomoses in the circle of Willis
Myogenic autoregulation - Ensures adequate cerebral blood flow despite changes in blood pressure
What factors influence blood flow through the brain, briefly explain each
Co2 - hypercapnia causes vasodilatation and the reverse vasoconstriction
Local metabolites such as k+ and adenosine are vasodilators. A decrease in oxygen partial pressure causes vasodilation too
