Capillaries I

Question 1

What is a solute and give some examples

Answer 1

A solute is any substance dissolved in the body fluid - oxygen, carbon dioxide, nutrients, amino acids, electrolytes, ions etc

Question 2

What are the 2 barriers to solute transport

Answer 2

• Cell membrane
• Capillary membrane

Question 3

Explain how metabolism facilitates the movement of solutes and fluids

Answer 3

• A H2O solution containing electrolytes, oxygen, glucose, amino acids, hormones, immune response etc for energy growth and repair
• These are used in metabolism and then a H2O solution containing metabolic end products re-enters circulation from the cells
• Cell metabolism is continually generating concentration gradients that facilitate this movement

Question 4

Describe the structure and function of the cell membrane

Answer 4

Structure -

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

Function -

• 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

State the two types of transport and describe them

Answer 5

Passive transport
- 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

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

Question 6

State and describe the types of passive transport

Answer 6

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

Question 7

Describe the structure of capillaries

Answer 7

• They connect terminal arterioles to venules - extension of inner lining of arterioles
• They have the smallest diameter of the blood vessels with an endothelium that is one cell thick and semi permeable

Question 8

Where are capillaries found

Answer 8

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

Question 9

What does solute exchange rely on

Answer 9

Passive diffusion

Question 10

What does fluid exchange rely on

Answer 10

Pressure gradients

Question 11

State briefly what the rate of solute transport depends on

Answer 11

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

Question 12

Why is passive diffusion over short distances

Answer 12

Time taken (t) for one randomly moving molecule to move a net distance (x) in one specific direction increases with the distance squared:

t = x^2/2D
D = diffusion coefficient for molecule within the medium

Question 13

What are the properties of solutes and membranes that affect transport

Answer 13

Properties of the solute

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

Properties of the membrane

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

Question 14

Describe Ficks law including the equation

Answer 14

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

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 15

Describe the 3 types of capillaries - where they are found and how their structure differs

Answer 15

Continuous capillaries
* Moderate permeability; tight gaps between neighbouring cells; constant basement membrane
* Blood-brain barrier
* Muscle, skin, fat, connective tissue

Fenestrated capillaries
* High water permeability, fenestration structures, modest disruption of membrane
* ‘High water turnover’ tissues eg. salivary glands, kidney, synovial joints, anterior eye, choroid plexus (cerebrospinal fluid), gut mucosa

Discontinuous capillaries
* Very large fenestration structures, disrupted membrane
* When movement of cells is required; RBCs in liver, spleen, bone marrow

Question 16

State 3 properties of the capillary that also impact solute transport

Answer 16

Intercellular cleft
10-20 nm wide

Caveolae & vesicles
Large pore system

Glycocalyx
- 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. The cells can regulate this and these processes are currently an active research area.

Question 17

Define permeability

Answer 17

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

Question 18

Explain how a porous membrane interferes with diffusion

Answer 18

• A porous membrane interferes with the diffusion of lipid insoluble solutes in many ways
• It 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
• All the factors affecting diffusion go into one term - permeability

Question 19

Describe how ficks law is different for a porous membrane

Answer 19

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 20

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

Answer 20

• Diffusion is going to be a passive process down the concentration gradient
• Filtration is through pores, gaps and fenestrations so a fluid moves solutes ( e.g. glucose)
• The concentration of solutes like glucose in the filtrate are much lower than our consumption per day, so diffusion is the dominant transport route

Question 21

Define filtration

Answer 21

Filtration is the bulk movement of fluid (water and solutes) - can move through gaps between endothelial cells or through fenestrations

Question 22

Describe the 3 ways that the diffusion rate is controlled

Answer 22

1. Fall in interstitial concentration
   During metabolism more solute is used up thereby increasing the concentration difference. Also, metabolism increases blood flow - increased O2 delivery, controlled by the arterioles
2. Recruitment of capillaries
   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
3. Blood flow
   More blood brings more solutes. Increased blood volume means less time for equilibration to occur across capillaries; eg. in flow limited difussion 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