Lecture 9: Blood volume Microcirculation & Fluid Exchange

Question 1

What is the function of capillaries?

Answer 1

-Contains little blood (only 5%)
-Site of water-nutrients-waste-gases exchanges with interstitial fluid and tissues
-Single layer of endothelial cells (no smooth or elastic) small diameter = very slow flow
-Dense networks: each cell of a tissue w/in 100 um of a capillary
-Receive blood from smallest arterioles
-Density of capillaries vary depending on the metabolic activity of the tissue (organ)

Question 2

What is transcapillary exchange?

Answer 2

-Diffusion: depends on capillary type and substance properties-follows gradient b/w blood and interstitial fluid
*Lipid soluble substances exchange freely across cell membrane (O2, CO2)
*Most capillaries have pores or clefts that allow transfer of water and lipid-insoluble molecules (Na+,Cl-,glucose, AA)
*Size and # of pores depend on the tissue
*Some capillaries are fenestrated (window)-vesciles fuse to form large gaps across endothelial cell membrane allowing water and water soluble macromolecules

Question 3

Explain Bulk flow=fluid exchange in more detail.

Answer 3

-Mass movement of eater and dissolved substances
-Takes place across the capillary walls
*Towards interstitial fluid=filtration
*Towards the intravascular fluid (Blood)=absorption
-Maintain fluid balance b/w intravascular and interstitial fluid

Dependant on:
-P gradients (hydrostatic and osmotic)
-Permeability of vessel (porosity)
-Size of diffusion surface (length of capillary and # of pores)
-Blood flow

Question 4

What are the 2 pressure gradients at the level of the capillary?

Answer 4

Hydrostatic pressure
-Filtration p
-P from the heart –> Arterioles –>cap
-Pressure in interstitial fluid is close to 0
-Pushes fluid out of vessel

Colloid osmotic pressure (oncotic)
-Due to big proteins that stay in vessel
-Counter balances hydrostatic pressure

Question 5

Explain starling forces=opposing forces

Answer 5

-Net movement of fluid depends on balance b/w filtration (hydrostatic) and osmotic (oncotic) P
-Hydrostatic P diff pushing fluid out
-Osmotic P diff pushing fluid in and remains constant along capillary length
-Along capillary
*In the first portion:hydrodtatic stringer (more fluid out)
*Towards the end: hydrostatic decrease = less fluid out, and if hydrostatic <osmotic = fluid moves in

Question 6

Where does the fluid go that leaves the arteriole?

Answer 6

-Net affect is generally accumulation of fluid into interstitial fluid
-Picked up by lymphatic system (embedded in capillary) drained into large veins called lymphatic drainage
-Fluid that remains in the blood collects into the venues and goes back to the heart
-If lymphatic system cants keep up—–> edema formation

Question 7

What does diffusion and bulk flow mean?

Answer 7

Diffusion= Nutrient and gas exchange
Bulk flow= stabilize blood volume using interstitial fluid as a buffer
*Blood loss= decrease in pressure = decreased hydrostatic = fluid enters back
*Excess fluid intake = increase pressure = increase in hydrostatic= fluid enters back
*Excess fluid intake=increase in pressure=increase in hydrostatic=fluid accumulates in interstitial
(Low Protein in plasma=decrease in osmotic=fluid out)
(High protein in plasma=increase in osmotic=fluid in)