The heart is a four-chambered muscular organ located in the chest. It has two atria that receive blood and two ventricles that pump blood. The heart pumps blood through two circuits - the pulmonary circulation and systemic circulation. It is regulated by the sinoatrial node which initiates heartbeats, and signals are conducted through the heart by specialized cardiac tissue to coordinate contractions of the atria and ventricles. The heart pumps around 5 liters of blood per minute through the body to supply oxygen and nutrients to tissues and remove carbon dioxide and waste.

1. Dr. K. Rama Rao
Department of Zoology
Phone: 9010705687
Structure and Function of heart
Dr. K. Rama Rao
Department of Zoology
Phone: 9010705687

2. The heart is located on the left side
of the chest, beneath the
breastbone. The heart is composed
of smooth muscle.
It has four chambers which
contract in a specific order,
allowing the human heart to pump
blood from the body to the lungs
and back again with high efficiency.

3. External Structure:
Human heart is four chambered, consisting
of two atria and two ventricles.
(i) Grooves (Sulci):
The left and right atria are separated
externally by a groove. The atria are
demarcated externally from the ventricles by
an oblique groove called atrioventricular
sulcus. These have coronary arteries,
through which the heart receives blood.

4. (ii) Atria (sing, atrium):
The left atrium is smaller than the right
atrium. The right atrium is a roughly
quadrangular chamber.
The superior vena cava, inferior vena cava
and coronary sinus open into the right
atrium.

5. (a) The superior vena cava carries blood
from the body’s upper region.
(b) The inferior vena cava is larger than the
superior and carries blood from the lower
body’s region.
(c) The coronary sinus carries the majority of
blood from the heart itself. The coronary
veins open into the coronary sinus.
(d) The left atrium receives oxygenated blood
from the lungs through two pairs of
pulmonary veins.

6. (iii) Ventricles:
There are present left and right ventricles
with thick walls. The wall of the right
ventricle is thinner than that of the left
ventricle.
The left ventricle is longer and narrower than
the right ventricle.

7. (iv) Pulmonary Trunk and Aorta:
The pulmonary trunk arises from the right
ventricle.
It divides into left and right pulmonary
arteries that carry deoxygenated blood to the
lungs.
The aorta arises from the left ventricle.

8. It is divisible into
the ascending aorta,
arch of aorta and
descending aorta.
The right and left
coronary arteries
arise from the
ascending aorta.

9. Internal Structure:
The internal structure of the heart can be
better studied by dissecting it from the
ventral side.
(i) Atria:
The two thin walled atria are separated
from each other by the interatrial septum.
The right atrium receives the openings of
superior vena cava, inferior vena cava and
coronary sinus. The opening of inferior
vena cava is guarded by Eustachian valve.

10. The left artrium
receives four
openings of
pulmonary
veins.

11. (ii) Bicuspid and Tricuspid Valves:
The artrioventricular opening between the
left atrium and the left ventricle is
guarded by the bicuspid valve, also called
mitral valve (having two flaps).
The right atrio-ventricular opening is
guarded by the tricuspid valve, as it has
three flaps.

12. (iii) Ventricles:
Attached to the flaps of the bicuspid and
tricuspid valves are special fibrous cords, the
chordae tendineae, which are joined to the
other ends with the special muscles of the
ventricular wall, the papillary muscles.
The chordae tendineae prevent the bicuspid
and tricuspid valves from collapsing back
into the atria during powerful ventricular
contractions.

14. (iv) Semilunar valves:
As stated in the external structure, the
pulmonary trunk and aorta arise from the
right and left ventricles respectively.
At the base of the pulmonary trunk and aorta
are located three half-moon shaped pockets
known as pulmonary semilunar valves and
aortic semilunar valves respectively.
These valves allow the free and forward flow
of blood, but prevent any backward flow.

15. The valves of
the heart are:
(a) Bicuspid valve,
(b) Tricuspid
valve,
(c) Aortic
semilunar valves
and
(d) Pulmonary
semilunar valves.

16. The heart collects blood through both the
atria and then distributes it through the
ventricles.
The action of heart includes contractions
and relaxations of the atria and ventricles.
A contraction of the heart is called a systole
and its relaxation a diastole.
The atria and ventricles contract
alternately.

17. The contraction of heart (systole) and the
relaxation of heart (diastole) constitute
the heart beat.
The contraction of atria is initiated and
activated by the sinoatrial node (SA
Node— pace maker) which spreads waves
of contraction across the walls of the atria
via muscle fibres at regular intervals.

