Capillaries I - Solute Exchange

Question 1

_____ creates a need to transport solutes and fluids.

Answer 1

Metabolism.

Question 2

What are the two types of transport in the body?

Answer 2

1. PASSIVE TRANSPORT: -
   
   • It is the movement of molecules DOWN a gradient.
   • does not require energy.
   • can be simple [specific channels or transmembrane molecules help with transport] (O2, CO2) or facilitated [transporter or channel is required but no external energy] (ions, glucose).
2. ACTIVE TRANSPORT: -
   
   • the movement of molecules AGAINST a gradient.
   • requires energy (uses ATP).
   • E.g. ATP dependent pumps, endocytosis, exocytosis

Question 3

What are the four passive transport processes (including examples of each)?

Answer 3

1. DIFFUSION: -
   
   • has a concentration gradient
   • eg. O2 uptake from the lungs to the blood.
2. CONVECTION: -
   
   • has a pressure gradient
   • eg. blood flow from the heart to the blood vessels
3. OSMOSIS: -
   
   • has an osmotic pressure gradient
   • eg. water uptake by cells
4. ELECTROCHEMICAL FLUX: -
   
   • has an electrical and concentration gradient
   • eg. ion flow through the action potential in a nerve

Question 4

Where does solute and fluid exchange take place?

Answer 4

• It takes place in the capillaries.
• These vessels connect arterioles to venules - they’re an extension of the inner linings of the arterioles.
• They are one cell thick and are semi-permeable.
• Capillaries are found near every cell but are found more in highly active tissue such as liver, muscles, heart, brain etc.

Question 5

What controls the rate of solute transport?

Answer 5

1. properties of passive diffusion (eg. rate, distance, concentration)
2. properties of solutes and membranes (eg. Fick’s Law).
3. properties of capillaries (affect movement)

Question 6

List some properties of passive diffusion.

Answer 6

• It doesn’t require energy (ATP)
• the molecules move randomly.
• they move from an area of high concentration to low concentration.
• it’s great for the transport of lipid-soluble solutes over very short distances (eg. O2, CO2).
• It can only be a short distance because the time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared.
• Larger/longer distance means longer diffusion time.
• Since the time increases exponentially, it’s best used for very short distances.
• t = (x²) / 2D.
  
  • Where,
  • D = diffusion coefficient for molecule within the medium. (eg. D for O2 in water and O2 in air are different).
  • x = Net distance.

Question 7

List some properties of solutes and membranes that affect transport.

Answer 7

1. PROPERTIES OF SOLUTES: -
   
   • Concentration gradient- supplies force for movement (higher concentration=faster)
   • Size of solute affects permeability (bigger the solute= slower across the membrane)
   • Lipid solubility of solute (lipophilic would get across easily vs lipophobic which will take longer)
2. PROPERTIES OF MEMBRANES: -
   
   • Membrane thickness/composition
   • Aqueous pores in the membrane- lipophobic molecules can diffuse quicker.
   • Carrier mediated transport
   • Active transport mechanisms

Question 8

What is Fick’s Law and what does it state?

Answer 8

• It describes diffusion ie. the movement of a molecule down the concentration gradient.
• It states that the solute movement {J_s} [mass per unit time (m/t)] can be determined by four factors: -
  
  • the diffusion coefficient of the solute (the ease of movement through the solvent) [D]
  • the area available for diffusion [A]
  • the concentration gradient (C1 - C2) [ΔC]
  • the distance (between C1 and C2) [x]

Question 9

List the three types of capillaries and describe them.

Answer 9

1. CONTINUOUS CAPILLARIES: -
   
   • They have moderate permeability with tight gaps between neighbouring cells and they have a constant basement membrane.
   • found in the blood-brain barrier, muscle, skin, fat, connective tissue
2. FENESTRATED CAPILLARIES: -
   
   • high water permeability, with fenestration structures throughout the cells (hence a moderate disruption of the membrane).
   • found in high water turnover tissues, such as salivary glands, kidneys, synovial joints (filled with synovial fluid), anterior eye, choroid plexus (spinal fluid), gut mucosa.
3. DISCONTINUOUS CAPILLARIES: -
   
   • very large fenestration structures throughout, and a disrupted basement membrane.
   • found where the movement of RBC’s is required, such as the liver, spleen and bone marrow

Question 10

List some properties of capillaries that affect transport.

Answer 10

1. INTERCELLULAR CLEFT: -
   
   • 10-20 nm wide.
   • allows solutes and fluids to move through between the cells
2. CAVEOLAE AND VESICLES: -
   
   • large pore system (endocytosis and exocytosis)
   • for the movement of large molecules (such as proteins) across the membrane
3. GLYCOCALYX: -
   
   • a negatively charged material that covers the endothelium on the lumen side, blocking solute permeation and access to transport mechanisms
   • it’s highly regulated
   • The glycocalyx is very dynamic and can be broken down and remade as required.

Question 11

What is permeability, and what is its relation with diffusion?

Answer 11

• Permeability is the rate of solute transfer by diffusion across a unit area of membrane per unit concentration difference (ie. how freely a solute crosses a membrane).
• A porous membrane interferes with the diffusion of lipid insoluble solutes in multiple ways: -
  
  • a reduction in the area for diffusion (A)
  • an increased path length through the membrane (x)
  • restricted diffusion in the pore produces hydrostatic issues (D)

Question 12

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

Answer 12

Diffusion.

Diffusion is going to be passive down its concentration gradient whereas filtration is going to be through pores, gaps and fenestrations etc, so as fluid moves through dissolved glucose goes with it.

Example - glucose transport from blood plasma to tissues…

Question 13

What increases diffusion rate?

Answer 13

1. INCREASED BLOOD FLOW: - increases solutes in capillaries. Increased blood flow means less time for equilibrium to occur across capillaries
2. FALL IN INTERSTITIAL CONCENTRATION: - Metabolism – more solutes used up- increases concentration difference. Metabolism increases blood flow- increased oxygen delivery, controlled by arterioles. increases the concentration difference
3. RECRUITMENT OF CAPILLARIES: - dilation of arterioles leads to an increased number of capillaries perfused which increases total surface area for diffusion (Ficks law)