Tunica Externa or Tunica Adventitia

(1) Tunica externa or tunica adventitia : Outermost, fibrous, made up of collagen rich connective tissue and less elastin fibres. The collagen fibres give strength to the blood vessels and prevent their overdilation.

(2) Tunica media : Middle, thickest, made up of smooth involuntary muscle fibres and elastin fibres. This layer is very much variable because number of elastin fibres and muscle fibres depend upon the position of blood vessels from the heart.

(3) Tunica interna or tunica intima : Innermost, thinnest, 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 vessel is called lumen.

Arteries Veins

Distributing (branching) system hence – Collecting system, hence, go on uniting ahead goes on branching to form ahead main branches towards heart.

Carry blood away from heart – Carry blood towards heart

Thick walled and less distensible – Thin-walled and more distensible

Lumen narrow, without valves – Lumen wider with many valves to prevent back flow of blood.

Blood flows within it by pumping action – Blood flows by the force of  general body of heart activity with low pressure

Contains about 15% of  blood at any time – Contains about 64% of  blood at any time

Carry oxygenated blood (except pulmonary artery) – Carry deoxygenated blood (except pulmonary vein)

Deeply situated in the body – Superficially situated in the body

Heart Rate

It is the number of heart beat per minute (72 in man);

Directly proportional to temperature and  physical activity; becomes double with the rise of  every 10°C but, caeses at 44°C.

Ca++ accelerates the heart rate while Na+ and K+ make it slow.

Increases also due to emotions, feeling, love, fear, pain and anger, etc.

Stroke volume (stroke output)

The volume of  blood ejected out by heart per beat i.e. = 70 ml.

Cardiac output (heart output)

The volume of  blood ejected per minute (heart rate x stroke volume i.e. 72 x 70 ml. = 5 litre) ; denoted as ml/100 gm of  body weight/minute (man 5 - 10 ml ; dog = 12 - 15 ml ; cat = 65 -

70 ml ). It increases by 20 to 30% at high altitude.

Entire blood of  our body is circulated within just more than a minute and during exercise within less than 15 seconds. (In open type circulation the velocity is very low i.e. in cockroach it takes about 3-8 minutes for entire blood to circulate)

Out of  the total blood pumped by heart about 5% is received by heart muscle itself; 15% by brain; 25% by liver; 25% by kidney; 15% by skeletal muscle and rest 15% by other organs.