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Cardiovascular System > Capillaries I & II > Flashcards

Capillaries I & II Flashcards

1
Q

How does metabolism affect solute exchange?

A

Metabolism creates concentration gradients to transport solutes and fluids for gaseous and nutrient exchange

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2
Q

What is the fate of O₂ , glucose, urea and CO₂ in the body?

A

O₂ and glucose are transported around the body

CO₂ and urea are removed

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3
Q

Describe the structure of cell membranes?

A
  • consist of 2 layers of ampiphatic phospholipids
  • polar phosphate heads (hydrophilic)
  • non polar fatty acid tails (hydrophobic)
  • form bilayers in solution
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4
Q

Outline the functions of cell membranes

A
  • providing support and function
  • cell to cell recognition (immune system)
  • controls what enters / leaves cells
  • regulates cell function (e.g. insulin mediated glucose
    uptake)
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5
Q

How does the membrane allow us to differentiate between cells?

A

The various membrane functions allow us to differentiate between cells

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6
Q

How do membranes act as barriers to transport?

A

it’s difficult to get solutes across the hydrophobic centres of the membrane

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7
Q

How does the structure of membranes aid transporting functions?

A
  • contains transmembrane proteins that span across the
    membrane and facilitate transport and signalling
  • carbohydrates anchor proteins and orient them

These complicated structures are a barrier and aid
movement across the membrane

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8
Q

What are the different types of transport?

A
  • passive

- active

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9
Q

What is passive transport?

A

Random movement of molecules down a gradient of:

  • concentration
  • pressure
  • osmotic
  • electrical

Passive - doesn’t require ATP
Works effectively over short distances - t is proportional to distance^2

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10
Q

Describe the features of passive diffusion?

A

Doesn’t require energy
Simple diffusion O₂ /CO₂
facilitated diffusion is via specific channels or transmembrane molecules (ions/glucose via GLUT)

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11
Q

What is active transport?

A

The movement of molecules against a gradient requiring energy (ATP)
- ATP dependent pumps, endocytosis + exocytosis

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12
Q

What are the different passive transport processes?

A

Diffusion

  • conc. gradients
    e. g. uptake of O₂ from lungs -> blood

Convection

  • pressure gradient
    e. g. blood flow from heart -> vessels

Osmosis

  • osmotic pressure graident
    e. g. H₂O uptake in cells

Electrochemical flux

  • electrical + concentration gradient
    e. g. ion flow during action potentials
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13
Q

Describe the structure of capillaries

A

Capillaries are made of endothelial cells - 1 cell thick

semi-permeable and amenable to certain types of diffusion over short distances

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14
Q

Where in the body are capillaries found?

A

Found near every cell in the body, but higher density in highly active tissues (muscle, liver, heart, kidney, brain etc.)

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15
Q

What is solute exchange?

A

Passive diffusion of O₂ , glucose, amino acids, hormones, drugs etc. between substances

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16
Q

What is the role of fluid exchange?

A

occurs along pressure gradients and regulates plasma and interstitial fluid volumes

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17
Q

Where does most exchange of solutes occur in the body?

A

Most exchange occurs at capillaries

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18
Q

Outline the vessel network solutes pass through

A

arteries branch into arterioles -> capillaries -> venules -> veins towards the heart

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19
Q

What is the role of capillaries in the network of vessels?

A

capillaries connect terminal arterioles to venules

- extension of inner lining of arterioles

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20
Q

What is the rate of solute transport dependent on?

A

Depends on properties of:

  • passive diffusion (conc. , rate & diffusion)
  • solutes + membranes (Fick’s Law)
  • Capillaries

combining all of these provides concept of permeability

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21
Q

What is permeability?

A

How easy it is for the solute to cross the membrane

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22
Q

Describe the movement of molecules during passive diffusion?

A

Molecules move at random (due to random thermal motion) from an area of high to low concentration, eventually reaching equilibrium

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23
Q

When is diffusion most effective?

A

Great for lipid-soluble solutes over short distances e.g. O₂ / CO₂

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24
Q

What is the relationship between time and diffusion rate?

A 
t ∝d² 
time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with distance squared (d²)
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25
Q

Whys is simple diffusion not applicable for whole body movement?

A

Movement across a capillary membrane will happen in fractions of a second, but around the body may take years

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26
Q

What are the solute properties that affect transport?

