Cardiac Physiology Notes

Cardiac Physiology

Describe the basic anatomy, function, and organization of the heart and the circulatory system.
Explain how the systemic and pulmonary circulations are linked physically and physiologically.
Describe the basic functional anatomy of the atrioventricular and semilunar valves and explain how they operate.
Diagram one-way flow through the heart and explain the relationship between pressure and valve function.

Cardiomyocytes:
- Striated, 10-20 µm wide, 50-100µm long
- 1-2 nuclei
- 30-35% of cellular volume is occupied by mitochondria
Conduction of action potentials to neighboring cells occurs through gap junctions in the intercalated discs
Ion channels: K+, Na+, Ca2+
Cardiac Contractility depends on:
- Size of [Ca^{2+}]transient
- Sensitivity of myofilaments to [Ca^{2+}]transient

Bundles of cardiac muscle are arranged spirally around the ventricle.
When they contract, they “wring” blood from the apex to the base where the major arteries exit.
Intercalated discs contain two types of membrane junctions:
- Mechanically important desmosomes that hold the cardiac cells together
- Electrically important gap junctions that link the cells of each chamber into a functional syncytium.

Cells (fibers)
- Skeletal muscle: Long tube-shaped, multinucleated cells, aligned with nearby cells
- Cardiac muscle: Long, branching cells with single nuclei aligned with nearby cells
Striations: Yes for both
Location of nuclei
- Skeletal muscle: Periphery of cell
- Cardiac muscle: Center of cell
T tubules
- Skeletal muscle: Part of triads at A-I junction
- Cardiac muscle: Part of dyads at Z disc
Sarcoplasmic reticulum
- Skeletal muscle: Abundant with two terminal cisterns in the triads
- Cardiac muscle: Less abundant with one terminal cistern per sarcomere in dyads
Distinctive structural features
- Skeletal muscle: Highly organized sarcomeres & triads
- Cardiac muscle: Intercalated discs with adhesion & gap junctions
Contraction mechanism(s): Similar to skeletal muscle for Cardiac Muscle
- Skeletal muscle: Ca^{2+} binding to troponin C exposes myosin binding site on actin
Connective tissue
- Skeletal muscle: Endomysium, perimysium, & epimysium
- Cardiac muscle: Endomysium, subendocardial, & subpericardial
Locations
- Skeletal muscle: Skeletal muscle, tongue, upper esophagus, eyes
- Cardiac muscle: Heart
Innervation
- Skeletal muscle: Motor for voluntary movement
- Cardiac muscle: Autonomic for involuntary pumping of blood
Growth/renewal
- Skeletal muscle: Hypertrophy, limited renewal involving satellite cells
- Cardiac muscle: Hypertrophy, little/no renewal

Includes:
- Superior Vena Cava
- Right Coronary Artery
- Left Coronary Artery
- Great Cardiac Vein

Pulmonary circulation
- Closed loop system carrying blood from the heart to the lungs
- Low Pressure – Low Resistance
Systemic circulation
- Circuit of vessels carrying blood between the heart and all body systems (except lungs)
- High Pressure – High Resistance

Backflow into veins from atria is not a significant problem because of:
1. Atrial pressures are not much higher than venous pressures
2. Sites where the venae cavae enter the atria are partially compressed during atrial contraction

Explain the ionic mechanism of pacemaker automaticity and rhythmicity and identify cardiac cells that have pacemaker potential and their spontaneous rate. Identify neural and humoral factors that influence their rate.
Describe the excitation-contraction coupling in cardiac contractile cells. Compare and contrast this behavior to skeletal muscle contractions.
Beginning in the SA node, diagram the normal sequence of cardiac activation (depolarization) and the role played by specialized cells. Predict the consequence of a failure to conduct the impulse through any of these areas.

Cardiac autorhythmic cells do not have a resting potential. Instead, they have a pacemaker potential.
Sodium and Potassium are essential for excitation
Calcium is responsible for contraction

Membrane clock mechanism
Ca^{2+} clock mechanism
Self depolarizing the membrane of autorhythmic cells to threshold
Ca^{2+} is released from the sarcoplasmic reticulum (SR)
Each time the cytosolic Ca^{2+} concentration rises, the Na^+-Ca^{2+} exchanger repetitively transports one intracellular Ca^{2+} ion out for every 3 extracellular Na^+ ions it moves in …part of slow depolarization to keep positive charge

Specialized non-contractile cardiac cells capable of auto-rhythmicity reside in the following locations:
- Sinoatrial (SA) Node
- Atrioventricular (AV) Node
- Bundle of His
- Purkinje Fibers

SA Node: 70-80 APs/min (Normal Pacemaker Activity)
AV Node: 40-60 APs/min (SA node damage, AV node takes over!)
Bundle of His: 20-40 APs/min (Impulse conduction between atria and ventricle is blocked)
Purkinje Fibers: 20-40 APs/min (Ectopic focus – abnormal excitation of Purkinje Fibers; 100-140 APs/min; premature ventricular contraction)