Comprehensive Anatomy and Physiology of the Human Heart

Location and General Anatomy of the Heart

Conceptual Overview: The human heart is a muscular pump described as a powerful engine that keeps the body continually supplied with blood.
Position in the Thorax:
- Location: Within the thoracic cavity, situated medially between the lungs.
- Region: Located in the mediastinum.
- Size: Approximately the size of a fist.
- Shape: It is broad at the top and tapers toward the bottom, which is referred to as the apex.

Chambers, Circulation, and Blood Flow

Dual System of Circulation: The human blood circulation is divided into two primary circuits:
- Pulmonary Circuit:
  1. Blood flows from the right atrium into the right ventricle.
  2. The right ventricle pumps blood into the pulmonary circuit via the pulmonary artery branches.
  3. Note: Blood in the pulmonary artery is low in oxygen ( $O_2$ ) but relatively high in carbon dioxide ( $CO_2$ ).
  4. Gas exchange occurs in the pulmonary capillaries: oxygen enters the blood, and carbon dioxide exits.
  5. Oxygenated blood (high in $O_2$ , low in $CO_2$ ) returns to the left atrium.
- Systemic Circuit:
  1. Blood enters the left ventricle from the left atrium.
  2. The left ventricle pumps blood into the systemic circuit (the rest of the body).
  3. Exchange occurs in the systemic capillaries: oxygen and nutrients move out of the capillaries to tissues; carbon dioxide and wastes move into the blood.
  4. Deoxygenated blood returns to the right atrium to repeat the cycle.

Membranes and Layers of the Heart Wall

Pericardium: The heart is surrounded by the pericardial membrane, which consists of three layers and the pericardial cavity.
Heart Wall Layers: The heart wall itself consists of three distinct layers:
- Epicardium: This layer is shared between the pericardial membrane and the heart wall.
- Myometrium (Myocardium): The muscle tissue of the heart. The transcript notes the "myometrium" in the left ventricle is significantly thicker than in the right ventricle.
- Internal Musculature Pattern: Cardiac muscle tissue exhibits a swirling pattern, which is essential for effective pumping.

Ventricular Differences and Comparative Anatomy

Muscle Thickness:
- The left ventricle is much thicker than the right ventricle.
- Functional Reason: While both ventricles pump the same volume of blood, the left ventricle must generate significantly greater pressure to overcome the high resistance of the systemic circuit.
Lumens: The lumen is the region inside each ventricle where blood is contained. During contraction (systole), the size of the lumens reduces compared to the relaxed state (diastole).

Internal Structure and Valvular Anatomy

Major Components: The internal anatomy includes four chambers, major vessels (and their early branches), and valves.
Septa:
- Interatrial Septum: Separates the two atria (often covered by the pulmonary trunk and aorta in anterior views).
- Atrioventricular Septum: Separates the atria from the ventricles.
Supporting Structures for Valves:
- Papillary Muscles: Muscles located in the ventricles.
- Chordae Tendineae: String-like fibers that attach the papillary muscles to the valve flaps.
- These structures are associated with the tricuspid valve (right side) and the mitral valve (left side).
Heart Valves:
- Atrioventricular (AV) Valves: Tricuspid (3 cusps) and Mitral/Bicuspid (2 cusps). These regulate flow from atria to ventricles.
- Semilunar Valves: Pulmonary valve and Aortic valve. These regulate flow from the ventricles into the great vessels.

Cardiac Muscle and Electrical Activity

Microscopic Anatomy of Cardiac Muscle:
- Myofibrils: Composed of myofilaments arranged into sarcomeres.
- T Tubules: Transmit impulses from the sarcolemma deeper into the cell.
- Mitochondria: Very numerous to provide energy for constant work.
- Intercalated Discs: Found at the junctions between cardiac muscle cells. They consist of:
  - Desmosomes: Provide structural linkage.
  - Gap Junctions: Facilitate electrical coupling and impulse transmission.

Conduction System of the Heart

Components of the Conduction Pathway:
1. Sinoatrial (SA) Node: The primary pacemaker.
2. Internodal Pathways: Connect the SA node to the AV node.
3. Atrioventricular (AV) Node: Located in the atrioventricular septum.
4. Atrioventricular Bundle (Bundle of His).
5. Right and Left Bundle Branches.
6. Purkinje Fibers.
7. Moderator Band: Transmits the impulse to the right papillary muscle.
Steps in Cardiac Conduction:
1. SA node and conduction system begin at rest.
2. SA node initiates an action potential that sweeps across the atria.
3. The impulse reaches the AV node, where there is a delay of approximately $100\,ms$ . This allows atria to finish pumping blood before ventricular contraction.
4. The impulse travels through the AV bundle and branches to the Purkinje fibers.
5. The impulse spreads to the contractile fibers of the ventricles.
6. Ventricular contraction begins.

