Arteries and Arterioles

What is the formula for flow?

Flow = Delta P / Resistance

What factors affect resistance? Are they direct or indirect?

• What can cause it to change?

Viscosity: Direct

• Change in % of RBCs

Length of Tube = Direct

• Can’t change

Radius of Tube (largest factor) = Inverse

• Contraction/relaxation of smooth muscle

Where does pressure drastically decrease? Why?

Arterioles due to high resistance

• Also due to low compliance since it doesn’t have elastic fibers and isn’t floppy like veins

What is compliance?

What is high compliance?

Compliance = Delta Volume / Delta Pressure

• I. E. how easily can a blood vessel be distended

High compliance = increase in blood = distended tube so there is not a lot of pressure change

What does the artery wall look like compared to the other blood vessel structures?

What factors give the artery it’s resistance and compliance?

Thicker walls and more muscle than veins

Elastic fibers unlike arterioles and veins (High compliance)

Have large radius (low resistance)

What type of vessels are large arteries?

What is their fucntions? (2)

Conductance vessels

• Rapid transit of blood from the heart to organs

• Pressure reservoir: driving force for blood during diastole

  • Due to high compliance so wall stretches and SA increases to limit the increase of systolic pressure and the energy is stored in the walls so elastic recoil can happen during diastole

  • Most blood goes out of arteries during diastole*

What would happen to the BP if the large arteries became stiffer?

What is pulse pressure? How would it change?

Would MAP change?

Systolic BP : Increase

Diastolic BP: Decrease

Pulse Pressure = Systolic - Diastolic = Increase

MAP doesn’t change

What is the formula for MAP?

Why is this?

MAP = 1/3 SP + 2/3 DP

Fraction of time spent in systole: 1/3

Fraction of time spent in diastole: 2/3

Why is MAP important?

Why is Pulse pressure important?

MAP: tells you whether or not you are perfusing your organs

Pulse Pressure: important predictor of mortality

What is the resistance and copmliance of arteriioles?

High resistance and low copliacne

What is the function of arterioles?

1. Transit for passage of blood from the larger arteries to capillaries

2. Control the amount of blood delivered to those capillaries and organs at any MAP

   1. Body can regulate how much blood an organ gets by changing the resistance of the arterioles to that specific organ

3. Help regulate arterial BP or MAP

What causes contraction or relaxation of arteriolar smooth muscle?

Arteriolar smooth muscle is tonically contracted

Local Controls:

1. Response to injury

2. Active hyperemia

3. Reactive hyperemia

4. Autoregulation

What is active hyperemia?

What does this cause in the blood vessels?

What are some examples?

When metabolic activity increases so does blood flow to that tissue

• Increase in CO2 and decrease in O2

• Localized relaxation of arteriolar smooth muscle = vasodilation = increased blood flow

Skeletal muscle = Exercise = 20-fold increase in blood flow

Cardiac muscle: Increased HR = 5-fold increase in coronary blood flow

What is the flow autoregulation mechanism?

What causes it?

1. An organ experiences a change in perfusion pressure

2. Blood flow to the organ changes

3. Within a few minutes the blood flow returns to normal

Due to metabolic factors similar to active hyperemia and myogenic response

What is the myogenic response with flow autoregulation?

1. Pressure it too high in brain

2. Stretch of arteriole

3. Vasoconstrict cerebral arterioles to reduce flow in brain

What is the mechanism of reactive hyperemia?

Loss of blood flow causes tissues to use up nutrients and accumulate waste

Localized relaxation of arteriole smooth muscle and vasodilation in ischemia tissue

What happens to blood vessels when you are cold?

How does this happen?

1. Cold so activate SNS to release NE

2. NE activates alpha 1 receptors

3. Usually due to hypothalamus