Regulation of Arteriolar Resistance Flashcards
What does Poiseuille’s law state?
When a fluid is pumped through a closed system, its flow is determined by the pressure developed by the pump and the resistance to that flow
What is resistance to flow caused by?
Frictional forces within the fluid
What does resistance to flow depend on?
Viscosity of the fluid and the dimensions of the tube
What is the equation relating flow, pressure and resistance?
Flow = change in pressure/resistance
Pressure = flow x resistance
What does varying the radius of the resistance vessels control?
Controls TPR and regulates MAP
What is the varying radius of the arterioles used to control?
Flow through the individual vascular beds
Also affects change in pressure
You can’t affect the flow through the vascular beds without affecting what?
MAP
In order to keep blood flow to each vascular bed sufficient and to keep MAP in the right range, what is needed?
Some resistance
How is resistance in vessels achieved?
Two levels of control over the smooth muscle surrounding the arterioles;
Intrinsic mechanisms, concerned with meeting the needs of each individual tissue
Extrinsic mechanisms, concerned with ensuring that the TPR of the whole body stays in the right range
How are extrinsic mechanisms causing resistance controlled neurally?
Neural control - sympathetic nerves release norepinephrine which binds to alpha-1 receptors and causes arterioles constriction, which decreases the flow through that tissue and increases TPR
Parasympathetic nerves usually have no effect
What is hyperaemia?
Increase in organ blood flow associated with increased metabolic activity of an organ or tissue
What is pressure autoregulation?
Maintenance of constant blood flow despite changes in MAP
What is reactive hyperaemia?
Large transient increase in blood flow that occurs following a brief period of ischaemia/release of occlusion (extreme version of autoregulation)
What is the basic sequence of events of the injury response?
Mast cells release histamine in response to an injury
This causes arteriolar dilation, increased blood flow and increased permeability
Aids the delivery of blood-borne leukocytes etc. to the injured area
What hormones affect the extrinsic control of resistance?
Epinephrine/adrenaline
Angiotensin II
Vasopressin
Atrial natriuretic peptide and brain natriuretic peptide
How does epinephrine affect resistance?
Released from adrenal medulla
Binds to alpha-1 receptors
Causes arteriolar constriction so tends to decrease flow through that tissue and increase TPR
In some tissues e.g. cardiac muscle it also activates beta-2 receptors to cause arteriolar dilation, increase flow and increase TPR
How does angiotensin II affect resistance?
Produced in response to low blood volume
Causes arteriolar constriction
Causes increased TPR
How does vasopressin affect resistance?
Released in response to low blood volume
Causes arteriolar constriction and increased TPR
How do ANP and BNP affect resistance?
Released in response to high blood volume
Causes arteriolar dilation and decreased TPR
What are the intrinsic control mechanisms that affect resistance?
Active hyperaemia
Pressure autoregulation
Reactive hyperaemia
Injury response
How does active hyperaemia affect resistance?
Increased metabolic activity causes increased concentration of metabolites
Triggers release of EDRF
Causes arteriolar dilation
Increased flow to wash out metabolites
Adaptation made to match blood supply to the metabolic needs of that tissue
How does pressure autoregulation affect resistance?
Decreased MAP causes decreased flow
Metabolites accumulate
Triggers release of EDRF
Arterioles dilate and flow restored to normal
Adaptation made to ensure that a tissue maintains blood supply despite changes in MAP
What are the main features of the coronary circulation?
Blood supply interrupted by systole
Has to cope with increased demand during exercise
Shows excellent active hyperaemia
Expresses beta-2 receptors
What are the main features of the cerebral circulation?
Needs to be kept stable at all times
Shows excellent pressure autoregulation
What are the main features of the pulmonary circulation?
Decreased oxygen causes arteriolar constriction
Opposite response to that of most tissue
This ensures that blood is directed to the best ventilated parts of the lungs
What are the main features of the renal circulation?
Main function is filtration which depends on pressure
Changes in MAP have big effects on blood volume
Shows excellent pressure autoregulation
