Cardiac Physiology: Contraction and Systolic Function

The cardiac cycle is divided into two parts:

1.) diastole

2.) systole

Diastole

period when ventricles fill

Systole

period when ventricles eject (pumping)

___________ is the relaxation phase and __________ is the contraction phase

Diastole; systole

Stroke volume

The amount of blood ejected from the heart in one contraction

Cardiac output

volume of blood, in liters, pumped by the left ventricle each minute

Cardiac index

cardiac output divided by body size/surface area

Four cardiac valves

1.) mitral valve

2.) tricuspid valve

3.) aortic valve

4.) pulmonary valve

What happens to the mitral and tricupsid valves during systole? Why?

they close to prevent backflow into the atria

What happens to the mitral and tricupsid valves during diastole? Why?

they open to allow the atria to collect with blood

What happens to the aortic and pulmonary valves during systole? Why?

they open because ventricular pressure becomes higher than pressure in the arteries

What happens to the aortic and pulmonary valves during diastole? Why?

they close to preserve blood pressure within the great vessels

Function of atria

act as reservoirs for ventricular filling

Atria fill effectively at very ________ venous pressures

low

Filling of atria occurs largely during...

ventricular systole at which time the atrium acts as a reservoir

During diastole, the ____________ valves are open and the atria __________ collect blood

atrioventricular; do not

At the end of diastole, atria...

contract, acting as pumps to help fill the ventricles

Function of ventricles

develop pressure needed to expel blood into the great vessels

The right ventricle ejects blood into the....

pulmonary circuit

The left ventricle ejects blood into...

the systemic circuit

Is arterial pressure higher for the pulmonary or systemic circuit?

systemic circuit

The pulmonary circuit has _________ systolic and diastolic pressures and __________ vascular resistance

The systemic circuit has _________ systolic and diastolic pressures and __________ vascular resistance

lower; lower

higher; higher

Four phases of the cardiac cycle:

1.) isovolumic contraction

2.) ejection

3.) isovolumic relaxation

4.) filling

Two systolic phases of the cardiac cycle

1.) isovolumic contraction

2.) ejection

Two diastolic phases of the cardiac cycle

1.) isovolumic relaxation

2.) filling

cardiac cycle: isovolumic contraction

initial phase of ventricular contraction in which pressure in the ventricle increases, but no blood is pumped or ejected from the heart

During isovolumic contraction, all valves are _______

closed

cardiac cycle: ejection

aortic valve opens and blood is ejected from the left ventricle through aorta to supply the body

What causes left ventricular ejection to occur?

when left ventricular pressure is greater than aortic pressure

Once ventricular ejection is complete, the pressure in the ventricle _________

decreases

Once pressure in the left ventricle becomes less than that of the aorta, the ______ valve ________

aortic; closes

cardiac cycle: isovolumic relaxation

All valves are closed and pressure in the ventricle decreases due to relaxation of myocytes

cardiac cycle: filling

Mitral valve opens and blood rushes into the left ventricle from the left atrium

What causes ventricular filling to occur?

pressure in the left ventricle becomes less than that of the left atria

Two parts of ventricular filling

1.) passive

2.) active

passive filling

period during diastole in which blood flows through the atria into the ventricles under relatively low pressure

During passive filling, the atria are not __________

contracting

active filling

atria contracts and squeezes remaining blood into the ventricles

What causes active filling to occur?

SA node firing

Myocardial contraction

process by which the heart muscle (myocardium) shortens and generates force to pump blood

Myocardial contraction is aka...

systole

Systole is associated with...

muscle tension and pumping of blood into the systemic circulation

Four essential components of myocardial contraction:

1.) actin

2.) myosin

3.) ATP

4.) calcium

Functional unit of the myocardium is called...

the sarcomere

Z disc

provides anchorage for actin filaments; separates each sarcomere

A band

where actin and myosin overlap

What triggers myocardial contraction to begin?

an electrical impulse at the SA node

Where does the electrical impulse from the SA node travel?

spreads through the atria, to the AV node, to the ventricles

What happens when the electrical impulse reaches sarcolemma of myocardial cells?

L-type Ca channels open and release small amounts Ca into the extracellular space

The small amount of Ca being released then causes...

a massive Ca release from the sarcoplasmic reticulum

Calcium-induced calcium release

Process in which Ca2 entry into a muscle fiber triggers release of additional Ca2+ from the sarcoplasmic reticulum

Once Ca is released from the sarcoplasmic reticulum, it targets...

troponin C complex on actin

Once Ca binds to troponin C, this...

releases troponin C from actin and frees up the myosin binding site to allow the power stroke to occur

In order for relaxation to occur, what must be removed?

