Cardiac Physiology: Contraction, Conduction, and Cycle

Intrinsic Regulation of Contraction

a) Relationship between length and tension

Muscle fiber (Frank-Starling Law) ownership of the heart to contract in proportion to its completeness (a more complete, higher volume of projection) to a level where more volume increases are not accompanied by spending.

Frank-Starling law: The higher the fill of the heart during diastole, the greater the force of contraction during systole. Therefore, tension increases as diastolic volume increases until it reaches a maximum, then it tends to decrease.

Speed Conduction in the Heart Muscle

Atrial and ventricular muscle fibers:

• 0.3 to 0.5 m/sec.
• 1/250 speed of large nerve fibers.
• 1/10 speed of skeletal muscle fibers.

Purkinje system: 4 m/sec.

The action potential can traverse the entire heart in 220 msec.

The contraction of the heart muscle normally lasts 300 msec.

Cardiac Cycle

The top beats until the beginning of the other.

• Delayed 1/sec. from stimulus to ventricular fibrillation

Period of relaxation: Diastole

Period of contraction: Systole

Emptying of Ventricles During Systole

i) Period of isovolumetric contraction: Begins with ventricular contraction (decrease in pressure of the ventricle).

• AV valves closing.
• It is expected an increase in pressure of the ventricle to open semilunar valves (aortic and pulmonary).
• Ventricular subscription there.

ii) Period of Expulsion: An increase in pressure of the ventricle above 80 mmHg, opening semilunar valves.

• Rapid ejection Period: 1st third empty # 70%.
• Slow expulsion period: 30% in the next 2 thirds.

iii) Period of isovolumetric relaxation: At the end of systole (relaxation of the ventricle) it decreases intraventricular pressure and closes semilunar valves.

• Ventricular muscles continue relaxing without ventricular volume change.
• AV valves are opened

iv) Volume Telediastolic, end-diastolic volume (amount to 110-120 ml).

• V. Heartbeat: volume of blood ejected by the ventricle in a heartbeat (about +- 70 ml)
• V. Telesystolic: volume remaining in each ventricle is 40-50 ml. The fraction of end-diastolic volume ejected is called expulsion or ejection fraction

Preload

The degree of muscle tension when it begins to contract. Diastolic pressure. Passive tension in the ventricular wall when the contraction starts and is determined by end-diastolic volume.

Afterload

The load against which the muscle exerts ventricular contractile force. Afterload is the pressure of the artery from the ventricle: "Pressure of blood against the ventricular contraction. It is governed by the Frank-Starling: a > preload > fiber elongation and > contractile force.

Cardiac Pumping Regulation

Intrinsic Regulation: Frank-Starling mechanism.

Extrinsic Regulation: Control of the heart by the SNS and PSNS.

Sinus Node

Specialized muscle tissue.

• Superolateral posterior wall of the right atrium
• Almost completely lacks contractile filaments.
• Its fibers connect directly with the atrial muscle fibers.
• Basically the intrinsic permeability of the fibers of the node Na.
• Every 2 beats increases in pressure of Mb
• Voltage threshold is -40 mv.

Located in the atrioventricular node in the atrial posterior wall right behind the tricuspid valve.

• A driving Delay
• Delay Conduction in the AV
• Allow adequate ventricular filling.
• From the sinus node to ventricular contractile muscle is 0.16 seconds.

Reason for slow conduction; few communicating junctions.

Transmission in Purkinje Fibers in the Ventricular System

Very large, P. Action 1.5 to 4.0 m/sec., 6 times greater than ventricular muscle and 150 times greater than some AV node fibers. Immediate transmission of the ventricular muscle. Increased permeability of gap junctions.

Transmission of Pulses in the Ventricular Cardiac Muscle

0.3 to 0.5 m/sec. From endocardium to epicardium only 0.03 sec.