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.
When does most of the coronary blood flow occur?
Almost exclusively in diastole, especially to the left ventricle.
When is it the most difficult for blood to flow through the coronary circulation?
Especially in the isovolumetric contraction stage and also systole, due to the contraction of cardiomyocytes putting pressure on and constricting the coronary arteries.
How is cardiac muscle adapted to increase its blood flow relative to skeletal muscle?
1. Smaller fibre diameter, therefore smaller diffusion distance
2. Higher capillary density
3. Constant perfusion of all capillaries
4. Continuous production of NO (vasodilator) by coronary endothelium which maintains a high basal blood flow
How is the coronary blood flow increased to meet extra demand?
Vasodilation due to metabolic hyperaemia (adenosine, increase in K+, CO2 and decrease in O2.
What factors, other than exercise, can trigger angina?
Stress, cold, infection etc... anything that causes sympathetic coronary vasoconstriction.
What is a systemic functional role dof skeletal muscle blood flow?
Important role in regulating arterial blood pressure
How does the baroreceptor reflex maintain blood pressure?
The baroreceptors detect a drop in blood pressure and lead to an increase in sympathetic outflow. This increases the sympathetic vasomotor tone (NA binding to alpha-1 receptors causes vasoconstriction), increasing peripheral resistance and causing an increase in arterial blood pressure.
How is skeletal muscle able to alter its blood flow so dramatically?
1. High vasomotor tone - permits lots of dilation
2. Half its capillaries are perfused at any one time, at rest - can increase its recruitment significantly
What controls vasodilation in skeletal muscle?
1. Metabolic vasodilators - K+, increased osmolarity, inorganic phosphates, adenosine, H+
2. Adrenaline - acting through B2 receptors
What are some of the systemic roles of the cutaneous circulation?
1. Temperature control (main function) - main heat dissipating surface
2. Maintaining blood pressure - e.g. vasoconstriction in shock
Give some examples of apical skin:
Areas with a high surface area to volume ratio e.g. ears, nose, toes
What is the main regulator of cutaneous circulation blood flow?
The sympathetic nervous system.
Most blood flow through the skin goes through which type of blood vessels?
Arterio-venous anastamoses, rather than capillaries, therefore it is not nutritive.
Non-apical skin loses heat by dilation of vasodilation of capillary beds and increased sweating. How does apical skin lose heat?
By shunting blood through arterio-venous anastamoses (low resistance shunt), as veins lie closer to the surface of the skin.
The main regulator of cutaneous blood flow is the sympathetic nervous system. What else mediates it?
It is not affected much by local metabolites, but is affected by mediators released from sweat glands (bradykinin?), causes vasodilation. Circulating vasodilators from other sources sometimes also increase skin blood flow.
Aterio-venous anastomoses are under what control?
Neural control - by sympathetic vasoconstrictor fibres. They are not regulated by local metabolites.
What is the cutaneous circulatory response to an decrease in core body temperature?
Sympathetic vasocontrictor drive to the AVAs is increased which results in decreased blood flow through the apical skin and less heat dissipation.