Calcium

Calcium is removed in two main ways:

1.) SERCA

2.) Na/Ca pump

SERCA

Ca ATPase pump of the sarcoplasmic reticulum; pumps Ca back into SR

Na/Ca pump

removes calcium out of the muscle cell in exchange for Na entering the cell

Steps of cardiomyocyte contraction

1.) Cell depolarizes

2.) Voltage change opens the long lasting Ca channels

3.) Extracellular Ca enters the cytoplasm via L-type Ca channels

4.) Small amount of Ca releases more Ca from the sarcoplasmic reticulum via ryanodine receptor

5.) Calcium binds to troponin C and troponin I is removed

6.) ATP is hydrolyzed

7.) Myosin head changes conformation and binds to actin to achieve the power stroke

8.) Sarcomere tenses and then shortens

9.) Ca leaves the cell via the SERCA pump and Na/Ca pump

10.) myocyte relaxes

Crossbridge cycle

Repeated attachment and detachment of myosin to actin triggered by arrival of Ca

Why does your heart beat faster during exercise and stress?

more myosin filaments attach to actin to produce a more powerful contraction

Myocardial contractility is aka...

inotropy

inotropy

inherent property of the muscle determining the rate (velocity) and magnitude (tension/force) of fiber shortening

inotropy is the ________ of contraction

strength

Contraction is an active process that requires ________

ATP

Contractility is an _________ characteristic of each cardiomyocyte

individual

Since contractility relates to each individual cardiomyocyte, this makes it is impossible to....

directly measure

Preload

end-diastolic myofiber stretch related to end-diastolic maximum filling volume

Afterload

all hemodynamic forces opposing ejection of blood (opposing contraction)

Four things that increase inotropy

1.) increased Ca

2.) increased preload

3.) increased heart rate

4.) increased sympathetic tone

Two things that decrease inotropy

1.) increased after load

2.) increased vagal tone

Increased Ca will increase the developed __________ in the heart

tension

*even when there is no preload in the heart, increase in Ca will increase contractility!

In the resting state, Ca concentration in the cytosol during systole is such that _______ of the contractile sites will be activated

half

However, during exercise, __________ stimulation occurs which __________ Ca release

sympathetic; increases

Two main factors that control myocardial contraction:

1.) Ca availability

2.) autonomic tone

Sympathetic nervous system effect on the heart

Increases heart rate and contractility

Parasympathetic nervous system effect on the heart

decreases heart rate and contractility

Two sympathetic nervous system signaling molecules

1.) norepinephrine

2.) epinephrine

Receptor for norepinephrine and epinephrine

B1 receptor

One parasympathetic nervous system signaling molecule

acetylcholine

Receptor for acetylcholine

Muscarinic M2 receptor

Two things autonomic tone can influence:

1.) inotropy

2.) chronotropy

chronotropy

heart rate

Increased sympathetic tone will ________ inotropy and chronotropy

Increased parasympathetic tone will ________ inotropy and chronotropy

increase

decrease

Preload

degree of stretch of the cardiac muscle fibers at the end of diastole

Preload is the stretch of cardiac muscle cells during...

ventricular filling

Preload is directly related to the Z-disc _________

length

Why does an increase preload increase contractility (2)?

1.) more stretch means more distance between the Z discs, meaning more opportunity for actin and myosin crossbridges to form

2.) when the muscle is stretched, troponin becomes more sensitive to Ca

Frank Starling Law

The greater the myocardial stretch, the stronger the heart's contraction, and greater cardiac output

Afterload

all hemodynamic forces opposing ejection of blood (opposing contraction)

The higher the after load, the higher the ________ that must develop _______ to sarcomere shortening

tension; prior

If afterload is high, contraction will be ________, and the distance of sarcomere shortening will be ________

short; low

Afterload relates to...

peak wall tension prior to ejection

Peak wall tension

highest tension in the ventricular wall during ejection

Peak wall tension occurs...

just before the onset of ventricular ejection (during isovolumic contraction)

Peak wall tension can be calculated using...

LaPlace's Law

LaPlace's Law

peak wall tension = pressure x ventricular radius / 2(ventricular wall thickness)

Three things that increase peak wall tension

1.) increased pressure

2.) increased ventricular radius

3.) decreased ventricular wall thickness

Greater pressure in the ventricle is related to ___________ peak wall tension

greater

Increased frequency of stimulation of cardiomyocytes ________ contractile force

increases

Bowditch-Treppe effect

increasing heart rate increases the strength of myocardial contraction