Venous return is defined as the flow of blood back to the heart. It is therefore important in maintaining normal circulation.
The heart is a myogenic pump, meaning it is responsible for its own stimulation to pump blood out to the rest of the body. In order for blood to be pumped out of the heart, enough blood must be returned to the heart so that it can be pumped around again in the next cardiac cycle.
This article will discuss factors which influence venous return.
Venous Pressure
Veins are blood vessels which return blood to the heart. The pressure in these veins is the driving force for the filling of the heart. This is known as venous pressure. Venous pressure is affected by two main parameters:
- The rate of blood entering the veins – this is linked to the peripheral resistance in the body.
When resistance is high, there is a slower rate of blood entering the veins, which causes a decrease in venous pressure.
When resistance is low, there is a faster rate of blood entering the veins, which will increase venous pressure.
- The rate at which the heart pumps out blood – this is linked to the cardiac output of the heart.
When cardiac output increases, blood is rapidly pumped out of veins, which reduces venous pressure (as it does not get a chance to rise).
When cardiac output decreases, blood backs up into the venous system. Therefore, the blood volume increases which raises venous pressure.
Factors Affecting Central Venous Pressure
Central venous pressure (CVP) is the blood pressure in the vena cava near the right atrium. Under normal circumstances, the CVP ranges from 2-6mmHg.
Veins are low pressure, low resistance vessels and have high capacitance. These properties allow veins to distend with the increasing pressure of blood, allowing them to maintain the heart’s venous pressure.
Additionally, veins have valves which act to maintain the unidirectional flow of blood. The competency of these valves is important in maintaining venous return as they ensure blood is always flowing towards the heart.
There are various factors which can affect venous pressure and venous return:
- Skeletal Muscle Pump – Peripheral veins work in concert with the muscular contraction to increase venous return to the heart. When muscles (such as the quadriceps) contract (during walking, running etc), the valves are forced open to increase the venous return.
Fig 1 Skeletal muscle pump of the venous system
- Respiration – During inspiration, venous return increases as the thoracic cavity’s pressure becomes more negative. This reduced intrathoracic pressure draws more blood into the right atrium. This results in greater venous return.
- Venous Compliance – Increased sympathetic activity will reduce venous compliance. This increases the venous pressure and venous return as when blood flow into the veins increases, it cannot dilate to accommodate the increased blood. Instead, pressure in the veins rises and blood flow through the vessels increases to empty the veins faster.
- Blood Volume – The greater the blood volume in the veins, the greater the blood flow and venous pressure. The heart can accommodate the increase in blood volume because of the Frank-Starling mechanism (the greater the stretch, the greater the contractility of the heart).
- The Heart- must be working efficiently to pump blood out of the veins and maintain CVP.
Clinical Relevance – Chronic Varicose Veins
In this condition, the competency of the valves in veins in compromised. This means the valves do not sufficiently close, allowing blood in the veins to flow backwards and accumulate in the veins. This can decrease the venous return.
This commonly affects the superficial veins of the legs, which look engorged and twisted. Blood can pool in the veins to cause bruising and ulceration of the tissue if the pressure becomes excessive.
Fig 2 – Diagram showing normal and varicose veins
