Osmosis, pressures and flow
The physical laws of body fluids: osmotic and oncotic pressure, the tonicity of infusions, the Starling balance in capillaries and how oedema forms, diffusion, and blood flow (flow, resistance, Poiseuille's and Laplace's laws).
- 32 cards
- 3 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Scenarios
- Understand
A few cards from the deck
Osmotic pressure: what is it?
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The pressure needed to stop water from crossing a membrane into a more concentrated solution.
It depends on the number of dissolved particles, not their size.
In nephrotic syndrome, blood albumin collapses and oedema appears. Why?
- Oncotic pressure falls: fluid is no longer held in the vessels
- Blood pressure rises too much
- Cells burst
- Lymph flows too fast
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Oncotic pressure falls: fluid is no longer held in the vessels
Albumin lost in urine tips the Starling balance outwards.
Oncotic pressure: what is it?
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The part of osmotic pressure due to plasma proteins, mainly albumin, which keeps water in the vessels.
Proteins barely cross the capillary wall.
Why does osmotic pressure depend on the number of particles, not their mass?
- Large particles are heavier
- Only ions count
- It depends on colour
- Each dissolved particle 'holds' water the same way, whatever its size
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Each dissolved particle 'holds' water the same way, whatever its size
One mole of NaCl gives two moles of particles (Na⁺ and Cl⁻).
Starling balance: what is it?
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The balance in capillaries between hydrostatic pressure pushing fluid out and oncotic pressure holding it in.
The small excess filtered out is returned by the lymph.
In right heart failure, the ankles swell. What mechanism?
- Albumin rises
- Venous pressure, hence capillary hydrostatic pressure, rises
- Lymph disappears
- The arteries block
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Venous pressure, hence capillary hydrostatic pressure, rises
Blood backs up before the heart and pushes fluid into the tissues, mostly low down by gravity.