Microcirculation and Fluid Balance

Understanding how water and substances cross the capillary wall
Familiarity with the Starling principle concerning fluid balance in microcirculation
The lymphatic system's function in regulating interstitial fluid balance
Identification of four key factors contributing to oedema formation

Composed of terminal arterioles, capillaries, and post-capillary venules
Key site for gas, water, nutrient, and waste exchange between blood and tissues
Forms a three-dimensional meshwork of blood vessels facilitating absorption and lymphatic flow
Oedema can arise from:
- Increased capillary hydrostatic pressure (e.g., heart failure, thromboembolism)
- Elevated capillary or venular permeability (e.g., inflammation)
- Decreased plasma oncotic pressure (e.g., malnutrition, burns, liver dysfunction)
- Lymphatic obstruction (e.g., lymph node removal, inflammation, parasitic invasion)

Pressure gradient exists in capillaries: ~30 mmHg at the arterial end and ~10 mmHg at the venous end
Net Filtration Pressure (NFP) is positive at the arterial end and negative at the venous end
Approximately 90% of fluid lost at arterial end returns via venous capillaries; remaining fluid is cleared by lymphatics

Essential for preserving fluid balance and transporting absorbed fats from the small intestine to the circulatory system
Transports foreign materials to lymph nodes for immune surveillance
Composed of lymphatic capillaries with intercellular clefts allowing one-way fluid entry
Fluid movement is driven by tissue compression and facilitated by lymphangions, which act as miniature hearts
Smooth muscle contractions in lymphatic trunks aid in fluid transport and require tissue compression between 40-50 mmHg to function effectively

Afferent and efferent lymphatics connect to major blood vessels, delivering lymph to either the thoracic duct or the right subclavian duct
Lymph nodes play a crucial role in reabsorbing some lymph fluid and facilitating immune responses