The Microcirculation Flashcards
Note bank this one too?
Okay
Harroop, slide 468
Which vessels make up the micro circulation?
1st order arterioles
Terminal arterioles
Capillaries
Pericytic venules
Venules
So basically arterioles, capillaries and venules
What is the aim of the cardiovascular system?
Adequate blood flow through the capillaries
Blood flow rate - volume of blood passing through a vessel per unit of time
What is the blood flow equation (darcys law)?
Flow rate (Q) = pressure gradient (delta P) / resistance (R)
Q=P/R
Delta P = pressure A - Pressure B
Increasing delta P will increase flow rate
Arterioles (pressure A) are the major resistance vessels and is pretty similar everywhere (MAP), pressure B is the venules at the end of the capillaries, these are considered to have negligible pressure (0mmHg) so the pressure gradient is basically MAP
The only factor that contributes to blood flow is resistance
Therefore F organ = P map / R organ
What is the equation for resistance in blood vessels?
R= (8Ln)/pi r^4
L is vessel length
n is eta, which is Viscocity
r is radius and had the largest effect
Halving the radius increase resistance x16 and decreasss flow x16
What are the effects of blood pressure and arteriolar vasoconstriction on pressure, rate and flow?
Increasing blood pressure: increases pressure, increases flow rate
Arteriolar vasoconstriction: increases resistance, decreases flow rate
Vasoconstriction and vasodilation alter the radius of the arterioles, what are the two functions of this?
- Match blood flow to the metabolic needs of specific tissues
Regulated by local (intrinsic) controls and independent of nervous or endocrine stimulation
May be chemically driven (increased metabolism, increased oxygen usage) -> This is called active hyperaemia
May be physically driven (blood temperature, stretch/distension due to raised BP) -> This is called autoregulation
- Help regulate systemic arterial blood pressure
Regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
What do some of the components of pressure force equations mean?
Cardiac output (Q) = blood pressure (MAP) / resistance (TPR)
This can obvs be rearranged
TPR - total peripheral resistance - sum of resistance of all arterioles present in systemic circulation
MAP - mean arteriole pressure - average pressure within venule system is negligible. Therefore the pressure differences is due to arterioles. It is an average
Regulation of arterial blood pressure is by the cardiovascular control centre in the medulla oblongata. It is facilitated by adrenoceptors
Alpha = constriction
Beta = dilation.
Vasoconstriction may also be hormonal: vasopressin, angiotensin II, adrenaline
What is the purpose of capillaries?
Delivery of metabolic substrates to the cells of the organism
To do this they are highly branched and there is a high density of capillaries
They are the perfect size to fit a red blood cell and ensure a short diffusion pathway (ficks law)
Skeletal muscle, myocardium and lungs have high capillary density. And brain
Skeletal muscle has a huge capacity but limited flow at rest, controlled by precapillary sphincter
What are the types of capillaries?
Continuous - majority. Water filled gap junctions between cells. Carrier proteins facilitate diffusion of metabolic substrates
Fenestrated - pores within the capillaries. Enables larger molecules to pass through. Larger gap junctions
Discontinuous - larger holes in endothelium. Important in bone marrow for leukocytes to enter circulation
Continuous capillaries are present in the blood brain barrier. Only they don’t have gap junctions but tight gap junctions. Tight regulation of substances entering the brain
What is bulk flow?
A volume of protein free plasma filters out of the capillary, mixes with the surrounding interstitial fluid and is reabsorbed
Pushed out due to a hydrostatic force
Brought in by an oncotic (plasma osmotic) force
Pressure inside > out : ultrafiltration
Pressure in
