Capillaries - Solute exchange

Question 1

Metabolism creates a need to transport solutes and fluids

Answer 1

• H2O solutions contain electrolytes, O2, glucose, amino acids, hormones, immune responses etc, these are for growth and repair.
• Cell metabolism generates conc gradients that facilitate movement of H2O solutions.

Question 2

Cell membrane = barrier to solute transport

Answer 2

• 2 layers of amphipathic phospholipids.
• Phosphate head is polar - hydrophilic.
• Fatty acid tails are non-polar - hydrophobic.
• These components form bilayers in solution.

Functions of cell membranes:
* Support and protection.
* Cell to cell recognition/cell signalling.
* Control of what goes in and out of the cell.
* Regulation of cell function.

Question 3

Active and passive transport

Answer 3

1. Passive transort - Movement of molecules DOWN the conc/pressure/osmotic/electrical gradient, without the requirement of energy.
   * Simple passive = O2/CO2
   * Facilitated = Ions, glucose
2. Active transport - Movement of molecules AGAINST a gradient, with the requirement of ATP.
   * ATP-dependent pumps, endocytosis, exocytosis

Question 4

Passive transport processes

Answer 4

• Diffusion - Conc gradient, O2 uptake from lungs into blood.
• Convection - Pressure gradient, blood flow from heart to blood vessels.
• Osmosis - Osmotic pressure gradient, water uptake by cells.
• Electrochemical flux - Electrical and conc gradient, ion flow across cell membranes.

Question 5

Solute and fluid exchange occurs at capillaries

Answer 5

• Terminal arterioles connected to venules.
• Smallest diameter blood vessel = 1 cell thick.
• Found in every cell in body but higher density near highly active tissues like muscles and the liver and heart.
• Solute exchange happens via passive diffusion: electrolytes, O2, glucose, amino acids, hormones and drugs.
• Fluid exchange happens aling pressure gradients.

Question 6

Rate of solute transport depends of 3 factors

Answer 6

• Properties of passice diffusion - conc, rate, distance.
• Properties of solutes and membranes - Fick’s law.
• Properties of capillaries - affect movement.

Question 7

Propertie of passive diffusion

Answer 7

• Doesnt require ATP.
• Random movement of molecules.
• Move from high conc to low conc area.
• Good for lipid-solube transport over short distances - like O2/CO2.

t = x^2/2D. t = time taken, x = distance, D = diffusion.
* Time taken for a molecule to move a certain distance increases as distance squared increases.

Question 8

Properties of solutes and membranes affecting transport

Answer 8

Properties of solute:
* Conc gradient.
* SIze of the solute.
* Lipid solubility of solute.

Properties of the membrane:
* Membrane thiclness.
* Aqueous pores in the membrane.
* Carrier - mediated transport.
* Active transport mechanisms.

Question 9

Fick’s law

Solute movement - mass per unit time m/t (Js) is determined by 4 factors.

Answer 9

Js = DA x ΔC/x
* D = diffuion coefficient of solute (ease of movement through solvent.
* A = area.
* ΔC = conc gradient (C1-C2)
* x = distance (between C1 and C2)

Js always has a negative value since solutes flow down a conc gradient.

Question 10

Types of capillaries

Answer 10

1. Continuous capillaries:
   * Moderate permability - tight gaps between neighbouring cells; constant basement membrane.
   * Blood-brain barier.
   * Muscle, skin, fat, connective tissue.
2. Fenestrated capillaries:
   * High water permability , fenestration structures/disruption of membrane.
   * High water turnover tissues such as salivary glands, kindey etc.
3. Discontinuous capillaries:
   * Very large fenestration.
   * When movement of cells is required, RBCs in liver, spleen, bone marrow.

Question 11

Properties of capillaries

Answer 11

• Intracellular cleft = 10-20 nm wide.
• Caveolae and vesicles = large pore system.
• Glycocalyx = Covers endothelium and blocks solute permeation and access to transport mechanisms - can be broken down and remade when required.

Question 12

What is permeability?

Answer 12

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

‘how freely a solute crosses a membrane’

• Reduction in area for diffusion (A)
• Increased path length through membrane (x)
• Restricted diffusion in pore produces hydrostatic issues (D)
• Js = -DA X ΔC/x
• ALL FACTORS AFFECTING DIFFUSION = PERMAILITY (A)
• Modification of fick’s law for porous membrane:
• Js = PAmΔC

Question 13

Summary of solite transport - diffusion and filtration

Answer 13