A
  • membrane thickness / composition
  • aqueous pores in the membrane
  • carrier mediated transport
  • active transport mechanisms
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27
Q

How does solute concentration affect diffusion?

A

[high solute] = faster diffusion

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28
Q

How does solute size affect diffusion rate?

A

The bigger the solute, the slower the transport across the membrane

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29
Q

How does charge / hydrophobicity affect diffusion?

A

Lipophilic solutes get across very easily but glucose would struggle as it’s large and charged.

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30
Q

What is the role of membrane aqueous pores?

A

Membrane aqueous pores act as carriers or active transport mechanisms

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31
Q

What is Fick’s Law?

A

Js = -DA ΔC / x

D - Diffusion coefficient for solute in solvent
A - area available for diffusion
ΔC - concentration difference
X - distance

(always a negative value)

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32
Q

What are the different types of capillaries?

A

Continuous capillaries
Fenestrated capillaries
Discontinuous capillaries

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33
Q

Describe the properties of continuous capillaries?

A
  • Tight junctions between the endothelial cells
  • very poor permeability as no space to move out from
    lumen into interstitial tissue.
  • Constant basement membrane, which all the cells
    adhere to has- no breaks in it.
  • Useful in blood brain barrier, doesn’t non-selectively
    allow things into the brain, but all entry is regulated.
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34
Q

What controls solute diffusion rates?

A

Permeability is the rate of solute transfer by diffusion across unit are of membrane per unit concentration difference - it’s a measure of how freely a solute crosses a membrane

Js = -PAm ΔC

Or
Js / Am = - PΔC

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35
Q

What is Starling’s Principle of solute exchange?

A

Fluid movement across the capillary wall is proportional to pressure difference across the capillary wall

Balance between pressures that favour filtration (hydrostatic - increased arterial BP)
and reabsorption (osmotic pressure due to plasma proteins, as long as they can’t pass through)
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36
Q

What are fenestrated capillaries?

A

Fenestrated structures are protein structures that act as a sieve for small molecules to pass through.

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37
Q

Describe the features of fenestrated capillaries

A
  • More permeable due to fenestrated structures (holes in
    the endothelial cells)
  • Permeable to water & small solutes (not bigger)
  • Good where a lot of water is required e.g. salivary
    glands - lots of water leaves the capillaries into the
    interstitial space to make saliva.
  • Present in all areas where fluid moves out of plasma.
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38
Q

What are the features of Discontinuous capillaries?

A
  • Allow everything through
  • Have very large fenestrations and disrupted basement
    membrane
  • Incredibly leaky so large molecules & some cells
    (rbc & immune cells) move out
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39
Q

Where are the different capillaries distributed in the body?

A

The 3 distinct types of capillaries, found in different parts of the body, depending on the specific functions of cells and tissues.

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40
Q

Describe the anatomy of capillaries

A

The capillary anatomy is a single layer of endothelial cells forming the endothelium.

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41
Q

What other capillary wall structural features influence solute transfer?

A
  • Intercellular cleft - 10-20 nm wide
  • Caveolae & vesicles - large pore system
  • Glycocalyx - covers endothelium, negatively charged
    material, blocks solute permeation and access to
    transport mechanisms, highly regulated
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42
Q

Describe the role of intercellular clefts in capillaries

A

Intercellular clefts are in between adjacent cells, not linked by tight junctions. This allows solutes and fluids to move between the cells,aiding permeability.

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43
Q

What is the role of Caveolae & vesicles?

A

Caveolae and vesicles is where substances are taken up on one side of the membrane, and moved to the other side => endocytosis & exocytosis

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44
Q

Explain the role of endocytosis & exocytosis in solute exchange at the capillaries

A

Endocytosis followed by exocytosis which will move the solute from the lumen across into the interstitial space…also related to selective large pores

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45
Q

What is the glycocalyx?

A

A negatively charged carbohydrate mesh that sits on the top of the endothelial cells next to the lumen

46
Q

What is the function of the glycocalyx?

A

Acts as an extra barrier - it is very dynamic can be broken down and remade as required. Cells can regulate this

47
Q

What is permeability?

A

The rate of solute transfer by diffusion across unit area of membrane per unit concentration difference

48
Q

How does a porous membrane affect diffusion?

