6 Microcirculation Flashcards
Q: What is blood flow rate? What is it determined by? (2)
A: volume of blood passing through a vessel per unit time
pressure difference in the vascular bed and vascular resistance
Q: What’s the equation relating blood flow rate with pressure gradient and vascular resistance? Relationship (2). How can it be rearranged?
A: F= pressure gradient/vascular resistance
(triangle P)/ R
Blood flow rate is directly proportional to pressure gradient and inversely proportional to vascular resistance
F(organ)= /\P(=MAP)
————–
R(organ)
Q: Summarise the arrangement of microcirculation. (5) What is the major determinant of the blood flow in the body?
A: 1 st order arterioles
terminal arterials (connects arterioles to capillary bed)
capillary
pericytic (post-capillary venule) = connects venule to capillary bed
venule
=> resistance of the arterioles in the organ
Q: What is resistance in terms of blood? What are the factors that affect it? (3) Describe them. What is resistance directly proportional to?
A: hindrance to the blood flow due to friction between moving fluid and stationary vascular walls
- Blood Viscosity = generally constant
- Vessel Radius - the narrower the vessel the more resistance there is = only way to really change resistance
- Vessel Length =fairly constant
1/r^4 (r = radius)
Q: How does increasing blood pressure affect:
pressure gradient?
vascular resistance?
force?
How does arteriolar vasoconstriction affect them?
A: increase
no change
increase
no change
increase
decrease
Q: What is normal blood pressure called? Where do the changes in pressure occur? What influences pressure in the capillary bed? What is the pressure at the end of the capillary bed? (2)
A: Mean Arterial Pressure (MAP) - in any artery in the body the blood pressure is around MAP
across arterioles
how dilated the vessels are
usually venous blood pressure= more or less 0 mmHg
Q: At what speed do you want blood going through the capillary bed? how?
A: pass relatively slowly to allow exchange of nutrients - so system of vessel dilation allows the blood to slow down
Q: What is the pressure going into a capillary bed? What is the pressure coming out? Means? How can the blood flow rate equation be rearranged?
A: around MAP
pretty much 0 mmHg
the pressure change in the blood passing through an organ is usually the same - around MAP
F(organ)= /\P(=MAP)
————–
R(organ)
Q: What is vascular tone? Explain why.
A: Arteriolar smooth muscle usually displays a state of partial constriction
They have to be partially constricted because then there’s a way of changing the blood flow up or down by constricting or dilating
Q: The radii of arterioles are adjusted independently to accomplish TWO functions. What are they? What regulates them?
A: 1. Match blood flow to the metabolic needs of specific tissues - this is regulated by local intrinsic control (independent of nerves and hormones - the tissue determines how much blood it needs)
- Help regulate arterial blood pressure - regulated by extrinsic controls
- neural
- hormonal
Q: What happens as a muscle becomes more active? (2) What detects this change? result? What is this process called?
A: metabolism increases and oxygen consumption increases
in the tissues and sends a signal to the arterioles to vasodilate -> (arterioles are responding to the local conditions) active hyperaemia
Q: What is active hyperermia?
A: an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue
Q: Give an example of how arterioles respond to a change in the physical environment. (4)
A: eg change in temperature
If blood temperature decreases, the microcirculation will detect this change and make the arteriolar smooth muscle contract so that less blood reaches the surface and so less heat is radiated away
Q: What is force equal to? How can it be changed to show what MAP is?
A: /\P (pressure gradient)
——
R (vascular resistance)
F=CO
/\P=MAP
R=TPR
CO=MAP/TPR
and
MAP=CO x TPR
Q: What is TPR?
A: Total peripheral resistance = the sum of the resistance of all arterioles in the body
Q: What structure is involved in neural regulation of arterial blood pressure? What occurs to increase bp? What facilitates constriction and dilation? 2 types.
A: cardiovascular control centre in the MEDULLA
vasoconstriction will take place thus reducing the blood flow to organs
ADRENORECEPTORS (By activating these receptors, the brain can control constriction and dilation)
- alpha=constriction
- beta=dilation
Q: What stimulates the hormonal regulation of arterial bp? How? (2) Include what each method produces. (2,2)
A: -brain can stimulate the release of various hormones that have an effect on blood pressure
- brain can mimic sympathetic nervous system to control heart by stimulating production of adrenaline and noradrenaline
- brain can control blood pressure by stimulating production of Vasopressin and Angiotensin II
Q: Summarise what affects arterioles/blood flow. (2-2,2) Reason?
