Capillaries II - Fluid Exchange

Question 1

What is the importance of fluid exchange?

Answer 1

• Fluid exchange is important for normal physiological function, for example, we need H2O for chemical reactions.
• Fluid re-absorption from the tissues to the blood can maintain circulation during a haemorrhage.
• Abnormalities in fluid exchange can lead to oedema / swelling.

Question 2

Briefly, describe fluid movement at the capillary wall in terms of hydraulic and oncotic pressure.

Answer 2

• The capillary wall is a semi-permeable membrane.
• Fluid moves across the membrane into the interstitial space due to blood flow, which exerts a hydraulic pressure.
• Large molecules (eg. plasma proteins) cannot pass through the membrane so they exert an osmotic pressure termed oncotic pressure.
• This creates a suction force to move fluid back into the capillary.
• Fluid movement across capillary walls depends on the balance between hydraulic and oncotic pressures across the capillary walls.

Question 3

What are the four pressures that determine filtration rate?

Answer 3

1. HYDROSTATIC PRESSURES: -
   
   • P_c: capillary blood pressure
   • P_i: interstitial fluid pressure
2. ONCOTIC PRESSURE: -
   
   • π_p: plasma proteins
   • π_i: interstitial proteins

Question 4

How do we calculate effective osmotic pressure?

Answer 4

• Effective osmotic pressure = σ x potential osmotic pressure.
• Where,
  
  • σ = Reflection coefficient = fraction of the osmotic pressure that is exerted
• Coefficient of 1 means the solute cannot pass through at all and a coefficient of 0 means it is freely permeable
• For example, the σ for plasma proteins is 0.9.
• This means that 10% of the plasma proteins are conducted across the capillary wall into the interstitial space.

Question 5

What is the fluid flux equation (Starling’s principle of fluid exchange)?

Answer 5

• J_v = L_p x A {(P_c - P_i) - σ (π_p - π_i)}
• Where,
  
  • Jv = Movement of fluid or ‘flux’
  • σ = Reflection coefficient = fraction of the osmotic pressure that is exerted
  • Lp = is the hydraulic conductance of the endothelium, ie. how leaky the endothelium is to the fluid.
  • A = the wall area.

Question 6

What do Starling’s forces normally favour in capillaries?

Answer 6

• Starling’s forces favour filtration in capillaries.
• Taking into account factors that affect both filtration and reabsorption, the balance is tipped into filtration (constantly).
• However, there are other factors involved, so the balance can change.
• Well-perfused capillaries will filter along their entire length.

Question 7

How does the lymphatic circulation relate to fluid exchange in capillaries?

Answer 7

• The lymphatic circulation returns excess tissue fluid / solutes back to the cardiovascular system (it drains into the vena cava).
• Lymph vessels have valves and smooth muscle.
• The spontaneous contraction of that smooth muscle contributes to lymph flow.
• Surrounding skeletal muscle contraction and relaxation also contributes to lymph flow.
• Lymph also contains immune cells, especially at the lymph nodes.

Question 8

What does the overall control of the extracellular fluid balance depend on?

Answer 8

1. capillary filtration
2. capillary reabsorption
3. the lymphatic system

Question 9

What happens in the capillaries during hypovolemia (decreased blood volume. I.e. during haemorrage)?

Answer 9

• A drop in venous return [preload] leads to a decrease in cardiac output (CO) leads to a drop in blood pressure (BP), as BP = CO x TPR.
• This means that the capillary blood pressure (Pc) is reduced.
• But plasma osmotic pressure will not change (as it still contains the same concentration of ions).
• With the reduced Pc, filtration may not occur throughout the capillary (the net balance is greater in favour of absorption than filteration).
• We would have interstitial fluid internally transfused into the blood.
• This would be life preserving.
• It would:
  
  • support CVP
  • increase CO
  • raise BP
  • leading to a greater BF

Question 10

Describe oedema and some of its causes.

Answer 10

• Oedema is an excess of fluid within the interstitial space.
• This indicates an imbalance between filtration, reabsorption and lymph function.
• CAUSES: -
  
  • increased capillary pressure, Pc
  • decreased plasma protein oncotic pressure, πp
  • an inflammatory response - they can increase the leakiness of capillaries by opening up bigger fenestrations.
  • lymphatic problems

Question 11

Describe how standing for long periods of time can cause oedema.

Answer 11

• When standing for too long, gravity pools your blood at your legs.
• This increases the venous pressure, causing a ‘backup’ pressure.
• This, in turn, leads to an increased Pc across the capillaries, which increases filtration.
• This means there is now more interstitial fluid in the legs, as indicated by the swelling.
• This situation could also be caused by deep vein thrombosis (DVT), as it also prevents venous return.

DVT meaning - It is a blood clot that develops within adeep vein in the body, usually in the leg.

Question 12

Describe how someone with low protein can develop oedema.

Answer 12

• There are several ways in which the body can not get enough proteins, such as malnutrition, Nephrotic Syndrome (urinary protein loss) and liver disease (where not enough endogenous albumin is produced).
• In each of these cases, there is a reduced plasma protein concentration.
• This means that there is a reduced plasma oncotic pressure, meaning there is a greater influence of Pc and πi.
• This causes a fluid efflux from the capillaries into the interstitial fluid, resulting in oedema. This is also known as Kwashiorkor.

Question 13

Describe how an inflammatory reaction can lead to oedema.

Answer 13

• When an inflammatory response is triggered, there is a large increase in capillary permeability.
• With increased Lp, there is an increased protein permeability (decreased πp) due to a decreased σ.
• With πp decreased, there is less reabsorption taking place, leading to oedema.