heart cardiac cycle

The Cardiovascular
System
A circulation consisting of …
– A pump (the HEART)
– A conducting system (blood vessels)
– A fluid medium (blood)
• Is specialized fluid CT
• Contains cells suspended in a fluid
matrix

Important BASIC “RULES” of the CV system:
This is independent of the OXYGEN content of the blood i.e the amount of HbO2.
1. By convention, arteries carry blood AWAY from the heart and
VEINS towards it.

2. The normal pattern of blood flow through the
vasculature is ….
• From the HEART → TISSUES in
ARTERIES
• “CONDUCTIING VESSELS”
• Distribution of nutrients etc. in
CAPILLARIES
• “EXCHANGING VESSELS”
• From the TISSUES → HEART
in VEINS
• “CAPACITANCE VESSELS”
An EXCEPTION to this pattern is called a PORTAL CIRCULATION.

The HUMAN heart has 4 interconnected chambers.
3. BLOOD FLOW through those chambers is
UNIDIRECTIONAL.

The HEART VALVES ensure unidirectional flow.
2 AV valves:
- Lt. AV (bicuspid/mitral)
- Rt. AV (tricuspid)
2 Semilunar valves:
- aortic valve
- pulmonary valve

Thus, the normal adult pattern of blood flow
would be …

“DOWNSTREAM”
“UPSTREAM”
X
X

Right atrium
Right ventricle
Papillary muscle
Chordae tendineae
Septum
AV valve structure

When pressure is greater “UPSTREAM, it
opens.
When pressure is greater “DOWNSTREAM” , it
closes. It does not open in the opposite direction;
that is, it is a 0NE-WAY VALVE.
4. All heart valves are PASSIVE.

“DOWNSTREAM”
“UPSTREAM”
When pressure is greater
“UPSTREAM, the valve is
OPEN.
When pressure is greater
“DOWNSTREAM” , the valves
is CLOSED.
It does not open in the
opposite direction; that is, it
is a 0NE-WAY VALVE.

What must the pressure relationships between
chambers or between chambers and draining
vessels be, in each of the following diagrams?

4. In an adult, blood
CANNOT flow from
the right side of the
heart to the left side
without LEAVING the
heart.
The RIGHT
VENTRICLE pumps
blood through the
PULMONARY
CIRCUIT and the LEFT
VENTRICLE pumps
blood through the
SYSTEMIC CIRCUIT.

5. BLOOD “flows” through the vasculature DOWN a
PRESSURE GRADIENT created by the contraction of the
heart.
We call this pressure the BLOOD PRESSURE or more
precisely the MEAN ARTERIAL PRESSURE (MAP).

6. MAP is the REGULATED VARIABLE in
the CARDIAC REFLEXES.
The GOAL of the CV
system is to MAINTAIN
MAP, i.e. maintain what
causes the blood to
flow.

The HEART is located in the THORACIC CAVITY.
It is found in a non-delineated space MEDIAL to the
LUNGS called the MEDIASTINUM.

Its BASE is located SUPERIORLY and its APEX, INFERIORLY.
It is ROTATED counterclock-wise so as to lie on its RIGHT SIDE.
Base
Apex

The BASE of the
heart lies at the
level of the 2nd
intercostal space at
midline. The APEX
lies at the level of
the 5th intercostal
space about 2.5-3”
to the LEFT of
midline.

The heart lies
in the
PERICARDIAL
CAVITY,
surrounded by
the a SEROUS
MEMBRANE
called the
PERICARDIUM.

Serous
membranes
consist of TWO
epithelial
“membranes”
facing each other
i.e. are DOUBLE
layered and
contain a fluid
filled space.

A section of the heart showing its three layers: epicardium,
myocardium, and endocardium
Myocardium
Endocardium
Epicardium
Parietal Pericardium
Dense fibrous layer
Areolar tissue
Mesothelium
Mesothelium
Areolar tissue
Connective tissues
Pericardial space
(contains serous fluid)
Muscular wall of the heart
consisting primarily of
cardiac muscle cells
Areolar tissue
Covers the inner surfaces of
the heart
Endothelium
Covers the outer surface
of the heart; also called
the visceral pericardium
The serous membrane that
forms the outer wall of the
pericardial cavity; it and a
dense fibrous layer form the
pericardial sac surrounding
the heart
The OUTER membrane is called the PARIETAL
PERICARDIUM, the INNER membrane the VISCERAL
PERICARDIUM and the space between them the
PERICARDIAL SPACE.

