Solutes and Fluid exchange (I & II)

Question 1

Name the 4 passive transport processes.

Answer 1

1) Diffusion: Down a concentration gradient e.g. O2 uptake from lungs into blood
2) Convection: Down pressure gradient e.g. blood flow from heart to vessels
3) Osmosis: Down an osmotic pressure (water) gradient e.g. water uptake by cells
4) Electrochemical flux: Down an electrical AND concentration gradient e.g. ion flow during an AP in a nerve

Question 2

What are the 3 different types of capillaries?

Answer 2

Continuous capillaries, fenestrated capillaries and discontinuous capillaries.

Question 3

Describe the structure and permeability of continuous capillaries. Give examples of where they are found.

Answer 3

They have a low to moderate permeability. Endothelial cells have tight junctions between them. Also the basement membrane which the endothelial cells are on is continuous producing a firm barrier against molecules moving out into the interstitial fluid from the plasma.
*These are present in the brain and nervous system (making up the blood brain barrier).

Question 4

Describe the structure and permeability of fenerstrated capillaries. Give examples of where they are found.

Answer 4

More permeable than continuous capillaries because the gaps between the endothelial cells in places involve fenestrations allowing certain molecules of the correct size to move out into the interstitial area.
*Found in exocrine glands, salivary/endocrine glands and other ‘high water turnover tissues’ such as: kidney, synovial joints, anterior eye, choroid plexus (CSF), gut mucosa.

Question 5

Describe the structure and permeability of discontinuous capillaries. Give examples of where they are found.

Answer 5

Discontinuous/Sinusoidal capillaries which have very large fenestrations between the endothelial cells, there is also breakdown of the basement membrane. This means lots of water and solutes can get through and even some plasma proteins. It is important when movement of cells is required e.g. RBCs in the liver, spleen and bone marrow.

Question 6

What are some of the other structural features of capillary walls which can influence solute transfer?

Answer 6

Intercellular clefts, Glycocalyx, Caveola - Vesicle system

Question 7

What is an intracellular cleft?

Answer 7

The gap between two endothelial cells (usually about 10-20nm wide), this can be tight or relaxed. It is a useful area/structure to move substances through. How tight these are will help determine permeability.

Question 8

What is the glycocalyx?

Answer 8

Covers the endothelium blocking protein access to transport mechanisms (like the clefts), it acts as a semipermeable membrane and regulates what enters through much more. Pore size can be regulated allowing different molecules through.

Question 9

Describe the Caveola - Vesicle system.

Answer 9

Allow dumping of large molecules from vascular side to interstitial space, they can do endocytosis and exocytosis to move large proteins.

Question 10

What 4 things control the rate of solute transport?

Answer 10

1) The properties of passive diffusion
2) Properties of solutes and membranes (Fick’s law)
3) Properties of the capillaries
4) The concept of permeability

Question 11

Give the equation which relates time taken (t) to net distance travelled (x).

Answer 11

t = x2 / 2D
D = Diffusion coefficient

Question 12

What is Fick’s Law?

Answer 12

Solute movement, which is mass per unit time m/t (Js), is determined by 4 factors:

Js = - D A (ΔC/x)

D = Diffusion coefficient of solute – ease through solvent
A = Area (greater area e.g. capillary recruitment = greater diffusion)
ΔC/x = Concentration gradient (C1 – C2) across distance x (greater = greater diffusion)
Js is a negative value because the substance is flowing down a concentration gradient

Question 13

Define permeability.

Answer 13

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

Question 14

What is Fick’s law when modified for a porous membrane?

Answer 14

Js = - PAm ΔC

Js = Rate of solute transport
Am = Surface area of capillary involved in transport – this involves pore size, length and diffusion coefficient
ΔC = Concentration gradient

Question 15

How does fast flow lead to exchange occurring throughout the length of the capillary?

Answer 15

Dilating our blood vessels which will increase the concentration of solutes in the capillaries resulting in more exchange of O2, CO2. The increased flow also allows less time for equilibration of O2/CO2 to occur, because if it moves too slowly by the time oxygen gets half way may start equilibriating and O2 starts moving back (flow-limited diffusion, if flow isn’t strong enough it limits diffusion).

Question 16

Why does O2 transport from blood to muscle increase during strenuous exercise?

Answer 16

Dilation in arterioles increases the number of capillaries perfused (blood flowing into capillaries that are not usually used, e.g. from 10 ->100). This increased recruitment means the total surface area (A) for diffusion is greater (Fick’s law) and it shortens the diffusion distance x, so we will get faster diffusion.

Question 17

Why is fluid re-absorption required during haemorrhage and shock?

Answer 17

We need to take fluid from interstitial space back into our plasma to maintain our cardiac output.

Question 18

What is hydraulic pressure responsible for?

Answer 18

Movement of water and low molecular weight molecules out of capillaries and into interstitial fluid.

Question 19

What does oncotic pressure mean and why is it formed?

