Download Document

Chapter 10
The heart as a pump
Section 1
The contractility of the heart
1. The ultrastructure of cardiac muscle




Sarcomere is similar with skeletal muscle
Intercalated disks, gap junctions between cells
Rich in mitochondria
Under-developed sarcoplasmic reticulum (SR),
contraction is dependent on [Ca2+]o (CICR)
2. The excitation-contraction coupling of cardiomyocyte
 The process which connects excitation and contraction
 Triggered by Ca2+ influx
 CICR, Ca2+ transient
 Cardiocyte contraction induced by Ca2+
 Ca2+ reuptake and relaxation: Ca2+ pump of SR
Membrane potential
ICal current
Ca2+ transient
Cell shortening
 Excitation-contraction of cardiomyocyte
Action potential of cardiomyocyte

Ca2+ influx via L-type Ca2+ channel

Ryanodine receptor of SR

CICR,Ca2+ release of SR

Ca2+ binds to troponin

Myofilament sliding

Cardiocyte contraction
The characteristis of heart contraction
1. Dependence on [Ca2+]o
Source of sachoplasmic Ca2+:
ICal (Ca2+ influx) (10%)
CICR: Calcium-induced calcium release (Ca2+ influx stimulates
ryanodine receptor) (90%)
2. “All or none” contraction (全或无式收缩）
The atrium and ventricle are two functional syncytia (机能合胞
体）, just like two cells.
3. No complete tetanus (不产生完全强直收缩)
ERP is very long, the heart is non-responsive to any stimulus
during contraction and early relaxation.
Section 2
The pumping function of the heart
 Cardiac cycle （心动周期）
 The cardiac events that occur from the beginning of
one heartbeat to the beginning of the next are
called cardiac cycle.
 Each cycle is initiated by spontaneous generation of
an action potential in the sinus node.
 The conduction delay in the A-V node (>1/10sec)
allows the atria to contract ahead of the ventricles,
thereby pumping blood into the ventricles before
the strong ventricular contraction begins. Thus, the
atria act as primer pumps (初级泵） for the ventricles.
The characteristics of a cardiac cycle
 Diastole is longer than systole
 Heart rate mainly affects diastole
Time sequence of the events in a cardiac cycle
Atrial systole

Ventricular systole
Isovolumic contraction phase
Rapid ejection phase
Reduced ejection phase

Ventricular diastole
Isovolumic relaxation phase
Rapid filling phase
Reduced filling phase
Figure 10-3 Events of the cardiac cycle for LV function, showing
changes in LA pressure, LV pressure, aortic pressure, ventricular
volume, ECG, and the phonocardiogram.
 Function of atria as primer pumps
 Atrial contraction usually causes an additional
25% filling of the ventricles; 75% of the blood
flows directly through atria into the ventricles.
 The heart usually can continue to operate even the
atria stopping work unless a person exercises.
 Heart sound （心音）
 The first heart sound: represents the beginning of
ventricular contraction.
 The second heart sound: represents the beginning of
ventricular dilation.
 The third heart sound: heard at the end of ventricular rapid
filling, caused by the vibration of ventricular wall and
papillary muscle, found in some children, young people and
HF. Gallop rhythm (奔马律): all the above 3 sounds appear.
 The forth heart sound (atrial sound): heard before the first
heart sound, caused by atrial contraction and ventricular
filling, usually can not be heard, found by phonocardiogram
at the decrease of ventricular wall compliance.
 Cardiac murmur （心脏杂音）
 Systolic murmur
 Diastolic murmur
 Phonocardiogram （心音图）
Section 3
The evaluation of cardiac pumping function
 Stroke volume （每搏输出量）
 The blood output in each contraction of LV or RV,
about 70ml at resting condition in an healthy adult.
 Ejection fraction （射血分数，EF）
EF =
stroke volume (ml)
ventricular end diastolic volume (ml)
 100%
 Minute volume (cardiac output)
（每分输出量, 心输出量）
Cardiac output = stroke volume  heart rate
 Cardiac index （心脏指数）
Cardiac output (L)
CI =
Body surface area (m2)
=
5 - 6 (L)
1.6 - 1.7 (m2)
= 3.0 – 3.5 L/min/m2
CI: 10 years old: 4
normal adult ≥ 3
80 years old 2
 Stroke work （每搏功，搏功）
Stroke work (J) = stroke volume (L)  ejection pressure +
kinetic energy of blood flow
心脏作功量
心室收缩一次所做的机械功称为搏功
v2
v1
W=∫ PdV（压力-容积功 ） ＋∑1/2v2dm （动力功）
V2为舒张期末容量， V1为收缩期末容量
v为血流速度，m为射出血液的质量
，P为心室内压，V为心室容量
W=PV＋1/2mv2
P为主动脉平均血压，V 为每搏输出量， v为平均血流速度
剧烈运动时m和v显著增大，
动力功不能忽略
W=PV
心脏作功效率（外功/心脏耗氧量）
心脏消耗内能（主要通过有氧代谢）

外能  热（降低作功效率）


功  内功（非有用功： 等容收缩、心率过快等）

外功（和泵血直接有关的有用功）
容量功：内功所占比例低，效率高
压力功：内功所占比例高，效率低
如动脉压升高，心室扩大
Section 4
Cardiac Reserve
 The capacity of cardiac output of a healthy adult:
 At rest: 5 L/min
 Maximum: 25-30 L/min
 Maximum of a well-trained athlete: 35 L/min
 The components of cardiac reserve
 Stroke volume reserve:
At rest: 70 ml
Maximum: 105-110 ml (including end diastolic
volume reserve 25 ml, end systolic volume reserve
15-20 ml)
 Heart rate reserve
Maximal HR: 160-180 beats/min
Too fast HR will decrease the cardiac output
Section 5
Factors that affect cardiac pumping function
 Preload （前负荷）
 Cardiac preload can be reflected by the ventricular
end diastolic pressure, or, initial length (初长度）.
 Heterometric autoregulation (Frank-Starling
mechanism of the heart) (异长自身调节）：
The greater the heart muscle is stretched during filling, the
greater the force of contraction and the greater the quantity
of blood pumped into the aorta.
In other words, within the physiologic limits, the heart
pumps all the blood that comes to it without allowing
excessive damming of blood in the veins.
 Ventricular function curve reflects the relationship
between preload and pumping function.
The curves
do not have
downward
branch
 The mechanisms of Frank-Starling law:
(1) The effective overlapping degree of thick
filaments and thin filaments of the sarcomere;
(2) The affinity of troponin to Ca2+ depends on the
length of muscle.
 The preload is affected by venous retuning volume
and end systolic ventricular volume:
Four factors determine the venous retuning volume:
ventricular filling time
venous blood retuning speed
The pressure of pericardial cavity
The compliance of the ventricle
 Afterload （后负荷）
 Cardiac afterload can be reflected by the stretching
force of the ventricular wall at systole, or the
peripheral resistance (外周阻力）.
 An increase in afterload will decrease cardiac
output
 Cardiac contractility （心肌收缩能力）
 An intrinsic factor
 Affected by neurohormonal factors
 Independent of pre- or afterload
 Heart rate （心率）
正常心脏
心脏扩大
Role of atria in the pumping funtion of heart
1. As a tract of blood returning
Increased ventricular pressure
Dysfunction of atrial contraction
Decrease in atrial compliance
↓
Decrease in veinous blood retuning
↓
Increase in atrial pressure
2. As a primary pump (初级泵)
(主动充盈作用, 在心室被动充盈障碍时尤为重要)