Location, and orientation of the heart in the thoracic cavity.

Full Answer Section

2. Pericardium and Heart Wall Layers:

• Pericardium: The heart is enclosed within a double-layered sac called the pericardium. The outer layer, the fibrous pericardium, is tough and dense, preventing overstretching of the heart. The inner layer, the serous pericardium, is further divided into two layers: the parietal pericardium (lining the fibrous pericardium) and the visceral pericardium (or epicardium), which covers the heart's surface. Between these two layers is the pericardial cavity, containing pericardial fluid that reduces friction during heartbeats.

• Heart Wall Layers: The heart wall itself has three layers:

  • Epicardium (Visceral Pericardium): The outermost layer, composed of connective tissue and covered by mesothelium.
  • Myocardium: The middle layer, composed of cardiac muscle. The arrangement of cardiac muscle fibers is complex, with superficial fibers oriented obliquely and deep fibers arranged circularly. This complex arrangement allows for efficient pumping action.
  • Endocardium: The innermost layer, lining the heart chambers and covering the heart valves. It's composed of endothelium (simple squamous epithelium) and underlying connective tissue.

3. Heart Chambers and Myocardium Type:

The heart has four chambers:

• Right Atrium: Receives deoxygenated blood from the body. Myocardium is relatively thin.
• Left Atrium: Receives oxygenated blood from the lungs. Myocardium is slightly thicker than the right atrium.
• Right Ventricle: Pumps deoxygenated blood to the lungs. Myocardium is thicker than the atria but thinner than the left ventricle.
• Left Ventricle: Pumps oxygenated blood to the body. Myocardium is the thickest of all chambers, reflecting its role in pumping blood against systemic pressure.

All chambers are made of cardiac muscle tissue.

4. Major Vessels and Blood Type:

• Right Side of the Heart:

  • Superior and Inferior Vena Cavae: Carry deoxygenated blood from the body to the right atrium.
  • Pulmonary Trunk: Carries deoxygenated blood from the right ventricle to the lungs.
  • Pulmonary Arteries (right and left): Carry deoxygenated blood to the lungs.

• Left Side of the Heart:

  • Pulmonary Veins (usually four): Carry oxygenated blood from the lungs to the left atrium.
  • Aorta: Carries oxygenated blood from the left ventricle to the body.  

5. Heart Valves:

• Tricuspid Valve: Located between the right atrium and right ventricle. It has three cusps and prevents backflow of blood into the right atrium during ventricular contraction. It opens when atrial pressure exceeds ventricular pressure and closes when ventricular pressure exceeds atrial pressure.

• Mitral Valve (Bicuspid Valve): Located between the left atrium and left ventricle. It has two cusps and functions similarly to the tricuspid valve, preventing backflow into the left atrium.

• Pulmonary Valve: Located at the exit of the right ventricle, at the beginning of the pulmonary trunk. It prevents backflow of blood into the right ventricle during ventricular relaxation. It opens when ventricular pressure exceeds pulmonary artery pressure.

• Aortic Valve: Located at the exit of the left ventricle, at the beginning of the aorta. It prevents backflow of blood into the left ventricle during ventricular relaxation. It opens when ventricular pressure exceeds aortic pressure.

The valves operate passively based on pressure differences across them. Chordae tendineae, which are fibrous cords, connect the cusps of the tricuspid and mitral valves to papillary muscles in the ventricular walls. These structures help prevent the valves from prolapsing into the atria during ventricular contraction.

6. Blood Flow Through Circulation:

• Pulmonary Circulation: Deoxygenated blood flows from the right ventricle through the pulmonary trunk and pulmonary arteries to the lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. Oxygenated blood returns to the left atrium through the pulmonary veins.

• Systemic Circulation: Oxygenated blood flows from the left ventricle through the aorta to the body's tissues. In the tissues, blood delivers oxygen and picks up carbon dioxide. Deoxygenated blood returns to the right atrium through the superior and inferior vena cavae.

