 Solution

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.