Heart and Cardiovascular System Anatomy

Overview of the Heart and Cardiovascular System

The cardiovascular system and the circulatory system are components of the body's transport infrastructure, though they differ in scope:
- Circulatory System: Consists of the heart, blood vessels, blood, and the lymphatic system.
- Cardiovascular System: Consists specifically of the heart, arteries, veins, and capillaries.
There are two major divisions of the cardiovascular system circuit:
- Pulmonary Circuit: Facilitated by the right side of the heart; it carries oxygen-poor and $CO_2$ -rich blood to the lungs for gas exchange and returns oxygen-rich blood to the heart.
- Systemic Circuit: Facilitated by the left side of the heart; it supplies oxygen-rich and $CO_2$ -poor blood to all organs and tissues of the body, including the branches of the aortic arch and the descending aorta.

Size, Shape, and Position of the Heart

Location: The heart is situated in the mediastinum, the space between the lungs.
General Structure:
- Base: The broad superior portion of the heart where the great vessels attach.
- Apex: The inferior end of the heart that tilts to the left and tapers to a point.
Physical Dimensions and Weight:
- Width at base: $3.5\,\text{in}$ .
- Length (base to apex): $5\,\text{in}$ .
- Depth (anterior to posterior): $2.5\,\text{in}$ .
- Weight: approximately $10\,\text{oz}$ .
Anatomical Context: The heart is positioned posterior to the sternum, anterior to the thoracic vertebrae, and rests superior to the diaphragm, roughly lining up with the 2nd rib at its base.

The Pericardium

The pericardium is a double-walled sac that encloses the heart. Its functions include allowing the heart to beat without friction, providing room for the heart to expand, and resisting excessive expansion.
Layers of the Pericardium:
- Parietal Pericardium: The outer layer consisting of an outer, tough, fibrous layer of connective tissue (fibrous pericardium) and an inner, thin, smooth, moist serous layer (serous pericardium).
- Pericardial Cavity: The space between the parietal and visceral layers, filled with pericardial fluid to reduce friction.
- Visceral Pericardium (Epicardium): A thin, smooth, moist serous layer that covers the surface of the heart wall.

Anatomy of the Heart Wall

The heart wall is composed of three distinct layers:
- Epicardium (Visceral Pericardium): The serous membrane covering the external surface of the heart.
- Myocardium: The thick, muscular middle layer responsible for contraction.
  - It includes a fibrous skeleton, which is a network of collagenous and elastic fibers.
  - Functions of the fibrous skeleton: provides structural support, serves as an attachment point for cardiac muscle, and acts as a nonconductor of electricity, which is critical for coordinating contractile activity.
- Endocardium: The smooth inner lining of the heart chambers.

Heart Chambers

The heart contains four chambers divided by a septum into right and left sides:
- Right and Left Atria:
  - These are the two superior "receiving chambers."
  - They receive blood returning to the heart from the body (right) or lungs (left).
  - Auricles: Ear-like flaps on the exterior of the atria that increase atrial surface area.
- Right and Left Ventricles:
  - These are the two inferior "pumping chambers."
  - They eject blood into the major arteries (pulmonary trunk and aorta).

External and Internal Heart Anatomy

External Anterior Features:
- Vessels: Aortic arch, superior vena cava, inferior vena cava, ascending aorta, pulmonary trunk, left and right pulmonary arteries/veins, and ligamentum arteriosum.
- Landmarks: Atrioventricular sulcus, anterior interventricular sulcus, and apex.
External Posterior Features:
- Vessels: Right and left pulmonary veins, pulmonary arteries, superior/inferior vena cava, and the aorta.
- Landmark: Posterior interventricular sulcus.
Internal Features:
- Fossa Ovalis: A depression in the right atrium (remnant of the fetal foramen ovale).
- Trabeculae Carneae: Internal ridges of myocardium in the ventricles.
- Interventricular Septum: The muscular wall separating the two ventricles.
- Papillary Muscles: Muscular projections on the floor of the ventricles associated with valve function.