• Coronary Circulation: This is the blood supply to the heart muscle itself. Branches of the aorta, the coronary arteries, supply oxygenated blood to the myocardium. Deoxygenated blood from the myocardium drains into coronary veins, which empty into the right atrium via the coronary sinus.

7. Cardiac Cycle:

The cardiac cycle refers to the sequence of events that occur during one heartbeat.

• Atrial Systole: The atria contract, pushing blood into the ventricles. The AV valves (tricuspid and mitral) are open.
• Ventricular Systole: The ventricles contract, pumping blood into the pulmonary trunk and aorta. The AV valves close (producing the first heart sound, "lub"), and the semilunar valves (pulmonary and aortic) open.
• Ventricular Diastole: The ventricles relax. The semilunar valves close (producing the second heart sound, "dub"), and the AV valves open, allowing blood to flow passively from the atria into the ventricles.
• Atrial Diastole: The atria relax.

The "lub-dub" heart sounds are associated with the closing of the heart valves. "Lub" corresponds to the closing of the AV valves at the start of ventricular systole, and "dub" corresponds to the closing of the semilunar valves at the start of ventricular diastole.

8. Cardiac Conduction System:

• Intrinsic Conduction System: This system is located within the heart itself and generates the heart's basic rhythm.

  • Sinoatrial (SA) Node: The pacemaker of the heart, located in the right atrium. It initiates the electrical impulses that cause heart contractions.
  • Atrioventricular (AV) Node: Located in the interatrial septum. It delays the impulse from the SA node, allowing the atria to contract before the ventricles.
  • Bundle of His (Atrioventricular Bundle): Located in the interventricular septum. It transmits the impulse from the AV node to the ventricles.
  • Right and Left Bundle Branches: Carry the impulse down the interventricular septum.
  • Purkinje Fibers: Spread throughout the ventricular myocardium, transmitting the impulse and causing the ventricles to contract.

• Extrinsic Conduction System: This system is outside the heart and modulates its activity.

  • Autonomic Nervous System: The sympathetic nervous system increases heart rate and contractility, while the parasympathetic nervous system decreases heart rate.  
  • Hormones: Epinephrine and norepinephrine (from the adrenal medulla) increase heart rate and contractility. Thyroxine (from the thyroid gland) also increases heart rate.

9. Control of Cardiac Output:

Cardiac output (CO), the amount of blood pumped by the heart per minute, is determined by heart rate (HR) and stroke volume (SV).

CO = HR x SV

The body controls CO through several mechanisms:

• Autonomic Nervous System: As mentioned above, the sympathetic and parasympathetic nervous systems regulate HR.
• Hormonal Control: Hormones like epinephrine and thyroxine influence both HR and SV.
• Frank-Starling Mechanism: This mechanism states that the force of ventricular contraction is proportional to the initial length of the muscle fibers. Increased venous return (and thus increased preload) stretches the ventricular muscle fibers, leading to a stronger contraction and increased SV.
• Baroreceptor Reflexes: These reflexes detect changes in blood pressure and adjust HR and SV accordingly to maintain blood pressure homeostasis.

10. Factors Affecting Heart Rate and Stroke Volume:

• Factors Affecting Heart Rate:

  • Autonomic Nervous System Activity: Sympathetic stimulation increases HR, while parasympathetic stimulation decreases it.
  • Hormones: Epinephrine, norepinephrine, and thyroxine increase HR.
  • Body Temperature: Increased temperature increases HR.
  • Age: Resting HR tends to decrease with age.
  • Exercise: Increases HR.

• Factors Affecting Stroke Volume:

  • Preload: The volume of blood in the ventricles at the end of diastole. Increased preload increases SV (Frank-Starling Mechanism).

Sample Answer

1. Size, Shape, Location, and Orientation of the Heart:

The heart is roughly the size of a clenched fist, shaped like a cone lying on its side. It's located in the thoracic cavity, specifically in the mediastinum (the space between the lungs). It's positioned obliquely, with its apex (pointed bottom) directed inferiorly and to the left, and its base (top) directed superiorly and to the right. This orientation means the right side of the heart faces more anteriorly, and the left side faces more posteriorly.