3- Microcirculation and the Lymphatic System: Capillary Fluid Exchange, Interstitial Fluid, and Lymph Flow Flashcards
Capillary permeability
- permeable to water, O2, CO2 and some small molecules (there is constant movement of water all along the capillary even if there is no net movement)
- impermeable to proteins (are transported through vesicles)
- protein is in high concentration in the blood and low concentration in the interstitial space a balance of forces is set up that retains fluid in the circulation
Microcirculation conforms to structure of tissue, explain 2 examples
- skeletal muscle: capillaries run b/w muscle fibers parallel to each other
- cardiac epithelium: very high metabolic activity, so it has very high density of capillaries for rapid delivery of nutrients and removal of waste products
size of arterioles and capillaries and RBCs
arterioles- 20 microns
capillaries- 5-9 microns
RBCs- 7-8 microns (they deform to fit into capillaries)
metarterioles vs venules and smooth muscle
How this affects capillary blood flow and what causes it
metarterioles- intermittent smooth muscle cells around vessel
venules- less smooth muscle but can still contract
- capillary blood flow is intermittent
- vasomotion causes intermittency: metarterioles and precapillary sphincters contract/relax
edema
movement of water out of the circulation and into the tissues
what determines vasomotion
- tissue O2 concentration
- low tissue O2 relaxes
- high tissue O2 contracts sphincters
billions of individual capillaries so despite intermittent opening and closing we consider
- average rate of flow
- average capillary pressure
- average rate of transfer of substances
Structure of capillary wall
single layer of endothelial cells and thin basement membrane
- Intercellular cleft (slit pore)
- 6-7 nm wide
- <1/1000 total surface area
- most water-soluble molecules and small solutes easily diffuse
- slightly narrower than diameter of albumin molecule - Plasmalemmal vesicles (aka caveolae)
- small packets of plasma or ECF that move through the endothelial cell
capillary location/area details
- nearly every cell is close to a capillary ~ within 20-30 microns
- 500-700 square meters (10^10 capillaries)
capillary exchange has 4 pathways
- Intercellular clefts (hydrophilic substances)
- Pinocytotic vesicles (large hydrophilic molecules)
- Fenestrae and aquaporins (large amounts of fluid)
- Transcellular (lipophilic substances)
Diffusion rate determined by
- concentration gradient
- surface area
- capillaries can dilate and constrict depending on how much O2 is needed in those tissues
- ex: skeletal muscle and heart capillaries open more during exercise - diffusion coefficient (determined by molecule and capillary type)
- brain: tight junctions, very closed for only small molecules
- liver: pores very open
- intestine: intermediate b/w liver and brain
- kidney glomerulus tufts: penetrate through middle of endothelial cells
Net transport in capillaries
higher to lower concentrations – down concentration gradient
- O2 tends to diffuse into tissues
- CO2 tends to diffuse into capillaries
high rate of diffusion- only slight gradient is sufficient for adequate transport
capillary beds differ dependent on size of pores
generally as size of molecule increases, relative permeability decreases
- liver is highly permeable to proteins
- kidney glomerulus is highly permeable to fluids/electrolytes
- capillary permability can be altered in disease states
Starling Forces
Typically flow out of capillary
- Capillary Pressure
- Interstitial fluid colloid osmotic pressure
Typically flow into capillary
- Plasma colloid osmotic pressure
- Interstitial fluid pressure
Interstitial space
- area b/w cells
- composed of collagen (high tensile strength), proteoglycans (tissue gel), and water
- interstitial fluid–> much like composition of plasma but with fewer proteins
“free fluid” is usually less than 1% but in edema 50% can be free
interstitial fluid pressure (numerically)
Typically a negative number and depends on how much fluid is in the space and how compliant the space is
ranges from -10 to +10 depending on the tissue (higher during edema)
-subcutaneous tissue: -2 to -6 mmHg
tightly encased tissue: -brain- +4 to +6 mmHg csf= 10 -kidney- +6 mmHg capsular pressure = 13
Plasma Colloid Osmotic Pressure (aka. oncotic pressure/ COP)
- due to balance of forces that attempts to abolish protein concentration gradient
- More plasma proteins – increased colloid osmotic pressure
- Fewer proteins – lower oncotic pressure
effects of different plasma proteins on COP
- osmotic pressure is determined by number of molecules dissolved and not by mass
- SO albumin is the most important contributor to the colloid osmotic pressure due to the number of albumin molecules present in the blood
Starling equilibrium for capillary exchange
Outward forces - Inward forces = Net force
(mean capillary pressure + interstitial fluid colloid osmotic pressure) - (plasma colloid osmotic pressure + interstitial free fluid pressure)
Filtration coefficient
greater in tissues with large pores, very low in brain and muscle
what could alter balance fluid balance?
- Increase capillary pressure could increase flow of fluid into interstitium – edema
- Decreased capillary pressure, fluid moves inward
- Increased capillary permeability, increased plasma proteins in interstitium, increased colloid oncotic pressure
- Dilution of plasma proteins, decreased plasma colloid oncotic pressure, fluid moves outward
Lymphatic system
- fluid + proteins + other large molecules carried away from interstitial spaces
- most fluid filtered is reabsorbed into venous ends of capillaries
- about 1/10 of the fluid enters lymphatic capillaries ~ 2-3 liters/day
- fluid pushes through one-way valves formed by endothelial cell flaps
- Lymph flows up thoracic duct from lower body and left side of body and empties into venous system at juncture of subclavian vein and left jugular.
- Right head and neck emptied into right lymph duct and right subclavian.
-Lymph similar in composition to interstitial fluid at first.
Absorption of fats from GI tract.
2/3 of total lymph derived from liver and intestines.
-Collects bacteria that are destroyed by lymph nodes.
Factors that influence lymph flow
- elevated capillary pressure
- decreased plasma COP
- increased interstitial fluid COP
- Increased permeability of the capillaries (to water which causes blood fluid to move out)
- Lymph pump (note role of valves)
- collecting lymphatics have myogenic responses
- contraction of surrounding muscles (lymph flow active during exercise)
- movement of body parts
- pulsations of adjacent arteries
- compression of tissues by external forces
main factors that determine lymph flow rate
- interstitial fluid pressure
- lymphatic pump activity
safety factors that prevent edema
- low compliance of tissues in negative pressure range
- lymphatic capacity for increasing flow
- dilution of interstitial protein
lymphatic capacity for increasing flow
- return the circulation fluids and proteins filtered from capillaries into the interstitium
- without with function, decrease plasma volume and edema occurs
- lymph flow increases 10-50 fold when fluid start to accumulate
- safety factor = 7 mmHg
