6.6 - Microcirculation Flashcards
1
Q
What is the order of vessels that blood flows through?
A
- 1st order arteriole –> terminal arteriole –> capillaries –> pericytic venule –> venule
- arterioles have more smooth muscle than venules, some areas of the arterioles can vasoconstrict more than the others
2
Q
What is the overall aim of the cardiovascular system?
A
Adequate blood flow through the capillaries
3
Q
What is blood flow rate defined as and the equation for it?
A
- blood flow rate - volume of blood passing through a vessel per unit time
- fluid circuit (Darcy’s Law): delta P = Q x R
- flow rate (Q) = pressure gradient (delta P) / resistance (R)
4
Q
What is pressure gradient?
A
Pressure gradient (delta P) = Pressure A - Pressure B
5
Q
What would increasing the pressure gradient do?
A
- increases flow rate
- increase pressure A by vasodilation and therefore increased blood flow rather than vasoconstriction (as although constriction would technically increase pressure, less blood flows through so less pressure)
6
Q
Why is a pressure difference so important?
A
- without it, blood would not reach tissue capillary beds
- pressure difference is the numerator of the flow rate equation so if pressure gradient was 0, flow rate would also be 0
7
Q
What is resistance to blood flow in capillaries?
A
- hindrance to blood flow due to friction between moving fluid and stationary vascular walls
- R = 8nL / (pi x r^4)
- vessel length (L) and blood viscosity (n) do not change much and are not significant
- vessel radius (r) can change and significantly impacts resistance
8
Q
What does increasing blood pressure do to pressure gradient, resistance of the vessel and flow to capillaries?
A
- increases pressure gradient
- no change to resistance of the vessel
- increases flow to capillaries
9
Q
What does arteriolar vasoconstriction do to pressure gradient, resistance of the vessel and flow to capillaries?
A
- no change to pressure gradient
- increases resistance of the vessel
- decrease in flow to capillaries
10
Q
Why are we interested in arterioles?
A
- they are the major resistance vessels
- important in determining blood pressure
11
Q
What is the typical pressure gradient in arterioles?
A
- pressure A = mean arterial pressure (MAP) = 93 mmHg
- pressure B = variable, depending on what happens to blood as it flows through the arteriole - around 37 mmHg, large pressure drop
12
Q
How do we work out the flow rate of blood to any organ?
A
- pressure gradient is normally constant (MAP) as pressure out is minimal, therefore pressure gradient is the same for all tissues
- resistance is the only variable that changes and therefore determines flow rate
- F(organ) = delta P (MAP) / R(organ)
13
Q
How do vasoconstriction and vasodilation affect resistance of the vessel and flow to capillaries?
A
- vasoconstriction decreases radius, increases resistance and decreases flow
- vasodilation increases radius, decreases resistance and increases flow
14
Q
What state of constriction/relaxation are arterioles normally in?
A
- arteriolar smooth muscle normally displays a state of partial contraction - called vascular tone
- allows it to both constrict and dilate further
- muscular people have a higher level of basal contraction
15
Q
What are the two functions of adjusting arteriole diameter?
A
- match blood flow to the metabolic needs of specific tissues (depending on body’s momentary needs)
- regulated by local (intrinsic) controls and independent of nervous/endocrine stimulation
- help regulate systemic arterial blood pressure
- regulated by extrinsic controls which travel via nerves or blood and are usually centrally coordinated
16
Q
What is an increase in blood in response to local need called?
A
- active hyperaemia - happens in muscles when exercising
- happens by vasodilation of arterioles
- can be chemically driven by increased metabolites (from respiration) or increased O2 usage
- sensed by arterioles locally and dilated as more glucose + O2 needed
17
Q
When would vasoconstriction of arterioles happen?
A
- if blood temperature is dropped or stretch increased (distension) due to increased blood pressure
- called myogenic autoregulation
- small intestine arterioles undergo myogenic vasoconstriction
- if tissues do not need blood from vasodilation, they constrict to reduce flow e.g. GI system
- increase in acute BP = reduced blood flow and increased resistance due to autoregulation
18
Q
How is systemic arterial blood pressure maintained/regulated - neural?
