Capillaries I - Solute exchange

Question 1

What are the 2 types of Extracellular fluid within the circulatory system?

Answer 1

Interstitial fluid and blood plasma

Question 2

Define the term “interstitial fluid”

Answer 2

Fluid that surrounds the cells and provides them with nutrients

Question 3

Define the term”Blood plasma”

Answer 3

Fluid within circulatory system

Question 4

Describe the structure of a cell membrane

Answer 4

Consist of two layers of amphipathic phospholipids.

Phosphate head of phospholipid is polar (hydrophilic) - Face outwards towards solution

Fatty acid tail of phospholipid is non-polar (hydrophobic) - Face inwards so they are shielded from solution by phosphate heads

Form bilayers in solution

Contain transmembrane proteins that facilitate transport and signalling.

Contain carbohydrates anchoring these transmembrane proteins and orientating them

Question 5

What are some of the functions of a cell membrane?

Answer 5

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 6

What is passive transport?

Answer 6

Movement of molecules down a gradient. Does not require energy

Question 7

What are the different types of passive transport and what gradients are required for them to occur?

Answer 7

Diffusion: Concentration gradient E.g. O2 uptake from lungs into blood

Convection: Pressure gradient E.g. blood flow from heart to blood vessels

Osmosis: Osmotic pressure gradient E.g. water uptake by cells

Electrochemical flux: Electrical and concentration gradient E.g. ion flow during an action potential in a nerve

Question 8

What are the 2 different types of diffusion?

Answer 8

Simple diffusion

Facilitated diffusion

Question 9

What is active transport?

Answer 9

Movement of molecules against a gradient. Requires energy (uses ATP)

Question 10

Describe some of the properties of the capillaries

Answer 10

Connect terminal arterioles to venules

Smallest diameter of all blood vessels

Semi-permeable

Made of one layer of endothelial cells

Found near every cell in the body but higher density in highly active tissue

Site of solute exchange (passive diffusion)

Site of Fluid exchange

Question 11

What is fluid exchange?

Answer 11

Fluid exchange involves regulation of plasma and interstitial fluid volumes to maintain blood volume and therefore blood pressure.

Question 12

What factors affect the rate of solute transport?

Answer 12

Properties of passive diffusion E.g. concentration, rate, distance

Properties of solutes and membranes and how they come together in Fick’s law

Properties of capillaries

Question 13

How do molecules of solute move around during passive diffusion?

Answer 13

Molecules move randomly due to their random thermal motion
Move from area of high concentration to area of low concentration – eventually this random movement will result in equilibrium being reached.

Question 14

Why is Passive diffusion only good for moving short distances?

Answer 14

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

Question 15

What is the equation for passive diffusion and what does each variable represent?

Answer 15

t = x^2/2D 
Where: 
t = time taken
x = distance 
D = diffusion coefficient

Question 16

Name some of the properties of the solute that affect solute transport and explain how they do this?

Answer 16

Concentration gradient - Higher concentrations of solute move across membrane faster

Size of the solute – The larger the solute the slower it moves across the membrane

Lipid solubility of solute (lipophilic, lipophobic nature) – Lipophilic solutes able to move across membrane easily while lipophobic solutes require aqueous protein channels

Question 17

Name some of the properties of the membrane that affect solute transport?

Answer 17

Membrane thickness/composition
No. of Aqueous pores in the membrane
Carrier-mediated transport
Active transport mechanisms

Question 18

According to Fick’s law what four factors affect diffusion of solute across a liquid?

Answer 18

Diffusion coefficient of solute (D) – ease of movement through solvent

Surface Area (A)

Concentration gradient, C1-C2 or ΔC

Diffusion distance Δx, (distance between C1 and C2)

Question 19

What is the equation for Fick’s law?

Answer 19

Js = - DA x ΔC/Δx

Question 20

State whether Js, mass of solute transferred by diffusion per unit time, is directly or inversely proportional to each of the variables of Fick’s law

Answer 20

Js is directly proportional to the concentration difference, the surface area and the diffusion coefficient.
Js is inversely proportional to the diffusion distance

Question 21

Why is there a negative sign at the start of the equation for Fick’s law?

Answer 21

Negative sign used to show that transport is “downhill” or down the concentration gradient.

Question 22

What are the 3 different types of capillaries?

Answer 22

Continuous capillaries
Fenestrated capillaries
Discontinuous capillaries

Question 23

Place the 3 types of capillaries in order of permeability - from most permeable to least permeable

Answer 23

Discontinuous capillaries
Fenestrated capillaries
Continuous capillaries

Question 24

Describe some of the properties of the continuous capillaries

Answer 24

Tight gaps between neighbouring endothelial cells

Comprised of a ring of 1-3 endothelial cells surrounded by a continuous basement membrane

Pericytes envelope outside of membrane – Regulate capillary diameter and structure

Special kind of continuous capillary, tight capillary, part of the Blood-brain barrier

Found in skeletal muscle, skin, fat, connective tissue and lung

Question 25

Why are continuous capillaries not very permeable?

