Page 215 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Ventricular Pressure–Volume phase, the pressure in the left ventricle rises
Relationships (all valves closed) until the diastolic aortic
pressure (80 mmHg in this case) is reached
The relationship between the volume (length) (! A1, point D). The aortic valve then opens.
and pressure (tension) of a ventricle illustrates During the ejection phase, the ventricular
the interdependence between muscle length volume is reduced by the stroke volume (SV)
and force in the specific case of the heart while the pressure initially continues to rise
(! p. 66ff.). The work diagram of the heart can (! p. 188, Laplace’s law, Eq. 8.4b: P tm " be-
be constructed by plotting the changes in cause r # and w "). Once maximum (systolic)
ventricular pressure over volume during one pressure is reached (! A1, point S), the volume
complete cardiac cycle (! A1, points A-D-S-V- will remain virtually constant, but the pres-
A, pressure values are those for the left ven- sure will drop slightly until it falls below the
tricle). aortic pressure, causing the aortic valve to
Cardiovascular System dicates the pressures that result passively (without decreases to (almost) 0 (! A1, point V). The
close (! A1, point K). During the isovolumetric
The following pressure–volume curves can be used to
phase,
relaxation
the
rapidly
pressure
construct a work diagram of the ventricles:
! Passive (or resting) pressure–volume curve: In-
ventricles now contain only the end-systolic
muscle contraction) at various ventricular volume
volume (ESV), which equals 60 mL in the il-
loads (! A1, 2; blue curve).
lustrated example. The ventricular pressure
! Isovolumic peak curve (! A1, 2, green curves):
rises slightly during the filling phase (passive
an isolated heart. Data are generated for various
volume loads by measuring the peak ventricular
8 Based on experimental measurements made using pressure–volume curve).
pressure at a constant ventricular volume during Cardiac Work and Cardiac Power
contraction. The contraction is therefore iso-
–2
volumetric (isovolumic), i.e., ejection does not take Since work (J = N · m) equals pressure (N · m =
place (! A2, vertical arrows). Pa) times volume (m ), the area within the
3
! Isotonic (or isobaric) peak curve (! A1, 2, violet working diagram (! A1, pink area) represents
curves). Also based on experimental measurements the pressure/volume (P/V) work achieved by
taken at various volume loads under isotonic
(isobaric) conditions, i.e., the ejection is controlled in the left ventricle during systole (13,333 Pa·
3
such a way that the ventricular pressure remains con- 0.00008 m = 1.07 J; right ventricle: 0.16 J). In
stant while the volume decreases (! A2, horizontal systole, the bulk of cardiac work is achieved by
arrows). active contraction of the myocardium, while a
! Afterloaded peak curve: (A1, 2, orange curves). much smaller portion is attributable to passive
Systole (! p. 190) consists of an isovolumic contrac- elastic recoil of the ventricle, which stretches
tion phase (! A1, A–D and p. 191 A, phase I) fol- while filling. This represents diastolic filling
lowed by an auxotonic ejection phase (volume work (! A1, blue area under the blue curve),
decreases while pressure continues to rise) (! A1,
D–S and p. 191 A, phase II). This type of mixed con- which is shared by the ventricular myo-
traction is called an afterloaded contraction (see also cardium (indirectly), the atrial myocardium,
p. 67 B). At a given volume load (preload) (! A1, and the respiratory and skeletal muscles
point A), the afterloaded peak value changes (! A1, (! p. 204, venous return).
point S) depending on the aortic end-diastolic pres- Total cardiac work. In addition to the cardiac
sure (! A1, point D). All the afterloaded peak values work performed by the left and right ventricles
are represented on the curve, which appears as a in systole (ca. 1.2 J at rest), the heart has to
(nearly) straight line connecting the isovolumic and
isotonic peaks for each respective volume load (point generate 20% more energy (0.24 J) for the pulse
A) (! A1, points T and M). wave (! p. 188, windkessel). Only a small
amount of energy is required to accelerate the
Ventricular work diagram. The pressure– blood at rest (1% of total cardiac work), but the
volume relationships observed during the car- energy requirement rises with the heart rate.
diac cycle (! p. 190) can be plotted as a work The total cardiac power (= work/time, ! p.
diagram, e.g., for the left ventricle (! A1): The 374) at rest (70 min –1 = 1.17 s ) is approxi-
–1
202 end-diastolic volume (EDV) is 125 mL (! A1, mately 1.45 J · 1.17 s –1 = 1.7 W.
point A). During the isovolumetric contraction
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.