Lecture 10 – Capillaries I: Solute exchange

Question 1

Metabolism (3)

Answer 1

Creates need for gaseous and nutrient exchange.
Continually generates concentration gradients that facilitates this movement.
Problem is the cell membrane as they act as barriers.
O2/aa/glucose in co2/urea out.

Question 2

Cell membranes (4)

Answer 2

Two layers of amphipathic phospholipids
Head is hydrophilic
Tail is hydrophobic
Contains transmembrane proteins that span across the membrane and facilitate transport and signalling
Carbohydrates anchor the proteins and orientate them

Question 3

Cell membrane functions (3)

Answer 3

Recognition immune system
Controls what enters and leaves ion movement in nerves
Regulate cell function e.g. insulin mediated glucose uptake

Question 4

Passive transport processes (4)

Answer 4

Diffusion

Osmosis

Convention- pressure gradient e.g. BF from heart to BV.

Electrochemical flux - electrical and chemical concentration gradient e,g, ion flow during action potential in a nerve.

Question 5

Rate of solute transport depends on 3 things

Answer 5

Passive diffusion - concentration/rate/distance
Properties of solute and membrane e.g Ficks law
Prosperities of capillaries e.g. affects movement and solute exchange between plasma and interstitial fluid.

Question 6

Formulas (2)

Answer 6

T is proportional to d2

t = x2 / 2D
t= time taken 
x= net distance 
D= diffusion coefficient is different for different media as solute moves in different ways

Question 7

Properties of solute affecting transport (3)

Answer 7

Concentration gradient
Size of solute
Lipid solubility of solute (lipophilic)

Question 8

Properties of membrane affecting transport (4)

Answer 8

Thickness/composition
Aqueous pores in membrane
Carrier mediated transport
Active transport mechanism

Question 9

Ficks law (2)

Answer 9

Measurement of flux, how much substance over time.
Js= -D A ( Delta C / x)
Js= solute movement, mass per unit time 
D diffusion coefficient of solute
DeltaC concentration gradient change 
x distance between C1 and C2

-ve value as you move down a concentration gradient (high to low)

Question 10

Types of capillaries (3)

Answer 10

Continuous
Fenestrated
Discontinuous

Question 11

Types of capillaries - Continuous (4)

Answer 11

Moderate to low permeability
Tight gaps between neighbouring cells,constant basement membrane
Blood brain barrier (tightest junction) - regulated passage of extracellular fluid in blood and brain
Muscle skin fat and connective tissue

Question 12

Types of capillaries - Fenestrated (3)

Answer 12

High water permeability
Modest disruption of membrane
High water turnover tissues e.g. salivary glands, synovial joints, kidneys

Question 13

Types of capillaries - DisContinuous (2)

Answer 13

Used when movement of cells are required e.g. bone marrow and spleen/RBCs in liver
Very large fenestration structures disrupted membrane

Question 14

Properties of capillaries which can influence solute transfer (3)

Answer 14

Intercellular cleft- 10-20nm wide, they are between adjacent cells that are not linked by tight junctions allows solute and small lipophibic molecules to move through cells.

Vesicles/Caveolae - large pore system, substances are taken up on one side (lumen) and moved to the other (interstitial fluid) endocytosis followed by exocytosis.

Glycocalyx - complex structure. Covers endothelium on the lumen side and acts as an additional barrier.
Negatively charged carbohydrate mesh , sits on top of endothelial cells, blocks solute permeation, additional barrier. Dynamics,

Question 15

Permeability (3)

Answer 15

Rate of solute transfer by diffusion across unit area of membrane per unit concentration difference.
All factors affecting diffusion go into one term which is permeability.
Modification of Ficks law for a porous membrane.
Js= -PAm DeltaC

Question 16

What is the dominant route of transport? (4)

Answer 16

Passive diffusion

E.g glucose concentration in plasma in 1g per litre, total volume in plasma is 8 litre. Max filtration of glucose is 8g a day out of 400g daily requirement.
Rest comes from passive disunion via GLUT transporter system.
Filtration occurs through pores/gaps and fenestrations.

Question 17

Factors controlling diffusion rate (3)

Answer 17

Blood flow
More blood means more solutes so increased BV means less time for equilibration to occur across capillaries.
Flow limited diffusion is slow flow in a long capillary, some diffusion. Occurs in sepsis where BP and BF is low and can lead to ischaemia.

Fall in interstitial concentration
During metabolism more solute is used up increasing concentration difference
Metabolism increases BP increases O2 delivery controlled by arterioles

Recruitment of capillaries
Dilation of arterioles leads to increased number of capillaries perfused increasing total surface area, Ficks law, shortens diffusion distance.