A

A porous membrane interferes with the diffusion of lipid insoluble solutes in multiple ways

Reduction in area for diffusion (A)
Increased path length through membrane (x)
Restricted diffusion in pore produces hydrostatic issues (D)

49
Q

What factors determine permeability (Fick’s Law)?

A

passive diffusion
solute properties
membrane properties
capillary types

50
Q

How does inflammation affect solute exchange?

A

During inflammation big gaps can form between cells causing large lipophobic proteins (plasma protein) to move out.

51
Q

What type of movement occurs for lipophilic molecules?

A

Transcellular diffusion (O₂ / CO₂)

52
Q

How do small lipohilic molecules move?

A

via intercellular channels or fenestrations

53
Q

How does water move around cells?

A

Water can move by a variety of means including specific water channels.

54
Q

Compare and contrast Diffusion and filtration

A

Diffusion is passive down a concentration gradient
Filtration is through pores, gaps and fenestrations etc.
so when fluid moves glucose moves with it

55
Q

How Describe the movement of fluid in our body

A

We have a constant cycling of fluid being filtered from our capillaries and joining the interstitial fluid, through the lymphatic system and eventually recycling it into the plasma.

56
Q

How much glucose is in our blood plasma?

A

Glucose concentration in plasma is 1 g / litre

57
Q

How much plasma filtrate flows into tissues every day?

A

Total volume of plasma filtrate flowing into tissues per day = 8 litres

58
Q

What is the maximum filtration of glucose a day?

A

Maximum filtration of glucose = 8 g / day

59
Q

Approximately how much glucose is consumed a day?

A

400 g/day

60
Q

How much of glucose transport does filtration account for?

A

Therefore filtration transport only accounts for 2% glucose transport

98% of glucose transport into interstitial space via passive diffusion – via GLUT transporter system

61
Q

What 3 factors control the rate of diffusion around the body?

A

Blood flow
Fall in [Interstitial]
Capillary recruitment

62
Q

Explain how Blood flow alters diffusion rate

A
  • more blood flow = more solutes in capillaries
  • more blood volume = less time for equilibrium to occur
    across capillaries

v. slow blood flow causes O₂ and CO₂ to exchange over a short space allowing them to reach equilibrium
=> less diffusion

63
Q

How does a fall in [interstitial] change diffusion rates?

A
  • more solute used in tissues = increased [ ] difference
  • metabolism increases blood flow + increased O₂
    delivery controlled by arterioles
64
Q

What pathological reason can cause decreased diffusion due to decreased blood flow?

A

Sepsis
- low blood flow & pressure
- tissue can become ischaemic
=> flow limited diffusion

65
Q

Explain how capillary recruitment alters diffusion rate?

A

Increase blood flow opens up more capillaries
- Not all the capillaries are full all of the time,
=> as blood flow increases more open up

Dilation of arterioles -> more perfused capillaries increasing, the total SA for diffusion

66
Q

How does exercise affect the rate of diffusion?

A

In exercise you go from 5 L/min to 20 L/min, reducing the distance between capillary and cell so diffusion occurs quicker.

67
Q

How does strenuous exercise affect O₂ transport?

A

O₂ transport from blood to muscle increases over 40x during strenuous exercise. We increase cardiac output (blood flow), using more O₂ (fall in tissue [interstitial]) opening up more capillaries (recruitment)

68
Q

What controls solute diffusion rates?

A

Permeability is the rate of solute transfer by diffusion across unit are of membrane per unit concentration difference - it’s a measure of how freely a solute crosses a membrane
Js = -PAm ΔC
Or Js / Am = - PΔC

Fluid exchange

69
Q

What are the different hydrostatic and oncotic pressures in a normal capillary bed ?

A 
Four pressures determine filtration (or reabsorption) rate
Pᵢ - interstitial fluid pressure 
Pᶜ - capillary BP
Πᵢ - interstitial proteins
Πp - plasma carotid osmotic pressure

Jv = LpA {(Pᶜ - Pᵢ) - (Πp - Πᵢ)}
Jv is proportional to hydraulic pressure difference - osmotic pressure difference

70
Q

How does hypovolemia affect the pressures in a capillary bed?

