The veins act as a blood reservoir, because of the ease of distending them. 55-60% of blood is in systemic veins which is important for blood pressure. The volume of blood returned to the right atrium of the heart per minute is called the venous return. Generally venous return is equal to cardiac output, due to the closed system of the circulation, except for minor transient periods. A number of factors affecting the venous return are: Smooth muscle: Due to presence of smooth muscle, the diameter of the veins can be altered by sympathetic discharge(normally vasoconstrictive), adrenaline and angiotensin II (cause venoconstriction). But because the diameters of veins are large, venoconstriction has little effect on peripheral resistance. Elasticity of the heart: If the elasticity decreases, then the ability to fill the heart is reduced. valves: one way valves in the veins (not in large veins) ease blood flow to heart. Muscle pump: When muscles, particularly leg muscles contract, the thin walled veins are squeezed and the blood is forced towards heart, when muscles relax, blood enters the veins, but only from the arterial side. This is an important mechanism for facilitating venous return. Respiratory pump: Breathing in causes negative pressure in thorax causing a suction effect, i.e. the pressure gradient pushing blood towards the right atrium is increased, thus more blood returns to the heart, which means that inspiration leads to an increase in venous return. The opposite occurs with expiration, and venous return is decreased. Gravity: Opposes return of blood from the periphery during sitting or standing. The effect is lost when we lie down. Blood pressure at venous end of capillary: An increased pressure at the venule end of the capillary reduces venous return, because the pressure difference between the right atrium and the peripheral veins is reduced. Similarly a decrease in pressure at the venule end would increase venous return. Right atrial pressure: Is the inflow pressure for the heart. It is a function of the amount of blood returned to the heart and the pumping ability of the heart. Its value at rest is normally close to zero but can also become negative (between -4 to 5 mmHg). Its value is increased by blood volume, a weak heart, rapid increase in venous return. Its value is decreased by lower blood volume and a strong heart. The lower the pressure the greater the venous return (due to suction effect), but there is a limit to the increase in venous return because veins are collapsible.
Valves aid in venous return by preventing the back flow of blood.
No, lift legs for venous return. Make sure legs are above heart. You do this for shock or bleeding.
increase venous return
The right atrium.
Increase venous returns increases end diastolic volume (EDV)
exhalation, the venous pump - muscle contraction, and sypmpathetic stimulation of the vascular bed
pressure gradient , gravity , skeletal muscle pump, thoracic pump and cardiac suction would be the mechanisms for venous return.
because the endothoracic pressure is decreased and the venous return to the heart is increased.
Return blood, most likely, refers to the venous blood that is returned to the heart.
When venous return is increased
the pumping action of the heart.
Contraction of veins would increase venous return to the right atrium. It would increase the efficiency of the heart pumping.
Venous return (VR) is the flow of blood back to the heart. Under steady-state conditions, venous return must equal cardiac output (CO) when averaged over time because the cardiovascular system is essentially a closed loop. Otherwise, blood would accumulate in either the systemic or pulmonary circulations.
Gravity does not assist in venous blood flow. If you really wanted gravity to help with venous blood flow you would have to hang in an inversion machine so you are upside down.
Venous return must always match cardiac output. So a CO of 7.5 l/m mill mean a VR od 7.5 l/m
Venous return controls EDV (end diastolic volume) and thus stroke volume and cardiac output. Venous return is dependent on: - blood volume and venous pressure - vasoconstriction caused by the sympathetic nervous system - skeletal muscle pumps - pressure drop during inhalation