Ans: 1. Structure of human heart:
Heart is the central pumping organ of blood. It is a muscular organ lies in the
thoracic cavity in the mediastinum. Its posterior end slightly turned to left. Heart is
mesodermal in origin and is covered by a double layered pericardium (outer fibrous
layer and inner serus layer).
The serus layer of pericardium is double layered- outer parietal layer and inner
visceral layer. Parietal layer is fused with fibrous layer of pericardium. visceral layer
is attached to outer surface of heart and forms epicardium. Between these two layers,
pericardial cavity is present and is filled with pericardial fluid. It reduces the friction
and protects the heart from shocks.
The wall of heart has outer epicardium, middle myocardium (formed by
cardiac muscles) and inner endocardium (formed by endothelial cells).
Human heart is formed by four chambers. They are anterior atria (left and right)
and posterior ventricles (left and right). Atria and Ventricles are separated by a deep
groove called coronary sulcus. The two ventricles are separated by inter ventricular
grooves posteriorly each atrium projects over the ventricle as auricular appendix.
a. Atria:
i. Atria (left and right) form the anterior part of the heart. Right atrium is larger
than left one.
ii. The two atria are separated from each other by an inter atrial septum. In
embryonic stage, it has a pore called foramen ovale. At the time of birth,
when lungs become functional, it is closed leaving a small depression called
fossa ovalis.
iii. Right atrium receives deoxygenated blood from different body parts through
three major veins, namely, two precaval veins and a post caval vein. Right
atrium also receives blood from wall of heart through a corononary sinus. Its
opening is guarded by a valve of Thebesius.
iv. Near the opening of post caval vein, a rudimentary Eustachian valve is
present. In embryonic stage it directs the blood from right atrium to left
through foramen ovale.
v. Left atrium receives Oxygenated blood from the lungs by two pairs of
pulmonary veins. They open into the left atrium through a common pore.
vi. In the right upper corner of right atrium, the pace maker called sinoatrial
node (S.A. node) is present.
vii. Atria and ventricles are separated from each other by an atrio ventricular
septum.
viii. In the lower left corner of the right atrium close to atrio ventricular septum
an atrio ventricular node is present.
b. Ventricles:
i. Ventricles (left and right) form the posterior part of heart.
ii. Left ventricle is large and has thick walls (than right ventricle) and the left and
right ventricles are separated by an interventricular septum.
iii. The inner surface of ventricles is raised into ridges called columnae cornae
(trabiculae cornae). Some of them are large and conical. They are called
Papillary muscles.
iv. Right atrium opens into right ventricle through right atrio ventricular aperture.
It is guarded by a Tricuspid valve.
v. Left atrium opens into left ventricle through left atrio ventricular aperture. It is
guarded by a Bicuspid valve or Mitral valve.
vi. Tricuspid Valve and Bicuspid Valve allow the blood to flow from atria to
respective ventricles only.
vii. Extending between tricuspid valve and bicuspid valve and papillary muscles,
there are collagenous cords known as Chordae tendinae. They prevent the
cusps of these valves to bulge into atria during ventricular systole.
viii. Arising from A.V. node, is a Bundle of His (A.V. Bundle). It extends into
inter ventricular septum, divides into two branches and then, they divide further
as purkinje fibres. They extend into the muscular walls of ventricles.
- S.A. node is formed by cardiac muscle cells (cardiomyocytes). It
has ability to generate action potentials without external stimuli.
Hence, it is called pace maker. As it is formed from muscle cells,
it is described as myogenic pace maker.
- A.V. node send the action potentials received from the S.A. node
into ventricular muscles. Hence A.V. node is a relay point.
c. Aortic arches: (Pulmonary arch and systemic arch)
i. Arising from the right ventricle, is a pulmonary arch. Its opening into right
ventricle is guarded by a pulmonary valve. It allows the blood to flow into pulmonary
arch only. Pulmonary arch transports the deoxygenated blood to lungs.
ii. Arising from the left ventricle, is a systemic arch. It turns to left side and
supplies oxygenated blood to different body parts through its branches - called
arteries. The opening of systemic arch into left ventricle is guarded by an
aortic valve. It allows the blood to flow into systemic arch only.
iii. Pulmonary valve and aortic valve are formed by three semilunar valves each.
iv. At the point of contact, the systemic and pulmonary arches are connected by
a fibrous strand called ligamentum arteriosum. It represents the remnant of
ductus arteriosus [In embryonic stage, systemic arch and pulmonary arch
are connected by ductus arteriosus].
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