A

Hypovolemia is caused by severe blood loss via haemorrhages (water, solutes, cells)

  • Osmotic pressure exerted by plasma proteins remains
    the same

Hydrostatic pressure decreases causing:
- less water/volume,
- less venous return
- preload decreases therefore by Starling’s law;
=> decreased SV reduces CO leading to decreased
arterial BP

Arterioles will constrict - capillaries closed off reducing BP
Constriction occurs to increase TPR, increasing BP via sympathetic output via baroreceptor reflex BP = CO x TPR and CO = BP x SV

71
Q

Do capillaries normally reabsorb or filter, and why?

A

Most capillaries filter > reabsorption
They hydrostatic pressure > osmotic pressure favouring filtration
Excess fluid returned via lymphatics system

72
Q

What are the lymphatics, and what is their role?

A

Accumulation of excess of fluid within the interstitial space - imbalance between filtration, reabsorption and lymph function

73
Q

What is oedema?

A

a condition characterised by an excess of watery fluid collecting in the cavities or tissues of the body.

74
Q

How do changes in Pc , πP, and inflammation cause oedema?

A

Anything increasing hydrostatic pressure or decreasing reabsorption, favouring filtration can lead to oedema and inflammation
Pᶜ Increasing capillary hydrostatic pressure favours filtration
Πp Decrease in plasma osmotic pressure favours filtration
Inflammation : increased blood flow, increased permeability, leakage
Venous return impairment causes build up of blood at venous end - decreases CO
Low plasma protein - liver failure causes low osmotic pressure

75
Q

What is the significance of fluid exchange?

A

Fluid exchange is important for normal physiological function
-> water is required for chemical reactions

76
Q

Describe the benefits of fluid reabsorption

A

fluid reabsorption from tissues to blood can maintain circulation during haemorrhage

77
Q

What are the consequences of fluid exchange abnormalities?

A

Abnormalities in fluid exchange can caus eoedema and tissue swelling

78
Q

Describe the structure of capillaries

A

Endothelial cells on the outside
followed by a basement membrane containing fibroblasts
epithelium lines the capillary
connective tissue with interstitial matrix

79
Q

Why is fluid movement able to occur at the capillary wall?

A

the capillary wall is a semi permeable membrane

80
Q

How is hydraulic pressure created

A

Fluid moves across the membrane into interstitial space due to blood flow which exerts hydraulic pressure

81
Q

How is oncotic pressure generated?

A

Large molecules (e.g. plasma proteins) aren’t able to pass through the membrane so exert an osmotic pressure which creates a suction force to move fluid into the capillary

82
Q

Describe the effects of increasing pressures and fluid movement

A

Increased hydraulic pressure
=> fluid moves into intertstitial fluid

Increased oncotic pressure
=> fluid moves into capillary

83
Q

What controls fluid movement across capillaries?

A

Fluid movement across capillaries depends on the balance between hydraulic and oncotic pressures

84
Q

What determines the filtration rate?

A

4 pressures determine the rate of filtration:

Oncotic pressures:
πp - plasma proteins; high osmotic pull against filtration
πᵢ - interstitial proteins; moderate osmotic pull out of
vessels

Hydraulic pressures:
Pc - capillary BP
Pi - Interstitial fluid pressure

85
Q

What is starling’s principle of fluid exchange?

A

Jv = LpA {(Pᶜ - Pᵢ) - 𝛔(Πp - Πᵢ)}

Pᵢ - interstitial fluid pressure 
Pᶜ - capillary BP
Πᵢ - interstitial proteins
Πₚ - plasma carotid osmotic pressure 
𝛔 - reflection coefficient (fraction of osmotic pressure exerted) for plasma proteins = 0.9
86
Q

Where does filtration occur in capillaries?

A

In well perfused capillaries filtration occurs across the entire length

87
Q

How does filtration differ at the arteriole and venous end?

A

The pressure drops as you move down the capillary so filtration decreases by the time you reach the venous end
Pᶜ and Πp alters along capillary length

88
Q

How is lymph related to fluid exchange?

A

Lymph becomes part of the interstitial fluid and later returned to the blood plasma

89
Q

What is the role of lymphatic circulation?

A

Returns excess tissue fluid / solutes back to the cardiovascular system (approx. 8 litres a day)

90
Q

How does lymph flow occur?