19. When the wave of contraction originating
from the sinoatrial node reaches the atrio-
ventricular node (AV Node— pace setter),
the latter is stimulated and excitatory
impulses are rapidly transmitted from it to
all parts of the ventricles via bundle of His
and Purkinje’s fibres.

20. These impulses stimulate the ventricles to
contract simultaneously.
The ventricles force bloohd trough long
system of arteries and hence must exert
great pressure on the blood.

21. Heart Beat:
What is heart beat? Heart beat is the
rhythmic contraction and relaxation of
the heart.
Each heart beat includes one systole
(contraction phase) and one diastole
(relaxation phase) of the heart to
distribute and receive blood to and from
the body.
The heart of a healthy person beats 72
times per minute (average).

22. Beating is an inherent capacity of the heart.
The heart of a resting human being pumps
about 5 litres of blood per minute.
This means that a quantity of blood equal to
the total amount contained in the body
passes through the heart each minute.
During exercise both the number of beats
per minute and the amount of blood
pumped per beat are greatly increased.

24. Types:
The heart beat is of two types:
Neurogenic and Myogenic.
The neurogenic heart beat is initiated by a nerve
impulse coming from a nerve ganglion. annelids
and most arthropods.
The myogenic heart beat is initiated by a patch of
modified heart muscle itself. It is found in hearts
of molluscs and vertebrates including human
beings.

25. Origin of heart beat:
The mammalian heart is myogenic, it is
regulated by the nerves.
The heart beat originates from the
sinoatrial node (SA Node)— pace maker,
which lies in the wall of the right atrium
near the opening of the superior vena cava.
The SA node is a mass of neuromuscular
tissue.

26. Sometimes, the SA-node may become
damaged or defective. So the heart does not
function properly.
This can be remedied by the surgical
grafting of an artificial pace maker in the
chest of the patient.
The artificial pace maker stimulates the
heart at regular intervals to maintain its
beat.

27. Conduction of heart beat:
Another mass of neuromuscular tissue, the
atrio-ventricular node (AV node) is
situated in the wall of the right atrium. The
AV node picks up the wave of contraction
propagated by SA node.
A mass of specialized fibres, the bundle of
His, originates from the AV node. The
bundle of His divides into two branches,
one going to each ventricle.

28. Within the mycocardium of the ventricles the
branches of bundle of His divide into a net
work of fine fibres called the Purkinje fibres.
The bundle of His and the Purkinje fibres
convey impulse of contraction from the AV
node to the myocardium of the ventricles.
Regulation of heart beat (= Regulation of
Cardiac Activity).

29. The rate of heart beat is regulated by
two mechanisms:
(i)Neural Regulation: “The cardiac centre
is formed of cardio-inhibitor and cardio-
accelerator parts.
(ii) Hormonal Regulation:
Adrenaline (epinephrine) and noradrenaline
(norepinephrine) hormones are secreted by
the medulla of the adrenal glands.
Noradrenaline accelerates the heart beat
under normal conditions

30. Thyroxine hormone
secreted by thyroid
gland increases
oxidative metabolism of
the body cells.
This requires more
oxygen and thus
indirectly increases
heart beat.

31. Heart Rate:
Human heart beats about 72 times per
minute in an adult person at rest.
This is called heart rate of that person. The
heart rate increases during exercise, fever,
fear and anger.

32. Because smaller animals have the higher
metabolic rate, their heart rate is higher
than larger animals.
An elephant has normal heart rate of about
25 per minute whereas mouse has a normal
heart rate of several hundred per minute.

33. Cardiac Output (Heart Output):
The amount of blood pumped by heart per
minute is called sraidiae output or heart
output.
Heart of a normal person beats 72 times
per minute and pumps out about 70 mL of
blood per beat.
Thus the cardiac output is 72 x 70 or 5040
mL per minute i.e., about 5 litres per
minute which is equivalent to the total
body blood volume (about 5.5 litres).

34. Pulse:
Pulse is the rhythmic contraction and
relaxation in the aorta and its main arteries.
Thus pulse is a wave of increase which
passes through arteries as the left ventricle
pumps its blood into the aorta.
Pulse is a regular jerk of an artery.
Therefore, it is also called arterial pulse.

35. The pulse rate are as follows:
(i) The pulse rate in children is more rapid
than in adults.
(ii) The pulse rate is more rapid in the
female than in the male.
(iii) When the person is standing up the
pulse rate is more rapid than when he/she
is lying down.

36. (iv) When any strong emotion is
experienced the pulse rate is increased, for
example, anger, excitement, fear, etc.
(v) Any exercise increases the rate of the
pulse.