Answer 25

The tight gaps between neighbouring endothelial cells and the fact that the continuous basement membrane doesn’t have any breaks in it both mean there’s no space to move out from lumen into interstitial tissue.

Question 26

Describe some of the properties of the fenestrated capillaries

Answer 26

Endothelium contains clusters of fenestrae, protein structures that look like small circular windows.

Each fenestra is bridged by thin membrane, the diaphragm, which look like cartwheels with spokes.

Found in tissues that specialise in fluid exchange E.g. salivary glands, kidney, synovial joints, anterior eye, choroid plexus

Question 27

Why do the fenestrated capillaries have a higher permeability than the continuous capillaries?

Answer 27

Because the diaphragm of each fenestra act like sieves allowing small molecules to pass through the capillary.

Question 28

Describe some of the properties of the discontinuous capillaries

Answer 28

Very large endothelial gaps, disrupted membrane – highly permeable even to plasma proteins
Found in organs where movement of cells is required; RBCs in liver, spleen, bone marrow

Question 29

What are the 3 structures of the capillaries that affect solute exchange?

Answer 29

Intercellular cleft
Caveolae & vesicles
Glycocalyx

Question 30

Explain what the intercellular cleft is and how it affects solute exchange

Answer 30

Small space between adjacent cells that are not linked by tight junctions. It allows small lipid-insoluble solutes and fluids to move through between the cells.

Question 31

Explain what Caveolae & vesicles are within the capillaries and how they affect solute exchange

Answer 31

Large pore system that transfers large molecules from the lumen across the capillary wall to the interstitial space.

Question 32

Explain what the Glycocalyx is and how it affects solute exchange

Answer 32

Negatively charged material that covers the endothelium and blocks solute permeation and access to transport mechanisms.
Very dynamic – can be broken down and remade as required, highly regulated – cells regulate break down/remaking of Glycocalyx

Question 33

Define the term permeability

Answer 33

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

Question 34

Name some of the ways a porous membrane interferes with the diffusion of lipid-insoluble solutes through it

Answer 34

Reduction in area for diffusion (A),
Increased path length through membrane (x),
Restricted diffusion in pore produces hydrostatic issues (D)

Question 35

When calculating rate of solute transport why is it convenient to put all of the membrane factors that affect diffusion into one value - Permeability (P)?

Answer 35

Because there are so many ways in which a porous membrane interferes with the diffusion of a lipid insoluble solute.

Question 36

If you modify Fick’s law to factor in the permeability of a porous membrane what does the equation become?

Answer 36

Js = - PAm x ∆C

Question 37

What do each of the variables in this modified Fick’s law equation represent?

Answer 37

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 38

What ways are there for large lipophobic molecules to move through the capillary membrane?

Answer 38

Through big gaps between endothelial cells
Through trans-cellular protein channels
Can be transported from the lumen to interstitial fluid by vesicles

Question 39

What ways are there for water molecules to move through the capillary membrane?

Answer 39

Through water channels within the capillary membrane

Through the diaphragm, membranes of the fenestrae

Question 40

What ways are there for small lipophobic molecules to move through the capillary membrane?

Answer 40

Through the diaphragm, membranes of the fenestrae

Through intercellular protein channels

Question 41

What ways are there for lipophilic molecules to move through the capillary membrane?

Answer 41

Diffuse straight through the capillary membrane

Question 42

Which is the more dominant mode of transport across the capillary membrane - diffusion or filtration and why?

Answer 42

Diffusion more dominant
Glucose concentration in plasma is 1 g / litre
So total volume of plasma filtrate flowing into tissues per day = 8 litres
Maximum filtration of glucose = 8 g / day
However, glucose consumption of human adult is 400 g / day
Therefore, filtration transport only accounts for 2% glucose transport
98% of glucose transport into interstitial space via passive diffusion – via GLUT transporter system

Question 43

What 3 factors control the diffusion rate?

Answer 43

Blood flow
Fall in interstitial concentration
Recruitment of capillaries

Question 44

Explain how blood flow affects diffusion rate

Answer 44

More blood flow brings more solutes into the capillaries. Also, Increased blood volume means there is less time for equilibration to occur across the capillaries so diffusion of 02/C02 continues for longer.

Question 45

Explain how a fall in interstitial concentration affects diffusion rate

Answer 45

During metabolism more solute is used up causing the decrease in interstitial conc. This increases the concentration gradient between the interstitial fluid and the lumen of the capillary thus increasing diffusion rate

Question 46

Explain how the recruitment of capillaries affects diffusion rate

Answer 46

Dilation of arterioles leads to increased number of capillaries perfused which increases total surface area for diffusion (Fick’s law). Also, this shortens diffusion distance between capillary and cell.