L18: Capillary And Lymphatic Function

Question 1

2 main mechanisms in which substance cross capillary wall

Answer 1

1. Diffusion (passive)
   - concentration gradient
   - surface area
2. Filtration (passive)
   - Hydrostatic force + Oncotic force
   - k ([Pc + πi] - [Pi + πc])
   - k: filtration constant (permeability x SA)
   - filtration (sum = +ve), absorption (sum = -ve)
3. main driving force out: Pc (since πi: low 0.1-5 mmHg)
   —> arterial + venous pressure
   —> post:pre-capillary resistance

—> Pc = [(Rv/Ra)xPa + Pv] / 1+(Rv/Ra) (no need to rmb)
—> Pa↑: ↑Pc (↑P gradient from artery to capillary —> more blood into capillary)
—> Pv↑: ↑Pc (↓P gradient from capillary to vein —> more blood stay in capillary)
—> Ra↑: ***↓Pc (more difficult for blood to go from artery to capillary —> less blood in capillary)
—> Rv↑: ↑Pc (more difficult for blood to go from capillary to vein —> more blood in capillary)

2. main driving force back in: πc: 25 mmHg (since Pi low: +1 mmHg or even negative)
   —> colloid osmotic pressure (non-diffusable large molecules)
   —> plasma protein —> draw water back into capillary

Question 2

Starling’s hypothesis (NOT Starling’s law)

Answer 2

• Arterial end: Pc high —> net filtration
• Venous end: Pc low —> net reabsorption
• πc remain the same throughout
• amounts of fluid filtered = amount of fluid reabsorbed
• true for most tissue
• some tissue only filtration: renal glomerulus (high Pc)
• some tissue only absorption: lung (low Pc)

Question 3

Congestive heart failure and Starvation / Nephrosis

Answer 3

Heart fail to pump out blood
—> Residual volume of heart ↑
—> Right atrial pressure ↑
—> venous return ↓
—> venous pressure ↑
—> capillary pressure ↑
—> capillary filtration ↑
—> oedema

Starvation / Nephrosis:
↓ plasma protein synthesis / ↑ plasma protein lost in urine
—> ↓ πc
—> ↓ reabsorption
—> filtration > reabsorption
—> oedema

Question 4

Lymphatics

Answer 4

• slow lymph flow (2-4L/day)
• clear excess fluid, protein, lipid, foreign material
• blind ends in interstitial space
• series of chambers with valves
• edges of epithelial cells not joined but freely overlap
• ↑ interstitial pressure allow valve to open
• ↑ lymphatic pressure close clave and trap fluid
• presence of fluid stretch wall —> contract and propel to next chamber
• 25 mmHg during contraction, 0 mmHg when not contracting