Human Circulatory System and Cardiac Anatomy

The Human Circulatory System

The circulatory system is a complex network responsible for transporting vital substances throughout the body. It consists of:

• 1. The Heart
• 2. Vessels (including arteries, veins and their branches: arterioles, venules, and capillaries)

Anatomy and Function of the Heart

The heart is the most important muscle in the body, located within the thoracic cavity. Positioned almost horizontally with an oblique axis, it functions through alternating periods of relaxation (diastole) and contraction (systole).

Structural Layers of the Heart

Anatomically, the heart consists of three distinct layers:

• Myocardium: The central layer consisting of involuntary striated muscle tissue.
• Endocardium: The interior layer consisting of a thin epithelium.
• Epicardium: The exterior layer. Additionally, a serosal layer called the pericardium wraps around the entire organ.

Cardiac Cavities and Valves

• Right Atrium: It connects with the right ventricle through a valve called the tricuspid valve, which prevents the backflow of blood. The superior and inferior vena cava reach the right atrium.
• Left Atrium: This cavity connects to the left ventricle through the mitral valve. Four pulmonary veins (two from each lung) reach the left atrium.
• Left Ventricle: From this ventricle, the aorta emerges to send oxygenated blood throughout the body. The aorta contains the aortic valve, which prevents blood from flowing back into the ventricle. The coronary arteries, which nourish the heart muscle, originate at the base of the aorta; partial or total blockage of these vessels triggers angina or myocardial infarction.
• Right Ventricle: From this ventricle, the pulmonary artery leads to the lungs. This vessel is regulated by the pulmonary valve.

Automatic Cardiac Operation

The heart's automatic operation is facilitated by a specific conduction system consisting of a series of nodes and channels. This operation must adjust to the body's physiological needs through the autonomic (vegetative) nervous system, which regulates the heart rate via:

• Sympathetic Nerve: Acts as an accelerator to increase heart rate.
• Parasympathetic (Vagus) Nerve: Acts as a brake to slow heart rate.

Classification of Blood Vessels

Arteries and Arterioles

These vessels consist of an inner lining called the endothelium, a tunica media with musculo-skeletal features that allow for vasodilation and vasoconstriction, and an outer layer containing the autonomic nerves responsible for blood vasomotricity.

Veins and Venules

Veins are less resistant and rigid than arteries. Crucially, the endothelium inside the lumen forms valves (shaped like birds' nests), which assist in venous return by preventing backflow.

Capillaries

As vessels transition into capillaries, they lose the structural characteristics of arteries and veins. They consist of a thin endothelium surrounded by external fibroelastic elements to facilitate nutrient and gas exchange.