5 - Pressure, Flow, and Resistance

Which tube has a greater flow (Q)?

Which tube has a greater flow (Q)?

How does pressure change going from the aorta to the vena cava?

Arteries

Arterioles

Capillaries

Venules

Veins

Arteries - Pressure high in the large, elastic arteries

Arterioles - Pressure falls off dramatically in the arterioles

Capillaries - Continues to fall

Venules - Continues to fall

Veins - Pressure very low in veins

How does resistance (R) change from one group of vessels to the next?

Arteries

Arterioles

Veins

Arteries - Very low resistance

Arterioles- High resistance

Veins - Low resistance

What is compliance?

How easily a blood vessel can distend

Depends on the stiffness of the blood vessel wall

Compliance equation

Compliance = change in volume/ change in transmural pressure

How does compliance change from one group of vessels to the next?

Arteries

Arterioles

Veins

Arteries - high compliance

Arterioles - low compliance

Veins - very high compliance

What is the artery wall like?

Arteries are under much higher pressure, so have thicker walls and more muscle than veins

Structure of a large artery

• Inner single layer of endothelial cells Aorta (endothelium)

• Many layers of smooth muscle, elastic fibers and connective tissue

• Outer layer of connective tissue

Identify what enables large arteries to have a low resistance and high compliance

A. Arterial wall contains a lot of elastic fibers

B. Arteries have a high pressure

C. Arteries have a large diameter or radius

Low resistance - C

High compliance - A and B

What is the function of arteries?

1. Rapid transit for passage of blood from the heart to organs

2. Pressure reservoir

Rapid transit for passage of blood from the heart to organs explained

Large diameter with low resistance

Called conductance vessels

Pressure reservoir explained

Driving force for blood during diastole

High compliance

Let’s look at what would happen during systole if the aorta was a rigid tube (not compliant)

Pressure increases from 80 mm Hg (during diastole) to 150 mm Hg (during systole)

Let’s look at what happens during systole when the aorta is a compliant tube (3)

Wall stretches (increases in surface area)

Limits the increase in systolic pressure

Maybe go from diastolic pressure of 80 to systolic pressure of 120

Another important thing that happens during systole: Some energy is stored

stored in the stretched wall

This energy can help maintain blood flow during diastole!

How blood continues to flow during systole? Diastole?

Systole:

• 70 mL goes in → 25 mL out

• Dialated

Diastole:

• 0 mL goes in → 45 mL goes out

• Contracts/ elastic recoil

Fluctuation of arterial pressure during the cardiac cycle

Pressure increases due to ejection of blood - Reaches systolic pressure

Arterial pressure slowly decreases during diastole - Reaches diastolic pressure

What will happened to blood pressure if the large arteries become stiffer (ie less compliant or lose some of their elasticity)? Compared to ‘normal’:

A. Systolic pressure will be lower

B. Systolic pressure will be higher

C. Systolic pressure will be the same

A. Diastolic pressure will be lower

B. Diastolic pressure will be higher

C. Diastolic pressure will be the same

B. Systolic pressure will be higher - Won't stretch so will increase

A. Diastolic pressure will be lower - Wont contract so will decrease

What will happened to pulse pressure if the large arteries become stiffer (ie less compliant or loose some of their elasticity)?

Pulse pressure will increase

Pulse pressure equation

Pulse pressure = systolic - diastolic

Effect of compliance on pulse pressure

Change with reduced compliance as with arteriosclerosis of the aorta

What will most likely happen to mean arterial pressure if the large arteries become stiffer (ie less compliant or lose some of their elasticity)?

A. Mean arterial pressure will decrease

B. Mean arterial pressure will increase

C. Mean arterial pressure will not change

C. Mean arterial pressure will not change

Mean arterial pressure (MAP) definition

average pressure in the arteries during one cardiac cycle

It is not the arithmetic mean of systolic and diastolic pressure

The negative feedback response is for MAP

to stay constant

MAP Equation

MAP = DP + 1/3 (SP – DP)

MAP = 1/3 SP + 2/3 DP

Why is mean arterial pressure (MAP) important?

Tells you whether or not you are perfusing your organs (supplying organs with blood)

Why is pulse pressure important?

It is an important predictor of mortality

Better indicator of arteriosclerosis than MAP

Let’s summarize. What is the function of the aorta and other large elastic arteries?

1. Conduit for blood

2. Act as a ‘pressure reservoir’ during diastole

3. Limit the increase in systolic pressure and limit the decrease in diastolic pressure

What is the wall of an arteriole like?

