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HDT 2.2 COPY > Exchange Across Capillaries and Venous Return > Flashcards

Exchange Across Capillaries and Venous Return Flashcards

1
Q

In what vessels does capillary take place?

A

capillaries

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2
Q

Capillaries are simple tubes of what cells?

A

endothelial

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3
Q

What is exchanged between capillaries?

A
  1. solutes - oxygen, glucose, amino acids etc and also waste products and metabolites such as CO2 and K
    1. by diffusion
  2. fluid - plasma minus plasma proteins pass across endothelial cells to interstitial space and back by filtration
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4
Q

What are three characteristics of capillaries?

A
  • endothelium (no vascular smooth muscle)
  • 3-6 micrometres diameter
  • discontinuous blood flow - can become more or less continuous
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5
Q

Arteriolar constriction leads to what in terms of arteriolar resistance and pressure in capillaries?

A

increased arteriolar resistance

pressure at capillary opening - lost when going through

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6
Q

Compare capillaries to arteries and veins in terms of surface area and speed of flow.

A

capillaries have the highest surface area and the lowest speed of flow

this means theres more time for exchange in the capillaries

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7
Q

What is the formula for rate of diffusion?

A

rate = P x (C1-C2) x A

C conc gradient

A surface area of capillaries

P permeability - increased during inflammation

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8
Q

Why does rate of diffusion increase when arteries dilate?

A

tissue blood flow increases

conc gradient steeper and more capillaries are better perfused with blood

eg functional hyperaemia

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9
Q

What forces does filtration across capillaries depend on?

A

hydrostatic pressure - fluid pressure

osmotic pressure - mainly exerted by proteins called oncotic pressure

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10
Q

Explain the starling forces across capillaries.

A

dependent on four different pressures

  • capillary hydrostatic pressure
  • tissue hydrostatic pressure
  • plasma oncotic pressure
  • tissue oncotic pressure
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11
Q

What is plasma oncotic pressure always?

What about tissue oncotic pressure?

A

POP 25mmHg

TOP 2-3mmHg

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12
Q

What happens when hydrostatic pressure is higher than oncotic pressure?

A

water pushed out

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13
Q

Why would there be changes in capillary hydrostatic pressure?

A

arteriolar dilatation/constriction

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14
Q

What happens when arteriolar dilatation occurs in terms of oncotic pressure?

Give an example of when this would occur

A

hydrostatic pressure higher along vessels, as theres less resistance meaning more energy to the blood

HP = OP point not reached, HP > OP along capillary instead

this means water keeps on being pushed out

net fluid out > net fluid in

eg cutaneous dilatation when hot - can cause oedema

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15
Q

What happens when arteriolar constriction occurs in terms of oncotic pressure?

Give an example of when this would occur

A

hydrostatic pressure lower along vessels, as theres more resistance meaning less energy to the blood

HP = OP reached early, HP > OP along capillary instead

net fluid in > net fluid out

eg in muscle, skin, GIT during haemorrhage

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16
Q

What happens when plasma oncotic pressure is increased?

Give an example of when this would occur

A

due to more plasma proteins

net fluid in > net fluid out

eg in dehydration helps to restore blood volume

17
Q

What happens when plasma oncotic pressure decreases?

A

net fluid out > net fluid in = oedema

eg in malnutrition, liver failure

18
Q

What happens when permeability to proteins is increased? When would this occur?

A

same as when plasma oncotic pressure decreased

eg in inflammation, infection, allergic responses (anaphylaxis)

proteins leak out - tissue oncotic pressure increased

gradient reduced between POP and TOP

oedema caused

19
Q

What is the net loss from CVS in terms of capillaries?

A

over 24 hours:

20 litres fluid filtered out of capillaries

16 litres filtered into capillaries

net loss 4L/24 hours

NB: lymphatic system returns excess fluid

20
Q

What do lymphatic collecting vessels drain back into?

A

back into CVS

via thoracis ducts which drain into subclavian vessels

21
Q

What lymph vessels enter the subclavians veins?

A

right lympatic duct enters right subclavian vein

thoracic duct enter left subclavian vein

22
Q

When does oedema occur physiologically and pathologically?

A

physiologically: outward filtration > lymph drainage
pathologically: vascular permeability increased

23
Q

An example of pathological oedema is lymphatics blocked. When would this occur?

A

consequence of surgical removal of lymph nodes = lymphodaema

24
Q

How do venules and veins have a reservoir function?

A

theyre normally elliptical when supine; can fill more and become more circular to store blood

determines filling of ventricles in diastole (EDV)

25
Q

What happens to collagen to increase blood volume in veins?

A

collaged pleated/folded when elliptical in adventitia

unfolded to store blood, increasing CVP

26
Q

How much of blood volume is in systemic venous vessels?

A

60%

this can increase or decrease

27
Q

What happens to arteries and vessels when standing?

A

gravity increases pressure in all vessels below heart

arteries: withstand strecth, increases leg artery pressure
veins: venous pooling, higher leg vein pressure = venous distension = but decreased CVP as blood redistibutes

28
Q

Why does heart failure increase CVP?

A

impaired ventricular contraction = venous distension due to blood backing up in venous vessels (particularly when lying) = increased CVP

= oedema in feet, legs, abdominal cavity (ascites)

29
Q

Why does increased venous pressure cause oedema?

A

it reduces rate of fall of CHP

net fluid out > in

30
Q

What are the active influences on central venous volume and CVP? What happens to them during exercise?

A
  1. sympathetic noradrenergic nerve fibres
    1. more NAd = venoconstriction = CVP higher
    2. less NAd = venodilatation = CVP lower
  2. skeletal muscle pump
    1. contraction of leg muscles pushes blood in veins towards heart = CVP higher
  3. respiratory pump
    1. inspiration pulls blood in veins towards heart = CVP higher

2 and 3 accenuated in exercise and impaired in elderly

31
Q

Why would you want a higher CVP during exercise?

A

higher CVP means higher venous return

32
Q

What does filling of central veins determine?

A

filling of right ventricle determines EDV = determines SV = SV determines changes in CO (HR x SV) = changes in ABP (CO x TPR)

HDT 2.2 COPY (18 decks)

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