Human Circulatory System Functions and Anatomy
The Circulatory System
The circulatory system carries nutrients and O₂ and collects waste products and CO₂. It is composed of:
The Heart
The heart is a muscular and hollow organ that drives blood. It is formed of three layers: the myocardium (muscle layer), the pericardium (outside), and the endocardium (inside). It is divided into four chambers: two atria and two ventricles, separated by the tricuspid valve (right) and the mitral valve (left). The vena cava and pulmonary veins reach the atria. Blood exits the ventricles through the aorta and pulmonary arteries.
Blood Vessels
- Arteries: These carry blood from the heart to the body, progressively branching into arterioles. They have three layers: connective tissue, muscle, and endothelium.
- Veins: These are blood vessels that carry blood back to the heart. They are formed by the junction of venules. Their wall thickness is smaller than that of arteries, and they have valves that prevent the backflow of blood.
Blood
Blood is a viscous liquid that travels through the inside of blood vessels (approximately 5.5 liters). It comprises:
- Plasma: A yellow liquid containing H₂O, mineral salts, nutrients, waste products, hormones, and proteins.
Movements of the Heart
The heart pumps blood throughout the body via two primary movements:
- Atrial Systole: Blood passes into the ventricles. The atrioventricular valves are open.
- Ventricular Systole: Blood moves through the arteries. The valves are closed.
- Diastole: Blood passes from the veins into the atria. The valves are closed.
- Ventricular Diastole: Blood moves through the atria to the ventricles. These valves open.
Related Cardiovascular Concepts
- Heart Rate: The number of times the heart beats per minute.
- Cardiac Output: The volume of blood driven per minute.
- Pulse: The wave generated in the walls of the arteries during ventricular systole.
- Blood Pressure (Tension): The force with which the blood hits the walls of the blood vessels. It is greater during systole than during diastole.
Blood Circulation Circuits
In humans, blood circulation occurs through two circuits:
Major Circulation (General/Systemic)
Blood leaves the left ventricle through the aorta and addresses all parts of the body except the lungs. The capillary blood gives O₂ and nutrients to cells and picks up CO₂ and waste substances. Afterward, the blood returns to the right atrium through the veins.
Minor Circulation (Pulmonary)
Blood enters the right ventricle and exits through the pulmonary arteries to the lungs. There, gas exchange occurs (CO₂ is released and O₂ is taken in), and oxygenated blood returns to the left atrium through the pulmonary veins, starting the cycle again.