Blood Vessels, Capillaries, Veins ,Anatomy Of  Human 

Heart (For Classes VII and X, CBSE Board)

(Compiled by PK Verma) --- Part 2 ----

Three varieties of blood vessels are - arteries,  veins and capillaries.  

Arteries are elastic vessels that transport blood   (oxygenated) away from the heart towards the other  body tissue. 

Pulmonary artery is the only artery which carries deoxygenated blood.

Aorta is the largest artery of the body which  originates from the heart and branches out into smaller arteries. 

The smallest arteries are called arterioles which branches into capillaries.

Arterioles receives blood from the arteries and  carries it to the capillaries.

Walls of artery consists of three layers-

-    Tunica intima - The innermost smooth layer lining  endothelium, connective tissue, elastic tissue.

-    Tunica media - A middle layer of smooth  involuntary muscle and elastic fibres.

-    Tunica externa - An external layer  consisting mainly of inelastic white fibres 

(collagen fibres).

Veins transport blood (deoxygenated) to the heart  (except pulmonary vein).

Pulmonary vein conveys oxygenated blood from  the lungs to the left atrium of the heart.

            Veins contain pocket valves which prevent the back  flow of blood.

             Largest vein in the human body is inferior vena  cava.

            Capillaries are extremely thin blood vessels with a diameter as small as 5­9 in man. 

            These form capillary plexus between arterioles on one end and venules on others. 

              Body contains three types of capillaries ­continuous capillary (found in brain,    

            connective tissue and skeletal muscle),  fenestrated capillary (found in endocrine      

             tissues, ciliary body of eye, etc), and  discontinuous capillary (found in liver        

             sinusoids). 

      Capillary beds are the sites where white blood cells are able to leave the blood                      and defend the body against harmful invaders.

            Human heart is situated in the thorax between the  lungs with its apex resting on the                    diaphragm. The wall of the heart  consists of three layers :

            the external covering layer or epicardium, the intermediate  cardiac  muscular  tissue                  or myocardium   and  the internal  layer  or endocardium in contact  with the                                  blood. 

             

            Myocardium does not fatigue due to alternate rest and activity and non formation of   

            lactic acid. 

            Heart is enclosed in a double fibro­serous sac called pericardium. 

            No neurons are present in the walls of heart. 

           The mammalian heart consists of 4 chambers ­ right and left atrium, right and left                        ventricle.

           The walls of the atria are thinner than those of   the ventricles.

           Right and left ventricles are separated by an interventricular septum. 

          Left ventricle is thicker than right ventricle  because it has to push blood to all the               body  parts at a much greater pressure.