08 Respiration and Circulation - part 14 - Blood vessels

08 Respiration and Circulation - part 14 - Blood vessels

Blood vessels :

• There are three main types of blood vessels in the human circulatory system viz - 

1. Arteries
2. Veins and
3. Capillaries.naazik   ,

1.Arteries :

• Carry blood from heart to various parts/organs of the body.
• Branch into arterioles and further into fine capillaries. 
• Normally carry oxygenated blood to all parts of the body (except the pulmonary artery which carries deoxygenated blood). 
• Usually situated deep in the body except a few like the radial, brachial, femoral, etc. which are superficially located.

T. S. of artery :

• its wall shows three layers - 

1. Tunica externa or tunica adventitia
2. Tunica media
3. Tunica interna or intima

Tunica externa : 

• Outermost layer
• Made up of thick, tough layer of collagen fibers. 

Tunica media  :

• Is made up of smooth muscles and elastic fibres. 
• This thick muscular and elastic layer makes the arterial wall pulsatile. 

Tunica interna :

• Innermost layer
• Is a single layer of flat compact endothelial cells surrounding the lumen. 
• The angular margin around the lumen shows tesselations. 
• Arterial lumen is devoid of valves and blood flows through it rapidly and with high pressure.

2. Veins :

• Veins are thin walled, mostly superficial vessels.
• Carry blood from the organs towards the heart. 
• The capillaries around the various organs join to form the veins. 
• Except for the pulmonary veins , other veins of the body carry deoxygenated blood towards the heart.

Portal vein :

• A portal vein e.g. hepatic portal vein, differs from the other normal veins.
• its starts as capillaries from one organ and capillarises in some intermediate organ e.g. liver, before taking the blood towards the heart.

Histology of veins :

• Also show the three layers like in the arteries-

1. Tunica externa,
2. Tunica media and  
3. Tunica interna. 

• However, thetunica media is comparitively thiner and their lumen is wide and narrow. 
• Internal valves at regular intervals can be seen. 
• Blood flows with flow pressure and the valves prevent backflow of blood.

3. Capillary :

• These are a network of minute blood vessels. 
• Thin walled having a single layer of flat squamous epithelium resting on a single basement membrane. 
• Mainly involved in exchange of materials.
• Wall of capillaries is formed of single layer of squamous epithelium and it is stretchable.
• Blood flows through the capillaries under high pressure. 
• Wall of capillaries bear small endothelial pores or fenestrae through which blood cells (WBCs) can escape by the process called as diapedesis.

Pulse : 

• It is a series of pressure waves that travel through the arteries due to ventricular systole. 
• It is the strongest in arteries closer to the heart and gradually becomes weak in arteries away from heart. 
• Can be felt easily in the superficial arteries like radial artery in the wrist and carotid artery in the neck. 
• Can be felt at particular points on the body. 
• All locations where the pulse can be felt are shown
• Pulse rate is equal to heart rate. 

Tachycardia : Pulse rate higher than normal (above 100 beats/min) is called tachycardia
Bradycardia : slower pulse rate (below 60 beats/min) than normal is called bradycardia.