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SGUL - Physiology 1 > Capillaries I: Solute Exchange > Flashcards

Capillaries I: Solute Exchange Flashcards

1
Q

What is a solute and give some examples?

A

Substance dissolved in body fluid - e.g oxygen, carbon dioxide, nutrients, electrolytes

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2
Q

What are the 2 barriers to solute transport?

A
  • Cell membrane
  • Capillary membrane
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3
Q

Describe the structure of the cell membrane

A
  • Consists of two layers of amphipathic phospholipids
  • Phosphate head is polar (hydrophilic)
  • Fatty acid tails is non-polar (hydrophobic)
  • Form bilayers in solution
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4
Q

Describe the function of the cell membrane

A
  • Provide support and protection
  • Cell-to-cell recognition – eg. immune system
  • Controls what enters or leaves the cell – e.g. ion movement in nerves
  • Regulates cell function – eg. Insulin-mediated glucose uptake
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5
Q

Describe passive transport

A
  • Movement of molecules DOWN a gradient (high to low) - concentration / Pressure / Osmotic / Electrical
  • Does not require energy
  • Simple (O2/CO2) OR facilitated (ions, glucose)
  • Molecules move randomly
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6
Q

Describe active transport

A
  • Movement of molecules AGAINST a gradient
  • Requires energy (uses ATP) - eg. ATP-dependent pumps, endocytosis, exocytosis
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7
Q

What gradient does diffusion use and give an example?

A

Concentration gradient eg. O2 uptake from lungs into blood

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8
Q

What gradient does convection use and give an example?

A

Pressure gradient eg. blood flow from heart to blood vessels

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9
Q

What gradient does osmosis use and give an example?

A

Osmotic pressure gradient eg. water uptake by cells

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10
Q

What gradient does electrochemical flux use and give an example?

A

Electrical and concentration gradient eg. ion flow across cell membranes

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11
Q

Describe the structure of capillaries

A
  • Connect terminal arterioles to venules - extension of inner lining of arterioles
  • Smallest diameter of the blood vessels
  • Endothelium that is one cell thick and semi permeable
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12
Q

Where are capillaries found?

A

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

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13
Q

What does solute exchange rely on?

A

Passive diffusion

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14
Q

What does fluid exchange rely on?

A

Pressure gradients

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15
Q

State briefly what the rate of solute transport depends on

A
  • Properties of passive diffusion - eg. concentration, rate, distance
  • Properties of solutes and membranes - eg. Fick’s law
  • Properties of capillaries - affect movement
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16
Q

What are the properties of solutes that affect transport?

A
  • Concentration gradient
  • Size of the solute
  • Lipid solubility of solute (lipophilic, lipophobic nature)
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17
Q

What are the properties of membranes that affect transport?

A
  • Membrane thickness/composition
  • Aqueous pores in the membrane
  • Carrier-mediated transport
  • Active transport mechanisms
18
Q

Describe Fick’s law

A
  • Describes how much of a substance is transported per unit time
  • Solute movement is mass per unit time determined by 4 factors
19
Q

Outline the equation for Fick’s Law

A

Js = - DA x (change in C / x)

D = Diffusion coefficient of solute – ease of movement through solvent
A = Area
ΔC = concentration gradient (C1-C2)
x = distance (between C1 and C2)
It has a negative value = flowing ‘down’ a concentration gradient

20
Q

Describe continuous capillaries.

A
  • Moderate permeability; tight gaps between neighbouring cells; constant basement membrane
  • Goes to blood-brain barrier, muscle, skin, fat, connective tissue
21
Q

Describe fenestrated capillaries.

A
  • High water permeability, fenestration structures, modest disruption of membrane
  • Found at ‘high water turnover’ tissues eg. salivary glands, kidney, synovial joints, gut mucosa
22
Q

Describe discontinuous capillaries

A
  • Very large fenestration structures, disrupted membrane
  • Found in structures when movement of cells is required; RBCs in liver, spleen, bone marrow
23
Q

State three properties of the capillaries that impact solute transport.

A
  • Intercellular cleft
  • Caveolae & vesicles
  • Glycocalyx
24
Q

Describe the intercellular cleft

A

10-20 nm wide
- allows solutes and fluids to move through between the cells

25
Q

Describe the caveolae and vesicles

A

Large pore system
- for the movement of proteins across the membrane e.g by exocytosis - moving proteins from inside the vessel into the interstitial fluid

26
Q

Describe the glycocalyx

A
  • Covers endothelium, negatively charged material, blocks solute permeation and access to transport mechanisms, highly regulated
  • The glycocalyx is very dynamic can be broken down and remade as required.
27
Q

Define permeability

A

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

28
Q

Explain how a porous membrane interferes with diffusion

A
  • Decreases the area for diffusion , there is increase path length through the membrane, and there is restricted diffusion in the pore which produces hydrostatic issues
29
Q

Describe how Fick’s law is different for a porous membrane

A

Js = - P Am ΔC

Where:
Js = Rate of solute transport
P = Permeability - involving pore size,
length, diffusion coefficient
Am = Surface area of capillary involved
in transport
∆C = Concentration gradient

30
Q

Explain whether diffusion or filtration is more prominent as a transport method

A
  • Diffusion is a passive process down the concentration gradient
  • Filtration is through pores, gaps, fenestrations so moves solutes like glucose
  • The concentration of solutes like glucose in the filtrate are much lower than consumption of glucose, so diffusion is the dominant transport route
31
Q

Define filtration

A

Bulk movement of fluid (water and solutes) - can move through gaps between endothelial cells or through fenestrations

32
Q

Describe the 3 ways that the diffusion rate is controlled

A

Fall in interstitial concentration
Recruitment of capillaries
Blood flow

33
Q

What is the effect of a fall in interstitial concentration?

A

During metabolism more solute is used up thereby increasing the concentration difference. Also, metabolism increases blood flow - increased O2 delivery, controlled by the arterioles

34
Q

What is the effect of capillary recruitment?

A

Dilation of arterioles leads to increased number of capillaries perfused which increases total surface area A for diffusion (Fick’s law). Also this shortens
diffusion distance between capillary and cell

35
Q

What is the effect of blood flow?

A

More blood brings more solutes. Increased blood volume means less time for equilibration to occur across capillaries.

eg. in flow limited diffusion where there is slow flow in a long capillary oxygen and carbon dioxide equilibrate over proximal section (no concentration gradient) and there is little diffusion the rest of the vessel

36
Q

What 2 things does metabolism of cells create?

A
  • A need for nutrient and gaseous exchange
  • Concentration gradients that facilitate this movement
37
Q

What is the main issue facing metabolism?

A

Membranes that act as a barrier to movement

38
Q

What type of particles can pass through the membrane?

A
  • Non-polar molecules are able to pass through
  • Polar molecules are unable to pass through due to the hydrophobic nature of the membrane. They rely on transporters.
39
Q

What are the layers of a capillary?

A

Cellular membrane, single layer of endothelial cells, a cellular basement membrane made of connective tissue

40
Q

What is the function of capillaries?

A
  • Solute exchange
  • Fluid exchange
  • Regulation of plasma and interstitial fluid
41
Q

When can flow limited diffusion occur and what can it lead to?

A
  • In sepsis when blood pressure and flow are low
  • Can lead to ischaemia

SGUL - Physiology 1 (68 decks)

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