Microcirculation

Question 1

anomalous blood viscosity

Answer 1

-not constant under all conditions- non-newtonian
-shear stress vs shear rate plot gives viscosity as slope
-physiological flow rates, blood is newtonian, linear slope that can extend to origin
-at lower flow rates, blood behaves anomalously, requires a threshold force to get moving
-polycythemia-abnormal increase in RBC proportion, elevated crit
-blood viscosity depends on fibrinogen, crit, vessel radius, velocity, temp
-increase in viscosity at low flow rates
increase in crit increases velocity

Question 2

blood viscosity

Answer 2

• depends on diameter of tube-fahraeus-lindquist effect
• diameter less than 0.3 mm, viscosity decreases
• low viscosity in small tubes due to axial streaming-red cells accumulate in axial lamina- mean transit time is faster than plasma
• lower crit in smaller vessels-lower viscosity
• forms rouleaux at low flow rates- increases viscosity and reduces flow- but effect above counteracts this

Question 3

non-ideal rheological effects

Answer 3

• formation of rouleaux
• axial streaming and plasma skinning
• cellular deformability
• fahraeua-lindquist effect
• determined by RBC because they take up most of the volume of the blood
• rouleaux break up outside microcirculation and decrease viscosit
• altered crit in small vessels from plasma skimming-breaks off branch point better than RBCs do

Question 4

microcirculatory unit

Answer 4

• extends from an ateriole to a venule
• arterioles have smooth muscle, venules have discontinuous layer of smooth muscle cells
• metarterioles are not present in all tissues, may provide a shunt that bypasses network
• precap sphincters control local flow within the cao network, not innervated, but are responsive to local conditions of oxygen, carbon dioxide, and acidity, dilate in response to hypoxia
• structure and function of microcirculation differs in tissues because:
• nutritional source and waste removal in most vascular beds
• filtration in renal glomeruli
• thermal regulation in skin

Question 5

y shaped branching point

Answer 5

• right receives most of flow
• left has variable flow rate
• central column of red cells

Question 6

continuous capillaries

Answer 6

• most common with interendothelial junctions 10-15 nm wide as in skeletal muscle
• junctions absent in brain cap, narrow tight junctions for blood-brain barrier

Question 7

fenestrated capillaries

Answer 7

• surround exocrine glands or epithelial membranes like small intestine
• endo cells have conduits that permit the flow of fluid and solutes across the capillary endothelial membrane

Question 8

discontinuous capillaries

Answer 8

-found in liver sinusoids, large gaps in endo cells

Question 9

Fick’s law of diffusion

Answer 9

• capillaries are efficient exchange sites where most substances cross by diffusion
• powerful mechanism for material exchange, no cell is more than 10 um from one
• four factors determine rate:
  1. concentration difference
  2. surface area for exchange
  3. diffusion distance
  4. permeability of the cap wall to the diffusing substance

Question 10

interstitial fluid

Answer 10

• interstitium is made of both solid and liquid phases and only a small fraction of the water is free to move
• difficult to measure Pif but in loose tissue it is negative while in rigid compartments and encapsulated organs it is positive
• Pif is sensitive to the addition of fluids to the interstitial compartment, can lead to disruption of the solid phase collagen fibers and proteoglycans

Question 11

lymph vessels

Answer 11

• pathway for larger molecule to reenter the circulatory system
• initial lymphatics similar to capillaries but with 1 way valves
• collecting lymphatics similar to small veins with sparse smooth muscle and secondary lymph valves
• nodes locating along path
• large collecting lymphatics drain into right and left subclavian veins
• 2-4 L/day

Question 12

edema

Answer 12

• excess salt and water in interstitial space
• caused by renal, cardiac, lung, hepatic disease
• L heart failure leads to pulm edema
• pulm hypertension causes pulm edema
• R heart failure leads to edema in lower extremities and abd viscera
• fluid from hepatic and intestinal capillaries may move from interstitium to peritoneal cavity-ascites
• plasma albumin is synthesized in liver, liver disease leads to hypoalbuminemia and peripheral edema
• SIADH-excess ADH from lung tumors-peripheral edema
• lymphatic blockage-malignant neoplasms may cause local edema upstream of the sites of blockage