Capillaries I: Solute Exchange

Question 1

What is a solute and give some examples?

Answer 1

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

Question 2

What are the 2 barriers to solute transport?

Answer 2

• Cell membrane
• Capillary membrane

Question 3

Describe the structure of the cell membrane

Answer 3

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

Question 4

Describe the function of the cell membrane

Answer 4

• 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

Question 5

Describe passive transport

Answer 5

• 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

Question 6

Describe active transport

Answer 6

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

Question 7

What gradient does diffusion use and give an example?

Answer 7

Concentration gradient eg. O2 uptake from lungs into blood

Question 8

What gradient does convection use and give an example?

Answer 8

Pressure gradient eg. blood flow from heart to blood vessels

Question 9

What gradient does osmosis use and give an example?

Answer 9

Osmotic pressure gradient eg. water uptake by cells

Question 10

What gradient does electrochemical flux use and give an example?

Answer 10

Electrical and concentration gradient eg. ion flow across cell membranes

Question 11

Describe the structure of capillaries

Answer 11

• 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

Question 12

Where are capillaries found?

Answer 12

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

Question 13

What does solute exchange rely on?

Answer 13

Passive diffusion

Question 14

What does fluid exchange rely on?

Answer 14

Pressure gradients

Question 15

State briefly what the rate of solute transport depends on

Answer 15

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

Question 16

What are the properties of solutes that affect transport?

Answer 16

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

Question 17

What are the properties of membranes that affect transport?

Answer 17

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

Question 18

Describe Fick’s law

Answer 18

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

Question 19

Outline the equation for Fick’s Law

Answer 19

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

Question 20

Describe continuous capillaries.

Answer 20

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

Question 21

Describe fenestrated capillaries.

Answer 21

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

Question 22

Describe discontinuous capillaries

Answer 22

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

Question 23

State three properties of the capillaries that impact solute transport.

Answer 23

• Intercellular cleft
• Caveolae & vesicles
• Glycocalyx

Question 24

Describe the intercellular cleft

Answer 24

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

Question 25

Describe the caveolae and vesicles

Answer 25

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

Question 26

Describe the glycocalyx

Answer 26

- 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.

Question 27

Define permeability

Answer 27

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

Question 28

Explain how a porous membrane interferes with diffusion

Answer 28

- 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

Question 29

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

Answer 29

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

Question 30

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

Answer 30

- 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

Question 31

Define filtration

Answer 31

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

Question 32

Describe the 3 ways that the diffusion rate is controlled

Answer 32

Fall in interstitial concentration Recruitment of capillaries Blood flow

Question 33

What is the effect of a fall in interstitial concentration?

Answer 33

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

Question 34

What is the effect of capillary recruitment?

Answer 34

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

Question 35

What is the effect of blood flow?

Answer 35

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

Question 36

What 2 things does metabolism of cells create?

Answer 36

- A need for nutrient and gaseous exchange - Concentration gradients that facilitate this movement

Question 37

What is the main issue facing metabolism?

Answer 37

Membranes that act as a barrier to movement

Question 38

What type of particles can pass through the membrane?

Answer 38

- 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.

Question 39

What are the layers of a capillary?

Answer 39

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

Question 40

What is the function of capillaries?

Answer 40

- Solute exchange - Fluid exchange - Regulation of plasma and interstitial fluid

Question 41

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

Answer 41

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