Exchange in, and control of, the Peripheral Circulation Flashcards
What is the point where one endothelial cell meets another known as?
Junction
What is a tight junction?
Endothelial cells are pressed very tightly together and this stops anything moving across.
Give an example of a tight junction.
Blood brain barrier.
In the vast majority of capillaries, there is a gap between the endothelial cells. What is this gap known as?
A cleft
What can be diffused over clefts?
Water and some dissolved solutes.
What cannot diffuse across a cleft?
Macromolecules, like proteins
How do macromolecules get transported across endothelial cells?
Via transcytosis.
What are the structures which carry macromolecules across during transcytosis known as?
Pores
What is between endothelial cells?
What is across endothelial cells?
Between= clefts
Across= pores
Name the three types of capillaries.
Continuous
Fenestrated
Discontinuous
Describe continuous capillaires.
No clefts or pores e.g. brain (hence the blood-brain barrier)
or
May only have clefts, like in muscle
Why is the blood brain barrier important?
Prevents things getting into the brain.
Protects brain from changes in potassium ion conc. which could alter the activity of the brain.
Describe fenestrated capillaries.
Clefts and pores
Specialised for fluid exchange
Give an example of fenestrated capillaries.
Intestine and kidneys
Describe discontinuous capillaries.
Has clefts and massive pores
Give an example of where you might find a discontinuous capillary.
Liver
Why are there discontinuous capillaries in the liver?
Massive pores which allow protein into the liver
Most exchange in capillaries happens via which process?
Diffusion
Describe the diffusion process of oxygen from capillaries to cells.
Oxygen moves down it’s conc. gradient into the extracellular fluid.
Oxygen moves from ECF to the cells, again going down it’s conc. gradient.
List the reasons why diffusion is suitable for capillaries.
Self-regulating
Non-saturable
Non-polar substances across the phospholipid membrane
Polar substances through clefts/pores
Describe what is meant by diffusion being a self-regulating process.
If oxygen levels are low, like during exercise, the conc. gradient will increase and more oxygen will flow from the capillary->ECF->cells
List some non-polar substances.
Oxygen and carbon dioxide
Give an example of a polar substance.
Water
How will carbon dioxide and oxygen get across?
Via dissuion.
How will water get across the membrane?
Via clefts and pores.
Give an example of a glucose mediated transporter.
Glucose transporter.
->brain required glucose but cannot diffuse across because of the blood brain barrier.
The majority of exchange happens via diffusion but how else can exchange occur?
In bulk flow
What is bulk flow determined by?
Starling’s forces
Which type of pressure can push water out of the clefts and pores of the endothelial cells in a capillary?
Hydrostatic pressure
Which type of pressure brings water back into the capillary via clefts and pores of the endothelial cell when concentration gets too high?
Osmotic pressure
How can we work out the balance between hydrostatic pressure and osmotic pressure?
By working out net filtration pressure
(Difference in hydrostatic pressure between capillaries and ISF) - (Difference in osmotic pressure between the capillaries and ISF)
On average, how much fluid is lost per day due to hydrostatic pressure?
20L
On average, how much fluid is regained per day due to osmotic pressure?
17L
What happens to the remaining 3L of fluid that was lost due to hydrostatic pressure?
It drains into the lymphatic system
Describe the journey taken by the fluid drained into the lymphatic system.
Fluid in lymphatic capillaries -> Drains to lymph nodes -> Drains to larger lymphatic vessels -> Makes it’s way back to the heart via vena cava and fluid is returned to the CVS
What happens if the lymphatic system isn’t working?
The extra fluid will form an oedema.
Define oedema.
Build up of excess fluid.
List the ways an oedema may form.
Raised CVP
Lymphatic obstruction
Hypoproteinaemia
Increased capillary permeability
Discuss how raised central venous pressure can cause an oedema.
Say the left ventricle isn’t pumping out blood but the right side of the heart is okay, there will be blood in the lungs.
This leads to an increased hydrostatic pressure in the capillaries and can lead to pulmonary oedema.
When may there by lymphatic obstruction?
Due to filariasis* or surgery
*parasitic disease caused by worms
What can cause hypoproteinaemia which can ultimately lead to oedema?
Nephrotic syndrome*, liver failure, malnutrition
*kidneys leaking protein in the urine
Why do we need protein to prevent oedema?
We need protein in capillaries to build up oncotic pressure to get water back in.
If there’s not enough, more fluid will end up in the lymphatic system as they don’t have osmotic pressure.
When may increased capillary permeability occur?
In those with rheumatoid arthritis.
RECAP- state Darcy’s Law.
Flow= change in pressure
—————————————
Resistance
When may the viscosity of your blood increase?
