Heart Anatomy and Physiology Practice Flashcards

Location and Orientation of the Heart

• General Anatomy: The heart is located in the chest area, specifically within the thoracic cavity, situated in the mediastinum.

• Cavity Specifics: It resides mostly within the pericardial cavity or space.

• Anatomical Boundaries: The heart extends from the $2^{nd}$ intercostal space down to the $5^{th}$ intercostal space.

• Positional Orientation:

  • It is not located in the midline.
  • Approximately $\frac{2}{3}$ of the heart's mass is positioned to the left of the midsagittal plane.
  • The heart is rotated within the chest; the right side is more anterior, while the left side is more posterior.

• Size and Shape:

  • It is roughly the size of a clenched fist.
  • The structure is cone-shaped.
  • Base: The wider, superior portion of the heart.
  • Apex: The pointy, inferior portion.

• Apex Directionality: The apex points inferiorly, anteriorly, and towards the left.

Clinical Correlation: The Apical Beat

• Location of the Apex: Clinically, the apex is found in the $5^{th}$ intercostal space along the midclavicular line (an imaginary line drawn downwards from the middle of the clavicle).

• Point of Maximal Intensity (PMI): Also known as the apical beat. When the heart contracts, the apex touches the anterior thoracic wall.

• Clinical Utility: A healthcare provider can feel this contraction by placing a hand on the chest at the midclavicular line in the $5^{th}$ intercostal space. This is one method used to determine heart rate, distinct from taking a pulse.

Coverings and Layers of the Heart Wall

• Pericardium: The heart is covered by a membrane called the pericardium, consists of two main components:

  1. Fibrous Pericardium: The tough, outer layer.
  2. Serous Pericardium: This includes the parietal layer and the visceral layer.

• Heart Wall Layers (Ordered from outside in):

  1. Epicardium: The outermost layer, which is identical to the visceral pericardium.
  2. Myocardium: The middle, muscular layer. The term "myo" signifies muscle.
  3. Endocardium: The innermost layer lining the chambers.

External and Internal Anatomy

• Chambers of the Heart: The heart consists of four chambers:

  • Atria: The two superior chambers (singular: atrium). These include the left atrium and the right atrium.
  • Ventricles: The two inferior chambers, consisting of the left ventricle and the right ventricle.

• Surface Features (Sulci): A sulcus is a shallow groove on the surface of the heart:

  • Coronary Sulcus: A groove located between the superior chambers (atria) and the inferior chambers (ventricles).
  • Interventricular Sulcus: A groove located between the two inferior chambers (ventricles).

• Septa (Partitions):

  • Interatrial Septum: The wall separating the two atria.
  • Interventricular Septum: The wall separating the two ventricles.

Heart Valves and Directional Flow

• Purpose of Valves: The primary function of heart valves is to prevent the backflow of blood, ensuring it flows in only one direction.

• Atrioventricular (AV) Valves: Located between the atria and ventricles:

  • Tricuspid Valve: Guards the opening between the right atrium and right ventricle. It features $3$ flaps.
  • Bicuspid Valve (Mitral Valve): Guards the opening between the left atrium and left ventricle. It features $2$ flaps (anterior and posterior).

• Semilunar Valves: Located between the ventricles and the major blood vessels:

  • Pulmonary Valve: Located at the beginning of the pulmonary trunk, exiting the right ventricle. It has $3$ flaps.
  • Aortic Valve: Located between the left ventricle and the aorta. It has $3$ flaps.
  • Naming Convention: Called "semilunar" because the valve cusps are shaped like half-moons.

Functional Roles of Chambers

• Receiving Chambers (Atria):

  • Right Atrium: Receives blood from the body.
  • Left Atrium: Receives blood from the lungs.

• Pumping Chambers (Ventricles):

  • Right Ventricle: Pumps blood to the lungs.
  • Left Ventricle: Pumps blood to the rest of the body.

Fetal Circulation vs. Adult Anatomy

• Fossa Ovalis and Foramen Ovale:

  • In Fetuses: There is an opening in the interatrial septum called the foramen ovale. This allows blood to bypass the lungs (which are not fully used for oxygenation in utero) by moving blood directly from the right atrium to the left atrium.
  • In Adults: After birth, this hole seals and leaves a shallow depression known as the fossa ovalis on the right side and the fossa lunata on the left side.

• Ductus Arteriosus and Ligamentum Arteriosum:

  • In Fetuses: The ductus arteriosus is a second bypass that connects the pulmonary trunk to the aorta.
  • In Adults: This structure closes after birth and becomes a fibrous band called the ligamentum arteriosum.

The Fibrous Cardiac Skeleton

• Composition: A dense region of fibrous connective tissue (mostly collagen fibers) located between the atria and the ventricles.

• Functions:

  1. Muscle Attachment: Provides the origin and insertion points for cardiac muscle fibers.
  2. Structural Support: Replaces and reinforces the structure around the four heart valves.
  3. Electrical Insulation: Acts as an electrical dead zone. It prevents action potentials from traveling directly from the atria to the ventricles (or vice versa), forcing the electrical signal through specific conduction pathways.

Coronary Circulation

• General Purpose: The heart muscle is too thick to receive nutrients solely through diffusion from the blood inside its chambers. Coronary circulation provides the necessary blood supply to the cardiac tissue.

• Coronary Arteries:

  • Right Coronary Artery (RCA): Branches include the marginal branch and the posterior interventricular branch.
  • Left Coronary Artery (LCA): Branches include the anterior interventricular branch and the circumflex branch.

• Coronary Veins:

  • Great Cardiac Vein: Runs next to the anterior interventricular artery and along the left circumflex.
  • Small Cardiac Vein: Runs along the right marginal artery.
  • Middle Cardiac Vein: Runs along the posterior interventricular artery.
  • Coronary Sinus: A large vein on the posterior of the heart below the left atrium where all cardiac veins drain. It opens directly into the right atrium.

Pathway of Blood Flow

1. Body (deoxygenated blood) via superior/inferior vena cava.
2. Right Atrium.
3. Tricuspid Valve.
4. Right Ventricle.
5. Pulmonary Valve.
6. Pulmonary Trunk and Pulmonary Arteries (Arteries carry blood away from the heart).
7. Lungs (Gas exchange occurs).
8. Pulmonary Veins (Veins carry blood toward the heart; these are oxygenated).
9. Left Atrium.
10. Bicuspid (Mitral) Valve.
11. Left Ventricle.
12. Aortic Valve.
13. Aorta.
14. Rest of the Body.

Microscopic Anatomy: Cardiac Muscle Cells

• Intercalated Discs: Specialized junctions between cardiac muscle cells containing:

  • Desmosomes: Structures that hold the cells together tightly against mechanical stress.
  • Gap Junctions: Channels that allow action potentials to transmit easily from one cell to the next. This ensures the heart contracts as a coordinated unit without needing individual nerve endings for every cell.

• Mitochondria and Metabolism:

  • Cardiac muscle cells contain an abundance of mitochondria, making up approximately $25\%$ of the cell volume (significantly higher than in skeletal muscle).
  • The heart is strictly dependent on aerobic respiration and requires a constant supply of oxygen.

• Clinical Implication (Heart Attack): Because the heart requires constant oxygen, blood vessel blockages (myocardial infarction) lead to muscle death within minutes. Rapid medical intervention is required to prevent massive injury.