Heart Valves and Blood Flow Regulation

The primary function of heart valves is to ensure a one-way flow of blood.
Atrioventricular (AV) Valves: Located between atria and ventricles.
- Right AV Valve (Tricuspid Valve): Has three cusps.
- Left AV Valve (Mitral or Bicuspid Valve): Has two cusps.
- Chordae Tendineae: Cords that connect the AV valves to the papillary muscles to prevent valve prolapse during ventricular contraction.
Semilunar (SL) Valves: Control blood flow into the great arteries.
- Pulmonary Semilunar Valve: Regulates flow from the right ventricle into the pulmonary trunk.
- Aortic Semilunar Valve: Regulates flow from the left ventricle into the aorta.
Mnemonic for Valve Order: "Tissue Paper My Assets" refers to Tricuspid, Pulmonic, Mitral, and Aortic.

Detailed Step-by-Step Path of Blood Flow

Superior and Inferior Vena Cava: Collects deoxygenated blood from the body.
Right Atrium (RA)
Tricuspid Valve
Right Ventricle (RV)
Pulmonary SL Valve
Pulmonary Arteries: Carries deoxygenated blood to the lungs.
Lungs: Blood picks up $O_2$ and drops off $CO_2$ .
Pulmonary Veins: Carries oxygenated blood back to the heart.
Left Atrium (LA)
Bicuspid (Mitral) Valve
Left Ventricle (LV)
Aortic SL Valve
Aorta and its Branches: Distributes oxygenated blood to the systemic circulation.
To Body Cells: Blood provides nutrients and oxygen to tissues.

Circulatory Pathways and Specialized Routes

Major Circuits:
- Pulmonary Circulation: Transports oxygen-poor, high- $CO_2$ blood to the lungs for oxygenation. Occurs on the right side of the heart.
- Systemic Circulation: Transports oxygen-rich blood through the left side of the heart to all body tissues via the aorta. Venous return from this circuit enters the right atrium.
Standard Vascular Route: Heart → Arteries → Arterioles → Capillaries → Venules → Veins → heart.
Specialized Circulatory Routes:
- Coronary Circulation: The blood supply dedicated to the heart muscle itself.
- Hepatic Portal System: A specialized systemic route where veins from the spleen and digestive organs are sent through the liver before returning to general circulation.
- Circle of Willis: A specialized arterial arrangement that provides blood to brain tissue.
- Fetal Circulation: Unique vessel arrangements in the fetus required for gas exchange and nourishment in the absence of lung function.

Types and Structure of Blood Vessels

Arteries:
- Composed of strong, elastic, muscular tissue (smooth muscle).
- Function under high pressure and carry blood away from the heart.
Arterioles: the thinnest divisions of arteries that lead into capillaries.
Capillaries:
- Microscopic vessels where oxygen and carbon dioxide gas exchange occurs.
Venules:
- Microscopic vessels that collect blood from capillaries and join to form veins.
Veins:
- Carry blood back toward the heart under low pressure.
- Act as a blood reservoir.
- Contain semilunar valves to prevent backflow and assist in moving blood.
Vessel Wall Structure (Large Vessels):
- Tunica Interna: Inner layer consisting of endothelium and a basement membrane.
- Tunica Media: Middle muscular layer.
- Tunica Externa: Outer layer containing collagenous tissue, a nerve supply, and the vasa vasorum (vessels that supply the walls of the vessel itself).
- Lumen: The hollow internal space where blood flows.

Major Systemic Arteries and Veins

Major Systemic Arteries (Supplying oxygen and nutrients):
- Vertebral, Subclavian, Axillary, Brachial, Descending Aorta, Intercostal, Renal, Ulnar, Radial, Deep Femoral, Femoral, Popliteal, Posterior Tibial, Anterior Tibial, and Dorsal Pedal.
Major Systemic Veins (Returning deoxygenated blood):
- Subclavian, Brachiocephalic, Axillary, Cephalic, Brachial, Basilic, Superior Vena Cava, Ulnar, Inferior Vena Cava, Hepatic, Renal, Radial, Femoral, Popliteal, Posterior Tibial, and Anterior Tibial.