Flashcards in CVS - special circulations Deck (47)
What are the major differences between the systemic and pulmonary circulations?
1. Systemic circulation is demand led whereas the pulmonary circulation is supply driven
2. Pulmonary system offers minimal flow resistance, operating as a low resistance, low pressure system
3. In the pulmonary circulation poor perfusion results in vasoconstriction, whereas in the systemic circulation it results in vasodilation.
What are the two circulations that supply the lungs?
1. The bronchial circulation - supplies the metabolic needs and is part of the systemic circulation
2. The pulmonary circulation - blood supply to alveoli for gas exchange
What causes the low resistance in the pulmonary system?
1. Short, wide vessels
2. Lots of capillaries (many parallel elements)
3. Arterioles have relatively little smooth muscle
How are the lungs adapted to promote efficient gas exchange?
1. Very dense network of capillaries in alveoli wall - large SA
2. Short diffusion distance (endothelium and alveoli epithelium together ~0.3 micrometres)
What is the optimal ventilation perfusion ratio?
How is the optimal ventilation perfusion ratio maintained in the lungs?
Blood is diverted away from alveoli that are not well ventilated. This is called hypoxic pulmonary vasoconstriction.
What is the most important mechanism maintaining pulmonary vascular tone?
Hypoxic pulmonary vasoconstriction
What are the causes of ventilation/ perfusion mismatch?
Many cardiovascular and respiratory disease cause this. Also physiological shunting of improperly oxygenated blood occurs due to a small mismatch caused by effect of gravity. Gravity increases the blood flow to the base of the lungs, whereas more air is delivered to the apices of the lungs.
What are some causes of chronic hypoxia?
The effects of altitude or chronic lung diseases such as emphysema.
How can chronic hypoxic pulmonary vasoconstriction lead to right ventricular failure?
It increases the resistance of the pulmonary circulation and therefore the pressure that the right ventricle requires to pump the entire cardiac output through the lung (chronic pulmonary hypertension). This leads to right ventricular hypertrophy and the high afterload on the right ventricle can lead to failure.
What is the effect of exercise on the pulmonary circulation?
1. Increased cardiac output
2. Small increase in pulmonary capillary pressure
3. Apical capillaries open up
4. Larger oxygen uptake in lungs
5. As blood flow increases capillary transit time is reduced, therefore less efficient gas exchange (but generally not enough to compromise gas exchange)
What is the biggest influence on capillary hydrostatic pressure in the systemic circulation?
Venous pressure - therefore hypertension does not result in peripheral oedema (silent killer), whereas heart failure increases venous pressure and therefore does cause peripheral oedema.
What minimises the formation of lung lymph?
Capillary pressure in the pulmonary circulation is lower than in the systemic circulation (but has the same plasma oncotic pressure). BUT oncotic pressure into the lung tissue is higher than that in the periphery. Together these two processes even out, so that filtration is roughly equal to reabsorption.
Although venous pressure is the biggest influence on capillary hydrostatic pressure. An increase in pulmonary arterial pressure can cause pulmonary oedema too. What can cause an increase in pulmonary arterial pressure?
Mitral stenosis and left ventricular failure - this increases the volume of blood left in the left side of the heart which has a knock on increase in pulmonary vein pressure and pulmonary arterial pressure.
Why do patient with pulmonary oedema prop themselves up with pillows at night time?
Due to the effect of gravity on hydrostatic pressure in the lungs. Pulmonary oedema occurs mainly at the base of the lungs in the orthostatic position but throughout the lungs in a supine position.
In general, how is pulmonary oedema treated?
1. Treat underlying cause
2. Relieve symptoms with diuretics
What percentage of the cardiac output does the brain receive?
What is the percentage oxygen consumption of cerebral grey matter at rest?
How does the cerebral circulation meet the brain's high demand for oxygen?
1. High capillary density: large SA, small diffusion distance
2. High basal flow rate (10x rest of body)
3. High oxygen extraction (35% above average)
Why is oxygen supply to the brain so important?
Neurones are very sensitive to hypoxia:
1. loss of consciousness - after a few seconds of cerebral ischaemia
2. Irreversible damage occurs in ~4 minutes
temporary loss of consciousness caused by a fall in blood pressure
What structural properties of the cerebral circulation help to ensure a secure blood supply?
Anastamoses between basilar and internal carotid arteries (Circle of Willis)
What functional properties of the cerebral circulation help to ensure a secure blood supply?
1. Myogenic autoregulation - during hypotension
2. Metabolic factors - affect blood flow
3. Brainstem regulates other circulations
Describe myogenic autoregulation in the cerebral circulation
Cerebral resistance vessels have a myogenic response to changes in transmural pressure. These maintain blood flow with changing blood pressure. Decreased blood pressure causes vasoconstriction and vice versa. This system fails if BP falls below 50mmHg.
Describe how metabolic factors affect cerebral blood flow
1. Cerebral vessels are very sensitive to changes in arterial pCO2. Increase in pCO2 (hypercapnia) causes vasodilation. Hypocapnia causes vasoconstriction.
2. An increase in K+ and adenosine and a decrease in pO2 also cause vasodilation
Why can panic hyperventialtion cause syncope?
Panic hyperventilation increases CO2 exhalation, therefore decreasing pCO2 in blood. This causes vasoconstriction of cerebral vessels and leads to cerebral ischaemia which causes temporary loss of consciousness after a few seconds. Upon which their breathing will return to normal.
Why do people blow into a bag when hyperventilating?
To reduce the risk of hypocapnia causing syncope.
What is Cushing's reflex?
Impaired blood supply to the vasomotor control regions of the brainstem (caused by an increase in intracranial pressure which impairs cerebral blood flow) increases sympathetic vasomotor activity in the systemic circulation which results in an increased arterial blood pressure and therefore help maintain cerebral blood flow.
What do the cerebral capillary endothelial cells allow through the blood-brain barrier?
Lipid soluble molecules like O2 and CO2 can diffuse freely, whereas lipid-insoluble molecules like K+ and catecholamines can't.