Ch. 13/14 Day 4 Flashcards
What are the physics of circulation?
Pressure, volume, flow, and resistance
What is mean blood pressure of the systemic circulation?
Ranges from just under 100 mmHg in aorta to a low of just a few mmHg in the vena cavae
Does blood flow up or down a pressure gradient?
Down
Pressure Differentials Drive Blood Flow
Pressure created by contracting muscles (friction) is transferred to blood
Driving pressure is created by the ventricles
BP affected by:
- If blood vessels dilate, BP decreases
- If blood vessels constrict, BP increases
- Volume changes affect BP in CV system
What is a gradient?
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)
Driving Force for Blood Flow
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)
If the tube has no pressure gradient, will it have a flow?
No
Resistance Opposes Flow
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
What determines resistance?
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
Poiseuille’s Law
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
Inverse Relationship between flow and resistance
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
Flow = deltaP/R
Flow of blood in CV system is directly proportional to the pressure gradient and inversely proportional to the resistance to flow (OPPOSITE EFFECTS)
Under normal conditions, the most important factor determining vascular resistance to flow is?
Vessel Diameter
What is the equation for resistance (R)?
R = 8Ln/(pi)r^4
L = vessel length stays constant
n = viscosity of blood stays constant
r^4 = vessel radius can significantly change
Flow rate is the ____, regardless of where you are in the system.
Same
Cardiac Output (CO)
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]
CO: Right Ventricle (RV) vs Left Ventricle (LV)
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
Regulation of Cardiac Rate
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
Regulation of Stroke Volume (SV)
Regulated by three variables:
- 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] - Total peripheral resistance: Frictional resistance in the systemic arteries
- -Inversely related to stroke volume
- -Called afterload - Contractility: strength of ventricular contraction
- -Stroke volume increases with contractility
Ejection Fraction
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
Preload
EDV (volume of blood in ventricles at end of diastole)
Afterload
Total peripheral resistance, inversely related to SV
Frank-Starling Law of the Heart
Increased EDV results in increases contractility and thus increased SV
Intrinsic Control of Contraction Strength
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
Extrinsic Control of Contractility
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
Venous Return
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.
Factors in Venous Return
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
What is the equation for cardiac output (CO)? KNOW FOR EXAM
CO = Stroke Volume (SV) * HR
