CVS 5 - Microcirculation Flashcards
(36 cards)
Describe the arrangement of microcirculation.
- 1st order arterioles
- Terminal arterioles
- Capillary
- Pericytic venule
- Venule
What is blood flow rate?
Volume of blood passing through a vessel per unit time.
F = P/R (pressure gradient / vascular resistance)
What is resistance?
Hindrance to blood flow due to friction between moving fluid and stationary vascular walls
What are the factors affecting resistance?
- Blood viscosity
- Vessel radius
- Vessel length
(blood viscosity and vessel length are generally constant)
What is resistance directly proportional to?
1 / r^4
If radius decreases, Resistance increases.
Which are the major resistance vessels?
Arterioles - where changes in BP occur
Blood pressure generated by the heart doesn’t change much in arteries
PRESSURE DIFFERENCE IS WHAT ALLOWS BLOOD TO REACH TISSUES
What is normal blood pressure called?
Mean Arterial Pressure (MAP). In arteries, BP is around MAP
Why do you want blood to go slowly through the capillary bed?
To allow nutrient exchange
Derive an equation knowing that initial pressure (A) is MAP and the final pressure (venous BP) is usually 0.
F (organ) = MAP / R (organ)
What is the major determinant of blood flow in the body?
The resistance of arterioles in the organ
What is vascular tone?
Arteriolar smooth muscle is usually partially constricted.
They have to be partially constricted to regulate blood flow by dilating or constricting
What are the 2 functions that require arteriolar radii adjustment?
- Matching blood flow to metabolic needs of tissue - LOCAL INTRINSIC CONTROL (independent of nerves and hormones)
- Aids in regulating artery BP - EXTRINSIC CONTROL
What is active hyperemia?
Increase in organ blood flow that is associated with increased metabolic activity of organ or tissue.
Arterioles respond to changes in chemical environment
How can arterioles respond to changes in the physical environment?
e.g. temperature
If blood temp decreases, microcirculation will make arteriolar smooth muscle contract. Less blood reaches surface. Less heat radiated away.
Describe autoregulation.
In the physical environment, not all tissues need more blood (CO is limited, so body must be selective).
Therefore, certain tissues (e.g. lungs, heart, muscle) may undergo Active Hyperemia and dilate. Other (less metabolically active) tissues won’t.
e.g. if BP increases, less metabolically active tissues will sense the vasodilation (and the STRETCH) and they will consequently undergo myogenic vasoconstriction, so more blood doesn’t go across the tissue where it is not needed.
Whole process is called auto regulation.
Give an equation that applies to the WHOLE of circulation.
CO = MAP/TPR (total peripheral resistance)
TPR = sum of resistance of all arterioles in the body
Which 2 pathways can influence arteriolar radii?
Neural and hormonal
Describe neural regulation of arterial BP
- Regulated by cardiovascular control system in medulla
- Vasonconstriction occurs to increase BP.
2 important receptors.
Beta receptor = on the heart. Important in speeding the heart up
Alpha receptor (important in microcirculation) = receptors that respond to noradrenaline and cause vasoconstriction.
Describe hormonal regulation of arterial BP.
There are potent vasoconstrictor hormones.
Vasopressin (Posterior pituitary)
Angiotensin (mainly lung)
Adrenaline/noradrenaline
What is the ultimate function of the CVS?
Capillary exchange - delivery of metabolic substrates to cells of organism
2 ways in which capillaries are perfectly designed for exchange.
- Very narrow walls - 1um thick
2. Extensive branching - no capillary more than a few um from a tissue cell
Why is capillary density important?
More metabolically active tissue means greater capillary density
Why do the myocardium and brain need a high capillary density?
They are particularly vulnerable to hypoxia.
Give an example of a poorly perfused tissue.
Adipose tissue
