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Arterial recoil, the muscle pump, respiratory movements, venous tone and the mechanics of cardiac filling assist blood circulation. The heart remains its principal continuing source of mechanical energy. These agencies differ in whether they store cardiac work, contribute new muscular work, or direct flow without supplying power.

During ventricular systole, the aorta and other large elastic arteries expand and store elastic potential energy. During diastole, their recoil continues to drive blood towards the peripheral vessels, while closure of the aortic valve limits return into the heart. This elastic reservoir smooths an intermittent cardiac input into a more continuous downstream flow. It does not manufacture extra energy: it releases part of the work performed during the preceding ejection.

Contraction of surrounding skeletal muscles compresses veins. Venous valves favour displacement towards the heart and limit backward flow when the compression is released. Alternating contraction and relaxation therefore produces a muscle pump, especially useful for return from dependent limbs. The added work comes from the contracting muscles; valves guide the displacement but supply no power themselves. Persistent compression without refilling is not equivalent to a series of pumping strokes.

Spontaneous inspiration supplies another assistance. Descent of the respiratory diaphragm raises abdominal pressure while expansion of the thorax lowers intrathoracic pressure. The resulting changes favour a pressure gradient towards the thoracic veins and right side of the heart. The effect depends on posture, the respiratory phase and whether vessels remain open. Positive-pressure inflation does not reproduce the same pressure changes as spontaneous inspiration.^[ref]

Venous smooth muscle can reduce the storage capacity of peripheral veins, transferring blood towards the central circulation. This changes the volume available for cardiac filling; it does not require a travelling wave of venous peristalsis. Relaxation and recoil of the heart, together with movement of the atrioventricular plane, also influence filling pressures. The older expression cardiac suction is therefore best interpreted through specific changes of chamber volume and pressure, rather than a separate force pulling uniformly through every vessel.

Gravity assists a downward column and opposes an upward one. For a complete circuit returning to its initial height it supplies no net gravitational work. Its important effects include hydrostatic pressure differences and dependent pooling. Likewise, the residual pressure gradient across the capillaries is chiefly the downstream consequence of the cardiac pump. Ordinary capillary action at a liquid meniscus is not a universal extra pump inside a closed, fluid-filled vascular network. Local vasodilation can improve perfusion by reducing vascular resistance, but it does not create the necessary pressure source.

With $Q$ the mean volumetric flow rate, $R$ an effective vascular resistance, and $\Delta P$ the driving pressure difference, the useful bookkeeping relation isIt separates an increase of driving pressure from a reduction of resistance. An instructive apparatus compares the outflow of a pulsatile pump through rigid tubing with its outflow through an elastic reservoir feeding the same resistance. Continued outflow between strokes demonstrates stored elastic potential energy. A separately compressed valved flexible tube demonstrates the additional muscle pump principle. Neither observation requires a mysterious attraction of blood by the tissues.

The heart supplies the principal continuing mechanical input to blood circulation; arterial recoil, respiratory movements and the action of skeletal muscles redistribute and supplement that input. They have different sources of energy and should not all be called independent propulsive powers. The pressure gradient remaining after passage through the capillaries is still a consequence of the upstream pump.

Arterial elasticity. During ventricular systole, ejected blood expands the aorta and large arteries, storing elastic potential energy in their walls. During diastole, recoil continues to drive blood into the peripheral vessels while closure of the aortic valve limits return towards the heart. This reduces the intermittency of flow downstream. The recoil is a temporary store of cardiac work, rather than a fresh source of energy. A vessel that is too rigid provides less of this smoothing; a compliant vessel with an open backward outlet would not provide the same useful result.

The skeletal-muscle pump. Contracting skeletal muscles compress adjacent veins. Venous valves make the displacement preferentially towards the heart; when compression ceases, refilling permits another stroke. Walking therefore aids return from dependent limbs. The source of the added work is muscle contraction, and the valves guide it without producing energy. Merely possessing valves cannot make a stationary column circulate.

The respiratory pump. During inspiration, falling intrathoracic pressure and displacement of the abdominal contents favour a pressure gradient from abdominal and peripheral veins towards the thoracic veins and right side of the heart. The respiratory diaphragm thus participates indirectly in venous return as well as pulmonary ventilation. The exact flow varies with posture, vessel collapse and the phase of the respiratory cycle; positive-pressure inflation does not reproduce the same mechanical circumstances as spontaneous inspiration.^[ref]

Venous tone and cardiac filling. Sympathetic contraction of venous smooth muscle can reduce venous storage capacity and transfer blood towards the central circulation. This alters the volume available for filling rather than requiring a wave of venous peristalsis. Cardiac relaxation, recoil and movement of the atrioventricular plane also affect filling pressure. Some traditionally described cardiac suction is therefore a feature of the cardiac cycle and mechanical coupling, not a separate organ pulling blood through every vessel.^[ref]

Gravity and supposed capillary attraction. Gravity assists a downward column but opposes an upward one. In a complete circuit returning to its original height it supplies no net gravitational work; its important practical effects include hydrostatic pressure differences and dependent venous pooling. Ordinary capillary action in an open narrow tube, driven by a liquid meniscus, is not a general pump in a fluid-filled capillary network. Nor does a vague attraction of blood by tissues replace a measurable pressure gradient. Local vasodilation can increase perfusion by lowering vascular resistance, but that change does not manufacture the necessary pressure source.

For a simple mean-flow description, with $Q$ the volume flow rate, $\Delta P$ the driving pressure gradient and $R$ the effective vascular resistance,This relation distinguishes a source of driving pressure from a change of resistance. In a lumped compliant arterial reservoir with vascular compliance $C$ and no inflow between beats, volume balance giveswhere $P_v$ is the downstream pressure. The exponential decay follows from stored volume leaving through the resistance. It explains continued flow during diastole without claiming that the idealization describes every reflected arterial wave.

A physical demonstration can compare a pulsatile pump feeding a narrow outlet through rigid tubing with the same pump feeding it through an elastic reservoir. Collecting the outflow over the pumping and resting phases reveals the reservoir's smoothing action; a side-arm manometer records its falling pressure. Separately compressing a valved flexible tube shows the role of a peripheral muscle pump. Keeping pump stroke and outlet resistance comparable prevents the apparatus from conflating a smoother flow with creation of extra energy.