Phys - Capillary Dynamics Flashcards
Primary function of the cardiovascular system
1.) Delivery of O2 and metabolic substrates 2.) Removal of waste products
What two steps are involed in vascular transport of substrates and waste products
1.) Convective blood flow to the capillaries 2.) Exchange of fluids and solutes between the capillaries and parenchyma (diffusion and osmosis)
Importance of the lymphatic system
very important for removing excess fluid and plasma protains that have filtered from the mircocirculation into the interstitial space = MAINTAINING INTERSTITIAL HOMEOSTASIS
What is the Interstitial space
Gel matrix composed of large mucopolysaccharides (primarily hyularonic acid) that hold everything together (tightly coiled and packed into a meshwork)
Functions of the interstitial gel matrix
1.) Fills the interstitial space around the tissue and parencyma (provides a pathway for diffusion) 2.) Fluid Trapping - prevents free flow of fluid throughout the body) 3.) Immobilization of trapped particulate matter and bacteria (minimizes the stread of infection)
Describe the characteristics of arterioles in microcirculation
1.) Greatest Resistance 2.) Greatest change in pressure (biggest drop)
what are exchange vessels
capillaries and small venules
Describe the characteristics of exchange vessels in microcirculation
1.) Largest cross sectional area 2.) large surface area 3.) Low pressure 4.) Low velocity
What makes microcirculation well suited for exchange of substrates and oxygen
Combination of low pressures, low flow velocity and large surface area
Describe the microcirculatory arcade
Tree like branching pattern. Terminal arterioles (metarterioles) to capillaries to small venules and ultimately to larger venules and veins.
How does change in arterial size change resistance
small changes result in BIG changes in resistance (1/r^4)
Composition of the capillary wall
single layer of thin endothelial cells overlying a basement membrane (basal lamina)
Intercellular clefts
endothelial cells in the capillary overlap and are attached through junctional complexes to form intercellular clefts - restrict the movement of larger solutes
what lines the capillary endothelium on the luminal side
glycolax “fuzzy coat”
List three ways solutes move across the capillary wall
1.) Diffusion 2.) Bulk Flow 3.) Vesicular transport
What is the most important means of soulute exchange
Diffusion (can occur in both directions)
What dictates capillary diffusion
Flicks law of diffusion
Flicks law of diffusion
smaller things have a smaller diffusion coefficient - Inversely proportional to the thickness of the barrier
What is bulk flow
When water moves out it washes solutes with it (solutes move with water out of the capillaries) through intercellular clefts
Where is bulk flow important
glomerular capillaries
What is vesicular transport
Movement of large macromolecules through pinocytotic channels. Occurs on both the luminal and abluminal surfaces and occurs in both directions.
How does water move across an exchange vessel
1.) Osmosis (both directions) 2.) Bulk flow (OUTWARD ONLY)
How does water move across the capillaries
1.) Transcellular route 2.) Paracellular route
Water: Transcellular route
through endothelical cells via water channels (eg: aquaporin 1 on the luminal membrae) High surface area
Water: Paracellular route
through intercellular clefts between the endothelial cells. LOW surface area
What is filtration
movement of water OUT of the capillaries
What is reabsorption
movement of water INTO the capillaries
Filtration vs Reabsorbtion… Which exceeds the other
Filtration exceeds reabsortion resulting in NET FILTRATION (goes into the lymph)
What are the driving forces for fluid movement
1.) Hydrostatic Pressures 2.) Oncotic Pressures
List the types of hydrostatic pressures
1.) Capillary pressure (Pc) 2.) Interstitial hydrostatic pressure (Pi)
List the types of oncotoc pressures
1.) Plasma oncotic pressure 2.) Interstitial fluid oncotic pressure
What determines the direction of fluid movement
the BALANCE between Hydrostatic and oncotic pressures
Capillary hydrostatic pressure (Pc) moves water in what direction
pushes water OUT of the capillary
Interstitial Hydrostatic pressure (Pi) moves water in what direction
Pulls water OUT of the capillary
Plama oncotic pressure moves water in what direction
pulls water INTO the cappillary
Interstitial fluid oncotic pressure moves water in what direction
pulls water OUT of the capillary
direction of water movement when hydrostatic pressure is greater than oncotic pressure
water moves out
direction of water movement when hydrostatic pressure is less than oncotic pressure
water moves in
Describe the change in pressure across the capillary tube
Pressure falls along the tube - 25-35 mmHg at the arteriolar end and 7-10 mmHg at the venular end
Changes in capillary hydrostatic pressure with increases in arterial pressure and increases in venous pressure
increas in arterial pressure with cause an increase in capillary pressure but not directly/by as much because it is blunted (multiplied by 0.3) however, changes in venous pressure directly increase the capillary pressure (85% of venous pressure reaches the capillaries)
Determinants of Pc
1.) Ratio of postcapillary and precapillary vascular resistance (~0.3) 2.) Arterial pressure (small effect due to 0.3) 3.) Venous pressure (greater effect)
How are capillaries protected from sudden changes in arterial pressure
myogenic vasoconstriction
What is Pc the measure of
the fluid pressure in the capillaries
what is Pi a measure of
the fluid pressure in the interstitial space
Interstitial hydrostatic pressure
Slightly atmospheric (averages between 0 and -6 mmHg) Due to normally “dry” interstitail space (maintained by lymphatics)
Capillary Filtration Coefficient
Kf : Describes the conductance (1/resistance) of the vessel wall to water
Capillary flow equation
Flow = (Pc-Pi)xKf
In terms of pressures when does water move out of the capillary
when capillary pressure is greater than interstitial pressure
Define osmotic pressure
Hydrostatoc pressure required to oppose the movement of water acorss a semipermeable membrane in response to a differnece in solute concentrations
What is the most important determinant of osmotic force
the number of osmotically active molecules in the solution
osmotic forces in the vascular bed are due to the presence of what
plasma proteins - large molecular size greater than that of the intercellular clefts. Capillaries are relatively impermeable to plasma proteins
what is oncotoc pressure
the osmotic pressure exerted by the plasma proteins
Majority of oncotic pressure is due to the presence of what
albumin (78%)
Donnan Effect
Since plasma proteins are negatively charged they attract cations (particularly Na) to balance the negative charge. The presence of Na increases the number of particles in solution contributing to the effective osmotic pressure
Reflection Coefficient
descrives the relative permeability of the exchange vessels to the plasma proteins 0-1 (1 is impermeable)
Net direction of oncotic forces (movement of water)
pulls water in
Conditions that increase capillary permeability (ex: allergic reactions or burns) will
disrupt the balance of forces across the capillary wall and will increase both the loss of plasma porteins and fluid filtration
Net Oncotic forces (movement of water)
pulls water in
Starling Landis Equation
Jv >0 there is net fluid FILTRATION Jv <0 there is net fluid REABSORPTION
average net fluid filtration in a 70 kg person? What is the fate of this fluid?
3 L/day or 2 ml/min - becomes lymph and is returned to central circulation
