Human Circulatory System - III

Blood vessels

Blood circulates in the body through closed vessels called blood vessels. These vary in size and function. There are three types of blood vessels:

a. Arteries

These carry blood away from the heart to the tissues.

b. Veins

These carry blood from the tissues to the heart.

c. Capillaries

These connect the arteries and veins.

Structures of artery and vein

Walls of arteries and veins are made up of three layers of tissue:

1. Tunica externa or Tunica adventitia

It is the outermost, fibrous layer made of collagen rich connective tissue. The collagen fibers give strength to the blood vessels and prevent their over dilation.

2. Tunica media

It is the middle, thickest layer made up of smooth involuntary muscle fibers and elastin fibers. This layer is very much variable because number of elastin fibers and muscle fibers depend upon the position of blood vessels from the heart.

3. Tunica interna or Tunica media

It is the innermost, thinnest layer made up of inner single layer of simple squamous epithelial cells called endothelium resting on a basement membrane and outer layer of elastic (yellow fibrous) connective tissue. The hollow space in the blood vessels is called lumen.

The amount of musculature and elastic tissue varies in the arteries and veins. In arteries, the tunica media consists of more layers of elastic tissue making them thick walled, whereas in veins, the tunica media consists of less layer making them thin walled. Therefore, when cut, veins collapse while the thicker arteries remain open.

Capillaries

These are the thin walled which form a very fine network of minute tubes. The lumen of capillaries is about 7-10 micrometer in diameter. Their walls are made up of a single layer of endothelial layer. The endothelium is very permeable and allows diffusion of dissolved      substances in and out of capillaries.

Differences between arteries and veins

Arterial Blood Circulation

Arterial system constitutes arteries. These are the blood vessels that carry the blood away from the heart to the various organs. Arteries carry pure blood except pulmonary artery. Arterial system includes two types of circulation:

A. Pulmonary Circulation

B. Systemic Circulation

A. Pulmonary Circulation

It is the circulation from heart to the lungs. Here, the right ventricle pumps de oxygenated blood into the pulmonary aorta to the lungs for oxygenation. Pulmonary aorta divides into two branches:

i. Left pulmonary artery

It carries de oxygenated blood to the let lung.

ii. Right pulmonary artery

It carries de oxygenated blood to the right lung.

B. Systemic Circulation

It is the circulation from heart to the different parts of the body. Here, the left ventricle pumps the oxygenated blood to the systemic aorta. It begins at the upper part of left ventricle and after passing upward for a short way, it bends (arches) backwards and to the left. It then descends behind the heart through the thoracic cavity.

As the aorta emerges from the heart, it gives off two coronary arteries which supply blood to the heart muscles.

Aortic arch gives three branches which supply blood to the anterior region of heart. These branches are:

1. Right Brachiocephalic artery

2. Left Common carotid artery

3. Left Subclavian artery

1. Right Brachiocephalic (=innominate) artery

It is the first branch arising from right side of aorta. It supplies the blood to the right part of the head, neck and fore limb by two branches:

a. Right common carotid

It runs upwards along with the trachea and supplies the blood to rhe right part of the head by two branches:

i. External carotid artery

It supplies blood to the right side of head and face.

ii. Internal carotid artery

It supplies blood to the brain, eyes, forehead and nose.

b. Right subclavian artery

It supplies blood to the right part of neck and forearm by following branches:

i. Right vertebral artery

It supplies blood to the neck region.

ii. Right auxillary artery

It supplies blood to the scapular region.

iii. Right brachial artery

It supplies blood to the arm by radial and ulnar arteries.

2. Left common carotid

It is second and middle branch of aortic arch. It runs upwards along the side of trachea. It distributes same as that of right common carotid. It divides into left external carotid and left internal carotid that supply blood to the left side of head and face.

3. Left subclavian artery

It is the third branch arising from the left side of aortic arch. It supplies blood to the left arm by similar type of branches as right subclavian giving off a vertebral artery for neck, auxillary artery for left shoulder and brachial artery for lower parts of the arm with left ulnar and radial arteries.

The aortic arch bends downwards and supplies the blood to the posterior parts of the body by following major arteries:

1. Inferior phrenic artery

It supplies the blood to the lower surface of diaphragm.

2. Coeliac artery

It is the branch which supplies the blood to the stomach and liver.

3. Superior mesentric artery

It is a thick, long and unpaired branch of aorta. It supplies blood to the various parts of small intestine such as duodenum, jejunum, ileum and proximal half of large intestine such as caecum and some parts of colon.

4. Renal arteries

A pair of renal artery supplies blood to the kidneys.

5. Superadrenal arteries

It arises from the renal arteries which supplies blood to the adrenal gland.

6. Genital arteries

A pair of genital artery supplies blood to the ovaries (in female) and to the testis (in male).

7. Lumbar arteries

Four pairs of lumbar arteries supply blood to the posterior body wall and vertebrae of posterior abdominal cavity.

8. Inferior mesentric artery

It supplies blood to the parts of large intestine such as colon and rectum.

9. Common iliac artery

In pelvic region, aorta is bifurcated into two common iliac arteries which supply to the each leg. Each common iliac divides into two branches:

i. External iliac artery

It runs obliquely downwards into leg of its side and is now called femoral artery. It continues own and bifurcates at the level of knee into anterior and posterior tibial arteries.

ii. Internal iliac(=hypogastric) artery

It is short and runs medially to supply the viscera, walls and organs of pelvic region.

In this way arteries carry blood away from heart.