Oxygen-rich blood enters the leg via the leg arteries, travel via the multiple branches of the major arteries until it enters the arterioles and then the capillaries. Here the exchange of gas, nutrients and waste between the blood in the capillaries and the tissue of the leg ensures that the leg structures are provided with an adequate oxygen and nutrients. Carbon dioxide and waste substances pass out of the tissue spaces and into the capillary blood where it collects into the venules, empties into the leg veins and returns to the heart.
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When blood flow within a leg vein is compromised, blood pools within the foot and lower leg. This is known as venous stasis in the legs. It essentially means that the circulation is sluggish and blood is moving too slowly or only small quantities of blood are able to exit the leg. For venous stasis to to occur, the blockage or valvular incompetence has to occur in one of the major veins of the leg.
Venous stasis affects the entire blood circulation of the leg to some extent and even the flow of blood within unaffected vessels is impaired. This ultimately affects the supply of nutrients and oxygen to the tissues of the legs. The causes of venous stasis are discussed further under Venous Insufficiency of the Legs.
Venous insufficiency is a condition where the flow of blood from the leg back to the heart is impaired. The leg veins carry oxygen deficient blood away from the legs and certain anatomical features and physiological mechanisms in the leg enable the unidirectional flow of blood to the heart.
This includes the :
- Musculovenous pump (leg blood circulation pump) where the contraction and relaxation of the leg muscles during activity compress the veins thereby pushing the blood up the vessel. The pulsating of the leg arteries also helps to keep blood flowing in the veins.
- Valves in the leg veins prevent backward flow as the blood travels back to the heart. If blood attempts to flow in the wrong direction within the vein (away from the heart) the valve closes and prevents this.
If the valves become incompetent, then blood may flow backwards (retrograde flow) and this is known as venous reflux. A blockage within a leg vein, usually by a blood clot, can also hamper the outflow of blood from the legs. Eventually blood will pool in the feet and lower legs and move very slowly or stay almost stationary. This sluggish circulation in the leg veins is known as venous stasis.
The vascular system is a continuous network. If the blood flow out of the legs are affected, then blood flowing into the legs will also be compromised and oxygen and nutrients cannot reach the tissue of the legs. Stasis of the blood also increases the chances of further and larger clot formation and eventually one of these clots can dislodge and block the arterial supply of a vital organ like the lung (pulmonary embolism).
What is peripheral arterial disease?
Peripheral arterial disease (PAD) is a condition where the the blood flow to the limbs, arm or leg, is impaired due to a narrowing of the arteries supplying these areas. It is more likely to affect the leg than the arm and has become a global problem due to the rise in atherosclerosis. Acute limb ischemia is the sudden occlusion of the artery as a result of clot. It needs immediate medical attention as it can lead to severe ischemia (tissue injury due to poor oxygen supply) and lead to necrosis and gangrene. This may ultimately result in a loss of the limb. Acute limb ischemia is discussed in detail under Blood Clot in the Leg.
The arteries to the legs carry oxygen-rich blood from the heart to the thigh, lower leg and foot. The main artery to the leg is the femoral artery. It is the continuation of the external iliac artery below the inguinal ligament. The external iliac artery is a branch of the common iliac artery which is formed when the abdominal aorta bifurcates (divides into two).
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