Cardiovascular System: Heart Anatomy and Function

• The heart's blood supply equals coronary circulation.

The Coronary Arteries

• The left and right coronary arteries originate at the aortic sinuses, at the base of the ascending aorta.
• High blood pressure and elastic rebound forces drives blood through the coronary arteries between contractions.
• Myocardial flow peaks during relaxation and stops during contraction.

Anatomy of the Heart

• The epicardium is the outer layer, also known as the visceral pericardium.
• The myocardium is the middle layer consisting of cardiac muscle tissue.
• The endocardium is the inner layer composed of simple squamous epithelium.

Cardiac Muscle Tissue

• Intercalated discs contain desmosomes and gap junctions.
• Intercalated discs convey the force of contraction.
• Intercalated discs propagate action potentials.

Structural and Functional Differences

• Cardiac muscle cells are 10-20 μm × 50-100 μm in size.
• Skeletal muscle fibers are 100 μm x up to 40 cm in size.
• Cardiac muscle cells contain typically 1 nucleus, but rarely 2-5.
• Skeletal muscle fibers contain multiple nuclei (hundreds).
• Both cardiac muscle and skeletal muscle fibers contain sarcomeres along myofibrils.
• Cardiac muscle cells have short T tubules and no triads formed with sarcoplasmic reticulum.
• Skeletal muscle fibers have long T tubules form triads with cisternae of the sarcoplasmic reticulum
• Cardiac muscle cells are abundant with 25% being cell volume.
• Skeletal muscle fibers are much less abundant.
• Cardiac muscle cells contains myoglobin, lipids, and glycogen.
• Skeletal muscle fibers contains, Little myoglobin, few lipids, but extensive glycogen reserves.
• Blood supply is very extensive to cardiac muscle cells.
• Blood supply is more extensive than in most connective tissues, but sparse compared with supply to cardiac muscle cells to skeletal muscle fibers.
• Cardiac muscle cells use aerobic metabolism, primarily using lipids and carbohydrates.
• Skeletal muscle fibers use aerobic metabolism, primarily lipid-based.
• Cardiac muscle cells use aerobic metabolism when active, primarily using lipids and carbohydrates.
• Skeletal muscle fibers us anaerobic metabolism when active, through breakdown of glycogen reserves.
• Cardiac muscle cell contractions include twitches with brief relaxation periods; long refractory period prevents tetanic contractions.
• Activity of somatic motor neuron generates action potentials in sarcolemma for skeletal muscle fiber contraction.
• Intercellular connections have a branching network with plasma membranes locked together at intercalated discs; connective tissue fibers tie adjacent layers together for cardiac muscle cells.
• Adjacent fibers are tied together by connective tissue fibers for skeletal muscle fibers.

The Conducting System

• Two types of cardiac muscle cells exist: cells of the conducting system and contractile cells.
• The cells of the conducting system control and coordinate the heartbeat.
• The contractile cells produce contractions.
• The sinoatrial, or SA, node is located in the wall of the right atrium.
• The atrioventricular, or AV, node is located at the junction between the atria and ventricles.

Prepotential or Pacemaker Potential

• As the resting potential of conducting cells gradually depolarizes toward threshold the SA nodes depolarize first, establishing the heart rate.
• Automaticity is when muscle contracts automatically.
• The SA node generates 80–100 action potentials per minute.
• The AV node generates 40–60 action potentials per minute
• Parasympathetic stimulation slows heart rate.
• Conducting cells interconnect SA and AV nodes.
• Conducting cells distribute stimulus through the myocardium.
• Internodal pathways are located in the atrium.
• The AV bundle, bundle branches, and Purkinje fibers are located in the ventricles.
• Step 1 in impulse conduction is when SA node activity begins and atrial activation also begins and is timed at 0 msec.
• Step 2 in impulse conduction is when the stimulus spreads across the atrial surfaces and reaches the AV node and is timed at 50 msec.
• Step 3 in impulse conduction is when there is a 100-msec delay at the AV node. Atrial contraction begins. and is timed at 150 msec.
• Step 4 in impulse conduction is when the impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers and, via the moderator band, to the papillary muscles of the right ventricle and is timed at 175 msec.
• Step 5 in impulse conduction is when the impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins and is timed at 225 msec.

Abnormal Pacemaker Function

• Bradycardia is an abnormally slow heart rate.
• Tachycardia is an abnormally fast heart rate.
• An ectopic pacemaker contains abnormal cells, generate a high rate of action potentials, bypasses the conducting system, and disrupts ventricular contractions.

Electrocardiogram - ECG or EKG

• The P wave equals atria depolarizing.
• The QRS complex equals ventricles depolarizing.
• The T wave equals ventricles repolarizing.
• The P-R interval goes from the start of atrial depolarization to the start of QRS complex.
• The Q-T interval goes from ventricular depolarization to ventricular repolarization

Contractile Cells

• Purkinje fibers distribute stimulus to contractile cells that make up most of muscle cells in the heart.
• The resting potential of ventricular cells is about -90 mV.
• The resting potential of atrial cells is about -80 mV.

Action Potential in Cardiac Muscle

• During rapid depolarization, Na+ enters. This lasts 3-5 msec and ends with closure of voltage-gated, fast, sodium channels.
• During the plateau phase, Ca2+ enters. This lasts ~175 msec and ends with closure of slow calcium channels.
• During repolarization, K+ is loss. This lasts 75 msec and ends with closure of slow potassium channels.

Absolute versus Relative Refractory Period

• Absolute refractory period is long and the muscle cannot respond.
• Relative refractory period is shorter and the response depends on degree of stimulus
• A long refractory period prevents summation and tetany.

Calcium Ions in Cardiac Contractions

• Contraction is produced by increase in calcium ion concentration
• Calcium enters through slow calcium channels during the plateau phase.
• Extracellular Ca2+ then triggers release of calcium reserves from sarcoplasmic reticulum (SR).
• As channels close, intracellular Ca2+ is absorbed by the SR or pumped out of the cell.
• Aerobic energy of the heart is from mitochondrial breakdown of fatty acids and glucose.
• Oxygen comes from circulating hemoglobin and cardiac muscles store oxygen in myoglobin.

Cardiac Cycle

• Cardiac cycle is the period between the start of one heartbeat and the beginning of the next.
• Systole is contraction.
• Diastole is relaxation.