S2: Capillaries: Solute Exchange

Question 1

Why is transport important?

Answer 1

• Metabolism creates a need to transport solutes and fluids:
• H20 solution containing O2, glucose, amino acids, hormones, immune responses, immune response etc.
• For energy, growth and repair
• H20 solution containing metabolic end products e.g co2 and urea

Question 2

Role of cell membranes

Answer 2

• Barrier to solute transport
• Provide support and protection
• Cell to cell recognition e.g. immune system
• Controls what enters or leaves the cell barrier e.g. ion movement in nerves
• Regulate cell function e,g, insulin mediated glucose uptake

Question 3

What is 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

What is passive transport?

Answer 4

· Movement of molecules DOWN a gradient
· Concentration / Pressure / Osmosis
· Does Not require energy
. Simple (O2/CO2) or facilitated (ions, glucose)

Question 5

What is active transport?

Answer 5

· Movement of molecules AGAINST a gradient
· Requires energy (uses ATP)
e.g. ATP-dependent pumps, endocytosis, exocytosis

Question 6

4 passive transport processes

Answer 6

1) DIFFUSION
Concentration gradient
e. g. O2 uptake from lungs into blood

2) CONVECTION
Pressure gradient
e. g. Circulation

3)  OSMOSIS
Osmotic pressure (water) gradient e. g. water uptake by cells

4) ELECTROCHEMICAL FLUX
Electrical and Concentration gradient
e. g. ion flow during an action potential in a nerve

Question 7

Describe capillaries

Answer 7

• Smallest diameter blood vessels
• Extension of inner lining of arterioles
• Endothelium only – 1 cell thick
• Semi-permeable

Capillaries are vessels that connect terminal arterioles to venules. They are found near every cell in the body but at higher density in highly active tissue (muscles, liver, heart, kidney and brain).

involved in solute and fluid exchange

Question 8

What is solute and fluid exchange?

Answer 8

Solute exchange (passive diffusion) : O2, glucose, amino acids, hormones, drugs

Fluid exchange (flow down pressure gradients) : regulation of plasma and interstitial fluid volumes

Question 9

What controls the rate of solute transport?

Answer 9

• Properties of passive diffusion
• Properties of solutes and membranes
• Properties of capillaries
• Concept of permeability

Question 10

Equation for diffusion

Answer 10

Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared 
	· t = x2 / 2D

D = diffusion coefficient for molecule within the medium e.g. D for O2 in water vs. O2 in air are different

Question 11

Name properties of solute and membrane than can affect transport

Answer 11

Properties of the solute:

• Concentration gradient
• Size of solute
• Lipid solubility of solute

Properties of the membrane:

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

Question 12

What is Fick’s Law?

Answer 12

How much of a substance is transported per unit of time
· Solute movement - mass per unit time m/t (Js)
It is determined by 4 factors

Js = - DA <i></i>

Question 13

What are the three types of capillaries?

Answer 13

1. Continuous capillaries
2. Fenestrated capillaries
3. Discontinuous capillaries

Question 14

Describe continuous capillaries

Answer 14

• Moderate permeability
  
  • Tight gaps between neighbouring cells
  • Constant basement membrane
    e. g. blood-brain barrier which keeps things in and out of brain (NT not moved out of brain)

Question 15

Describe fenestrated capillaries

Answer 15

• High water permeability
  
  • Fenestration structures
  • Modest disruption of basement membrane
    e.g. ‘high water turnover’ tissues such as
    salivary glands, kidney, synovial joints, choroid plexus (cerebrospinal fluid)

Question 16

Describe discontinuous capillaries

Answer 16

• Very large fenestration structures
  
  • Disrupted basement membrane
  • Large substances like plasma proteins and cells can move out and in
    e. g. When movement of cells is required such as RBCs in liver, spleen, bone marrow, Plasma proteins made in liver enter the blood circulation through discontinuous capillaries

Question 17

3 other structural features of capillary walls that can influence solute transfer

Answer 17

Intercellular cleft: 10-20 nm wide

Glycocalyx: covers endothelium on lumen side, -ve charged material, blocking solute permeation and access to transport mechanisms and it is highly regulated acting as a barrier
· increasingly considered important to control movement of molecules -

Caveolae and vesicles: large pores system

Question 18

What is permeability?

Answer 18

Permeability is the rate of solute transfer by diffusion across unit area of membrane per unit concentration difference
’how freely a solute crosses a membrane’

Question 19

What is the modified Fick’s law for a porous membrane?

Answer 19

Because a porous membrane interferes with the diffusion of lipid insoluble solute in multiple ways (Fick’s law doesn’t really work for ion channels)

Js = - PAm <i></i>

Question 20

Which is the dominant route of solute transport: diffusion or filtration?

Use glucose as an example

Answer 20

Diffusion

xample : glucose transport from capillaries into tissues can potentially occur by either passive diffusion or filtration through fenestrations/intercellular gaps etc.

e.g. filtration only accounts for 2% of glucose transport - Therefore, 98% of glucose transport into interstitial space via passive diffusion – via GLUT transporter system

Question 21

What controls diffusion rate?

Answer 21

• Increased blood flow because increased concentration of solutes in capillaries means more exchange of O2/ CO2 occurs along capillaries in the lungs. There is then less time for equilibration of O2/CO2 to occur between interstitial spaces and capillaries – which would reduce exchange.
• Fall in interstitial concentration increases conc. difference due to more solute used (e.g. metabolism). Also, metabolism increases blood flow - metabolic hyperaemia - ­increases O2 delivery
• Recruitment of capillaries.
  Dilation of arterioles - ­ increases number of capillaries perfused
  -Increases total surface area A for diffusion (Fick’s law)
  Shortens diffusion distance x (faster diffusion)