The pericardium
STABILIZES the
position of the heart
and associated
vessels within the
mediastinum.
The SEROUS FLUID in
the pericardial space
LUBRICATES and
eliminates FRICTION
between the heart and
adjacent tissues as
the heart beats.

Externally, the CORONARY SULCUS
marks the separation between the atria
and ventricles.

Similarly, the ANTERIOR & POSTERIOR
INTERVENTRICULAR SULCI indicate the
location of the interventricular septum.

Commonly, significant
accumulations of WAT
(structural fat) are found in
these sulci, …
Mesothelium
(visceral pericardium)

… as are the
major cardiac
arteries and veins.

Most of the
heart consists
of CARDIAC
MUSCLE.
This muscular
component is called the
MYOCARDIUM. It is
made up of cardiac
muscle cells or
CARDIOMYOCYTES.

The
ORIENTATION
of the cardiac
muscle cells
means that the
contracts with
a “wringing”
motion from
APEX to BASE.

Fibrous CT (fibrous trigone) anchors the valves
and almost completely separates the atrial
myocardium from the ventricular myocardium.

The internal surfaces of the
heart are lined by a simple
squamous epithelium
called the ENDOCARDIUM.
This is continuous with the
ENDOTHELIUM that lines
the blood vessels.

A section of the heart showing its three layers: epicardium,
myocardium, and endocardium
Myocardium
Endocardium
Epicardium
Parietal Pericardium
Dense fibrous layer
Areolar tissue
Mesothelium
Mesothelium
Areolar tissue
Connective tissues
Pericardial cavity
(contains serous fluid)
Muscular wall of the heart
consisting primarily of
cardiac muscle cells
Areolar tissue
Covers the inner surfaces of
the heart
Endothelium
Covers the outer surface
of the heart; also called
the visceral pericardium
The serous membrane that
forms the outer wall of the
pericardial cavity; it and a
dense fibrous layer form the
pericardial sac surrounding
the heart
Thus, the point of a
pin entering the
pericardial cavity
from the outside ….

4. The heart relies completely on
AEROBIC RESPIRATION for its
energy, and is unable to pump
sufficiently in an ISCHEMIC
(oxygen-deprived) environment.
The heart, as a muscle, pumps
continuously throughout life and is
adapted to be highly resistant to fatigue.
2. Cardiomyocytes contain large
numbers of mitochondria, enabling
continuous aerobic respiration and
production of ATP.
3. Cardiac muscle also contains
MYOGLOBIN, an oxygen-storing
protein.
1. Cardiac muscle has an extensive
supply from the CORONARY
ARTERIES that provide nutrients
and oxygen.

The CORONARY CIRCULATION is the
circulation to and from the heart itself.
The largest
elements of this
circulation are
visible on the
surface of the
heart, deep to the
epicardium.

The CORONARY ARTERIES
are the initial branches of the
AORTA.
They originate practically
behind the cusps of the aortic
valve!

The heart is supplied with blood from the
LEFT & RIGHT CORONARY ARTERIES.

Branches of
these arteries
penetrate the
myocardium.

Consequently, only a small portion of the
cardiac vasculature is visible on the
surface of the heart.

Generally speaking, the left CA supplies
the LEFT side of the heart and the right
CA the RIGHT.

The CORONARY SINUS receives most of
the venous return from the heart.

In a cardiac bypass, blood flow is restored by “detouring”
around a blockage in a coronary artery.

In the fetal circulation,
blood is pumped by
the heart TO the
TISSUES and FROM
the TISSUES back to
the heart.
However, the
PULMONARY circuit is
essentially inoperative
since …..
THE FETAL CIRCULATION

…..the fetus obtains oxygen and nutrients from the mother
through the PLACENTA via the UMBILICAL CORD.
There is no direct contact between fetal and maternal blood.

Umbilical
cord
An UMBILICAL/PLACENTAL
VEIN delivers oxygenated
blood TO the fetus and 2
UMBILICAL ARTERIES return
venous blood FROM the fetus
to the placenta.

On entering the fetus, the
umbilical vein
BYPASSES the fetal liver
via the DUCTUS
VENOSUS.
It then joins the fetal
INFERIOR VENA CAVA.
Thus the oxygenated
blood from the placenta
MIXES with fetal venous,
deoxygenated blood.

