Exchange in, and control of, the Peripheral Circulation Flashcards
What is the function of capillaries ?
They are specialised for exchange
What are some properties of capillaries ?
There are lots of them: - Every tissue within 100mm They are thin-walled: - Presents a small diffusion barrier They have a small diameter: Big surface area: volume ratio
What are continous capillaries ?
No clefts or pores e.g. Brain (hence blood brain barrier)
Lefts only e.g. Muscle and most other capillaries
What are fenestrated capillaries ?
Clefts and pore e.g. Intestine and kidney, specialised for fluid exchnage
What are discontinous capillaries ?
Clefts and massive pores e.g. Liver
How does most exchange happen ?
Most exchange is via diffusion:
- Self-regulating
- Non-saturable
- Non-polar substances across the phospholipid membrane
- Polar substances through clefts/ pores
Where is there carrier mediated transporters ?
Glucose transporter in the brain
What is bulk flow determined by ?
Starlings forces:
- Capillary hydrostatic pressure v ISF hydrostatic pressure
- Plasma osmotic pressure v ISF osmotic pressure
- Net filtration pressure = (HC - HIF) - (pP - pIF)
What is oedema ?
Accumulation of excess fluid
What can oedema be due to ?
Lymphatic obstruction - e.g. Due to filariasis, surgery Raised JVP - e.g. Due to ventricular failure Hypoproteinaemia - e.g. Due to nephrosis, liver failure, nutrition Increased capillary permeability - inflammation, e.g. Rheumatism
How is MAP calculated ?
MAP = CO X TPR
What does the arteriolar radius affect ?
Arteriolar radius affects through individual vascular beds, and it affects mean arterial pressure
You cannot affect one without the other
How can we keep the blood flow to each vascular bed sufficient and keep MAP in the right range ?
This is achieved by two levels of control over the smooth muscle surrounding arterioles:
- Local (intrinsic) mechanisms- concerned with meeting the selfish needs of each individual tissue - Central (extrinsic) mechanisms- concerned with ensuring that the total peripheral resistance (MAP) of the whole body stays in the right ball park
How does active hyperaemia trigger an increase in local metabolism ?
Increase metabolic activity causes increase in conc of metabolites
Triggers release of paracrine signal (e.g. EDRF/NO)
Cause arteriolar dilation
Increase flow to wash out metabolites
An adaption to match blood supply to the metabolic needs of that tissue
How does pressure autoregulation cause a decrease in perfusion pressure ?
Decrease MAP causes decrease flow
Metabolites accumulate
Triggers release of paracrine signal
Arterioles dilate and flow is restored to normal
An adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
How does active hyperaemia trigger an occlusion of blood supply ?
This causes a subsequent increase in blood flow
An extreme version of pressure autoregulation
What does the injury response aid ?
Delivery of blood born leukocytes etc to injured area
What is the effect of sympathetic nerves on central neural control ?
Release noradrenaline
Binds to alpha1-receptors
Cause arteriolar constriction
Therefore decrease flow through that tissue and tends to increase TPR
What is the effect of parasympathetic nerves on central neural control ?
Usually no effect
Genitalia and salivary glands are the exception
How does adrenaline effect central hormonal control ?
Released from adrenal medulla
Binds to alpha1-receptors
Cause arteriolar constriction
Therefore decrease flow through that tissue and tends to increase TPR
In some tissues e.g. Skeletal and cardiac muscle, it also activates B2-receptors
Cause arteriolar dilation
Therefore increase flow through that tissue and tends to decreases TPR
Significance re exercise
