Microcirculation Flashcards
What is the overall aim of the cardiovascular system?
adequate blood flow through capillaries
What is blood flow rate?
volume of blood passing through a vessel per unit of time
What is Darcy’s Law / Fluid Circuit?
ΔP = Q x R ΔP = change in pressure / pressure gradient Q = flow rate R = resistance
How is Pressure gradient (ΔP) calculated?
Pressure gradient (ΔP) = Pressure (A) - Pressure (B)
What is the result of increasing ΔP?
increases flow rate
What is resistance (R)?
hindrance to blood flow due to friction between moving fluid + stationary vascular walls
What is the equation for R (resistance)?
R = 8Lη/πr^4
• L = vessel length
• η = blood viscosity
• r = vessel radius
What happens to resistance and blood flow rate when vessel radius is halved?
increases resistance by 16 times + decreases flow by 16 times
How would increase in blood pressure affect pressure gradient?
increases - arriving blood will be higher in pressure
How would increase in blood pressure affect flow rate?
increases - higher blood pressure = more flow
How would arteriolar vasoconstriction affect resistance?
increases - resistance increases with vasoconstriction
How would arteriolar vasoconstriction affect flow rate?
decreases - due to increase in resistance
How do you calculate the flow rate of any organ?
F (organ) = ΔP (MAP) / R (organ)
Why is pressure difference important in the arterioles?
without pressure difference, blood would not reach tissue capillary beds
What happens to vessel radius when vasoconstriction occurs?
decreases
What happens to vessel radius when vasodilation occurs?
increases
What happens to resistance when vasoconstriction occurs?
increases
What happens to resistance when vasodilation occurs?
decreases
What happens to flow rate when vasoconstriction occurs?
decreases
What happens to flow rate when vasodilation occurs?
increases
What is vascular tone?
arteriolar smooth muscle normally displays a state of partial constriction
For what 2 functions are the radii of the arterioles adjusted independently?
- match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
- help regulate systemic arterial BP
How are the 2 functions of the arterioles regulated?
- local (intrinsic) controls + independent of nervous or endocrine stimulation
- extrinsic controls which travel via nerves or blood + are usually centrally coordinated
How can blood flow meet metabolic needs of tissue chemically?
chemically driven by increasing metabolites + O2 usage - causes active hyperaemia
What is active hyperaemia?
vasodilation of arterioles
How can blood flow meet metabolic needs of tissue physically?
physically driven by decrease in blood temp + increases stretch (distention) due to increase blood pressure - caused myogenic autoregulation
What is myogenic autoregulation?
vasoconstriction of arteries
What is the equation for cardiac output (which is flow rate) involving BP and resistance?
F = ΔP (MAP) / R
Q or F = Cardiac Output
MAP = Blood Pressure
R = Total Peripheral Resistance
How is arterial blood pressure regulated neurally?
cardiovascular control centre in medulla
How is arterial blood pressure regulated hormonally?
vasoconstriction caused by:
• vasopressin / ADH secreted by pituitary
• angiotensin II from lungs
• adrenaline/noradrenaline from adrenals
What is the purpose of capillary exchange?
delivery of metabolic substrates to the cells of the organism
What are the general dimensions of a capillary?
7µM lumen diameter
1µM cell width
What are capillaries specially designed for?
Ideally suited to enhance diffusion by Fick’s Law
• minimise the diffusion distance
• maximise the surface area and time for diffusion
What tissues have denser capillary networks? Examples?
Highly metabolically active tissues:
• Skeletal muscle = 100cm2/g
• Myocardium/Brain = 500cm2/g
• Lung = 3500cm2/g
What is special about the capillaries in skeletal muscle?
huge capacity but limited flow at rest
What are the 3 types of capillaries?
- continuous
- fenestrated
- sinusoid / discountinued
What are features of continuous capillaries?
continuous basement layer + endothelial layer w/ intracellular clefts
What are features of fenestrated capillaries?
fenestrations in the endothelial layer + continuous basement membrane - allows large molecules to pass through
What are features of discontinued capillaries?
intracellular gaps + incomplete basement membrane - allows proteins and even blood cells to pass through
What is bulk flow?
volume of protein-free plasma filters out fo the capillary, mixes with the surrounding interstitial fluid + is reabsorbed
How does hydrostatic pressure affect capillaries?
pushing force - pushes plasma out of capillary
How does oncotic pressure affect capillaries?
pulling force - pulls plasma to be reabsorbed into the capillary
What was Starling’s theory on capillary fluid movement?
hydrostatic pressure of the blood in the capillaries =
osmotic attraction of the blood for the surrounding fluids
What causes Ultrafiltration in the capillaries?
pressure in capillaries > in interstitial fluid
What causes Reabsorption in the capillaries?
inward driving pressures > outward pressure across capillary
What is more effective; ultrafiltration or reabsorption?
ultrafiltration
Why is it important that ultrafiltration is more effective?
any fluid lost is picked up by lymph vessels + returned to circulation to maintain BP
What happens if there is more ultrafiltration than reabsorption?
fluid accumulates in IF
Why is the lymphatic system important when ultrafiltration > reabsorption?
drains the fluid accumulated
What happens if the lymphatic system fails?
rate of production of fluid > rate of drainage so OEDEMA can occur
Where can continuous capillaries be found?
- fat
- muscle
- nervous system
Where can fenestrated capillaries be found?
- intestinal villi
- endocrine glands
- kidney glomeruli
Where can discontinuous capillaries be found?
- liver
- bone marrow
- spleen
