Mammalian Heart Anatomy

• The mammalian heart is a complex organ responsible for pumping blood throughout the body.
• Its structure is intricately linked to its function, ensuring efficient circulation.
• Adaptations at both macroscopic and microscopic levels enable the heart to meet the body's metabolic demands.

Heart Anatomy

• The heart is a muscular organ located in the thoracic cavity, between the lungs.
• It is enclosed in a double-layered sac called the pericardium, which protects the heart and reduces friction.
• The heart consists of four chambers: two atria (right and left) and two ventricles (right and left).
• The atria are the receiving chambers, while the ventricles are the pumping chambers.
• The right atrium receives deoxygenated blood from the body via the superior and inferior vena cavae.
• The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
• The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery.
• The left ventricle pumps oxygenated blood to the body via the aorta.
• Valves within the heart ensure unidirectional blood flow:
  • The atrioventricular (AV) valves (tricuspid on the right, mitral/bicuspid on the left) prevent backflow from ventricles to atria.
  • The semilunar valves (pulmonary and aortic) prevent backflow from arteries to ventricles.
• The heart wall consists of three layers:
  • The epicardium (outer layer)
  • The myocardium (middle, muscular layer)
  • The endocardium (inner layer)
• The myocardium is the thickest layer and is responsible for the heart's pumping action.
• The thickness of the myocardium varies among the chambers, with the left ventricle being the thickest due to its higher workload.
• The heart has its own blood supply through the coronary arteries, which originate from the aorta.

Heart Physiology

• The heart functions as a dual pump, with the right side pumping blood to the lungs (pulmonary circulation) and the left side pumping blood to the body (systemic circulation).
• The cardiac cycle consists of two main phases: systole (contraction) and diastole (relaxation).
• During atrial systole, the atria contract, forcing blood into the ventricles.
• Ventricular systole follows, where the ventricles contract, pumping blood into the pulmonary artery and aorta.
• During diastole, the heart relaxes, allowing the chambers to fill with blood.
• Heart sounds are produced by the opening and closing of the heart valves.

Cardiac Muscle

• The myocardium is composed of cardiac muscle cells (cardiomyocytes).
• Cardiomyocytes are striated, similar to skeletal muscle, but have distinct features:
  • They are shorter and branched.
  • They have a single, centrally located nucleus.
  • They are connected by intercalated discs, which contain gap junctions for rapid electrical communication.
• Cardiac muscle has intrinsic contractility, meaning it can contract spontaneously without external stimulation.
• The sinoatrial (SA) node, located in the right atrium, is the heart's natural pacemaker.
• The SA node generates electrical impulses that spread throughout the heart, initiating and coordinating heart contractions.
• The atrioventricular (AV) node delays the impulse slightly before transmitting it to the ventricles via the bundle of His and Purkinje fibers, ensuring coordinated atrial and ventricular contractions.
• The autonomic nervous system (sympathetic and parasympathetic) modulates heart rate and contractility.
• Hormones, such as epinephrine, can also affect heart function.

Adaptations for Function

• The heart's structure is highly adapted to efficiently pump blood.
• The four-chambered design allows for complete separation of oxygenated and deoxygenated blood, maximizing oxygen delivery to tissues.
• The thick myocardium of the left ventricle enables it to generate the high pressure required to pump blood throughout the systemic circulation.
• The valves ensure unidirectional blood flow, preventing backflow and maintaining efficient circulation.
• The coronary arteries provide the heart muscle with a constant supply of oxygen and nutrients.
• Intercalated discs with gap junctions allow for rapid and coordinated contraction of the myocardium.
• The heart's intrinsic conduction system (SA node, AV node, bundle of His, Purkinje fibers) ensures rhythmic and coordinated contractions.
• The heart's ability to respond to nervous and hormonal stimuli allows it to adjust its output to meet the body's changing metabolic needs.
• High mitochondrial content within cardiomyocytes ensures a constant supply of ATP for the energy-demanding process of muscle contraction.
• The presence of specialized proteins like myosin and actin in cardiac muscle allow for forceful contractions.

Microscopic Adaptations

• Abundant mitochondria: Cardiac muscle cells have a high density of mitochondria to meet the constant energy demands of contraction.
• Extensive capillary network: Ensures efficient oxygen and nutrient delivery to cardiomyocytes.
• T-tubules: Invaginations of the cell membrane that allow rapid transmission of action potentials into the cell.
• Sarcoplasmic reticulum: A specialized endoplasmic reticulum that stores and releases calcium ions, which are essential for muscle contraction.
• Intercalated discs:
  • Desmosomes: Provide strong adhesion between cells.
  • Fascia adherens: Anchor actin filaments to the cell membrane.
  • Gap junctions: Allow direct electrical communication between cells, enabling coordinated contraction.

Macroscopic Adaptations

• Size: The heart's size is proportional to body size and metabolic demands.
• Shape: The conical shape of the heart is optimized for efficient pumping.
• Position: Location in the thoracic cavity provides protection and allows for efficient connections to major blood vessels.
• Valve structure: The design and placement of the valves are crucial for ensuring unidirectional blood flow and preventing backflow.
• Myocardial thickness: Variations in myocardial thickness among the chambers reflect their different workloads.

Adaptations to Specific Physiological Demands

• Exercise: During exercise, the heart rate and stroke volume increase to meet the increased oxygen demands of the muscles.
• Pregnancy: During pregnancy, the heart increases its output to support the growing fetus.
• High altitude: At high altitude, the heart may enlarge to compensate for the lower oxygen availability.
• Disease: Various heart diseases can lead to structural and functional adaptations, such as hypertrophy (enlargement) or dilation (stretching) of the heart chambers.