Regulation of Arteriolar Resistance Flashcards
Define Darcy’s Law.
- defines the ability of a fluid to flow through a porous media
- the velocity of flow of a liquid through a porous medium due to difference in pressure is proportional to the pressure gradient in the direction of flow
Define Poiseuille’s Law.
- the velocity of flow through a tube is directly proportional to the fourth power of the radius of the tube, and inversely proportional to the length and viscosity of the tube
What is the equation involving mean arterial pressure?
MAP = CO x TPR
- CO = cardiac output
= TPR - total peripheral resistance
(pressure = flow x resistance
How do you control the resistance in a vessel?
- control the radius
Name and describe the two levels of control over the smooth muscle surrounding arterioles.
- Intrinsic mechanisms - concerned with meeting the selfish needs of each individual tissue
- Extrinsic mechanisms - concerned with ensuring that the total peripheral resistance of the body stays with the desired range
Describe the neural aspect of extrinsic control.
Sympathetic nerves - release noradrenaline - binds to alpha 1 receptors - causes arteriolar constriction - therefore, reduces flow through that tissue and tends to increase total peripheral resistance Parasympathetic nerves - usually no effect
Describe the hormonal aspect of extrinsic control. Be specific to epinephrine.
Epinephrine
- released from adrenal medulla
- binds to alpha one receptor
- causes arteriolar constriction
- therefore, reduces flow through that tissue and teds to increase total peripheral resistance
- in skeletal and cardiac muscle, it also activated beta two receptors
- causes arteriolar dilation
- therefore, flow increases through that tissue, and tends to decrease total peripheral resistance
Describe the hormonal aspect of extrinsic control. Include all expect epinephrine.
Angiotensin II
- produced in response to low blood volume
- causes arteriolar constriction
- therefore, increases total peripheral resistance
Vasopressin (antidiuretic hormone)
- released in response to low blood volume
- causes arteriolar constriction
- therefore, increases total peripheral resistance
Atrial natriuretic peptide and brain natriuretic peptide
- released in response to high blood volume
- causes arteriolar dilation
- therefore, reduces total peripheral resistance
Descibe the mechanism of active (metabolic) hyperaemia in intrinsic control.
- increase metabolic activity cause and increase in the concentration of metabolites
- triggers release of EDRF
- cause arteriolar dilation
- increased flow to wash out metabolites
- an adaption to match blood supply to the metabolic needs of that tissue
Describe the mechanism of pressure (flow) autoregulation in intrinsic control.
- a decrease in mean arterial pressure causes a decrease in blood flow
- metabolites accumulate
- triggers release of EDRF
- arterioles dilate and flow is restored to normal
- an adaption to ensure that a tissue maintains its blood supply despite changes in mean arterial pressure
Describe the mechanism of reactive hyperaemia and the injury response in intrinsic control.
Reactive hyperaemia
- occlusion of blood supply causes a subsequent increase in blood flow
- an extreme version of pressure autoregulation
The injury response
- mast cells recruit histamine
- arteriolar dilation
- increase in blood flow, therefore an increase in permeability
- aids delivery of blood born leucocytes etc to injured area
What happens to blood supply in coronary arteries during systole?
- blood supply is interrupted
What is special about cerebral circulation?
- needs to be kept stable
- shows excellent pressure autoregulation
What is special about pulmonary circulation?
- a decrease in oxygen causes arteriolar constriction
- ensures that blood is directed to the best ventilated parts of the lung
