The heart is in the thoracic cavity within the mediastinum between the left and right pleural cavities and protected by the ribs from about the third to the sixth intercostal spaces. The dorsal aspect is horizontally in line with the middle of the first rib and the ventral aspect is on the sternum. The long axis of the cardiac silhouette is oriented vertically in the horse, almost vertically in ruminants, and progressively more obliquely in the pig, dog, and cat. The dorsal part of the heart is known as the base and is formed by the atria and the major vessels entering (veins) and leaving (arteries) the heart. The major vessels tend to hold the heart in a relatively fixed position dorsally while ventrally it is free within the pericardial sac.

The heart is actually two separate pumps: a right heart that pumps blood through the lungs, and a left heart that pumps blood through the peripheral organs. In turn, each of these hearts is a pulsatile two-chamber pump composed of an atrium and a ventricle. Each atrium is a weak primer pump for the ventricle, helping to move blood into the ventricle. The ventricles then supply the main pumping force that propels the blood either (1) through the pulmonary circulation by the right ventricle or (2) through the peripheral circulation by the left ventricle.

The cardiovascular system has two circulations in series:

  • (i) the Pulmonary circulation composed of the right atrium (RA), right ventricle (RV) and lungs; and
  • (ii) the Systemic circulation composed of the left atrium (LA), left ventricle (LV), and the systemic organs.

Each circulation has three major divisions:

  • (i) the distribution system (ventricles, arteries, and arterioles),
  • (ii) the perfusion/exchange system (capillaries), and
  • (iii) the collecting system (venules, veins, and atria).

[ A Portal system is defined as two capillary beds connected in series between an artery and a vein.

  • In the renal circulation, there is a glomerular capillary bed between two arterioles and another normal capillary bed between an arteriole and a venule.
  • Another portal system is shown in the digestive venous circulation as it drains into the liver via the portal vein. Additionally, the liver has an arterial supply for delivery of oxygen and nutrients.
  • A third portal system occurs in the hypothalamus/pituitary area of the cranial blood flow. ]

Physiology of Cardiac Muscle:

The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers. Cardiac muscle is striated in the same manner as in skeletal muscle. Further, cardiac muscle has typical myofibrils that contain actin and myosin filaments almost identical to those found in skeletal muscle; these filaments lie side by side and slide along one another during contraction in the same manner as occurs in skeletal muscle. But in other ways, cardiac muscle is quite different from skeletal muscle, as we shall see.

Myocardial cells: Functional syncytium(Present), Morphological syncytium(Absent).

Atrial functional syncytium is present from Ventricular functional syncytium by- Annulus fibrosi(Physical attachment of atrium & ventricles)

[Annulus fibrosi: A fibrous structure electrically insulates the atrium from the ventricles and physically forms the attachment rings called Annulus fibrosi.

An opening exits in the fibrous plate for the Atrio-ventricular bundle to conduct impulses from the atrial to ventricles]

Myocardial Cells

Spontaneous depolarization:- The normal pacemaker of the heart is SA node.(Right atrium)

Spontaneous phase-4 depolarization of these cells initiates an action potential that is conducted throughout the heart. Other slower pacemakers are located in the AV node & the His-Purkinje system.

Slower pacemaker may capture normal pacemaker rhythm if they fire more rapidly or normal pacemaker becomes slower.

Myocardial cells are capable of transmitting action potential that can pass from cells to cells through gap jn.

White muscle fibers are found in Eye ball,Gastronemius,Biceps brachii.In these fibres anaerobic energy production is seen.


Propagation of action potentials throughout the heart required depolarization of a cardiac cell from arresting state to an excited state.

Propagation of cardiac action potential requires the flow of electrical current(charge).

Charge carriers: Na+, K+ , Ca++

Anions are generally large charge proteins that cannot easily diffuse across cell membranes, except Cl-

Flow of charge depends on-

i.Diving force: a) Potential difference & b) Concentration difference

ii.Path [Channel]: Voltage gated, Time depended, State of Phosphorylation

  • Membrane potential b/w the inside & the outside the cells=Electrical diving force
  • Concentration difference b/w the inside & outside the cell=Chemical diving force

Generation of an Action Potential:-

i.Generation of Polarizes state



(i) Generation of a polarized state: Polarization of the cardiac cell results from a selective permeability to K+, which allows K+ to flow out of the cell down its concentration gradient.

K+ efflux: Intracellular Positive ion balance alters—à Generation of Negative intracellular potential–à Negative potential reduce K+ efflux diving force= K+ equilibrium Potential

Thus, the resting cardiac cell is polarized, in that the interior of the cell represents a negative pole, whereas the exterior represents a positive pole.

  • If positive charge is added to the inside of the cell, the cell becomes less polarized or Depolarized.
  • Conversely, if positive charge is removed from the cell, it becomes more polarized, or hyperpolarized.
  • Finally, if a cell has been depolarized (by adding positive charge), removal of that positive charge will cause the cell to repolarize.

(ii) Depolarization:


Slow & Fast Response Action Potentials: (Speed of the Depolarization)

Fast Responses: Atrial,Ventricular myocardium,His-Purkinje system

Slow Responses: SA node, AV node.

Fast Responses Slow Responses
RMP= -90mV No RMP actually, -65mV
Depolarization by slow K+ influx & then fast K+ efflux. Depolarization by slow Ca+ influx.

No Na+ channel present.

Repolarization by slow K+ efflux & then fast K+ efflux. Repolarization by K+ efflux.
Present in: Atrial,Ventricular myocardium,His-Purkinje system Present in : SA node, AV node.
Gap JN” are more in numbers Gap JN” are less in numbers


Ventricular Hypertrophy:

1.Concentric hypertrophy:-

i.Wall thickened increased

ii.Chamber volume same,due to pressure overload

2.Eccentric hypertrophy:-

i.Little wall thickness increased,due to volume overload

ii.Chmaber volume increased.



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