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Mammalian Heart Structure
The heart is the major
organ of the
circulatory system
It is a fist-sized muscular
pump consisting of
four chambers
Superior
Vena
Cava
AORTA
The left side of the
heart pumps oxygenated
blood out into
the body’s arteries via
the aorta
The human heart
recirculates the entire
blood volume (5 dm3)
every minute when
the body is at rest
Pulmonary
Artery
The ability of the
heart to perform such
work is due to the
presence of specialised
cardiac muscle in
its walls
The job of the heart is to
pump blood around
two separate circuits
The left side of the
heart receives oxygenated
blood from the lungs via the
pulmonary veins
Coronary
Arteries
Inferior
Vena
Cava
Deoxygenated blood
returns to the right side
of the heart via the
vena cava
Deoxygenated blood
is pumped to the
lungs via the
pulmonary artery
Heart muscle receives
its own supply of
blood from the
coronary arteries
Mammalian Heart Structure
Aorta
Venae cavae
Semilunar valves
Pulmonary artery
Pulmonary veins
Left atrium
Right atrium
Bicuspid valve
Tricuspid valve
Right ventricle
Septum
(dividing wall)
Left ventricle
Cardiac muscle
Cardiac Muscle
Cardiac muscle tissue consists of branched, faintly striated fibres (cells), each of
which contains a centrally placed nucleus
Thickenings of
the plasma
membrane
between
individual
cardiac muscle
cells form
intercalated
discs
Cardiac muscle tissue forms networks due to its branching structure
When one muscle fibre is electrically excited, an impulse is quickly
transmitted to neighbouring fibres such that impulses spread
in all directions when cardiac muscle is stimulated
This photomicrograph of cardiac muscle tissue shows the branching nature
of the fibres, the intercalated discs and the nuclei
Nuclei
Intercalated
disc
Mammalian Heart Structure
The mammalian heart is
a muscular pump that consists
of four chambers
Two upper chambers, called the
atria, are thin walled cavities that
receive blood from veins
Two lower chambers, called the
ventricles, are thick walled cavities
that receive blood from the atria and
pump blood away from the heart
Right
The cavity of the heart is
atrium
divided completely by a
partition called the septum
Left
atrium
The muscular walls of the
heart are referred to as the
myometrium and consist of
specialised cardiac muscle cells Right
ventricle
The thicker walled structure of the left
ventricle is a consequence of the distance
over which it is required to pump blood
Left
ventricle
Septum
The direction of blood flow through the heart is maintained be valves
Between the right atrium and the right
ventricle is the tricuspid valve
This valve prevents the backflow
of blood from the right ventricle
to the right atrium
Aorta
Pulmonary
Artery
Between the left atrium and the left
ventricle is the bicuspid valve or
mitral valve
This valve prevents the
backflow of blood from the
left ventricle to the left
atrium
Right
atrium
Tricuspid
valve
The bicuspid and tricuspid
valves are collectively
known as the atrio-ventricular Right
ventricle
valves or AV valves
Pocket-shapes valves known as semilunar valves are
located at the base of the arteries responsible for
transporting blood away from the heart
Left
atrium
Semilunar
valves
Bicuspid
or Mitral
valve
Left
ventricle
Heart Valves
Pulmonary semilunar valve
Aortic semilunar valve
Bicuspid or mitral
valve
Tricuspid valve
Thicker-walled
left ventricle
Thinner-walled
right ventricle
HEART VALVES VIEWED FROM ABOVE
The Cardiac Cycle
The average human heart rate at rest is 72 beats a minute
Each heart beat lasts for approximately 0.8 seconds at rest
The sequence of events taking place during one complete heartbeat is called the
cardiac cycle
A single heartbeat is divided into two major phases known as systole and diastole
Systole describes periods when the heart is contracting and Diastole describes
periods when the heart is relaxing
The Cardiac Cycle
The average human heart rate at rest is 72 beats a minute
Each heart beat lasts for approximately 0.8 seconds at rest
The sequence of events taking place during one complete heartbeat is called the
cardiac cycle
A single heartbeat is divided into two major phases known as systole and diastole
Systole describes periods when the heart is contracting and Diastole describes
periods when the heart is relaxing
The Cardiac Cycle
Since the heartbeat is a cycle of events, there is no absolute first or last phase
This account begins with the
phase of late diastole when
both the atria and the
ventricles of the heart
are relaxed
During late diastole, all
chambers of the heart
are relaxed with the
atrio-ventricular valves
(AV valves) open and
the semi-lunar valves
closed
Blood flows passively
from the atria, through
the open AV valves, into
the ventricles that stretch
to accommodate the
extra volume of blood
This phase of the heartbeat is known as
passive filling of the ventricles
Passive filling of the ventricles
continues until the ventricles
have filled to about 70% of
their capacity to hold blood
The increasing volume of
blood in the ventricles
presses against the AV
valves and they begin to
drift towards a
closed position
At this point both of the
atria contract, rapidly
propelling blood into the
ventricles which stretch
to accommodate their
full capacity for blood
This phase of the heartbeat
is known as
atrial systole
The Cardiac Cycle
At the end of atrial
systole, the volume of
blood in the ventricles
is such that the AV
valves are forced closed
There is now a situation
where the AV and the
semilunar valves are
both closed, the atria
are relaxed and the
ventricles enter a
phase of contraction
or systole
AV valves
shut
When the rising pressure
exceeds that in the aorta
and pulmonary arteries,
the semilunar valves are
forced open and blood is
ejected from the heart
The Cardiac Cycle
As the ventricles relax, closure of the semilunar valves occurs due to a
brief backflow of blood from the aorta and pulmonary arteries
The pressures in the
ventricles continue to fall
and reach very low values
When the pressures
in the ventricles fall
below those of the
atria, the AV valves open
This phase of the heartbeat
is known as
ventricular
relaxation
(Early Diastole)
The Cardiac Cycle
As the ventricles relax, closure of the semilunar valves occurs due to a
brief backflow of blood from the aorta and pulmonary arteries
The pressures in the
ventricles continue to fall
and reach very low values
This phase of the heartbeat
is known as
ventricular
relaxation
(Early Diastole)
When the pressures
in the ventricles fall
below those of the
atria, the AV valves open
Passive filling of the ventricles (late diastole) occurs
as the cycle begins again
Late Diastole &
Atrial Systole
Ventricular Systole
(Isometric Phase)
AV valves close
at the end of
atrial systole
AV valves open,
semi-lunar valves
closed
Passive filling
of the ventricles
followed by
atrial systole
All valves closed
as the ventricle
muscles contract
without shortening
(Isometric Contraction)
Pressure builds
up in the
ventricles
Ventricular
Relaxation
Semi-lunar valves
close as the ventricles
begin to relax
Pressure in the
ventricles falls
to a very low
value and the
AV valves open
Ventricular Systole
(Ejection)
Semi-lunar valves
open and blood
is ejected into
the aorta and
pulmonary artery
Muscles shorten
as they contract