Capillary Solute and Fluid Exchange II Flashcards
Fluid movement across capillary walls depends on the balance between which 2 main pressures, each of which is subdivided into 2 distinct pressures?
- Hydraulic pressure - blood flow exerts pressure forcing fluid into the interstitial fluid.
- Oncotic pressure - an osmotic pressure exerted by plasma proteins and other large molecules that draws fluid into the capillary.
Capillary filtration rate is determined by which 4 pressures, 2 of which are classed as osmotic and 2 as hydrostatic?
Osmotic pressures:
- Πp - plasma proteins
- Πi - interstitial proteins
Hydrostatic pressures:
- Pc - capillary blood pressure
- Pi - interstitial fluid pressure
Describe Starling’s principle of fluid exchange.
Jv (capillary filtration rate) = Lp A { (Pc - Pi) - σ (πp - πi) }
Jv is proportional to hydraulic pressure difference - osmotic pressure difference.
Describe and explain the terms “Lp”, “A” and “σ” in Starling’s principle.
Lp - hydraulic conductance of the endothelium - how leaky the endothelium is to fluid.
A - wall area.
σ - reflection coefficient - fraction of the osmotic pressure that is exerted. Effective osmotic pressure = σ x osmotic pressure.
σ = 0.9 for plasma proteins under normal conditions, i.e. 10% of plasma proteins are conducted across capillary wall.
Describe the typical values of the 4 pressures of Starling’s principle and how this influences capillary solute exchange.
Pc = 35, Pi = 1 πp = 25, πi = 10
(Pc - Pi) - σ (πp - πi) = (35 - 1) - σ (25 - 10)
Thus, Starling’s forces normally favour filtration.
Explain the difference between colloid osmotic pressure and oncotic pressure.
The colloid osmotic pressure is the osmotic pressure exerted by a compartment containing proteins that cannot leave that compartment. Both the capillary blood and interstitial fluid have colloid osmotic pressures, and the difference between them is the oncotic pressure. Thus, the oncotic pressure is the net osmotic pressure favouring reabsorption. As the interstitial fluid osmotic pressure is virtually negligible, the oncotic pressure is effectively the same as the colloid osmotic pressure of the plasma.
Regarding capillary filtration, what does it mean if the reflection coefficient (σ) of a capillary is 0?
This means that 100% of plasma proteins are able to leak out of the capillary, and thus none are “reflected” back into the lumen. The capillary is maximally permeable and there will therefore be virtually no oncotic pressure.
Well-perfused capillaries filter along their entire length. Give typical net filtration pressures at the arterial and venous end of a well-perfused capillary.
Arterial end - 22 mmHg
Venous end - 2 mmHg
Why does net filtration pressure decrease further towards the venous end of the capillary?
Pc is highest at the arterial end - 35 mmHg. At the venous end it is around 15 mmHg. This drop in Pc reduces the net filtration pressure moving along the capillary.
Lymphatic circulation returns excess tissue fluid/solutes back to the CVS. What forces drive lymph flow?
Lymph vessels have valves and smooth muscle. Spontaneous smooth muscle contractions contribute to flow.
Contraction of surrounding skeletal muscle also drives lymph flow.
In patients with hypovolaemia following haemorrhage, 500ml of interstitial fluid is “internally transfused” into the blood over 0.5 hours. Explain how and why this process occurs.
Hypovolaemia means reduced venous return to the heart. This leads to a smaller EDV, resulting in reduced SV and therefore reduced cardiac output. This drop in CO consequently leads to a drop in BP. The initial drop in BP stimulates sympathetic nerve-induced vasoconstriction of pre-capillary arterioles, which reduces pressure downstream.
Thus, low BP due to hypovolaemia coupled with sympathetic activity results in decreased Pc along the entire capillary length, favouring reabsorption rather than filtration. There may be a minimal amount of filtration at the start of the capillary but further along there will be reabsorption of fluid into the capillary. This reabsorption restores blood volume, CVP, EDV, SV and cardiac output, eventually returning BP and flow to normal levels.
Define oedema.
Oedema is fluid accumulation in the interstitial space due to an imbalance between filtration, reabsorption and lymph function.
Name the 4 main causes of oedema.
- Increased Pc.
- Decreased πp.
- Inflammatory response.
- Lymphatic problems.
Deep vein thrombosis (DVT) causes swelling and inflammation of the affected leg. Explain why.
DVT prevents venous return, causing increased venous pressure in the affected area. This causes a “back-up” of pressure because arterial blood accumulates without adequate drainage, leading to increased Pc across capillaries. Net filtration is increased, causing oedema - accumulation of fluid in the interstitial space. This manifests as swelling of the limb.
What are the possible causes of increased capillary pressure?
- Standing up for long periods.
- DVT.
- Cardiac failure.
