SN3
SHORT NOTE
5M
★ High-yield
Define oedema. Describe the Starling forces and enumerate the mechanisms by which oedema is produced.
Definition
Oedema = abnormal accumulation of fluid in the interstitial space or body cavities.
- Clinically detectable only after about 3–5 litres have accumulated
Starling Forces at the Capillary
| Force | Arteriolar end | Venular end | Direction |
|---|---|---|---|
| Capillary hydrostatic pressure | 32 mmHg | 15 mmHg | Out |
| Plasma oncotic pressure | 25 mmHg | 25 mmHg | In |
| Interstitial hydrostatic pressure | −3 mmHg | −3 mmHg | Out |
| Interstitial oncotic pressure | 8 mmHg | 8 mmHg | Out |
| Net | +13 (filtration) | −7 (absorption) | — |
Mechanisms and Causes
| Mechanism | Causes |
|---|---|
| ↑ Capillary hydrostatic pressure | Heart failure, venous obstruction, DVT, pregnancy, salt and water retention |
| ↓ Plasma oncotic pressure (hypoalbuminaemia) | Nephrotic syndrome, liver failure, malnutrition (kwashiorkor), protein-losing enteropathy |
| ↑ Capillary permeability | Inflammation, burns, allergy, sepsis, snake bite |
| Lymphatic obstruction | Filariasis (elephantiasis), post-mastectomy, malignant infiltration |
| Na+ and water retention | Renal failure, hyperaldosteronism, steroids |
Types
- Pitting — low protein content; heart failure, nephrotic syndrome
- Non-pitting — high protein content; lymphoedema, myxoedema
- Localised — DVT, inflammation, filariasis
- Generalised (anasarca) — cardiac, renal, hepatic
Safety Factors Against Oedema
- Low interstitial compliance — negative interstitial pressure
- ↑ lymphatic flow (up to 10–20×)
- Washdown of interstitial protein → ↓ interstitial oncotic pressure
- Together these absorb about 17 mmHg of extra capillary pressure before oedema appears
Applied Aspects
- Cardiac oedema → dependent, starts at ankles, worse in the evening
- Renal (nephrotic) oedema → periorbital, worse in the morning
- Pulmonary oedema — the most dangerous; occurs when pulmonary capillary pressure exceeds 25–28 mmHg
All questions in General Physiology & Body Fluids
- Q1Describe the structure of the cell membrane. Classify the mechanisms of transport across it and describe passive transport in detail. ★
- Q2Define active transport. Classify it and describe the sodium–potassium pump with its functions. Add a note on secondary active transport. ★
- Q3Enumerate the body fluid compartments with their volumes and ionic composition. Describe the methods used to measure them. ★
- Q4Define resting membrane potential. Describe its ionic basis and the factors responsible for it. Add a note on the Nernst equation. ★
- Q5Define homeostasis. Describe the components of a control system and explain negative and positive feedback with examples. ★
- SN1Define osmosis and osmotic pressure. Explain tonicity with examples. ★
- SN2Write a short note on the Gibbs–Donnan equilibrium.
- SN3Define oedema. Describe the Starling forces and enumerate the mechanisms by which oedema is produced. ★
- SN4Classify cell junctions and describe their functions.
- SN5Write a short note on facilitated diffusion. ★
- SN6Enumerate the second messenger systems and describe the cyclic AMP pathway. ★
- SN7Define apoptosis. Differentiate it from necrosis and describe its pathways.