Inner endothelium

Lumen

One to a couple of layer of smooth muscle cells

What is the function of arterioles? (3)

1. Transit for passage of blood from the large arteries to capillaries in an organ

2. Control the amount of blood delivered to those capillaries (or the tissue) at any given MAP

3. Help regulate arterial blood pressure or MAP

Change in P definition

Pressure reservoir - “arteries”(delta P is the same for all organs)

Variable-resistance definition

outflow tubes (“arterioles”)

How does the body ensure that a particular organ is supplied with sufficient blood to meet its metabolic demands?

A. By changing mean arterial pressure

B. By changing the resistance of the veins

C. By changing the resistance of the

arterioles that supply that organ

D. By changing the resistance of the large,

elastic arteries such as the aorta

E. By changing the resistance of the

capillaries in that organ

C. By changing the resistance of the

arterioles that supply that organ

How resistance changes organ blood flow

1. Delta P doesn’t change

2. Resistance in some vessels changes

3. Flow to those organs change

What causes contraction or relaxation of arteriolar smooth muscle to change resistance and hence blood flow?

Arteriolar smooth muscle is tonically contracted (independent of neuronal or hormonal stimulation)

Local control

Extrinsic control (ANS and Hormones)

Local controls of arteriolar smooth muscle tone or resistance examples (3)

1. Active hyperemia

2. Reactive hyperemia

3. Autoregulation

Endothelium releases thromboxane A2

Active hyperemia

Blood flow response to increased tissue metabolic activity

Reactive hyperemia

Blood flow response to blood vessel occlusion

Autoregulation

Blood flow response to changes in perfusion pressure

Active hyperemia, When metabolic activity increases

blood flow to the tissue increases

How blood flow changes with active hyperemia

If a tissue increases its metabolic activity:

• Uses more O2, nutrients (Decrease O2)

• Generates more CO2, waste products (increase CO2)

Localized relaxation of arteriolar smooth muscle, vasodilation and increased blood flow

Flow autoregulation mechanism (4)

1. An organ experiences a change in perfusion pressure (Delta P) →

2. Blood flow to the organ changes →

3. Autoregulation →

4. Within few minutes blood flow returns to normal

Flow autoregulation example

1. Blood pressure decreases →

2. Blood flow to an organ (e.g. the brain) decreases →

3. Autoregulation →

4. Blood flow to the brain goes back to normal

Within a range of MAP, despite changes in perfusion or driving pressure

blood flow to the brain is constant

Autoregulation involves two mechanisms

1. The same metabolic factors as active hyperemia

2. The myogenic response → Some blood vessels act as stretch receptor

The myogenic response with flow autoregulation

1. Pressure is too high (too high a blood flow) →

2. Stretch the arteriole →

3. Vasoconstrict cerebral arterioles to reduce flow to the brain

And vise versa

A tourniquet is place on the arm. The arm starts turning blue. The tourniquet is removed. What happens to the arm?

C. Slowly goes back to its normal color

D. Turns bright red

E. Turns a darker blue

D. Turns bright red

Reactive hyperemia

Mechanism of reactive hyperemia

Loss of blood flow causes the tissues to:

• Use up all of the O2, nutrients (Decreased O2)

• Accumulate more CO2, waste products (Increase CO2)

Localized relaxation of arteriolar smooth muscle and vasodilation in the ischemic tissue

Extrinsic control of arteriolar vasodilation and vasoconstriction (2)

1. Sympathetic nervous system

2. Other Hormones

Extrinsic control of arteriolar vasodilation and vasoconstriction (SNS) (2)(1)(1)

A. Helps regulate blood pressure (or MAP)

• Mostly postganglionic sympathetic neurons and NE

B. Involved with increasing or decreasing blood flow to select tissues

• There is a differential distribution of alpha 1 and Beta 2 receptors on vasculature – EPI and NE

Extrinsic control of arteriolar vasodilation and vasoconstriction (Other hormones) (2)

• Angiotensin II, Vasopressin, Atrial natriuretic peptide (ANP)

• Are more involved with blood pressure (or MAP) regulation

What happens to blood vessels when you are cold? (5)(1)

1. Cold →

2. Activates SNS to release NE →

3. NE activates alpha 1 receptors on skin and gut→

4. Vasoconstriction to reduce heat loss

5. Can be shut off by the hypothalamus

Note: Smooth muscle on cutaneous arterioles contain predominantly alpha 1 receptors

When you exercise, there can be a very large increase in blood flow to skeletal muscle. How can that happen? (4)

1. Hot →

2. Active hyperemia + EPI from adrenal glands →

3. EPI activates Beta 2 receptors on skeletal muscle →

4. Vasodialation to increase heat loss

Let’s summarize. What is the function of the arterioles? (3)

1. Deliver blood to capillaries

2. Responsible for the relative blood flow to an organ at any given mean arterial pressure

3. Help to determine MAP