Circulatory System

The circulatory system is the transport system that supplies oxygen and nutrients to the tissues and returns carbon dioxide to the lungs. The human circulatory system comprises systemic circulation, pulmonary circulation, and lymphatic circulation.

Functions of the Circulatory System

1. Transport of oxygen from the lungs to the tissues, and of carbon dioxide from the tissues to the lungs.
2. Transport of metabolic waste products from the body tissues to the kidney for their removal.
3. Transport of nutrients absorbed from the intestine to the liver and body tissues.
4. Regulation of body temperature by altering blood flow to the skin.
5. Distribution of hormones from the endocrine glands to their target tissues.

Blood Vascular System

A well-developed blood vascular system is present in higher invertebrates from Annelida and Echinodermata, and also in all chordates. It is mainly of two types:

a) Open type of circulatory system In higher invertebrates like prawn and insects, the blood does not remain confined to blood vessels but flows freely in the open spaces of the body cavity, bathing the tissues directly.

b) Closed type of circulatory system The blood vascular system of humans consists of the heart, blood vessels, and blood. This is a closed system, that is, blood always remains within blood vessels. Blood is carried from the heart to various tissues by the arteries and returns to the heart from the tissues by the veins.

Heart

The heart is a thick, muscular, contractile, automatic pumping organ of the blood vascular system. The wall of the heart is made of cardiac muscle fibers, while internally it has a number of chambers interconnected with one another.

There are two kinds of heart:

a) Neurogenic heart In this type, the initiation of the heartbeat is under the control of a ganglion (nervous control), so once removed from the body it stops beating. Example: insects.

b) Myogenic heart The rhythmic contraction takes place in the heart and is initiated by the cardiac muscle itself. Example: hearts of vertebrates.

Location and External Structure

The human heart is a hollow, conical, triangular, muscular organ located under the breastbone, between the lungs, in the thoracic cavity. It is located in the mediastinum and fits in the cardiac notch of the left lung. It usually lies slightly to the left of the midline of the thoracic cavity, with its apex resting on the diaphragm.

• It looks like a blunt cone, about 12 cm in length and 9 cm in width.
• The weight of the human heart varies from 230 to 280 gm in females, and 280 to 340 gm in males.
• The heart has three layers, namely the endocardium, myocardium, and pericardium.
• The endocardium lines the inner cavity of the heart and is made up of thin squamous epithelium.
• The myocardium is the thick muscular layer made of cardiac muscle, which is involved in contraction.
• The pericardium is the outer layer, and it is divided into two layers, the inner visceral and the outer parietal pericardial layers, forming a cavity called the pericardial cavity.
• The cavity between the two layers is filled with a fluid called pericardial fluid, which lubricates the heart and protects it from mechanical shock.
• The heart has four chambers: two auricles (atria) and two ventricles.
• Externally, the auricles can be separated from the ventricles by a circular groove called the coronary sulcus.
• The ventricles are more thick-walled than the auricles, and are separated from each other by the interventricular septum.