What are microcirculations?
The microvessels of the microvasculature present in organ tissues
What is the pathway of blood through a microcirculatory unit? (5)
- 1st order arteriole
- Terminal arteriole —> precapillary sphincter
- Capillaries
- Pericytic venule
- Venule
What is Darcy’s Law?
Equation for a fluid circuit:
ΔP = Q x R
- ΔP —> pressure gradient
- Q —> flow rate
- R —> resistance
What is flow rate and which 2 factors affect it?
Volume of blood passing through a vessel per unit time
1. Pressure gradient - direct
2. Resistance - inverse
—> Q = ΔP/R
- ΔP across organ —> 93 ∵ MAP = 93, MVP negligible
in organ —> 56 ∵ MAP = 93, capillary P = 37
What is resistance and which 3 factors affect it?
Force against blood flow due to friction from vascular walls
1. Vessel length (L) —> constant
2. Vessel radius (r) —> main factor
- via vasoconstriction/dilation
3. Blood viscosity (η) —> usually constant
—> R = 8Lη / πr^4
What are the 2 functions of arterioles?
Major resistance vessels —> control blood flow:
1. Match blood flow to specific tissue’s metabolic
needs
- intrinsically controlled by tissue
- active hyperaemia —> chemical
myogenic autoregulation —> physical
- Regulate whole systemic arterial blood pressure
- extrinsically controlled by brain
- neural
hormonal
What is the equation for resistance?
R = 8Lη / πr^4
- L —> vessel length
- η —> blood viscosity
- r —> vessel radius
What is vascular tone?
Arteriolar smooth muscle is always in a state of partial constriction
—> allows for both dilation and constriction to occur
What is active hyperaemia?
Tissue’s chemical control of its blood flow:
- Inc metabolic activity of tissue —> inc metabolites
and O2 usage —> vasodilation of tissue’s arterioles
What is myogenic autoregulation?
Tissue’s physical control of its blood flow:
- Dec temp —> restrict blood flow to regulate blood
temp —> vasoconstriction of tissue’s arterioles
- Inc blood pressure —> inc arteriole distension —>
some tissues don’t need more blood flow —>
vasoconstriction of tissue’s arterioles (to inc R and
counteract) eg. gut during exercise
What is the equation for the blood flow of the whole cardiovascular system?
Q = MAP / TPR
- Q —> cardiac output
- MAP —> mean arterial pressure
- TPR —> total peripheral resistance
What is the equation for the blood flow of an organ?
F(organ) = MAP / R
- MAP —> mean arterial pressure
- usually 93
How is overall arterial blood pressure controlled by the brain? (2)
- Neural
- Baroreceptors sense inc BP —> CVS control centre
in medulla senses —> sympathetic response —>
vasoconstriction of arteries (eg. gut, skeletal
muscle) —> doesn’t affect brain/heart/lungs
- Baroreceptors sense inc BP —> CVS control centre
- Hormonal
- Pituitary gland —> ADH
- Lungs —> angiotensin II
- Adrenal glands —> adrenaline
—> noradrenaline
How are capillaries adapted to enhance Fick’s Law?
Inc conc grad —> inc diffusion
1. Dec diff distance —> 1µm width of wall
2. Inc SA —> 7µm diameter (tiny) —> squish RBCs
3. Extensive —> every cell close enough for exchange
- more metabolically active tissue —> inc
capillary density
What is Fick’s law?
Rate of diffusion of a substance across unit area is proportional to the concentration gradient
How is cardiac output distributed around the body at rest?
Overall = 5L/min
- Gut —> 1
- Heart —> 0.25
- Kidneys —> 1
- Brain —> 0.75
- Skin —> 0.25
- Bone —> 0.15
- Skeletal muscle —> 0.75
How does exercise affect cardiac output?
Inc cardiac output - overall —> 25L/min
- Gut dec —> 1 to 0.75
- Heart inc —> 0.25 to 1.25
- Bone inc —> 0.15 to 0.25
- Skeletal muscle —> 0.75 to 20
Overall = 5L/min
- GI —> 0.75
- Heart —> 1.25
- Kidneys —> 1
- Brain —> 0.75
- Skin —> 0.25
- Bone —> 0.25
- Skeletal muscle —> 20
What are the 3 types of capillaries?
- Continuous —> only gap junctions between cells
- eg. lungs
- blood brain barrier —> tight junctions - Fenestrated —> fenestrae (80nm) between cells
- eg. glomeruli - Discontinuous —> large gaps between cells
- eg. bone marrow, liver
What is bulk flow?
Process of plasma fluid leaking out of vessels and returning via Starling’s forces:
1. Ultrafiltration - hydrostatic pressure pushes out
—> mixes with interstitial fluid
2. Reabsorbed - oncotic pressure pulls in
- Overall - hydrostatic: 32 —> 15 mmHg
- oncotic: 25 constant
∴ start —> Δ9
end —> Δ8
ultraflitration > reabsorption —> net loss
(Δ1) —> lymphatic system returns fluid
(right lymphatic duct —> right subclavian
vein, thoracic duct —> left subclavian vein)
