Veins are the blood vessels which carry the blood from peripheral tissues towards heart. They carry the deoxygenated blood, which is bluish in color and for the same reason veins appear blue.
- Veins are thin-walled, being thinner than the arteries.
- Their lumen is larger than that of the accompanying arteries.
- Veins have valves which maintain the unidirectional flow of blood, even against gravity.
- Since the venous pressure is low (7 mm Hg) the valves are of utmost value in the venous return. However, the valves are absent in:
- The veins of less than 2 mm diameter.
- The venae cavae.
- The hepatic, renal, uterine, ovarian (not testicular), cerebral, spinal, pulmonary, and umbilical veins.
- The muscular and elastic tissue content of the venous walls is much less than that of the arteries. This is directly related to the low venous pressure.
- Large veins have dead space around them for their dilatation during increased venous return. The dead space commonly contains regional lymph nodes.
Structure of Veins:
Veins are made up of usual three coats which are found in the arteries. But the coats are ill-defined, and the muscle and elastic tissue content is poor. In poorly developed tunica media, the amount of collagen fibers is more than the elastic and muscle fibers. The adventitia is thickest and best developed. The smooth muscle is altogether absent in:
- The veins of maternal part of placenta;
- The cranial venous sinuses and pial veins;
- The retinal veins;
- The veins of cancellous bone; and
- The venous spaces of the corpora cavernosa and corpus spongiosum.
Blood and Nerve Supply of Veins:
The larger veins, like the arteries, are supplied with nutrient vessels called vasa vasorum. But in the veins, the vessels may penetrate up to the intima, probably because of the low venous pressure and the low oxygen tension.
Nerves also are distributed to the veins in the same manner as to the arteries, but arc fewer in number.
Factors Helping in Venous Return:
- Overflow from the capillaries pushed from behind by the arteries.
- Negative intrathoracic pressure sucks the blood into the heart from all over the body.
- Gravity helps venous return in the upper part of the body. .
- Arterial pulsations press on the venae comitantes intermittently and drive the venous blood towards the heart.
- Venous valves prevent any regurgitation (back flow) of the luminal blood.
- Muscular contractions press on the veins and form a very effective mechanism of venous return. This becomes still more effective. within the tight sleeve of the deep fascia, as is seen in the lower limbs. The calf muscles (soleus) for this reason are known as the peripheral heart. Thus the muscle pumps are important factors in the venous return.