Microcirculation Flashcards
microciculation
diameters less than 100 micrometers
arterioles
5-100 micrometers
thick layer of smooth muscle
terminal arterioles
arterioles that end in capillaries
metarterioles
thinner and less well-developed layer of muscle
allow blood to pass around capillary bed
diameter can be varied, thus their resistance can be varied (Poiseuille)
capillaries
4-8 micrometers
single layer of endothelial cells
lack muscle
nutrient excahnge
precapillary sphincter
band of smooth mm before individual capillaries
on/off switch
postcapillary venules
venules
15-20 micrometers & 35-45 micrometers
thin walled, highly compliant, lack mm
capacitance vessels - stretch!
arteriovenous anastomosis / av shunt
highly muscular
blood flow directly from arterial to venous system w/o entering capillaries
not in all tissues
4 ways substances cross the endothelium of a capillary
1) diffusion - ie O2 and CO2
2) bulk flow - driven by P generated by heart pumping
3) pinocytosis - for lrg proteins
4) active transport - into and out of the endothelial layer
- most occurs by diffusion created by concentration gradient
enormous surface area of capillaries = slow mvmt
hydrostatic presure
created by hearts pumping and applying pressure to the blood
drives fluid across capillaries
osmotic (oncotic) pressure
generated by concentration of impermeable solutes (water follows these solutes)
salts, albumin
hydrostatic pressure w/in capillary
35 at arterial end
15 at venous end
main force to move stuff OUT
pressure diff drives the flow
osmotic (oncotic) pressure
main force to move stuff IN
albumin, globulins and fibrinogen (25mmHg pressure)
Note: capillaries filter only 1% of fluid so loss of plasma doesn’t raise oncotic pressure
tissue hydrostatic pressure
maybe 0 or less due to sucking action of lymphatics
tissue osmotic pressure
small amt of protein can get through capillary wall and cause an osmotic gradient, however most is returned to blood via lymphatics
1-2mmHg
starlings law of the capillary
Q (fluid that leaves) =
k((Pc+Oncotic i) - (Pi +Oncotic capillary))
OUT - IN
k- tissue specific constant
renal system; filtration/absorption
blood enters at high pressure (45mmHg) so large amts of fluids filtered out, some components are recovered by the kidney the rest makes urine
pulmonary circ; filtration/absorption
blood enters at Pressure < plasma oncotic pressure
pulmonary capillaries absorb fluid (pulmonary secretions
nutritional edema
plasma protein conc drops = > plasma osmotic pressure falls (main force keeping fluid in capillaries) arterial pressure remains normal ascites
local control of blood flow
1) metabolic; Inc levels of metabolic wastes -> vasodilation -> Inc blood flow
wastes; adenosine, lactic acid, K, H, CO2
Lack of O2 also initiates this too
2) myogenic - smooth mm response
inc P -> mm contract -> Inc R
dec P -> mm relax -> dec R
imp in standing up, keeps Q similar over a lrg range of BPs (preventing damage, allowing adequate Q), auto reg, no need for CNS imput
central control of blood flow
CNS - urgent situations
1) humoral - peptides released into blood plasma
(atrial natriuretic peptide, angiotensin II, epinephrine, nitric oxide, endothelin)
2) neural mech - direct innervation
innervation of small art and ven by SNS
NT - NE, ACh, Sub P, Neuropeptide Y
main mech - regulate blood flow to diff regions of the body
atrial natriuretic peptide (ANP)
causes excretion of Na and water, also causes vasodilation to decrease TPR
humoral mech of central control
angiotensin II
potent vasoconstrictor
stimulates aldosterone synthesis and release for retention of Na (and thus water)
epinephrine
released from adrenals vis SNS
vasodilation in liver and skel mm
increases heart rate
nitric oxide (NO)
gas produced by endothelium
short half-life
causes vascular relaxation
endothelin (ET-1)
potent vasoconstrictor from endothelium
