Chapter 09: The Heart as a Pump

Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/neurotransmitters-at-smooth-muscle.html

The Heart as a Pump

The heart functions as a remarkably efficient dual-pressure pump designed to drive continuous, unidirectional blood circulation throughout the human body. it is divided into two distinct functional units operating in series: the right heart, which receives oxygen-depleted blood from systemic circulation and propels it into the low-resistance pulmonary circuit for gas exchange, and the left heart, which collects oxygenated blood from the lungs and pumps it under high pressure through the systemic arterial system to nourish peripheral tissues. Each pump consists of an atrium that acts primarily as a weak primer pump to facilitate ventricular filling, and a powerful ventricle that supplies the main force required to propel blood forward. The mechanical efficiency of this pumping system relies heavily on the specialized structure of cardiac muscle, which forms a functional syncytium, allowing electrical action potentials to spread rapidly and induce coordinated, rhythmic atrial and ventricular contractions. Furthermore, the orderly movement of blood without backflow is maintained by a system of specialized, pressure-sensitive heart valves including the atrioventricular and semilunar valves that open and close passively in response to pressure gradients generated during each cardiac cycle.

• The heart functions as two separate pumps.
• The right heart pumps blood through lungs.
• The left heart pumps blood through organs.
• Each side contains an atrium and ventricle.
• Atria act as weak primer pumps.
• Ventricles supply the main pumping force.
• Continuous electrical signals cause cardiac rhythmicity.

Types of Cardiac Muscle

• Atrial muscle contracts similar to skeletal muscle.
• Ventricular muscle also contracts like skeletal muscle.
• Contraction duration is much longer than skeletal muscle.
• Conductive fibers contract feebly with few fibrils.
• Conductive fibers generate and conduct action potentials.

Anatomy of Cardiac Muscle

• Muscle fibers form an interconnecting latticework pattern.
• Cardiac muscle is striated like skeletal muscle.
• Myofibrils contain sliding actin and myosin filaments.
• Intercalated discs separate individual cardiac muscle cells.
• Gap junctions at discs allow rapid ion diffusion.
• Action potentials travel easily from cell to cell.

Action Potentials in Cardiac Muscle

• Ventricular action potential averages about 105 millivolts.
• Resting intracellular potential is about −85 millivolts.
• Peak intracellular potential reaches about +20 millivolts.
• Membrane remains depolarized during a plateau phase.
• Plateau lasts for about 0.2 seconds.
• Contraction lasts up to 15 times longer than skeletal muscle.

Causes of Long Action Potential and Plateau

• Fast sodium channels open briefly during the initial spike.
• Slow calcium channels open and stay open longer.
• Influx of calcium and sodium maintains the plateau.
• Calcium influx directly activates the contractile process.
• Membrane permeability to potassium decreases fivefold initially.
• Decreased potassium outflux prevents early repolarization.
• Slow channels close after 0.2 to 0.3 seconds.
• Increased potassium outflux rapidly ends action potential.

Velocity of Signal Conduction in Cardiac Muscle

• Conduction velocity in atrial and ventricular muscle is 0.3 to 0.5 m/sec.
• It is about 1/250 the speed of large nerve fibers.
• It is about 1/10 the speed of skeletal muscle.
• Conduction velocity in Purkinje fibers reaches up to 4 m/sec.

Refractory Period of Cardiac Muscle

• Cardiac muscle cannot be re-excited during its refractory period.
• Normal ventricular refractory period lasts 0.25 to 0.30 seconds.
• The relative refractory period lasts an additional 0.05 seconds.
• Strong signals can excite muscle during relative refractory periods.
• The atrial refractory period is shorter, lasting about 0.15 seconds.

Excitation-Contraction Coupling

• Action potentials travel into fibers through transverse (T) tubules.
• T tubule signals stimulate sarcoplasmic reticulum calcium release.
• Calcium diffuses to myofibrils to catalyze sliding filaments.
• Calcium ions also enter directly through T tubule membranes.
• T tubule calcium opens ryanodine receptor channels in sarcoplasmic reticulum.
• Sarcoplasmic reticulum of cardiac muscle is less developed.
• Cardiac T tubule diameter is 5 times larger than skeletal muscle.
• The tubule volume is 25 times larger than skeletal muscle.
• Electronegative mucopolysaccharides inside T tubules bind calcium stores.
• Heart contraction depends strongly on extracellular calcium concentration.
• Hearts in calcium-free solutions quickly stop beating.
• At the plateau end, calcium influx stops immediately.
• Calcium pumps back via calcium-ATPase and sodium-calcium exchangers.

Duration of Contraction

• Contraction starts a few milliseconds after action potential begins.
• Contraction duration depends on action potential duration.
• Atrial contraction lasts about 0.2 seconds.
• Ventricular contraction lasts about 0.3 seconds.

Cardiac Cycle

• One cardiac cycle spans the start of one beat to the next.
• The sinus node spontaneously generates each cardiac cycle action potential.
• The sinus node sits in the superior lateral right atrium wall.
• A 0.1-second conduction delay occurs passing into ventricles.
• Delay allows atria to contract before ventricular contraction.
• Atria function as primer pumps for ventricles.

Diastole and Systole

• Diastole is the heart relaxation and filling period.
• Systole is the heart contraction period.
• Total cardiac cycle duration equals inverse of heart rate.
• At 72 beats/min, cycle duration is about 0.833 seconds.

Effect of Heart Rate on Cardiac Cycle

• Increased heart rate decreases total cardiac cycle duration.
• Relaxation phase (diastole) shortens more than contraction phase.
• At 72 beats/min, systole is 40 percent of the cycle.
• At triple normal rate, systole is 65 percent of cycle.
• Very fast heart rates prevent complete chamber filling.