5. Microcirculation Flashcards
What is the starting point of microcirculation?
- Arterioles
- Branch off the artery
- Divert blood towards the organ
What is the ‘blood flow rate’ (F) and the equation (relating it with pressure and resistance)?
- Volume of blood passing through a vessel per unit time
- F = ΔP / R
- directly proportional to pressure gradient
- inversely proportional to vascular resistance
What is ‘resistance’?
Hindrance to the blood flow due to friction between moving fluid and stationary vascular walls
What 3 factors affect resistance?
- Blood viscosity
- Vessel Radius (narrower => more resistance)
- Vessel length
How does F, ΔP & R change with increasing BP and arteriolar vasoconstriction?
- ↑BP = ↑F, ↑ΔP
* ↑ Arteriolar Vasoconstriction = ↑R, ↓F
How does the BP generated by the heart and the MAP have an influence on the microcirculation?
- BP generated by heart - does not change much
- Blood pressure in any artery around the body is around the MAP
- MAP (going in) in arterioles but pressure in capillaries influenced by dilation
- Allows blood to pass slowly to allow exchange of nutrients
- End of capillaries - venous pressure, around 0 mmHg
- Resistance of arterioles in the organ is the main determinant of blood flow in the body
- Pressure difference allows blood to reach tissue capillary beds
What is the state of the arteriolar smooth muscle and why?
- Partial constriction - vascular tone
* Allows space for changing the blood flow by constricting or dilating
What can the adjustment of the radii of arterioles be used to accomplish?
- Match blood flow to the metabolic needs of specific tissue - local intrinsic control (independent of nerves and hormones)
- Help regulate arterial blood pressure - extrinsic controls
What is active hyperaemia?
- Muscle becomes more active (↑ metabolism and oxygen consumption)
- Change detected in tissues and signal is sent to the arterioles to vasodilate - more blood flow
- Response to local conditions
- in other words: “an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue”
What happens in the microcirculation if blood temperature decreases?
- Arteriolar smooth muscle contracts
- Less blood reaches the surface - less heat radiated away
- This is why cold is applied to a damaged area as it reduces blood flow => swelling
What is myogenic vasoconstriction?
- Type of autoregulation
- When BP increases (e.g. during exercise) more blood is needed in some places than others
- Active hyperaemia in lungs, heart and muscles
- Myogenic vasconstriction in tissues that don’t need as much blood (not all the blood can reach every tissue so an appropriate distribution is needed)
- MV increases resistance to match the increase in pressure, so the flow rate returns to normal
How can F = ΔP / R be substituted into an equation describing cardiac output?
CO = MAP / TPR (total peripheral resistance)
Describe the neural regulation of arterial blood pressure (extrinsic)
• Regulated by the cardiovascular control centre in the medulla (of the brain)
• To increase BP - brain secretes noradrenaline via sympathetic nervous system
• Adrenoreceptors
- Alpha - constriction of arterioles
- Beta - increases heart rate
• Blood flow to organs is reduced
• Brain will always try to receive the normal amount of blood to survive - so, some organs may receive very little blood if blood loss occurs
Describe the hormonal regulation of arterial blood pressure (extrinsic)
• The brain can mimic the sympathetic nervous system to control the heart
- stimulates the production of adrenaline and noradrenaline
• Can act on arterioles
- stimulates the production of vasopressin and angiotensin II
• Vasopressin comes from the posterior pituitary
• Angiotensin II (mainly) comes from the lungs
What is the pressure entering and leaving the arterioles?
- 93 mmHg entering
* 37 mmHg leaving
Describe the hormonal regulation of arterial blood pressure (extrinsic)
• The brain can mimic the sympathetic nervous system to control the heart
- stimulates the production of adrenaline and noradrenaline
• Can act on arterioles
- stimulates the production of vasopressin and angiotensin II
• Vasopressin comes from the posterior pituitary
• Angiotensin II (mainly) comes from the lungs
What is the pressure entering and leaving the arterioles?
- 93 mmHg entering
* 37 mmHg leaving
When is capillary density important?
- More metabolically demanding tissue - greater capillary density
- e.g. skeletal muscle, myocardium, brain, lungs
- Myocardium and brain are vulnerable to hypoxia so this is important
- Lung has this to maximise gas exchange, not metabolic activity
What 3 things do the capillaries aim to achieve with their design?
- Minimise diffusion distance
- Maximise surface area
- Maximise diffusion time
When is capillary density important?
- More metabolically demanding tissue - greater capillary density
- e.g. skeletal muscle, myocardium, brain, lungs
- (Myocardium and brain are vulnerable to hypoxia so this is important)
Why is adipose tissue poorly perfused?
Nothing that the adipose tissue does that needs a rapid response
What is special about the perfusion of the skeletal muscle?
- High capillary density
- Large number of these are shut off at rest
- Precapillary sphincter shuts off arterioles/capillaries
What are the features of fenestrated capillaries?
- Leakier capillaries
- Slightly bigger holes - fenestrae (around 80nM) in cells
- Allow larger substances to pass e.g. glomerulus
What are the features of discontinuous capillaries?
- Large holes in the capillary between the endothelial cells
* Important in the bone marrow - white cells have to get into blood
What are the features of fenestrated capillaries?
- Leakier capillaries
- Slightly bigger holes - fenestrae (around 80nM)
- Allow larger substances to pass e.g. glomerulus
What type of capillary do you mainly find as part of the blood-brain barrier and why?
(• continuous)
• No water-filled gap junctions between endothelial cells
• Really tight gap junctions instead
• Substances have to diffuse across endothelial cells - access to the brain tightly regulated and it is more protected
• Brain has the fewest blood borne agents
What happens to lipid soluble molecules in the blood-brain barrier?
- Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells
- Specific carrier proteins remove them
What happens to lipid soluble molecules in the blood-brain barrier?
- Lipid soluble molecules can diffuse through the lipid bilayer in endothelial cells
- Specific carrier proteins remove them
What is bulk flow?
A volume of protein-free plasma that filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
What are starling forces?
• The 2 main pressure affecting the movement of fluid in and out of the capillary:
- Hydrostatic pressure
- Oncotic pressure
How does the pressure change across the capillaries and why?
• Higher pressure at the resistance vessels (arteriolar end)
• Pressure drops considerably at the venular end
• Oncotic pressure doesn’t change
• Balance of Starling forces determine movement in and out:
- Arteriolar end: hydrostatic > oncotic - ultrafiltration
- Venular end: oncotic > hydrostatic - reabsorption
• Some fluid might not return to the capillary (e.g. 9 mmHg leaves, 8 mmHg returns) and is drained by the lymphatic system
How does the pressure change across the capillaries and why?
- Higher pressure at the resistance vessels (arteriolar end)
* Pressure drops considerably at the venular end
Where do you find lymphatic capillaries?
By normal blood capillaries
Does the circulation of blood and the lymphatic system form a closed loop?
- Circulation of blood forms a closed loop
* Lymphatic system doesn’t - they are blind ended
How does fluid move through the lymphatic vessels?
- Lymphatic pressure (skeletal muscles, respiratory pump)
* Valves ensure flow in one direction
At which vessels does the lymph all come together and drain into?
Thoracic duct => right lymphatic duct => right subclavian vein => left subclavian vein
How much fluid is returned to the circulatory system by the lymphatic system per day?
3L
What can the parasitic blockage of lymph nodes lead to?
- Oedema
- rate of production of fluid > rate of removal of fluid
- e.g. filariasis => elephantiasis
