Chapter 10 The heart as a pump

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), cont

raction 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 calle

d 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) all

ows the atria to contract ahead of the ventricles, the

reby pumping blood into the ventricles before the st

rong 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 d

irectly 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 ventricul

ar contraction.

The second heart sound: represents the beginning of ventric

ular dilation.

The third heart sound: heard at the end of ventricular rapid

filling, caused by the vibration of ventricular wall and papill

ary 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 fil

ling, usually can not be heard, found by phonocardiogram a

t 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 ）

Minute volume (cardiac output)　（每分输出量 , 心输出量） Cardiac output = stroke volume heart rate

stroke volume (ml)

ventricular end diastolic volume (ml) 100%EF =

Cardiac index （心脏指数）

Cardiac output (L)

Body surface area (m2)CI =

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 ， P 为心室内压， V 为心室容量 v 为血流速度， m 为射出血液的质量

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

v2v1

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 mech

anism 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 le

ngth 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 )