Membrane Potentials and Ion Movement

SA Node (Conductive Cells):
- Prepotential: A slow influx of sodium ions ( $Na^+$ ) until threshold is reached.
- Depolarization: Rapid influx of ions after threshold.
- Repolarization: Return to the starting state.
- Note: Conductive cells lack a true resting potential, allowing for spontaneous depolarization.
Cardiac Contractile Cells:
- Plateau Phase: A long phase during depolarization caused by the influx of calcium ions ( $Ca^{2+}$ ).
- Refractory Period: Extended duration allows the cell to contract fully before another electrical event can trigger it, preventing tetany.
- Contrast: Significantly longer action potential than skeletal muscle.

Electrocardiogram (ECG/EKG)

Setup: A standard 12-lead ECG involves 6 electrodes on the chest and 4 on the limbs.
Waves and Segments:
- P Wave: Represents atrial depolarization.
- QRS Complex: Represents ventricular depolarization (and masks atrial repolarization).
- T Wave: Represents ventricular repolarization.
- Intervals/Segments: PR, QT, QRS, ST intervals; P-R and S-T segments.

Detailed Phases of the Cardiac Cycle

1. Initial Diastole:
- Both atria and ventricles are relaxed.
- AV valves (tricuspid and mitral) are open; semilunar valves are closed.
- Ventricular Filling: Approximately $70\% - 80\%$ of filling occurs passively during this phase.
2. Atrial Systole:
- Follows the P wave of the ECG.
- Atria contract toward the septum, pumping the remaining blood ( $20\% - 30\%$ ), often called the "atrial kick."
- At the end of this phase, ventricles contain the End Diastolic Volume (EDV) or preload, which is approximately $130\,mL$ in a resting, standing adult.
3. Ventricular Systole (Phase 1): Isovolumic Contraction
- Follows the QRS complex.
- Ventricular pressure rises; AV valves close (producing the S1 sound).
- Semilunar valves remain closed because pressure is not yet high enough to open them.
- Volume remains constant during this initial pressure spike.
4. Ventricular Systole (Phase 2): Ventricular Ejection
- Ventricular pressure exceeds pressure in the pulmonary trunk and aorta.
- Semilunar valves open, and blood is ejected.
- Stroke Volume (SV): The amount of blood pumped, normally $70\,mL - 80\,mL$ .
- End Systolic Volume (ESV): The blood remaining in the ventricle after ejection, normally $50\,mL - 60\,mL$ .
5. Ventricular Diastole (Phase 1): Isovolumic Relaxation
- Follows the T wave.
- Ventricular pressure drops; blood flows back toward the heart, closing semilunar valves (producing the S2 sound).
- Dicrotic Notch: A small dip in blood pressure tracings caused by the closure of semilunar valves.
- AV valves remain closed; volume remains constant.
6. Ventricular Diastole (Phase 2): Late Ventricular Diastole
- Pressure in ventricles drops below atrial pressure.
- AV valves open, and blood flows into the ventricles from the atria and major veins.

Heart Sounds and Auscultation

S1 (Lub): Sound of closing AV valves during ventricular contraction.
S2 (Dub): Sound of closing semilunar valves during ventricular diastole.
Murmur: An unusual sound caused by turbulent blood flow.
Auscultation: Using a stethoscope to listen to these sounds. There are 4 primary locations on the chest, each facilitating the hearing of a specific valve.

Cardiac Physiology and Output Calculations

Cardiac Output (CO): The amount of blood pumped by each ventricle per minute.
- Formula: $CO = HR \times SV$
- Mean CO: Approximately $5.25\,L/min$ (Range: $4.0\,L/min - 8.0\,L/min$ ).
Heart Rate (HR): Measured in beats per minute (bpm).
Stroke Volume (SV): Amount of blood pumped per contraction.
Ejection Fraction: The portion of blood pumped out of the heart.
- Formula: $\frac{SV}{EDV}$
- Mean: $58\%$ (Range: $55\% - 70\%$ ).

