Endocrine System and Heart: Anatomy & Physiology Study Notes

Endocrine System

Overview of the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. It plays a crucial role in maintaining homeostasis, growth, metabolism, and reproduction.

• Hormones: Chemical messengers produced by endocrine glands that regulate activities in target organs.

• Regulation: Hormone secretion is controlled by feedback mechanisms, primarily negative feedback.

• Major Glands: Hypothalamus, pituitary (anterior/posterior), thyroid, adrenal, pancreas, gonads, etc.

• Pathway: Hypothalamus → Pituitary (anterior/posterior) → Target organ → Hormone

• Feedback: Positive/negative feedback mechanisms regulate hormone levels.

Additional info: Hormones act on specific target cells with appropriate receptors, and their effects can be widespread or highly localized.

Heart

Circulation and Blood Flow

The heart is a muscular organ responsible for pumping blood throughout the body via the circulatory system. It ensures the delivery of oxygen and nutrients and the removal of waste products.

• Circulation: Refers to the movement of blood (oxygenated and deoxygenated) through the heart and body.

• Pathway of Blood: Blood flows through a series of chambers and valves in a specific sequence.

• Oxygenation: Blood is oxygenated in the lungs and deoxygenated in the tissues.

• Widow Maker: Refers to the left anterior descending (LAD) artery; blockage can be fatal.

• Heart Muscle: Does not regenerate after injury (e.g., heart attack).

Heart Valves

Heart valves ensure unidirectional blood flow and prevent backflow by opening and closing in response to pressure changes.

• Atrioventricular (AV) Valves:

  • Tricuspid valve: Between right atrium and right ventricle (three cusps).

  • Bicuspid (mitral) valve: Between left atrium and left ventricle (two cusps).

• Semilunar Valves:

  • Pulmonary semilunar valve: Between right ventricle and pulmonary trunk.

  • Aortic semilunar valve: Between left ventricle and aorta.

• Chordae Tendineae: Anchor AV valve cusps to papillary muscles, preventing valve prolapse.

• Valve Function: No valves between major veins and atria due to inertia and heart contraction compressing venous openings.

Pathway of Blood Through the Heart

Blood flows through the heart in a specific sequence, ensuring separation of oxygenated and deoxygenated blood.

• Right Side:

  • Superior/inferior vena cava → Right atrium → Tricuspid valve → Right ventricle → Pulmonary valve → Pulmonary arteries → Lungs

• Left Side:

  • Pulmonary veins → Left atrium → Mitral valve → Left ventricle → Aortic valve → Aorta → Systemic circulation

• Equal Volumes: Both sides pump equal volumes; systemic circuit is high pressure, pulmonary is low pressure.

Coronary Circulation

Coronary circulation supplies blood to the heart muscle itself, ensuring its metabolic needs are met.

• Arteries:

  • Right and left coronary arteries arise from the base of the aorta.

  • Arteries contain anastomoses (junctions) for collateral circulation, but not enough to prevent damage from occlusion.

  • Left coronary artery: Supplies interventricular septum, anterior ventricular walls, left atrium, and part of left ventricle.

  • Right coronary artery: Supplies right atrium and most of right ventricle.

• Veins:

  • Coronary sinus: Main vein draining into right atrium.

  • Great, middle, and small cardiac veins: Drain different regions of the heart.

  • Anterior cardiac veins: Drain directly into right atrium.

Comparison: Skeletal vs. Cardiac Muscle

The following table summarizes the key differences between skeletal and cardiac muscle:

Feature	Skeletal Muscle	Cardiac Muscle
Structure	Long, cylindrical, multinucleate	Short, branched, 1-2 nuclei
Junctions	No gap junctions	Gap junctions at intercalated discs
T tubules	Abundant, narrower	Fewer, wider
Sarcoplasmic reticulum	More elaborate	Less elaborate; no terminal cisterns
Source of Ca2+	Sarcoplasmic reticulum only	Sarcoplasmic reticulum and extracellular fluid
Pacemaker cells	No	Yes (autorhythmicity)
Supply of ATP	Aerobic and anaerobic (fewer mitochondria)	Aerobic only (many mitochondria)

Electrical Activity and Heartbeat Coordination

The heart's electrical system ensures coordinated contraction and efficient pumping.

• Pacemaker cells: Specialized cells with unstable resting membrane potentials (pacemaker potentials).

• Intrinsic conduction system: Network of autorhythmic cells that initiate and distribute impulses.

• Gap junctions: Allow rapid spread of electrical impulses.

• Autonomic nervous system: Modifies heart rate and force via sympathetic (increases) and parasympathetic (decreases) input.

Sequence of Excitation

1. Sinoatrial (SA) node: Pacemaker; initiates impulse (~75x/min).

2. Atrioventricular (AV) node: Delays impulse (~0.1s) to allow atrial contraction.

3. Atrioventricular (AV) bundle (Bundle of His): Only electrical connection between atria and ventricles.

4. Right and left bundle branches: Carry impulses through interventricular septum.

5. Subendocardial conducting network (Purkinje fibers): Distribute impulse through ventricles.

Action Potentials in Cardiac Muscle

• Contractile muscle fibers: Responsible for pumping action; have a plateau phase in action potential.

• Phases:

  1. Depolarization: Na+ influx

  2. Plateau: Ca2+ influx prolongs depolarization

  3. Repolarization: K+ efflux

• Benefit: Longer action potential ensures efficient ejection of blood and prevents tetanic contractions.

Electrocardiography (ECG/EKG)

ECG is a graphic recording of the heart's electrical activity, representing all action potentials at a given time.

• P wave: Depolarization of SA node and atria

• QRS complex: Ventricular depolarization and atrial repolarization

• T wave: Ventricular repolarization

• P-R interval: Beginning of atrial excitation to beginning of ventricular excitation

• S-T segment: Entire ventricular myocardium depolarized

• Q-T interval: Beginning of ventricular depolarization through ventricular repolarization

Mechanical Events of the Heart

• Systole: Period of heart contraction

• Diastole: Period of heart relaxation

• Cardiac cycle: All events associated with blood flow through the heart during one heartbeat

Formulas and Equations

• Cardiac Output (CO): The amount of blood pumped by each ventricle in one minute. $CO = HR \times SV$ Where:

  • $CO$ = Cardiac Output

  • $HR$ = Heart Rate (beats per minute)

  • $SV$ = Stroke Volume (mL per beat)

Additional info: The notes above are expanded with standard academic context for clarity and completeness, including definitions, examples, and a summary table for muscle comparison.