The Heart and Its Chambers

The heart is a four-chambered organ roughly the size of a clenched fist, centrally located in the chest.
Each heart consists of two atria (upper chambers) and two ventricles (lower chambers), separated by a muscular wall called the septum.
The right atrium receives oxygen-poor blood from the body, while the left atrium receives oxygen-rich blood from the lungs.
The right ventricle pumps oxygen-poor blood to the lungs, and the left ventricle pumps oxygen-rich blood to the rest of the body.

Major blood vessels include the vena cava (largest vein), pulmonary artery, pulmonary veins, and aorta (largest artery).
The vena cava directs oxygen-poor blood back to the heart; the pulmonary artery transports this blood to the lungs.
Pulmonary veins bring oxygen-rich blood from the lungs to the left atrium, while the aorta distributes this blood to the body.
Coronary arteries branch from the aorta to supply blood to the heart itself.
Valves, including atrioventricular (tricuspid and bicuspid) and semilunar valves, prevent backflow of blood throughout the heart's chambers and vessels.

The circulatory system is closed, meaning blood remains within vessels during its journey.
Three main types of blood vessels exist: arteries, veins, and capillaries.
Arteries have thick walls to withstand high blood pressure and primarily transport oxygen-rich blood away from the heart.
Veins have thinner walls, are more prone to collapse, and carry oxygen-poor blood back to the heart, equipped with valves to prevent backflow.
Capillaries are the smallest vessels, one cell thick, facilitating the exchange of gases and nutrients between blood and cells.

Blood consists of plasma (55% of blood weight) and corpuscles (45% of blood weight).
Plasma is a straw-colored fluid containing water, proteins, and small inorganic substances.
Corpuscles include red blood cells (RBCs), white blood cells (WBCs), and blood platelets (thrombocytes), produced in red bone marrow.

RBCs are biconcave disks without a nucleus, carrying oxygen due to the presence of hemoglobin.
WBCs defend against infections by secreting antibodies and engulfing pathogens.
Blood platelets are essential for clotting to prevent excessive bleeding.

The heart's chambers work together to maintain blood flow and circulation.
Blood vessels play distinct roles based on their structure and function.
The blood transports vital substances and plays a crucial role in the immune response.