Ch. 13/14 Day 4

Question 1

What are the physics of circulation?

Answer 1

Pressure, volume, flow, and resistance

Question 2

What is mean blood pressure of the systemic circulation?

Answer 2

Ranges from just under 100 mmHg in aorta to a low of just a few mmHg in the vena cavae

Question 3

Does blood flow up or down a pressure gradient?

Answer 3

Down

Question 4

Pressure Differentials Drive Blood Flow

Answer 4

Pressure created by contracting muscles (friction) is transferred to blood

Driving pressure is created by the ventricles

BP affected by:

1. If blood vessels dilate, BP decreases
2. If blood vessels constrict, BP increases
3. Volume changes affect BP in CV system

Question 5

What is a gradient?

Answer 5

Change or difference in the magnitude of a given parameter, e.g. pressure, in one location w/ respect to another location

Fluid flow through a tube depends on the pressure gradient - fluids flow from high to low pressure (pressure gradient)

Question 6

Driving Force for Blood Flow

Answer 6

Flow through a tube is directly proportional to the pressure gradient

• -Flow = (delta)P
• -the higher the pressure gradient, the greater the fluid flow
• -fluid flows only if there is a positive pressure gradient (deltaP)

Question 7

If the tube has no pressure gradient, will it have a flow?

Answer 7

No

Question 8

Resistance Opposes Flow

Answer 8

Flow through a tube is inversely proportional to resistance

• -flow (Q) = 1/R, where R = resistance
• -if resistance increases, flow decreases
• -if resistance decreases, flow increases

Question 9

What determines resistance?

Answer 9

R = 8Ln/(pi)r^4 or R = Ln/r^4 –> hydraulic resistance

Resistance is proportional to length (L) of the tube (blood vessel)
–resistance increases as length increases

Resistance is proportional to viscosity (n), or thickness, of the fluid (blood)
–resistance increases as viscosity increases

Resistance is inversely proportional to tube radius to the fourth power
–resistance decreases as radius increases

Question 10

Poiseuille’s Law

Answer 10

Combine resistance (R = Ln/r^4) w/ the effect of pressure difference (deltaP) on fluid flow rate (Q = deltaP)

deltaP = Q * R where Q = flow rate, R = resistance

So Q = deltaP/R = deltaPr^4(pi)/nL(8)
–Poiseulle’s Law

Question 11

Inverse Relationship between flow and resistance

Answer 11

Small change in radius has a large effect on resistance to blood flow

• -vasoconstriction is a decrease in blood vessel diameter/radius and decreases blood flow
• -vasodilation is an increase in blood vessel diameter/radius and increases blood flow

Question 12

Flow = deltaP/R

Answer 12

Flow of blood in CV system is directly proportional to the pressure gradient and inversely proportional to the resistance to flow (OPPOSITE EFFECTS)

Question 13

Under normal conditions, the most important factor determining vascular resistance to flow is?

Answer 13

Vessel Diameter

Question 14

What is the equation for resistance (R)?

Answer 14

R = 8Ln/(pi)r^4

L = vessel length stays constant

n = viscosity of blood stays constant

r^4 = vessel radius can significantly change

Question 15

Flow rate is the ____, regardless of where you are in the system.

Answer 15

Same

Question 16

Cardiac Output (CO)

Answer 16

Volume of blood pumped each minute by each ventricle

(KNOW THIS)
CO = Stroke Volume (SV) * HR

• -average HR = 70 bpm
• -average SV = 70-80mL/beat
• -average CO = 5,500mL/minute [equivalent to total blood volume]

Question 17

CO: Right Ventricle (RV) vs Left Ventricle (LV)

Answer 17

In each cycle, lungs get 100% of CO from RV, while other organs share output of LV

Therefore, pulmonary circulation has low R, low P, and high Q.

MAPpulmonary = 10-20 mmHg
MAPsystemic = 70-105 mmHg

Question 18

Regulation of Cardiac Rate

Answer 18

Spontaneous depolarization occurs at SA node when HCN channels open, allowing Na+ in. [recall slide # 42]

A. Open due to hyperpolarization at the end of the preceding action potential (based upon slope of pacemaker potential)
B. Sympathetic norepinephrine and adrenal epinephrine keep HCN channels open, increasing heart rate.
C. Parasympathetic acetylcholine opens K+ channels, slowing heart rate
D. Controlled by cardiac center of medulla oblongata that is affected by higher brain centers
E. Actual pace comes from the net affect of these antagonistic influences
–Positive chronotropic effect - increases rate
–Negative chronotropic effect - decreases rate

Question 19

Regulation of Stroke Volume (SV)

Answer 19

Regulated by three variables:

1. End diastolic volume (EDV): volume of blood in the ventricles at the end of diastole
   - -Sometimes called preload
   - -Stroke volume increases with increased EDV. [Frank-Starling]
2. Total peripheral resistance: Frictional resistance in the systemic arteries
   - -Inversely related to stroke volume
   - -Called afterload
3. Contractility: strength of ventricular contraction
   - -Stroke volume increases with contractility

Question 20

Ejection Fraction

Answer 20

Normally, about 60% of the EDV is ejected – ejection fraction

1) Ejection fraction remains constant over a range of EDVs, such that SV ↑’s as EDV ↑’s
2) Thus strength of ventricular contraction must increase as EDV increases

Question 21

Preload

Answer 21

EDV (volume of blood in ventricles at end of diastole)

Question 22

Afterload

Answer 22

Total peripheral resistance, inversely related to SV

Question 23

Frank-Starling Law of the Heart

Answer 23

Increased EDV results in increases contractility and thus increased SV

Question 24

Intrinsic Control of Contraction Strength

Answer 24

Due to myocardial stretch:

• -Increased EDV stretches the myocardium, which increases contraction strength.
• -Due to increased myosin and actin overlap and increased sensitivity to Ca2+ in cardiac muscle cells

Adjustment for rise in peripheral resistance:

• -Increased peripheral resistance will decrease stroke volume
• -More blood remains in the ventricles, so EDV (what’s left in ventricles) increases
• -Ventricles are stretched more, so they contract more strongly

Question 25

Extrinsic Control of Contractility

Answer 25

Contractility – strength of contraction at any given fiber length

Sympathetic norepinephrine and adrenal epinephrine (positive inotropic effect) increase contractility by making more Ca2+ available to sarcomeres. Also increases HR.

Parasympathetic acetylcholine (negative chronotropic effect) will decrease HR which will increase EDV –> increases contraction strength –> increases SV, but not enough to compensate for slower rate, so CO decreases

Question 26

Venous Return

Answer 26

EDV controlled by factors that affect venous return:

• -Total blood volume
• -Venous pressure (driving force for blood return)

Veins have high compliance - stretch more at a given pressure than arteries (veins have thinner walls).

Veins are capacitance vessels – 2/3 of the total blood volume is in veins

They hold more blood than arteries but maintain lower pressure.

Question 27

Factors in Venous Return

Answer 27

Δ P between arteries and veins (about 10mm Hg)

Δ P in venous system - highest P in venules vs. lowest P in venae cavae into the right atrium (0mm Hg)

Sympathetic nerve activity to stimulate smooth muscle contraction and lower compliance

Skeletal muscle pumps

Δ P between abdominal and thoracic cavities (respiration)

Blood volume

Question 28

What is the equation for cardiac output (CO)? KNOW FOR EXAM

Answer 28

CO = Stroke Volume (SV) * HR