If you were dehydrated
What happens to the resistance of a tube if you half the radius?
Resistance increases by 2 to the power of 4 = 16 fold
Substituting Poiseuille’s Law into Darcy’s Law gives us the following equation-
Flow= (Change in pressure) x (radius to the power of 4) x pi
————————————————————————————–
Viscosity x (length of capillary) x 8
We do not need to get hung up over the details of this equation and you don’t need to learn it. However, what is the key takeaway message?
Changing to radius has a large impact on flow.
How can we calculate mean arterial pressure?
MAP= CO X TPR
(reminder -> CO= cardiac output, TPR= total periphery resistance).
What happens if MAP is too low?
Decreased perfusion through vascular beds, brain will notice this and can cause dizziness.
What happens if MAP is too high?
No immediate effects but high BP over time increases risk of hypertension and risk of a stroke
(h.e. knowledge coming in clutch here hehe).
Varying the radius of resistance vessels can control which things?
- Regional redirection of blood flow
- Total periphery distance, regulating MAP
Define active hyperaemia.
Increased blood flow
What is the cause of active hyperaemia?
Increased metabolic activity.
Describe what happens when metabolite production is increased in the blood.
Increase in metabolic activity causes an increase of metabolites produced.
Triggers release of paracrine signal (e.g. EDRF/NO), causing arterial dilation to make sure the blood flow matches the rate of metabolites and i able to wash them away.
Name four types of local/intrinsic controls for blood flow.
- Active hyperaemia
- Pressure/flow autoregulation
- Reactive hyperaemia
- Injury response
What is the trigger for pressure/flow autoregaultion?
Decrease in perfusion pressure.
What is the trigger for active hyperaemia?
Increase in local metabolism.
Describe what happens in pressure autoregulation when there is an increase in MAP.
Increases causes decrease of blood flow.
Metabolites accumulate.
Triggers release of paracrine signal (e.g. EDRF/NO)
Arterioles dilate and flow is restored to normal
What is the trigger for reactive hyperaemia?
Occlusion of blood supply.
What does an occlusion of blood supply do to blood flow?
Increases blood flow
Discuss what happens in the injury repsonse.
Response to stubbing your toe, for example.
- C-fibres are activated- will send active potential via spinal cord to brain
- Action potential fired down collateral branches causing activation of substance P
- Substance P acts on mast cells and causes them to release histamine
- Histamine causes smooth muscle to relax causes arterioles to dilate, increasing blood flow.
- Injury site goes red
- Increased blood flow can help to get leukocytes to site of injury to attack any invading pathogens.
Please read through this and write it out :)
In fight or flight, what is released from sympathetic nerves?
Noradrenaline
What does noradrenaline bind to and what does this cause?
Binds to α1 receptors
Causes arteriolar constriction, decreasing blood flow
What does a decrease in blood flow do to TPR and MAP?
Increases both
What is the effect of parasympathetic nerves on blood flow?
Usually no effect as don’t innervate blood vessels
Where in the body is there an effect based on parasympathetic nerves?
Genitalia and salivary glands (↑ flow)
What is released for the central hormonal control of blood flow?
Adrenaline
Where is adrenaline released from?
Adrenal medulla
Where does adrenaline bind and what is the effect on blood flow?
Binds to α1 receptors
Causes arteriolar constriction
Therefore ↓ flow through that tissue and tends to ↑ TPR and ↑ MAP
What would happen in some tissues like skeletal or cardiac when adrenaline is released?
Also binds and activates B2 receptors.
Causes arteriolar dilation
Therefore ↑ flow through that tissue and tends to ↓ TPR
What happens to the coronary circulation during systole?
Very low blood flow, almost cut off as heart contracts.
What happens to coronary blood flow during diastole?
Blood flow greatly increased
How can the heart deal with the demand of oxygen during exercise despite the blood supply being interrupted during systole?
->Good active hyperaemia and is sensitive to small changes in metabolites
-> Expresses many Beta 2 receptors which allow adrenaline to bind, allowing smooth muscle relaxation and arteriole dilation.
What type of local/intrinsic control is the cerebral circulation very good at?
Pressure autoregulation
In the pulmonary circulation, what will a decrease in oxygen do?
Causes arteriolar constriction
(opposite to most tissues)
Why does a decrease in oxygen lead to arteriolar constriction in the pulmonary circulation?
It ensures the blood is directed to the best ventilated parts of the lungs
(ventilation-perfusion matching discussed in resp. lectures!)
What is the main job of the renal circulation?
Filtration
What is filtration dependant on?
Pressure
Filtration rate in the renal circulation can be kept pretty constant despite change in MAP. Why?
Excellent pressure autoregulation.