This is the reason that
the higher affinity of
HbF for oxygen is
advantagous!
PO2 umbilical artery = 35 mm Hg
PO2 adult arterial blood = 100 mm Hg

This “mixed” blood
enters the fetal heart
via the RIGHT
ATRIUM.
A small portion of it
passes to the RIGHT
VENTRICLE and
completes the
PULMONARY
CIRCUIT, re-entering
the heart via the
LEFT ATRIUM.

Alternatively, rather than
flowing to the LUNGS via
the PULMONARY
TRUNK, blood may flow
DIRECTLY into the fetal
AORTA via the DUCTUS
ARTERIOSUS.

….. OR, blood can
pass directly from the
RIGHT ATRIUM to the
LEFT ATRIUM via the
FORAMEN OVALE,
located in the
interatrial septum.

However it gets
there, blood
from the
AORTA is
distributed to
the fetal
tissues.
It RETURNS to
the PLACENTA
via the 2
UMBILICAL
ARTERIES.

Remnants
of these
fetal
structures
remain in
the adult.

Most of the
heart consists
of CARDIAC
MUSCLE.
This muscular
component is
called the
MYOCARDIUM.

Striated
muscle
Unstriated
muscle
Skeletal
muscle
Cardiac
muscle
Smooth
muscle
Voluntary
muscle
Involuntary
muscle
ANATOMICALLY, cardiac muscle is like skeletal muscle.
PHYSIOLOGICALLY, it is smooth muscle.

Cardiac muscle is STRIATED.
• Contains SARCOMERES
• Contains ACTIN, MYOSIN
and TROPONIN
• Contraction explained by
the SLIDING FILAMENT
MODEL
• Excitation/contraction
coupling via Ca++

Cardiac muscle cells are interconnected via
INTERCALATED DISCS.

GAP JUNCTIONS located
in the INTERCALATED
DISCS cause the
myocardium to function
as a SYNCYTIUM.

Cardiac muscle cells are INVOLUNTARY, i.e.
their CONTRACTION is regulated ….
• INTRINSICALLY by PACEMAKER
activity

Cardiac muscle cells are INVOLUNTARY, i.e.
their CONTRACTION is regulated ….
• EXTRINSICALLY by the ANS and
ENDOCRINE SYSTEMS

Cardiac muscle is an EXCITABLE tissue!!!
It consists of 2
populations of
excitable cells –
-Contractile
cardiomyocytes
-Pacemaker
cardiomyocytes

Contractile cells
make up the bulk of
the myocardium.
They are specialized
for contraction.

Pacemaker cells are contained
within the INTRINSIC
CONDUCTION SYSTEM.
They are specialized for
conduction, and are responsible
for the COORDINATED contraction
of the contractile cells.

PACEMAKER action potentials initiate the action potentials
in CONTRACTILE cells, which result in their
CONTRACTION.
They are thus referred to as PACEMAKER POTENTIALS.

Since the pacemaker
cardiomyocytes are linked
by GAP JUNCTIONS to the
contractile
cardiomyocytes …
…when the pacemaker
cells depolarize, so do
the contractile cells, AT
THE SAME TIME AND
RATE!
Thus the INHERENT pacemaker activity of these
cardiomyocytes determines the INTRINSIC CONTRACTILE
RATE OF THE HEART!

2. AT ANY MOMENT IN TIME the membrane potential is produced
by the WEIGHTED AVERAGE of all the ions in DISEQUILIBRIUM to
which it is PERMEABLE.
1. PACEMAKER CELLS have an inherently UNSTABLE membrane
potential that DEPOLARIZES over time.
This is called a PACEMAKER or PREPOTENTIAL.
The pacemaker potential gradually
becomes less negative until it reaches
THRESHOLD and triggers an ACTION
POTENTIAL.

3. DURING THE PREPOTENTIAL the membrane’s permeability to
THREE IONS- Na+, K+ and Ca++ - changes AND SO DOES THE
VALUE OF THE MEMBRANE POTENTIAL.
1. K+ permeability DECREASES throughout.
2. Na+ permeability INCREASES slightly.
NET EFFECT: the membrane potential “drifts” towards a more positive value.

