Download Cardiac Muscle Mechanics

PHYSIOLOGY 1
LECTURE 24
CARDIAC MUSCLE
MECHANICS
CARDIAC MUSCLE
MECHANICS

Cardiac Muscle is unique in several aspects
from both skeletal and smooth muscle. First
of all cardiac muscle operates on the left
hand ascending limb of the active tension
curve so that stretching cardiac myocytes
results in increased force of contraction
(Frank-Starling law of the heart). Second due
to the low concentration of Ca++ ion muscle
contraction can be enhanced in both strength
and speed by increased Ca++ influx
(Contractility or Inotrophy).
CARDIAC MUSCLE MECHANICS
The isometric and isotonic condition

Cardiac muscle goes through the same
series of isometric and isotonic events
as skeletal muscle. Therefore, the
cardiac cycle (Heart beat) has two
phases which are considered to
isometric (Isovolumic contraction, and
Isovolumic relaxation) while all the
other phases are considered to be
isotonic in nature.
CARDIAC MUSCLE MECHANICS
The isometric and isotonic condition

Active tension curve- Due to the heavy
amounts of connective tissue the heart’s
sarcomere length is shorter than that of
skeletal muscle (1.6 mm vrs. 2.2 mm).
This means that the heart operates on
the left hand ascending limb of it’s
active tension curve. Note - There is no
right hand descending limb to the
cardiac tension curve for this reason.
CARDIAC MUSCLE MECHANICS
Review of Skeletal muscle Tension

In skeletal muscle the
length tension
relationship is controlled
by the split into the
contractile element
(active tension) plus the
series and parallel
elastic elements
(passive tension) which
equals to total tension.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension

In cardiac muscle the
resting sarcomere
length is 1.6 mm due to
the large amount of
connective tissue. This
places the resting
sarcomere length on
the left hand ascending
limb of the active
tension curve.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension

The passive tension
curve in cardiac
muscle begins with
the active tension
curve and increases
rapidly eliminating
the right hand
descending limb of
the active tension
curve.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension

The cardiac muscle total
tension curve then is
nearly a straight line
steadily increasing.
This means that
stretching the cardiac
muscle sarcomeres
results in an increased
force and speed of
contraction (FrankStarling law of the
heart.
CARDIAC MUSCLE MECHANICS
Cardiac muscle Length-Tension


Frank-Starling Law
of the Heart
The heart pumps
whatever is
presented to it
(Venous return).
Stretch of the
cardiac sarcomeres
causes an increase
in force and speed
of contraction.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

There are two types of load cardiac muscle is
presented with. First is afterload or the load
the heart must pump against in this case it is
aortic artery pressure or pulmonary artery
pressure. In order for the semilunar valves to
open the ventricles of the heart must
generate greater pressures than the afterload.
In the second case the heart must move
blood or preload, we will be calling this
venous return.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

The isotonic
condition -
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
B. Affect of changing afterload (Aortic
Pressure or Diastolic Pressure) constant preload
 The pressure in the left ventricle must
exceed aortic pressure in order to open
the aortic semilunar valve. Therefore,
as aortic pressure rises, such as in
hypertension, then the heart must work
harder to overcome the afterload.

CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

In general the
cardiac muscle load
to velocity of
shortening curve is
similar to the
skeletal muscle
curve.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

As afterload on the
heart increases the
velocity of
shortening
decreases.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship
C. Affect of changing preload constant afterload
 Preload on the heart is the venous
return. As venous return increases
stretching the sarcomeres, which in turn
by Frank-Starling the heart increases
force of contraction. In effect this shifts
the velocity of shortening to load curve
to the right increasing the speed of

CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

As preload is
increased the
velocity of
shortening to load
curve is shifted to
the right. FrankStarling Law of the
heart. Hence,
increased velocity of
contraction for any
given load.
CARDIAC MUSCLE MECHANICS
Cardiac Contractility

D. Concept of contractility or inotrophy

Contractility or inotrophy is an increase in
cytosolic calcium concentration. This
phenomenon is under the control of the
sympathetic nervous system. As NE is
released it activates both the b1 and a1
receptors which activate both cAMP and IP3
second messengers which cause
phosphorylation of calcium channels and
thus, increased Ca++ influx. Therefore,
more Troponin is activated - > force & Vel.
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship


Concept of
Contractility The heart can also
increase both the
force and velocity of
contraction by
increasing it’s
efficiency through
sympathetic
stimulation.
(catacholamines)
CARDIAC MUSCLE MECHANICS
Cardiac Load-Velocity Relationship

An increase in sympathetic stimulation
of the heart increases cytosolic Ca++
ion concentration thereby, increasing
both the force and velocity of shorting
of cardiac muscle. This makes the
heart more efficient and increases
performance, but also increases cardiac
work.
CARDIAC MUSCLE MECHANICS
Cardiac Pressure-Volume Loops

Pressure volume loops are an excellent
way of illustrating cardiac performance
through out the cardiac cycle. They can
be used to illustrate contractility,
afterload, and preload effects on
cardiac performance. The drawback is
that they can only be utilized on one
side of the heart therefore, the
following discussion involves only the
left ventricle but the right is similar.