The Heart as a Pump
Starting with the Most Basic of Principles
All functional devices work on the principal that they receive process and export.
A pump is a mechanical device that moves gases or fluids. In order for a pump to be effective it must pump out what it receives.
In order for the heart to be effective in the body it needs to pump out what the body needs and able to receive what is returned to it.
The heart is a positive displacement pump meaning that it causes the blood to move by first trapping a fixed amount of blood in the chambers, and then forcing the blood into the outflow system by contraction. It accomplishes this by a reciprocal motion of relaxation and contraction. The pumping chamber reduces its volume by muscular contraction, while the receiving chamber increases its volume by relaxation. The contraction forces the blood to move forward, and the relaxation creates a relative vacuum allowing blood to be sucked into the heart.
The heart consists of two such pumps that are connected in series allowing for a pulmonary circulation and a left sided systemic circulation
The power added to the fluid flow by the pump (Po), is defined using SI units by:
PO is = ρ is the fluid density (kg/m3) X g is the gravitational constant (9.81 m/s2) X H is the energy Head added to the flow (m) X Qis the flow rate (m3/s)
- PO is the output power of the pump (W)
- ρ is the fluid density (kg/m3)
- g is the gravitational constant (9.81 m/s2)
- H is the energy Head added to the flow (m)
- Q is the flow rate (m3/s)
Power is more commonly expressed as kW (103 W) or horsepower (multiply kW by 0.746), H is equivalent to the pressure head added by the pump when the suction and discharge pipes are of the same diameter. The power required to drive the pump is determined by dividing the output power by the pump efficiency
In an average lifetime, the heart beats more than two and a half billion times. Think about this number a few times – imagine what an unbelievable engineering feat nature has accomplished. In fact it is not just a pump that pumps in and out. There is second to second, and beat to beat variation so that it continuously changes its patterns of contraction and relaxation, always accommodating bodily needs. The heart circulates the body’s blood supply about 1,000 times each day, circulating 5-6000 litres of blood through 62,000 miles (length of two and half times around the earth). An average adult body contains about five liters of blood.
The beat to beat to beat variation allows the output from the left to be reconciled with what is coming back in the return, so that congestion does not occur. In addition physiology is a dynamic process and the demands of each system in the body changes based on special needs and activities. The heart also has to accomodate to these changing needs distributing a fixed amount of blood to the regions that need it most at any one time. During excercise for example the skeletal muscle is in great demand, while the gastrointestinal system is quiet. Blood has to be shunted from the one to the other. The controlling mechanisms at both the local and systemic levels are simply elegant, complex, but effective.