A
- extrinsic controls which travel via nerves or blood and are usually centrally coordinated
- neural - cardiovascular control centre in medulla –> vasoconstriction
19
Q
How is systemic arterial blood pressure maintained/regulated - hormonal?
A
- renin from adrenal glands causes angiotensinogen –> angiotensin I, then ACE in vasculature of lungs and kidneys –> angiotensin II (increases blood volume and has a pressor effect i.e. vasoconstriction) –> increased BP
- ADH and adrenaline increase water reabsorption in kidney to increase BP and ADH also has potent effect on local vasoconstriction when it binds to vascular smooth muscle cells
20
Q
What is the equation for blood pressure?
A
Blood pressure (MAP) = cardiac output (Q) x total peripheral resistance (TPR)
21
Q
What is the purpose of capillaries?
A
The purpose of capillary exchange is the delivery of metabolic substrates to the cells of the organism (which is the ultimate function of the CVS)
22
Q
What is the capillary lumen and cell width?
A
- 7 um lumen diameter
- 1 um cell width
23
Q
Why is capillary density important?
A
- so that they are ideally suited to enhance diffusion through Fick’s law
- minimising diffusion distance
- maximising surface area and time for diffusion
24
Q
Which tissues have denser capillary networks?
A
- highly metabolically active tissues have denser capillary networks
- skeletal muscle - 100cm2/g
- myocardium/brain - 500cm2/g
- lung - 3500cm2/g
- skeletal muscles is lower as ability of body to adapt and repair is large
- skeletal muscle has large capacity, but limited flow at rest
25
How are capillaries structured?
- continuous - cells have small H2O-filled gap junctions between them, but the vessel is called continuous
- ions and small molecules can leak through these gaps in the capillary
- cells are covered with basement membrane which is continuous
- fenestrated - some cells have holes called fenestrae which allow molecules >80nM to pass through - these still have continuous basement membrane - e.g. glomerulus in kidney
26
What are discontinuous capillaries?
- they have fenestrae but also breaks in the basement membrane
- allow large molecules to diffuse out e.g. in bone marrow, where WBCs leave
- common in liver (highly metabolically active) - liver also has sinusoids which are discontinuous structures
27
What type of capillary is the blood brain barrier?
- continuous - protects the brain as it only allows certain molecules to enter the brain
- neonates and premature babies have much leakier BBB so more susceptible to CNS infections secondary to infection in blood
28
What is bulk flow?
A volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF), and is reabsorbed by capillaries (by oncotic pressure due to higher concentration of plasma proteins in vessels)
29
What determines fluid accumulation in tissues?
- Starling's hypothesis
- balance of hydrostatic and oncotic forces
- 'whereas capillary pressure determines transudation, the osmotic pressure of the proteins of the serum determines absorption'
30
When does ultrafiltration occur?
If pressure inside the capillary > in IF
31
When does reabsorption occur?
If inward driving pressures > outward pressures across the capillary
32
What are the hydrostatic and oncotic pressures like at the arteriole and venule ends of capillaries?
- hydrostatic pressure = -2 due to extracellular fluid exerting pressure into vessel
- oncotic pressure = 0 due to us assuming there are no proteins outside the vessel
- combining hydrostatic and oncotic pressures give net pushing force at arteriole end of 9mmHg and net pulling force of fluid back into capillary at venule end of 8mmHg
- the significance of the fact that ultrafiltration is more effective than reabsorption is a net loss of 1mmHg fluid --> lymphatics
33
What do lymph vessels look like?
- they are discontinuous
- as fluid moves into the lymphatic system, this creates a gradient for the movement of lymph
- lymphatic endothelium with openings
- surrounded by tissue cells and interstitial fluid
- anchoring filaments
34
Does the lymphatic system have a pump to induce flow?
No
35
Where does lymph drain into?
Right lymphatic duct drains into thoracic duct and then into right and left subclavian veins
36
How much lymph travels through the lymphatic system a day?
3L
37
What is elephantiasis?
- if rate of production > rate of drainage --> oedema ensues
- parasitic blockage of lymph nodes can cause it
