Human Circulatory and Excretory Systems Explained
Blood Components and Function
Blood is a red liquid that circulates through blood vessels, carrying oxygen, nutrients, carbon dioxide, and collected waste products. It consists of plasma (55%) and blood cells (45%), totaling approximately 5 liters in the average adult.
Plasma Composition
Plasma is a yellowish liquid containing water, salts, minerals, hormones, and proteins.
Blood Cells: Types and Roles
Blood cells are formed in the bone marrow.
Red Blood Cells (Erythrocytes)
These are the most abundant blood cells. They are small, disc-shaped cells containing an important protein called hemoglobin (which contains iron atoms). Hemoglobin binds to oxygen in the lungs and transports it to the capillaries, where it releases the oxygen to reach all body cells.
White Blood Cells (Leukocytes)
Leukocytes defend the body against infectious agents such as bacteria, viruses, and various parasites. There are several types:
Granulocytes
These are cells containing vesicles (granules) that engulf foreign particles.
Lymphocytes
These produce antibodies to prevent the spread of microorganisms and are also involved in the destruction of cancer cells.
Monocytes
These are large cells responsible for destroying large particles.
Platelets (Thrombocytes)
Platelets are small cell fragments responsible for plugging wounds and initiating blood clotting.
Lymph and Interstitial Fluid
Lymph is a yellowish liquid formed from intercellular fluid. It contains lymphocytes and also aids the body’s defense system. Lymph circulates through lymph vessels.
Interstitial Fluid is the liquid that surrounds and bathes the body’s cells.
Blood Vessels: Structure and Types
Blood vessels are muscular tubes that distribute blood throughout the body. There are three main types: arteries, veins, and capillaries.
Arteries
Arteries are blood vessels that carry blood away from the heart to the tissues. Arteries leaving the heart branch into smaller arterioles, becoming increasingly thin until they become capillaries. Capillaries facilitate the exchange of gases and materials between blood and cells. After this exchange, the blood, now carrying waste products, returns through capillaries that merge into venules, then into larger veins, and finally reaches the heart. Arteries are elastic and resilient vessels.
Veins
Veins are blood vessels that carry blood towards the heart. They generally have thinner walls than arteries and possess small valves to prevent the backflow of blood.
Capillaries
Capillaries are the finest ramifications of blood vessels, forming an intricate network. Their walls consist of a single layer of endothelial cells. The exchange of substances between blood and the extracellular fluid occurs through these thin walls.
The Heart: Anatomy and Function
The heart is a muscular organ responsible for pumping blood. It is surrounded by a double membrane called the pericardium. The space between the two layers is the pericardial space. The main tissue composing the heart muscle is called the myocardium. The heart has its own circulatory system: the coronary arteries and veins, which supply the oxygen and nutrients needed for its continuous activity.
The two upper chambers of the heart are the atria. Their walls are relatively thin. The left atrium receives oxygenated blood from the pulmonary veins, while the right atrium receives deoxygenated blood from the vena cavae. The ventricles are the two lower chambers, possessing thicker walls than the atria. Blood leaves the heart from the ventricles into the arteries: from the left ventricle into the aorta and from the right ventricle into the pulmonary artery.
The right atrium communicates with the right ventricle through the tricuspid valve. The left atrium communicates with the left ventricle through the mitral valve (also known as the bicuspid valve). Blood flows from the ventricles into the arteries by crossing the arterial valves (aortic and pulmonary valves).
Excretion: Removing Metabolic Waste
The cells of our bodies produce waste substances during their metabolism that must be eliminated, as their accumulation would be toxic. The main waste products are:
- Carbon Dioxide (CO2): Expelled by the respiratory system.
- Urea: A nitrogen-rich compound produced during the catabolism (breakdown) of proteins, eliminated by the urinary system.
- Bilirubin: A compound generated from the degradation of hemoglobin, released into bile by the liver.
The Urinary System Components
The urinary system is formed by:
Kidneys
Organs where urine is formed.
Ureters
Two tubes, each about 25 cm long, responsible for carrying urine from the kidneys to the bladder.
Urinary Bladder
A muscular pouch with an expandable wall that stores urine. It can hold approximately 0.5 liters of urine.
Urethra
A tube that connects the bladder to the outside of the body, allowing urine to be expelled.