An Introduction to Cardiovascular System

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Cardiovascular system (CVS) consists of blood, heart and the blood vessels.

1. Heart:

Heart serves as a ‘pump’ to impart pressure to blood and to establish pressure gradient for blood flow . Heart rate is 72 beats/min (it beats 3 billion times in life !!).

2. Blood vessels:

Blood vessels serve as passageways for blood circulation. Length of blood vessels is nearly 100,000 km or 60,000 miles.

3. Blood:

Blood is the transport medium. Volume of blood is nearly 5.5 liters.  Cardiac output is about 5 l/min, 14000 l/day or 10 million l/yr.

Thus in the ‘cardiovascular system’ the heart serves as the pump and blood vessels constitute the circulatory system. There are two closed circuits

Systemic circulation
Pulmonary circulation

The Heart

Heart comprises of atrial syncytium and the ventricular syncytium. Fibrous insulator exists between atrium and ventricle. It is the first organ to start functioning in embryo (3 weeks of development).

Heart is cone shaped, about the size of closed fist. Its size is 12 cm x 9 cm x 6 cm (long) (wide) (thick). Its mass is 300 grams in male and 250 grams in females.

The heart rests on the diaphragm. In lies inside mediastinum, between vertebral column and sternum.

The heart consists of three layers, endocardium, myocardium and the epicardium. Fibrous skeleton of heart consists of dense connective tissue. Four fibrous (connective tissue) rings, surround valves, and merge with interventricular septum. This fibrous skeletion serves several functions:

Foundation for valve attachment
Point of insertion of cardiac muscle
Prevents overstretching of valves
Serves as electrical insulator

Valves of the heart are formed by dense connective tissue covered by endothelium. These open and close in response to pressure changes and ensure one-way flow of blood.
Pericardium or pericardial sac normally contains 5-30 ml of clear fluid.

Cardiac Muscle

Cardiac muscle consists of two kinds of muscle cells:

Contractile cells constitute 99% atrial and ventricular muscles (Myocardium)

Conducting or Autorhythmic cells form the remainder 1%.

Myocardium consists of interlacing bundles of Y-shaped muscle cells, spirally arranged around the circumference of the heart. These are the branching fibers, interconnected by intercalated discs.

Two types of ‘Membrane Junctions’ exist: desmosmes and gap junctions.

Cardiac muscle has actin and myosin filaments. It has low resistance intercalated disks (1/400 the resistance of cell membrane). No gap junctions exist between atrial and ventricular syncytium.

Abundant mitochondria are present in cardiac muscle (up to 40% of cell volume). This is because heart depends on aerobic energy. Cardiac muscle also has abundant myoglobin to store oxygen and facilitate oxygen transport to mitochondria.

No new cardiac muscle is produced after infancy. The muscle only ‘hypertrophies’ (increases size of muscle fibers). Actin and myosin filaments are present in ‘SARCOMERE’

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The writer enjoys medical education and has special interest in community medicine.