Capillaries 2

Question 1

Why is fluid reabsorption from tissue important

Answer 1

Fluid re-absorption from tissue to blood is important for maintaining circulation volume
levels, for example during haemorrhage. Abnormalities in fluid exchange can lead to oedema (tissue
swelling).

Question 2

What causes lyphoedema

Answer 2

Lymphoedema is caused by the removal of lymph nodes or damage to them due to
radiotherapy.

Question 3

Where does fluid exchange occur and where does the movement occur

Answer 3

Fluid exchange occurs between the capillaries and interstitial fluid. This movement occurs across the
endothelium lining of the capillaries and the endothelial cells. This movement can also be through
the diffusion of solutes (see previous lecture). Fluid movement occurs across the capillary
membrane.

Question 4

How is hydrostatic pressure formed across the system

Answer 4

Blood pressure creates a difference in hydrostatic pressure across the system. This means
there is generally a higher blood pressure inside the capillary than in the interstitial fluid meaning
fluid moves out.

Question 5

Compare and describe the albumin concentration

Answer 5

Solute composition of plasma is relatively similar with some important exceptions.
Albumin is a protein that is in relatively low concentrations that is present in plasma but not
interstitial fluid. This is because albumin cannot pass through the capillary membrane due to its size.

Question 6

How is albumin also important

Answer 6

The presence of a solute in plasma not present in interstitial fluid creates an osmotic pressure
gradient (sometimes termed oncotic pressure) that moves water into plasma in the capillary.
These
two forces must be in balance to ensure fluid volumes stay in balance ( hydrostatic and oncotic )

Question 7

What happens if hydrostatic pressure is increased or oncotic pressure decreases

Answer 7

Any increase in hydrostatic

pressure or decrease in oncotic pressure will lead to excess filtration and potentially oedema.

Question 8

What is hydrostatic pressure split into

Answer 8

The 2 hydrostatic and osmotic pressures are both composed of subcomponents. Hydrostatic pressure
has a larger capillary blood pressure component (PC) and a smaller interstitial fluid pressure (Pi)
component.

Question 9

Describe the interstitial pressure

Answer 9

This interstitial fluid pressure is very small and can even be slightly negative.

Question 10

What is the osmotic pressure split into

Answer 10

The
osmotic pressures are set by the osmotic pressure of the plasma proteins (πp) and the osmotic
pressure of interstitial proteins (πi).

Question 11

What gives the osmotic pressure direction

Answer 11

There are lower concentrations of interstitial fluid proteins than
plasma proteins giving the osmotic pressure a direction.

Question 12

Describe starlings principle of bulk movement

Answer 12

Starling’s principle of fluid exchange
discusses bulk movement. It states that bulk movement is proportional to hydraulic pressure
difference balanced out against osmotic pressure difference.

Question 13

What is hydraulic conductance

Answer 13

Hydraulic conductance of the

endothelium (how leaky the endothelium is to fluid) is also considered in this equation.

Question 14

What is the starling equation

Answer 14

Sigma is the
reflection coefficient (fraction of the osmotic pressure that is exerted) and A is the surface area of the
wall. The full equation is JV = LpA {(Pc – Pi) – σ(πp – πi)}. We consider Lp and A to be constant meaning
this equation is dependent on the hydrolytic and osmotic pressures. The reflection coefficient
determines how easily a particles can cross a membrane.

Question 15

Describe the lymphatic system

Answer 15

This is composed of specialised vessels that collect interstitial fluid
and drain it back into circulation. These specialised lymph vessels have valves that ensure one-way
flow and go through lymph nodes that are involved in immunity.

Question 16

What does overall control of extracellular fluid depend on

Answer 16

Overall control of extracellular fluid

balance depends on capillary filtration, capillary reabsorption and the lymphatic system.

Question 17

Describe the pressure at the arterial end to venous

Answer 17

At the arterial end of a capillary, the hydrostatic pressure is down from around 100 mmHg when it
was at the aorta, to 35 mmHg. As the blood continues along the capillary, further pressure is lost
until the venous end of the capillary is reached. This loss in pressure means that less plasma passes
out of capillaries to become interstitial fluid as more pressure is lost. This is because the osmotic
pressure does not change along the capillary. Any change in these pressures can lead to problems.

Question 18

Describe what happens during a heamorrage

Answer 18

During haemorrhage for example, blood volume is decreased. This means venous return is reduced
and this affects preload. Preload is reduced due to less filling of the heart and the result is a decrease
in cardiac output. Cardiac output drop causes a drop in blood pressure and this means the blood
pressure at the arterial end of the capillary in this circulatory system with haemorrhage will be lower
than the blood pressure at the arterial end of a capillary in a circulatory system that is normal. In the
case of haemorrhage, there is initially some filtration however, the blood pressure drops so low that
osmotic pressure is now greater than blood pressure meaning reabsorption occurs. This is useful as
this mechanism can compensate for some loss of blood.

Question 19

What is the cause of oedema

Answer 19

Oedema is caused by too much filtration.
This is because venous return is suppressed meaning there is an increase in pressure in the venules
upstream of the region of suppression. This means that the hydrostatic pressure at the venule end of
the capillary remains at a somewhat higher level meaning there is more filtration at the venous end
and therefore more filtration in general. Oedema is in fact any excess fluid within the interstitial
space and is caused by any imbalance between filtration, reabsorption and the lymphatic system.
This means increased capillary pressure is not the only cause but other causes include decreased
plasma protein oncotic pressure, inflammatory response and lymphatic problems.

Question 20

What can deep vein thrombosis cause

Answer 20

Increased capillary pressure clinically can be caused by deep vein thrombosis. This disrupts venous
return and therefore increases venous pressure upstream of the clot causing oedema.

Question 21

What can cause oedema

Answer 21

Standing up

for long periods of time also causes oedema due to gravity backing up venous return.

Question 22

What causes decreased plasma oncotic pressure

Answer 22

Decreased
plasma oncotic pressure can be caused by malnutrition or malabsorption meaning not enough
protein intake to make plasma proteins.

Question 23

What does nephrotic syndrome cause

Answer 23

Nephrotic syndrome results in urinary protein loss. This can
still somewhat be compensated by liver protein production.

Question 24

What shortages does liver disease cause

Answer 24

Liver disease can result in shortages in
the production of plasma proteins like albumin and this also affects protein concentration in plasma
and therefore oncotic pressure.

Question 25

What does inflammation cause in capillaries

Answer 25

Inflammation causes an increase in capillary permeability. The result
is an increase in protein leakage which decreases oncotic pressure.

Question 26

How is inflammation triggered

Answer 26

Inflammation is triggered by
things like chemicals, insect bites, nettle stings, infection, physical trauma and autoimmune disease.
Lymphatic problems are anything that affects the lymphatic system. Lymphatic obstruction that
affects drainage can be caused by insect bites (e.g. elephantitis where larvae migrate to lymphatic
system, grow, mate and form nests that block lymph drainage). Lymphoedema is caused my
lymphatic removal that is a side effect of surgery to treat testicular cancer and other cancers.