3. As membrane potential approaches -50 mV, Ca++ (T) channels open.
4. As membrane potential approaches -40 mV Ca++ (L) channels open.
This brings the membrane to threshold. The L-type channels open
EXPLOSIVELY and the membrane rapidly DEPOLARIZES.

N.B. UNLIKE neurons,
depolarization in
pacemaker
cardiomyocytes is
due to an influx of
Ca++ NOT Na+!

4. At maximum depolarization, the L-type channels
CLOSE and the K+ channels OPEN, initiating
REPOLARIZATION.

The pacemaker cells are arranged in an
interconnected pathway called the INTRINSIC
CONDUCTION SYSTEM.
SA node

Each population of pacemaker cells in the intrinsic
conduction system has its own inherent rate of
depolarization.
• SA node – 100 times/minute
• AV node – 40-60 times/minute
• Purkinje fibers – 30-40 times/minute

The fastest depolarizing element in the conduction
becomes THE pacemaker for the entire myocardium.
The pacemaker activity
of the slower
depolarizing
elements is inhibited.
This is called
OVERDRIVE
SUPPRESSION.

If the fastest group of cells is inhibited, the
NEXT fastest group becomes THE pacemaker!

If the “linkage” between the SA node and the
pacemaker cells in the ventricles is broken,
atria and ventricular myocardia beat
independently.
This is called a HEART BLOCK.

An ECTOPIC FOCUS is a group of cells that
transiently depolarizes more rapidly than the
normal pacemaker.
In a normal, healthy heart this is usually SELF-LIMITING.
Causes include: fatigue, caffeine, ANS irregularities

Transmission of the
cardiac impulse through
the heart, showing the
time of appearance (in
fractions of a second
after initial appearance at
the sinoatrial node) in
different parts of the
heart.
© 2005 Elsevier
Although all regions of the
myocardium are functionally united
by this system….

… passage of the beat through the fibrous
trigone DELAYS transmission of the beat by
about 200 msec.
Thus the atria depolarize and contract about 200
msec. BEFORE the ventricular myocardium.

Pacemaker cell
Contractile cell
Action potentials in CONTRACTILE cardiomyocytes is
characterized by a prolonged PLATEAU PHASE.

Contractile cardiomyocyte
• Prolonged Ca++ INFLUX
following depolarization
produces a broad
PLATEAU phase.
• These voltage-gated
channels are termed L-
type calcium channels
(L for Long lasting)

Action potentials & contraction in skeletal muscle

The difference in
duration of the
action potential
(10 msec.) and
contraction (100
msec.) in
skeletal muscle
permits
SUMMATION,
TETANUS and
eventual
FATIGUE.

Action potentials & contraction in contractile
cardiomyocytes
In cardiac
muscle the
duration of the
action potential
extends
through most
of the
contraction.

Consequently, SUMMATION and its
consequences cannot occur in cardiac muscle.

Of course, all this electrical activity IS
measurable.
The ECG (EKG)
represents the
SUMMATION of
all of the
electrical
activity
associated with
one
“heartbeat”.

Electrodes placed on the
skin detect and amplify
the minute electrical
activity occurring in the
heart.

– P wave
• atrial depolarization
– QRS complex
• vent. depolarization
– T wave
• vent. repolarization
– PR interval
• AV nodal delay
– QT segment
• ventricular systole
– TQ interval
• ventricular diastole
A normal ECG (EKG)

The Cardiac Cycle:
Systole and diastole

The Cardiac Cycle
Heart rate = 72 bpm
- 72 cardiac cycles/minute
- each cardiac cycle = 0.83 seconds
- or about 800 msec.

• ELECTRICAL ACTIVITY precedes CONTRACTION.
• Atrial SYTOLE lasts about 100 msec.
• Ventricular SYSTOLE lasts about 300 msec.
• At this rate of contraction, the ENTIRE heart is in
SIMULTANEOUS DIASTOLE for the last half of the cycle.

A CARDIAC CYCLE consists of one set of
atrial and ventricular systoles and diastoles.
Since both “sides” of the heart and doing the same thing at the same
time, we can deal with a “half heart”.
By convention, this is the LEFT side or SYSTEMIC PUMP.
By convention, one assumes a normal heart “at rest”.