A

lymph vessels have valves and smooth muscle

Spontaneous smooth muscle and surrounding skeletal muscle contraction & relaxation contributes to lymph flow

91
Q

What does the overall control of extracellular fluid balance depend on?

A
  • capillary filtration
  • capillary reabsorption
  • lymphatic system uptake
92
Q

How does Starling’s Law affect fluid balance?

A

Starling’s Factor determine changes in fluid balance within:

  • circulation
  • interstitial fluid
  • lymphatic system
93
Q

What is the effect of hypovolemia on fluid exchange?

A

Low capillary pressure Pc
Decreased blood volume -> drop in preload
Decreased Contraction force
Decreased diastolic pressure
Starling’s Law doesn’t help - CO decreases
Haemorrhage leads to drop in BP affecting capillaries

94
Q

What is the benefit of interstitial fluid moving into the blood after a haemorrhage?

A

life preserving

  • supports CVP
  • increases CO
  • Raises BP
  • Greater Blood flow
95
Q

What causes the interstitial fluid to move into blood after haemorrhage?

A

Caused by sympathetic nerve induced vasoconstriction of pre-capillary arterioles leading to a drop in downstream capillary pressure

96
Q

Explain in terms of pressure how interstitial fluid is able to move into blood following a haemorrhage?

A

Filtration occurs to begin with, followed by reabsorption as:
Osmotic pressure > hydraulic pressure
- useful self correcting mechanism during low BP allowing more fluid to be absorbed back into circulation => increasing Blood volume

97
Q

How does the movement of interstitial fluid into blood relieve haemorrhage?

A

Leads to greater preload -> greater CO -> returns BP to normal maintaining blood flow to the heart and brain

98
Q

How does a drop in cardiac output affect capillary pressure?

A

Drop in Co leads to a drop in BP causing a reduction in Pc (capillary pressure)

99
Q

What is oedema?

A

Excess fluid within the interstitial space due to the imbalance between filtration, reabsorption and lymph function

100
Q

What are the causes of oedema?

A

Increased capillary Pc
Decreased πₚ
Inflammatory Response
Lymphatic Problems

101
Q

What may be the causes of Increased Capillary pressure?

A
  • dependent gravitational oedema - prolonged orthostasis
  • DVT - deep vein thrombosis
  • Cardiac Failure
102
Q

What is DVT?

A

Deep vein thrombosis is the prevention of venous return, increasing venous pressure causing ‘back up’ of pressure leading to increased Pc across capillaries and increased filtration

103
Q

How can [plasma protein] affect venous return?

A 
reduced [plasma protein] 
-> reduced plasma oncotic pressure
=> greater Pc and πi influence 
-> fluid efflux from capillaries into interstitial fluid 
=> causes oedema
104
Q

What pathological causes may result in low protein oedema?

A

Malnutrition / malabsorption
- not enough protein intake to make plasma proteins

Nephrotic Syndrome
- urinary protein loss, replaced by liver production

Liver Disease
- not enough endogenous albumin produced

105
Q

What is Kwashiorkor?

A

a malnutrition disease linked to decreased osmotic pressure

106
Q

How does low [plasma protein] increase filtration?

A

Less plasma proteins in plasma = less osmotic force to absorb fluid from interstitial fluid
Balance is shifted towards hydraulic pressure, increasing filtration

107
Q

What is the cause of inflammatory mediated oedema?

A

Swelling triggered by local chemical mediators of inflammation causing a large increase in capillary permeability -> increased Lp (hydraulic conductance of endothelium)

108
Q

What is increased protein permeability during inflammatory mediated oedema caused by?

A 
chemicals
insect bites
infection 
physical trauma
autoimmune diseases
nettle stings
109
Q

What is lymphatic obstruction caused by?

A

filariasis / elephantiasis - nematode infestation larvae migrate to lymphatic system to grow, mate and form nests blocking lymph drainage

110
Q

What is the cause of lymphatic removal?

A

Lymphodema caused by surgery to treat testicular cancer

111
Q

What are the 2 main capillary functions?

A

Solute exchange

  • nutrition of tissue, hormone & drug delivery
  • individual molecules move down diffusion gradient
  • obey Fick’s Law
Fluid exchange 
- regulation of plasma & IF volumes
- bulk flow of all molecules together down pressure 
  gradients 
- obey Starling's Law

Cardiovascular System (16 decks)

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