The Heart: Anatomy and Function

The Heart

• A muscular organ, about the size of a fist, slightly left of the chest center.
• Functions as a pump for propelling oxygen-rich blood to body tissues.
• It also maintains blood pressure.

Macroscopic Anatomy

• Key components include autorhythmic conducting cells which regulate heart rhythm.
• Cardiac muscle cells (myocardial cells) are also key as they are responsible for contractions.
• Epicardium: Outer layer, protective.
• Endocardium: Inner lining of chambers and valves.
• Cardiac skeleton: Connective tissue framework.
• Valvular tissues: Ensure one-way blood flow.

Histology

• Autorhythmic Cells (Conducting Cells): Electrical conduction system.
• Includes SA node, AV node, and other pathways for coordinating heartbeats.
• Cardiomyocytes (Contractile Cells): Specialized muscle cells, contract in response to electrical impulses.

Electrical or Conducting System

• SA Node is the natural pacemaker, establishing heart rhythm and rate.
• Internodal Pathways conduct electrical signals from the SA node to the AV node, triggering contractions.
• AV Node receives signals from the SA node and delays signal to ensure proper timing for contraction.
• AV Bundle (Bundle of His) transmits signals from the AV node down into ventricles via right and left bundle branches.
• Purkinje Fibers spread throughout the ventricular myocardium, initiating ventricular contractions.

Purkinje Fibers

• Larger than regular cardiac cells, they accommodate rapid impulse conduction.
• More euchromatic due to higher glycogen content and fewer myofibrils.

Cardiac Muscle Cell Characteristics

• Myocardium is the thickest layer.
• It's responsible for contractile function.
• Thicker in ventricles for greater pumping force.
• Structure consists of small, branching cylindrical cells that are typically uninucleated with possessing a central nucleus

Tunica Externa (Adventitia)

• The outer layer of connective tissue provides vessel support.
• It also supplies nerves and blood vessels (vasa vasorum).

Types of Blood Vessels

• Arteries carry blood away from the heart.
• They have thicker walls to withstand high pressure and lack valves.

Types of Vessels

• Elastic Arteries are large and rich in elastic fibers, absorbing heartbeats' pressure fluctuations (e.g., aorta).
• Muscular Arteries are medium-sized and control blood flow via smooth muscle contraction (e.g., renal artery).
• Arterioles are the smallest and regulate flow into capillaries through vasoconstriction/vasodilation.

Capillaries

• They are key for the exchange of gases, nutrients, and waste between blood and tissues.
• They are composed of only the tunica intima (endothelium) for ease of exchange.

Types of Capillaries

• Continuous Capillaries feature tight junctions, found in most tissues/organs.
• Fenestrated Capillaries contain pores, allowing more permeability in kidneys/intestines.
• Sinusoids have large openings that allow proteins/blood cells to pass, typically found in the liver, spleen, and bone marrow.

Veins

• Veins carry blood toward the heart.
• They have larger diameters and thinner walls compared to arteries and accommodate lower pressure.
• They contain valves to prevent backflow.
• Approximately 70% of the body's blood is stored in veins.

Types of Veins

• Venules are small and collect blood from capillary beds.
• Medium-Sized Veins collect blood from venules and typically contain valves.
• Large Veins carry blood directly to the heart with a greater number of vasa vasorum compared to arteries.

Medical Correlations

• Atherosclerosis is where plaque builds up in arteries, leading to narrowed arteries and restricted blood flow.
• Hypertension is elevated blood pressure often from increased peripheral resistance, is a potential complication for heart and kidney.
• Aneurysms are localized enlargements or bulges in arterial walls due to weakness.
• Varicose Veins are enlarged veins due to weakened valves, leading to blood pooling.