A: intrinsic (local)
-chemical eg metabolic activity
-physical eg stretch
=> match blood flow to metabolic needs
extrinsic
-neural - sympathetic output
-hormonal - adrenaline, ATII
=> regulate arterial BP
Q: What is capillary exchange? In terms of the CVS?
A: delivery of metabolic substrates to the cells of the organism
ultimate function of the cardiovascular system
Q: How are the capillaries designed for substance exchange? (2)
A: Very narrow walls - 1 micrometre thick
Extensive branching - no capillary is more than a few micrometres away from a tissue cell
Q: To enhance diffusion what do you want to do? (3) Law?
A: enhance diffusion (via Fick’s law)
- Minimise diffusion distance
- Maximise surface area
- Maximise diffusion time
Q: Why is capillary density important? Name 4 structures with high capillary density.
A: the more metabolically demanding a tissue is, the greater the capillary density
- Skeletal muscle
- myocardium
- brain
- lungs
Q: Why does the myocardium and brain need high capillary density? Why not adipose tissue? Skeletal muscle?
A: they are particularly vulnerable to hypoxia
is poorly perfused because there is nothing that the adipose tissue does that needs a rapid response
has a high capillary density but a large number of these capillaries are SHUT OFF at rest
Q: What shuts off arterioles and capillaries?
A: PRECAPILLARY SPHINCTER
Q: What are the 3 main types of capillaries? Most common type?
A: Continuous -
MOST substances move through the endothelial cells **
Fenestrated
Discontinuous
Q: Describe continuous capillary abundance. Describe their structure.
A: vast number of capillaries have a CONTINUOUS structure
There are small water-filled gap junctions between the endothelial cells which allows the passage of electrolytes and other small molecules
Q: What are fenestrated capillaries? Structure? allow?
A: leakier capillaries
They have slightly bigger holes called FENESTRAE which allow larger substances to pass through
Q: What’s the structure of discontinuous capillaries? Importance?
A: LARGE holes in the capillary
This is particularly important in the bone marrow where white cells have to get into the blood
Q: What do you not have in the brain? What do you have instead? Result? Some areas have?
A: In the brain you DO NOT have water-filled gap junctions between the endothelial cells
Instead you have really TIGHT gap junctions
-the access of substances to the brain is tightly regulated
discontinuous blood-brain barrier - in these areas it returns to having water-filled gap junctions
Q: Describe the tight regulation of substance movement in the brain. Benefit?
A: Anything that wants to get to the brain has to diffuse across the endothelial cells so the brain is far more protected from certain substances than other parts of the body
Q: What is bulk flow?
A: a volume of protein-free plasma filters out of the capillary, mixes with the surrounding interstitial fluid (IF) and is reabsorbed
Q: What are the 2 main pressures which affect the movement of fluid in and out of the capillary? What are these?
A: Hydrostatic Pressure
Oncotic Pressure
These are STARLING FORCES
Q: What is oncotic pressure?
A: an osmotic force due to protein in the capillary drawing water back in
Q: What happens as blood moves through the capillaries in terms of pressure? (2) Oncotic pressure? Balance? Fluid return?
A: -arteriolar end of capillary = highest pressure
-the blood moves through the capillaries and comes to the venular end, the pressure will DROP considerably
Oncotic pressure is THE SAME the whole way because the plasma protein concentration doesn’t really change
Depending on the balance of the two Starling forces you get movement of fluid in or out
some fluid that doesn’t return to the capillary -> extra fluid is drained by the lymphatic system
Q: What causes fluid to leave the capillaries? Enter?
A: Arteriolar end - hydrostatic pressure > oncotic pressure = fluid leaves the capillary
Venular end - oncotic pressure > hydrostatic pressure = fluid enters the capillary
Q: Is the circulation of blood a closed or open loop circuit? What about the lymphatic system?
A: closed
not closed <= have lymphatic capillaries which are BLIND ENDED
Q: What prevents backwards flow in lymphatic capillaries? What drives movement? 2 examples. What are lymph vessel walls?
A: valves
lymphatic pressure (e.g. muscle pressure, lungs inflating and deflating)
single layer of endothelial cells
Q: Lymph drains into which 4 places. How much fluid is returned to the circulatory system by the lymphatic system per day?
A: -Thoracic Duct
- Right Lymphatic Duct
- Right Subclavian Vein
- Left Subclavian Vein (veins in the lower neck)
3L
Q: What is oedema? What causes it? Example disease.
A: Rate of production of fluid > Rate of removal of fluid = OEDEMA
Can be caused by parasitic blockage of lymph nodes
For example filaria can cause ELEPHANTIASIS
