Human Circulatory System: Blood, Heart, Vessels
The Circulatory System
The circulatory system has several functions:
- Brings food and oxygen to the cells.
- Collects metabolic wastes (removed by the kidneys as urine).
- Exchanges air (rich in carbon dioxide) in the lungs.
All this work is done by the blood, which constantly circulates. Additionally, the circulatory system:
- Is involved in the body’s defense system.
- Regulates body temperature.
- Transports hormones.
Blood
Blood is the fluid circulating throughout the body via the circulatory system. This system comprises the heart and a network of tubes or vessels (blood vessels).
Blood is a liquid tissue composed of water, organic and inorganic substances (mineral salts) dissolved in blood plasma. There are three types of blood cells: red cells, white cells, and platelets.
Blood Plasma: The liquid part of blood; salty, yellowish, and carries nutrients and waste.
Red Blood Cells (Erythrocytes or RBCs): Carry molecular oxygen (O2). They are biconcave disc-shaped, small (about seven microns in diameter), and lack a nucleus. Red cells contain hemoglobin, which transports oxygen from the lungs to the cells. Insufficient production results in anemia.
White Blood Cells (Leukocytes): Important for the immune system; cleaning (phagocytes) and defense (lymphocytes). Larger than red blood cells but less numerous. They destroy microbes and dead cells, and produce antibodies.
Platelets: Tiny cell fragments that plug wounds and prevent bleeding.
The Heart
The heart is a fist-sized, hollow organ in the thoracic cavity, between the lungs and diaphragm. It has three layers: endocardium, myocardium, and pericardium.
The heart is divided into two halves (right and left), which don’t communicate. The right half contains oxygen-poor blood, while the left half contains oxygen-rich blood.
Each half has an upper chamber (atrium) and a lower chamber (ventricle). Atrioventricular valves control blood flow between atria and ventricles.
The heart pumps blood throughout the body in phases: atria fill, then contract; ventricles fill, then contract, pushing blood into arteries. The heart beats about seventy times per minute, pumping about 10,000 liters of blood daily.
Blood Vessels
Blood vessels (arteries, capillaries, and veins) distribute and collect blood. Arteries carry blood from the heart to organs; capillaries allow gas and substance exchange; veins return blood to the heart.
Arteries
Thick, elastic vessels originating in the ventricles. Blood circulates through them under pressure.
- Pulmonary artery: Carries blood to the lungs.
- Aorta: Branches into other major arteries, including:
- Carotid: Oxygenated blood to the head.
- Subclavian: Oxygenated blood to the arms.
- Hepatic: Oxygenated blood to the liver.
- Splenic: Oxygenated blood to the spleen.
- Mesenteric: Oxygenated blood to the intestine.
- Renal: Oxygenated blood to the kidneys.
- Iliac: Oxygenated blood to the legs.
Capillaries
Very thin vessels where arteries divide and rejoin to form veins.
Veins
Thin-walled, slightly elastic vessels that collect blood and return it to the heart.
The superior vena cava receives blood from the head and upper limbs; the inferior vena cava receives blood from the legs, kidneys, liver, etc. Pulmonary veins bring oxygenated blood from the lungs to the left atrium.
Lymphatic System
Lymph is a colorless liquid (blood plasma and white blood cells). Lymphatic vessels have valves and lymph nodes (armpits, groin, neck, etc.) that produce white blood cells.
