Terms in this set (5)
- pressure gradient. 7-13 mm hg pressure towards heart. lower neard vena cava.
- gravity. drains head and neck.
- skeletal muscle pump limbs. valves push blood towards heart when squeezed by skeletal muscle.
- thoracic pump. abdominal to thoracic cavity.
- exercise. increased venous return.
Venous return (VR) is the flow of blood back to the heart. Increased pulmonary venous return to the left atrium leads to increased filling (preload) of the left ventricle, which in turn increases left ventricular stroke volume by the Frank-Starling mechanism.
The muscle “pump” refers to the temporary increase in muscle size that occurs when you lift weights, especially when you use higher reps and shorter rest periods. The net effect is that blood is being pumped into your muscles faster than it can leave, which makes the blood “pool” in your muscles, and gives you a pump.
Venous Return. Venous return to the right atrium is the most important factor determining cardiac output, provided both ventricles and the pulmonary circulation are normal. This increases the pressure gradient and venous return. Under normal conditions, the right ventricle and pulmonary circulation are highly compliant
Skeletal muscles also play a key role in the movement of blood around the body. Veins embedded within a muscle are compressed during contraction of that muscle, causing an increase in blood pressure due to the presence of one-way valves within the veins. This increase in pressure drives the blood towards the heart.
Calf muscle pump is the motive force enhancing return of venous blood from the lower extremity to the heart. It causes displacement of venous blood in both vertical and horizontal directions, generates ambulatory pressure gradient between thigh and lower leg veins, and bidirectional streaming within calf perforators.
Factors Affecting Venous Return
- MSFP. Volume. e.g. Haemorrhage, resuscitation. Compliance.
- RAP. Respiratory pump. Negative intrathoracic pressure reduces RAP, improving venous return. Positive pressure ventilation. Pericardial compliance. Constriction.
- Resistance to Venous Return. Posture. Vascular compression. Obesity. Pregnancy.
According to Starling's law of the heart, in a normal heart the cardiac output is directly proportional to the venous return. This, person 2 has the greater venous return. ventricular filling decrease with increased heart rate; person 1 has the lower heat rate and therefore the longer ventricular filling time.
Decreased venous compliance.
Sympathetic activation of veins decreases venous compliance, increases central venous pressure and promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanism, which increases the total blood flow through the circulatory system.5), and as a result, 95% of venous return comes from muscle tissue where venous oxygen content is very low. Thus, with increasing exercise intensity, the relative amount of venous blood returning to the heart from active striated muscle increases so that mixed venous oxygen content decreases as shown in Fig. 4.
A decrease in cardiac output either due to decreased heart rate or stroke volume (e.g., in ventricular failure) results in blood backing up into the venous circulation (increased venous volume) as less blood is pumped into the arterial circulation. The resultant increase in thoracic blood volume increases CVP.
The increase in blood volume raised mean systemic pressure and shifted the venous return curve to the right in a parallel manner. Notice that, at each level of right atrial pressure, the rate of venous return was greater at higher levels of mean systemic pressure, due to the greater pressure gradient for venous return.
For example, if systemic venous return is suddenly increased (e.g., changing from upright to supine position), right ventricular preload increases leading to an increase in stroke volume and pulmonary blood flow. In this way, an increase in venous return can lead to a matched increase in cardiac output.
Venous tone is a property of the venous system which reflects the venous resistance and pressure. An increased venous tone caused by venoconstriction leads to an increased net capillary filtration by increasing the venous resistance and venous pressure.
Respiration – During inspiration, venous return increases as the pressure in the thoracic cavity becomes more negative. This will increase the venous pressure and venous return as when flow into the veins increases, it cannot dilate to accommodate the increased blood.
During respiratory inspiration, the venous return transiently increases because of a decrease in right atrial pressure. The opposite occurs during expiration.
In a healthy person, blood pressure is greater at the aorta than at the inferior vena cava. Blood, like other fluids, moves along a pressure gradient from areas of high pressure to areas of low pressure. A hot day adds to this effect, because the loss of body water through sweating reduces blood volume.
Sympathetic activation of veins decreases venous compliance, increases central venous pressure and promotes venous return indirectly by augmenting cardiac output through the Frank-Starling mechanism, which increases the total blood flow through the circulatory system. Respiratory activity.
Second, PEEP increases intrathoracic pressure, particularly when used in focal processes. This decreases venous return and cardiac output with subsequent adverse effects on systemic blood pressure and tissue oxygen delivery.
Continuous positive airway pressure, like positive end-expiratory pressure (PEEP), increases lung volume and pleural pressure and usually decreases venous return. The dogma that PEEP decreases venous return by decreasing the pressure gradient driving blood from the systemic vessels to the heart appears to be untrue.
During respiratory inspiration, the venous return transiently increases because of a decrease in right atrial pressure. The opposite occurs during expiration.
Although it fluctuates during inspiration and expiration, intrapleural pressure remains approximately –4 mm Hg throughout the breathing cycle.
Effects of Gravity on Venous Return. Gravitational forces significantly affect venous return, cardiac output, and arterial and venous pressures. This shift in blood volume decreases thoracic venous blood volume (CV Vol) and therefore central venous pressure (CVP) decreases.