Neural Regulation of the Heart

Control Center: Two paired cardiovascular centers located in the medulla oblongata.
Sympathetic Control (Cardioaccelerator Regions):
- Impulses travel via cardioaccelerator nerves to the SA node, AV node, atria, and ventricles.
- Neurotransmitter: Norepinephrine (NE).
- Mechanism: Binds to beta-1 receptors, opening $Na^+$ and $Ca^{2+}$ channels.
- Effect: Shortens repolarization, speeds up depolarization, increases HR.
- Clinical Note: Beta blockers treat hypertension by blocking these receptors.
Parasympathetic Control (Cardioinhibitory Centers):
- Impulses travel via the Vagus Nerve (Cranial Nerve X).
- Neurotransmitter: Acetylcholine (ACh).
- Mechanism: Opens ligand-gated potassium ( $K^+$ ) channels.
- Effect: Slows spontaneous depolarization, extends repolarization, decreases HR.
Autonomic Tone: At rest, both centers provide slight stimulation to maintain a baseline.
Cardiac Plexus: A network of nerve fibers near the base of the heart where both sympathetic and parasympathetic fibers flow.

Sensory Input and Cardiac Reflexes

Visceral Receptors:
- Proprioceptors: Monitor physical activity levels.
- Baroreceptors: Stretch receptors monitoring blood pressure. Located in the aortic sinus, carotid bodies, venae cavae, and pulmonary vessels.
- Chemoreceptors: Monitor levels of $O_2$ , $CO_2$ , $H^+$ , and lactic acid.
- Limbic System: Responds to emotions and anticipation of exercise.
Specific Reflexes:
- Baroreceptor Reflex: Maintaining homeostasis based on firing rates related to blood pressure.
- Atrial (Bainbridge) Reflex: Adjusts HR based on blood flow return to the atria.

Factors Affecting Heart Rate and Force

Factor	Effect (Increase HR/Force)	Effect (Decrease HR/Force)
Nerves	Cardioaccelerator (Norepinephrine)	Cardioinhibitor/Vagus (ACh)
Proprioceptors	Increased firing (Exercise)	Decreased firing (Rest)
Chemoreceptors	Low $O_2$ , High $CO_2$ , $H^+$ , Lactic acid	High $O_2$ , Low $CO_2$ , $H^+$ , Lactic acid
Baroreceptors	Decreased firing (Low BP/Volume)	Increased firing (High BP/Volume)
Therapeutic/Drugs	Nicotine, Caffeine	Opiates, Tranquilizers
Hormones	Catecholamines (Epi/NE), Thyroid (T3/T4)	Decreased Thyroid levels
Electrolytes	High $Ca^{2+}$ , Low $K^+$ , Low $Na^+$	Low $Ca^{2+}$ , High $K^+$ , High $Na^+$
Temperature	Increased body temperature	Decreased body temperature

Regulation of Stroke Volume (SV)

SV is dependent on three primary factors:

Preload: The stretch on the ventricles before contraction (highly dependent on EDV).
- Filling Time: Shorter filling time (higher HR) leads to lower EDV and lower preload.
- Frank-Starling Mechanism (Starling’s Law of the Heart): Within limits, the greater the stretch of the ventricular muscle, the more powerful the contraction, which increases SV.
Contractility: The inherent strength of the contraction.
- Positive Inotropic Factors: Factors that increase contractility (e.g., Sympathetic stimulation, NE/Epi, thyroid hormones, high $Ca^{2+}$ , glucagon).
- Negative Inotropic Factors: Factors that decrease contractility (e.g., Parasympathetic stimulation, hypoxia, high $K^+$ , calcium channel blockers, abnormal pH).
Afterload: The tension the ventricles must develop to pump blood against vascular resistance.
- Increased resistance (e.g., vessel constriction or valve stenosis) increases afterload and decreases SV.

Reflexive Responses to Hemodynamic Changes

Response to Decreasing Blood Flow/Pressure (Low CO):
- Baroreceptors: Sense decreasing stretch $\rightarrow$ Suppress parasympathetic $\rightarrow$ Increase HR and SV.
- Chemoreceptors: Sense low $O_2$ /high $CO_2$ $\rightarrow$ Increase sympathetic stimulation $\rightarrow$ Increase HR and SV.
- Result: Blood flow/pressure increases; homeostasis is restored.
Response to Increasing Blood Flow/Pressure (High CO):
- Baroreceptors: Sense increasing stretch $\rightarrow$ Increase parasympathetic stimulation $\rightarrow$ Decrease HR and SV.
- Chemoreceptors: Sense high $O_2$ /low $CO_2$ $\rightarrow$ Suppress sympathetic stimulation $\rightarrow$ Decrease HR and SV.
- Result: Blood flow/pressure decreases; homeostasis is restored.