Physiology 10 Flashcards
Total body water of 70 kg young man? What does total body water consist of? (2) What is extracellular fluid composed of? (2)
- 60% (~42 L)
- Intraceullular fluid (2/3) and extraceullular fluid (1/3)
- Interstitial fluid (75%) and plasma volume (25%)
What is interstitial fluid? What is its role? What does this have t diffuse from?
- Fluid bathing the body cells
- Acts as the go-between blood and body cells - delivers oxygen and nutrients to body cells
- Have to diffuse from intravascular compartment to reach body cells
What are capillaries composed of? What is their function? What does blood flow in capillaries depend on?
- Single layer of endothelial cells
- Allow rapid exchange of gases, water & solutes with INTERSTITIAL FLUID
- Blood flow in the capillaries depends on the contractile state of the arterioles
What is the function of terminal arterioles? What regulate flow only some tissues? Example?
- Regulate regional blood flow to the Capillary Bed (CB) in most tissues
- ‘Precapillary Sphincters’ regulate flow in few tissues
- e.g. mesentery
Is blood flow through capillary bed fast or slow? Why? What do capillaries unite to form?
- Slow
- To allow adequate time for exchange
- Post-capillary venules
Explain the transport of substances across capillary wall (5)
- Small water-soluble substances pass through water-filled capillary pores e.g. Na+, K+, glucose, amino acids
- Lipid soluble substances pass through the endothelial cells e.g. O2, CO2
- Exchangeable proteins moved across by vesicular transport
- Plasma proteins generally cannot cross capillary wall (usually stay within the blood)
What does fluid movement across capillary wall depend on? What does movement of gases and solutes follow?
- Pressure gradient (bulk flow)
* Fick’s Law of Diffusion (i.e. downhill)
What is transcapillary fluid flow driven by? What is transcapillary fluid flow?
- Passively driven by pressure gradients across the capillary wall
- Ultra-filtration i.e. exchange across the capillary wall of essentially protein-free plasma
What is Net Filtration Pressure? What also affects net fluid filtration?
- NFP = forces favouring filtration - forces opposing filtration
- A filtration coefficient (Kf)
What are forces involved in transcapillary flow known as? What are forces favouring filtration? (2) Forces opposing filtration? (2)
- Starling Forces
Favouring
- Pc - capillary hydrostatic pressure
- nI - interstitial fluid osmotic pressure
Opposing
- nC - capillary osmotic pressure
- Pi - Interstitial fluid hydrostatic pressure
NOTE: n = pi
Look at picture showing forces affecting arteriolar and venular end of capillary (pic)
…
What is the most important force favouring filtration? What does it occur as a result of?
- Capillary hydrostatic pressure
* Results from blood pressure itself - BP falls from arterioles to capillaries - this forces fluid outside capillary
What is osmotic pressure inside the capillary? Why?
- ~25 mmHg
* Due to presence of plasma proteins
What is another way of writing NFP = forces favouring filtration - forces opposing filtration? What is NFP at arteriolar end? Venular end?
- NFP = (PC + ni) - (nC + Pi)
- NFP arteriolar = (35 + 1) - (25 +1) = +10 mmHg
- NFP venular = (17 + 1) - (25 +1) = -8 mmHg
What do Starling forces favour at each capillary end?
Starling forces favour filtration at arteriolar end, reabsorption at venular end