Under these circumstances the HEAERT RATE is about 72
beat/min (bpm).
- 72 cardiac cycles/minute
- each cardiac cycle = 0.83 seconds
- or about 800 msec.
• ELECTRICAL ACTIVITY precedes CONTRACTION.
• Atrial SYTOLE lasts about 100 msec.
• Ventricular SYSTOLE lasts about 300 msec.
• At this rate of contraction, the ENTIRE heart is in SIMULTANEOUS DIASTOLE
for the last half of the cycle.

The cardiac cycle begins with VENTRICULAR FILLING.
What is the valvular configuration?

Because the pressure in the atrium
is greater than the pressure in the
ventricle, the AV valves are OPEN.
Because the pressure in the aorta is
greater than the pressure in the
ventricle, the aortic valve is CLOSED.

The RATE of ventricular filling is
PASSIVE and NON-UNIFORM.
Initial filling is rapid and then slows.

At the end of
ATRIAL
DIASTOLE, the
SA node fires and
the atria
depolarize and
CONTRACT.

Right
atrium
Right
ventricle
Passive
Left
ventricle
Left
atrium
Atrial contractionVentricular filling
A B
Atrial contraction

This adds a small, additional amount of
blood to the ventricle (~ 15 mls).
This is the END DIASTOLIC VOLUME (EDV)
EDV = volume of blood in ventricle at end of
(ATRIAL) diastole (~ 130 mls)

About 100 msec.
later, the
VENTRICLES
depolarize and
CONTRACT.

The contraction of the ventricles raises
the ventricular pressure ABOVE the
atrial pressure, and the AV VALVES
CLOSE.

The 1st HEART SOUNDS occur as
the AV VALVES close.

However, the pressure in the
ventricles is still LOWER than
that in the aorta, so the aortic
valves remain closed.
ISOVOLUMETRIC VENTRICULAR
CONTRACTION

As the pressure in the ventricles exceeds
that in the aorta, the AORTIC VALVE OPENS
and blood enters the aorta.
VENTRICULAR EJECTION
Since the pressure in the ventricle is
still greater than the pressure in the
atrium, the AV valve remains closed.

The volume of blood remaining in the
ventricle AFTER ejection i.e. after
ventricular systole is called the
END SYSTOLIC VOLUME.
The volume of blood ejected into the
aorta is the STROKE VOLUME.

– EDV = volume of blood in ventricle at end of
diastole (~ 130 mls)
– ESV = volume of blood in ventricle at end of systole
(~ 65 mls)
– SV = volume of blood ejected from heart each cycle
SV = EDV – ESV

If the EDV = 135 mls and the SV = 65 mls, then the
EJECTION FRACTION = 65/135 = 48%
This can also be expressed as a fraction
(the EJECTION FRACTION).

As the ventricles
begin to relax into
diastole, the
pressure falls
BELOW the
pressure in the
aorta and the
AORTIC VALVES
CLOSE.

The 2nd HEART SOUNDS occur as
the SEMILUNAR VALVES close.

The pressure in the ventricles
is still greater than the
pressure in the atria, so the
AV valves still remain
CLOSED.
The heart is thus in
simultaneous diastole with ALL
VALVES CLOSED.
ISOVOLUMETRIC
VENTRICULAR RELAXATION

As the pressure in the
ventricles rapidly declines
during diastole, it falls BELOW
that in the ATRIA and the AV
VALVES OPEN.
Since the pressure in the atria is
BELOW that in the aorta, the aortic
valve remains closed.
VENTRICULAR FILLING begins as
the CARDIAC CYCLE repeats itself.

Fetal Hb, with its
HIGHER AFFINITY
O2, compensates
for the lower PO2.

Cardiac Muscle Cells/cardiomyocytes
• Characteristics of
Cardiac Muscle Cells
– Small size
– Single, central
nucleus
– Branching
interconnections
between cells

Orientation of contractile cardiac fibers
Cardiomyocytes spiral
superiorly, from the
apex to the base.

The PULMONARY circulation is a low pressure system, the
SYSTEMIC circulation a high pressure system.
Whether WITHIN the heart and in BOTH the SYSTEMIC and
PULMONARY circulations, BLOOD ALWAYS FLOWS DOWN
A PRESSURE GRADIENT.

heart cardiac cycle

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (20)

Similaire à heart cardiac cycle

Similaire à heart cardiac cycle (20)

Dernier

Dernier (20)

heart cardiac cycle