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What are veins?
Our cardiovascular system is a closed circuit system, which comprises of arteries and veins. Arteries (except pulmonary artery) carry oxygenated blood from the heart to the tissues, while the veins (except pulmonary vein) bring back oxygenated blood to the heart. Hence, the arteries appear red and the veins appear bluish in color. Veins are thinly walled compared to the arteries because the venous system is typically a low-pressure system. The heart pumps blood into the arteries. This explains the high- pressure in the arterial system, but veins lack a pumping system and rely on passive factors like contractions or surrounding muscles for pressure to pump the blood. Moreover, venous return functions against gravitational force and hence, veins have valves to prevent the backflow of blood.
Anatomy of veins
Veins range from being just 1 to 1.5 mm in diameter. The smallest veins in the human body are known as venules.They receive blood from the capillaries and open into veins which finally carry deoxygenated blood into the systemic veins, i.e. the venae cavae.
Types of veins:
- Superficial veins: These veins are situated just beneath the skin. These can be seen as bluish cords under the skin. Superficial veins bring back about 10% of the blood back to the heart.
- Deep veins: These veins are situated deeper within the muscles and run close to a corresponding artery with the same name; e.g., coronary arteries and coronary veins. The deep veins bring back about 90% of the blood to the heart. The deep veins, under the action of muscle contractions, are able to drain the blood back to the heart.
- Systemic veins: These veins carry deoxygenated blood from the rest of the body to the heart e.g., venae cavae.
- Pulmonary veins: Veins in the lungs, an exception to the rule, pulmonary veins carry oxygenated blood to the left atrium of the heart.
- Perforating (communicating) veins: These connect superficial veins to the deep veins, thereby forming a network of veins.
- Structure of veins : Veins comprise of three layers of tissues, from outside to inside, they are known as tunica adventitia, tunica media and tunica intima.
They contain muscle and elastic tissue, but these are structurally ill-defined. Since the venous system is a low-pressure system, veins are devoid of an elastic membrane. This prevents them from adapting to pressure changes.
Veins have unidirectional flow valves, unlike the arteries. This prevents the backflow of blood even in the standing position when blood is circulated anti-gravity.
How does the venous system work?
Several mechanisms are involved in the efficient return of blood from the lower limbs toward the heart (termed venous return).
They are enlisted as under:
vis a tergo pressure (pressure from behind): The heart pumps blood with pressure into the arteries (120 mmHg). Since the circulatory system is a closed circuit, this very pressure helps in venous return.
Intrathoracic pressure variation: The pressure variation inside the chest ensures that there is no accumulation of blood in the circulatory system or in the lung circulation.
Arterial pulsation: The pulsations in the artery help to drive the venous blood towards the heart. This mechanism is important in those veins that flow close to an artery.
Venous valves: These prevent backflow and help push the blood against gravity.
Calf muscle contraction: The contractions of muscles around the veins exert an active pressure on the veins. This mechanism is most appreciable in the lower limb veins. The soleus muscle in the calves helps in pumping the venous blood upward. The flow is always towards the heart owing to these valves. Thus, this muscle is also known as a peripheral or second heart due to its importance in venous circulation.
Foot pump: Another mechanism called foot pump helps pump the blood upwards when we walk.
Conversely, it can be derived from the understanding of this physiology that the diseases of the venous circulation system arise out of the failure of any of these mechanisms. For example, long standing or sitting leads to lack of calf muscle contractions and hence, the stasis (or stagnation-related) diseases can be seen as occupational hazards in those who stand or sit for long durations or frequent travelers.
Click here to know about diseases of the veins