L26 Microcirculation Flashcards
Small arterioles
Highly innervated by SNS nerves (release norepinephrine)
Contain vascular smooth muscle (VSM)
Site of resistance
Terminal arterioles
Contain VSM
Less innervation by SNS nerves than small arterioles
Greater control by local vasoactive substance
Site of resistance
Divide to give rise to capillaries
Undergo vasomotion- periodic constriction and relaxation
Metarterioles
In some tissues act as thoroughfare vessels from small arterioles to venules
Capillaries branch from metarterioles
Capillaries have precapillary sphincters at origin - cuff of VSM
Blood flow can bypass capillaries
Capillaries
Major site for exchange
Postcapillary venules
Some exchange also may occur
Non-nutritive flow
Flow through metarterioles
Bypass capillaries
Little exchange occurs (walls a little too thick)
Nutritive flow
Flow through capillaries
Active tissue requires greater blood flow
Arterioles and precapilary sphincters dilate
Increased flow through capillaries
Arteriolar radius
Determines overall blood flow to a particular vascular bed
Precapillary sphincters
Determine which capillaries are perfused
3 types capillaries
Continuous
Fenestrated
Discontinuous
Continuous capillaries
Most common
Found in muscle, skin, lung, fat, connective tissue, neural tissue
Tight junctions in intercellular clefts btw endothelial cells
Hydrophilic molecules must pass through clefts or small pores
Fenestrated capillaries
Leaky
Found in kidney, intestines, endocrine glands, and joints
Endothelial cell later contain perforations called fenestrae
Enhances permeability to water and hydrophilic molecules
Discontinuous capillaries
Found in bone marrow, liver, spleen
Wide gaps btw adjacent endothelial cells
Highly permeable for large molecules (proteins) and water (trafficking of proteins)
Lipophilic substances
Can diffuse through the plasma membranes of capillary endothelial cells
Hydrophilic substances
Mainly diffuse through pores in capillary wall
Surface area for diffusion of
Lipophilic substances is generally much greater than the surface area for diffusion of hydrophilic Substances
Smaller subs diffuse easily (small radii relative to pore) Na+, glucose
Larger molecules (proteins) diffuse poorly
Capillary walls are very permeable to
Water
Water transport across capillary walls depends on hydrostatic and osmotic forces in capillary and interstitial fluids - starling forces
Balance of forces determine direction and magnitude of flow
Filtration
Movement of fluid out of the capillary
Absorption
Movement of fluid into the capillary
4 starling forces that control filtration and absorption
- Capillary hydrostatic P (Pc)
- Interstitial fluid hydrostatic P (Pi)
- Plasma oncotic P (pic)
- Interstitial oncotic P (pii)
Capillary hydrostatic P
Pc
Depends on pre and post capillary resistance and pressures
Favors filtration - movement of fluid out of capillary into the interstitium
30-40mmHg
Pressure drops from arteriole to venule end (friction, viscosity) 1.5mmHg
Variable (other 3 constant)
Determinants of Pc
Pa- arterial P
Ra- arteriolar resistance
Rv- venular resistance
Pv- venous P ( major contributor of edema)
Plasma oncotic pressure
Pic
Osmotic pressure due to plasma proteins which cannot leave capillary lumen due to low permeability
25-30mmHg
Favors absorption- movement of fluid into the capillary from the interstitium
This is a major pressure , along with Pc
Interstitial hydrostatic pressure
Pii
Produced by fluid located in intermolecular spaces of interstitial matrix
P depends on V of fluid in interstitial space as well as compliance of space
-5 to 10 mmHg
Think of it like a back P
Interstitial oncotic P
Pii
Some plasma protein escapes the capillary lumen
Protein in interstitial space exerts an osmotic P, generally quite small
1-5mmHg, can be 0
Net filtration pressure
NFP
Balance of hydrostatic and oncotic forces
Sign determines direction
Pos = filtration Neg = absorption
Rate of fluid movement
Kf x NFP
Kf (ml/mmHg/min) - filtration coefficient and reflects the capillary permeability
Increased capillary pressure
Favors filtration
Vasodilator
Decreased capillary pressure
Favor absorption
Vasoconstrictor
Raise BP
Decreased plasma protein level
Liver not functioning well
Passing proteins in urine (nephrotic syndrome)
Steady line decreases
Favors filtration (filtration phase much large)
Major transfer of fluid from inter vascular space to interstitial space - leads to edema
Why doesn’t fluid accumulate in interstitial space?
Lymph vessels collect interstitial fluid and return it to the bloodstream through subclavian veins
Lymphatic system
Network of one way vessels that returns fluid/lymph back into blood
Functions:
Returns filtered fluid
Transport of absorbed fats
Return of filtered protein
Filtration from capillaries = 20L/day
Reabsoprtion back to capillaries = 17L/day
3L/day enter lymphatic system
What causes edema?
Increased filtration (filtration grossly outstripping absorption)
Or
Impaired lymphatic drainage
