Understanding Human Body Systems: Circulation and Excretion
Classified in Biology
Written on in 
English with a size of 2.58 KB
Circulatory System
Blood is a viscous, red, and saline liquid (approximately 5 liters). It is composed of:
- Plasma (55%): A yellowish liquid made of water and substances such as nutrients and hormones.
- Blood Cells: Produced in the bone marrow, there are three types:
- Red blood cells (erythrocytes): Transport oxygen from the lungs to the cells using a red protein called hemoglobin.
- White blood cells (leukocytes): Essential components of the immune system.
- Platelets: Fragments of cells that function to stop bleeding.
Blood Functions
- Transport nutrients
- Transport oxygen and carbon dioxide
- Transport waste products
- Regulate body temperature
- Participate in the immune system
- Prevent hemorrhages
Blood Vessels
- Arteries: Transport blood from the heart to the organs. They have thick, elastic walls to resist blood pressure.
- Veins: Transport blood from the organs to the heart. They have thinner walls and valves to prevent backflow.
- Capillaries: Small vessels connecting arteries and veins with thin walls to permit gas exchange.
The Cardiac Cycle
The heart pumps blood through contraction (systole) and relaxation (diastole) in three steps:
- The heart muscle relaxes and blood enters the atria; valves are closed.
- The atria contract, valves open, and blood flows into the ventricles.
- The ventricles contract, valves close, and blood exits through the arteries.
The Double Circulation
The human circulatory system is closed (blood remains within vessels) and double (blood passes through the heart twice to complete a full circuit of the body).
Excretory System
This system expels waste products from the organism through:
- Sweat: Through the skin.
- Urine: Through the urinary system.
- CO2: Through the lungs.
- Feces: Non-digested food.
Urinary System
- Kidneys: Filter the blood and produce urine.
- Ureters: Tubes connecting the kidneys to the bladder.
- Urinary bladder: A sac that stores urine.
- Urethra: The duct that expels urine; in males, it is longer and shared with the reproductive system.