Answer 19

Large molecules (e.g. plasma proteins) cannot cross into interstitial fluid. So, plasma proteins like albumin stay in the blood plasma. Because plasma proteins cannot move from blood plasma they create an osmotic gradient. There will be high plasma proteins in the blood plasma and low plasma proteins in the interstitial fluid – this means water wants to move from interstitial space back into plasma, osmotic pressure is what is pulling it in. This type of osmotic pressure is called oncotic pressure.

Question 20

What are the 4 pressures which determine the filtration rate? Give the direction the fluid moves in for each one.

Answer 20

Osmotic pressures
PIp - plasma proteins (moves into capillary)
PIi - interstitial proteins (moves out of capillary)
Hydrostatic pressures
Pc = Capillary blood pressure (moves out of capillary)
Pi = Interstitial fluid pressure (moves into capillary)

Question 21

What is the equation for Jv (flux of fluid)? Describe what the components of the equation mean.

Answer 21

Jv = LpA { ( Pc - Pi ) - σ(PIp - PIi) }
says Jv is proportional to {hydraulic pressure difference (Pc – Pi) – colloid osmotic pressure difference (πp – πi)}.
Lp = Hydraulic conductance of endothelium, in other words how leaky is the endothelium to fluid.
A = Wall area (is there recruitment of capillaries).

Question 22

What does the sigma in the Jv equation stand for?

Answer 22

σ = Reflection Coefficient, this is how much of the osmotic pressure is actually being exerted based on how much plasma proteins can get through the endothelium. So a reflection coefficient of 0.9 means about 10% of plasma proteins can get through (are being conducted). If there is a coefficient of 1, it means no plasma proteins can get through so the plasma proteins will be exerting their full osmotic pressure.

Question 23

Do Starling’s forces favour filtration or reabsorption?

Answer 23

Capillary pressure is generally much larger due to force of heart and πp is much higher because majority of plasma proteins are in the plasma as capillary is not that leaky.
Starling’s forces normally favour filtration of fluid (i.e. movement out of plasma).

Question 24

What part of the Jv equation would you use to calculate the net pressure for fluid leaving the capillary? What can you use this number for?

Answer 24

σ (PIp -PIi) + Pi
This can be used with the capillary pressure along the capillary (usually 35mmHg at the beginning) to see whether the blood is being filtered or reabsorbed (i.e. is it well perfused). Number is usually 13, therefore at the beginning net filtration pressure is 22mmHg.

Question 25

What is the lymphatic system responsible for?

Answer 25

Returns excess tissue fluid/solutes back to the cardio-vascular system (into great veins).

Question 26

What causes lymph flow in the lymphatic system?

Answer 26

Lymph vessels have valves and smooth muscle, the spontaneous contractions of smooth muscle contribute to lymph flow. Surrounding skeletal muscle contractions/relaxation also help lymph flow.

Question 27

What 3 factors does the control of ECF balance depend on?

Answer 27

Capillary filtration
Capillary reabsorption
Lymphatic system

Question 28

What happens to filtration when patient has Hypovolaemia?

Answer 28

To restrict the amount of blood lost there is vasoconstriction in the capillaries. This reduces the BP at the arterial end. Note that the pressure pushing fluid back into the capillaries stays the same, this leads to fluid being reabsorbed into the capillaries at the venous end because it decreases and BP already started off low.

Question 29

What happens if you clamp the upper arm and increase pressure to 40mmHg (possibly representing a DVT)?

Answer 29

This will put pressure on the venous system, this causes an increase in capillary pressure. As a result, more fluid moves out of the plasma into the interstitial fluid and there is an increase in forearm volume – i.e. swelling in the arm. This represents oedema.

Question 30

List potential causes of oedema?

Answer 30

Increase capillary pressure (Pc)
Decreased plasma protein oncotic pressure (πp)
Inflammatory response
Lymphatic problems

Question 31

What are the potential clinical scenarios where there is decreased plasma osmotic pressure (πp) and how does this cause oedema?

Answer 31

Malnutrition/malabsorption, hepatic failure (not enough albumin produced), nephrotic syndrome (urinary protein loss).
Reduced plasma protein concentration, as a result we will have a reduced plasma oncotic pressure. Hence there will be alower difference between πi (interstitial oncotic pressure) and πp (plasma oncotic pressure).
There will be far less force sucking fluid back into plasma to balance the output by capillary pressure. This means there is increased filtration = Oedema.

Question 32

How does increasing capillary permeability lead to oedema?

Answer 32

We are changing (increasing) the LP value of Starling’s Principle. This means it is easier for fluid to move into the interstitial space AND there will be more easy movement of plasma proteins into the interstitial space, this means there will be a change in the balance of osmotic pressure. Usually osmotic pressure in plasma is dominant drawing fluid in, but now because the capillary is so leaky the balance will change and difference between them will change (get closer to cancelling out) so there will be increased fluid drive out of the capillary.
**Increased Lp, Increased protein permeability (PIi), decreased Sigma

Question 33

Describe filariasis and elephantitis?

Answer 33

Caused by a nematode infestation. The larvae grow/mate/form nests and block lymphatic drainage. This causes the oedema, particularly in legs due to effect of gravity increasing Pc.