Microcirculation Flashcards
The exchange of gases across the capillary wall occurs via
diffusion
Lipid soluble gases
O2, CO2. diffuse THROUGH endothelial cells
Water, glucose, amino acids
are not lipid soluble gases; cannot pass through endothelial cells via diffusion
must pass through aqueous clefts between endothelial cells, so limited by surface area
hydrostatic and osmotic pressure
sterling forces
Simple diffusion is driven by
partial pressure gradients of the individual gases.
Rate of simple diffusion is determined by
partial pressure of gases and the surface area available for diffusion
in capillaries the driving force across the endothelia include
hydrostatic and osmotic forces
Sterline equation =
Fluid =
(Cap Hyd presh - Interstish hydrostatic presh) - (capillary oncotic presh - interstitial oncotic presh)
tells you what direction the movement of fluid will go
it’s always (C-I)-(C-I)
fluid OUT of the capillary is called
filtration
fluid INTO the capitally is called
absorption
Sterling equation constant is what?
K, the magnitude of fluid movement. it is a hydrolytic conductance (Water permeability)
a + sterling value indicates ___ and a - sterling value indicates
filtration and absorption
hydraulic conductance is not influenced by things such as
hypoxia, metabolites, arteriolar resistance
hydraulic conductance IS influenced by
capillary injury
Capillary hydrostatic pressure
determined by venous and arterial pressures
closer to arterial pressure
influenced by changes in venous pressure more than arterial pressure
what happens to interstitial fluid that leaves the capillaries?
it has to be returned to the capillaries or removed by the lymphatic system
what can alter capillary hydrostatic pressure?
enema
elevated by venous pressure during heart failure
influenced by a decrease in albumin (starvation and liver failure)
what can alter interstitial hydrostatic pressure?
edema
can be restricted by lymphatic flow or increased driving out of capillary
can be restricted by lymph flow or inflammation
Interstitial capillary pressure is usually
0 or a little bit negative
What two big things generally favor filtration?
decreased capillary oncotic pressure and increased venous pressure
edema: definition
forms when volume of interstitial fluid, due to filtration out of the capillaries) exceeds the ability of lymphatics to return it to circulation
Autoregulation: which organs exhibit autoregulation and what is it
maintenance of a constant blood flow. certain organs have intrinsic mechanisms that attempt to maintain the blood flow to meet their metabolic needs under changing conditions
kidneys
brain
heart
skeletal muscle
blood flow to an organ or tissue is proportional to its _____
and this is described by
metabolic requirements
active hyperemia—> the flow of blood to tissues based on their needs
reactive hyperemia
increase in blood flow in response to or reacting to a prior period of decreased blood flow.
so let’s say there was a period of arterial occlusion, reactive hyperemia would be the response afterwards
Myogenic hypothesis
can be invoked to explained autoregulation but doesnt explain hypermia (either forms)
this hypothesis states that when smooth muscle is stretched, it causes contraction
Law of laplace for a cylinder
T = P x r
T = wall tension P = pressure r = radius
this formula explains the myogenic hypothesis. tension decreases when R or P decrease etc.
metabolic hypothesis
can be invoked to explain each of the phenomena of local control of blood flow
basic premise of this theory is that O2 delivery = O2 consumption by tissue by altering alterial resistance, which in turns alters blood flow
metabolic hypothesis: what is the ultimate result?
the theory is that metabolic activity causes the production of “vasodilator metabolites” like CO2, H, K, lactate, and adenosine
these cause vasodilation which allows increased blood flow, allowing delivery of O2
vasodilator metabolites
produced by metabolically active tissues: include CO2, lactate, adenosine, H, K, which cause vasodilation, allowing more blood to come and supply more O2
which theory is best described by the metabolic hypothesis?
active hyperemia
describe how the metabolic hypothesis best describes active hyperemia
strenuous exercise: as skeletal muscle undergoes rapid cross cycling, the tissues required to meet the energy requirements are producing metabolites like lactate: these cause increased vasodilation, allowing delivery of more blood and thus O2 for electron capture
Extrinsic control of regional blood flow is by
neural control, specifically sympathetic (I think)
histamine and its relation to hydrostatic pressure
vasoactive substance
released in response to trauma, causes arterial vasodilation and venous vasoconstriction
net effect: LARGE increase in capillary hydrostatic pressure
increases Kf
bradykinin
like histamine, causes arterial vasodilation and venous vasoconstriction
localized edema
‘arteriolar vasodilation’
increases Kf
serotonin
released in response to blood vessel damage and causes local vasoconstriction (in an attempt to reduce blood flow and blood loss)
Shear force in capillary beds
if vascular bed dilates in response to metabolic need, increased blood flow through arteries causes shearing–or friction–against vessel walls, compelling them to release NO to induce relaxation which would allow them to augment the downstream metabolic effect
Prostaglandins
prostacyclins - relaxation
thromboxane - constriction
Angiotensin II and vasopressin
vasoconstrictors that increase TPR
what is the parasympathetic control of vasculature entail?
there are no parasympathetics
Coronary circulation: main types of control
almost entirely metabolic, sympathetics are limited
Two most important metabolic factors for coronary flow include
hypoxia and adenosine
Cerebral circulation control factors
controlled almost entirely by local metabolites
exhibits autoregulation and active and reactive hyperemia
most important regulator is CO2 (or H, same thing)
an increase in PCO2 causes an increase in H, therefore a decrease in pH and a
