The Urinary and Circulatory Systems

The Nephron

The nephron is the functional unit of the kidneys, responsible for filtering blood and producing urine. Each kidney contains over one million nephrons.

Parts of the Nephron

• Bowman's Capsule: Surrounds a tiny network of capillaries.
• Tubule: Shaped like a handle, the tubule of several nephrons empties into a larger tube called a collecting duct.
• Collecting Ducts: Move urine into the renal pelvis.

Kidney Function

1. Filtration: Molecules exit the bloodstream and enter the Bowman's capsule, forming filtrate.
2. Reabsorption: As filtrate travels through the tubule, some molecules are reabsorbed by the capillaries. Glucose, 99% of water, a small amount of urea, and salts return to the blood.
3. Excretion: The remaining filtrate forms urine, which is excreted from the body.

Composition of Blood

Blood Plasma

• Liquid part of blood with a yellow appearance.
• Mostly water (91%).
• Contains a variety of dissolved molecules.

Blood Cells

• Red Blood Cells: Cells without a nucleus, double-concave disc shape, contain hemoglobin for oxygen transport. Live for approximately 4 months.
• White Blood Cells: Various types, all contain a nucleus, protect the body from infection by surrounding and digesting pathogens. Live for approximately 1 day.
• Platelets: Cell fragments that form small groups, help to stop bleeding by accumulating at the site of injury and stimulating the formation of fibers to create a clot. Live for approximately 1 week.

Functions of Blood Components

• Plasma: Transports nutrients and waste products.
• Red Blood Cells: Transport oxygen. High oxygen concentration results in bright red blood, while low concentration results in dark red blood.
• White Blood Cells: Protect the body from infection by surrounding and digesting pathogens like bacteria, and producing substances that help destroy foreign material.
• Platelets: Help to stop bleeding by forming clots at the site of injury.

Blood Vessels

• Arteries: Carry blood away from the heart to the organs. They have strong, elastic walls to withstand high blood pressure.
• Veins: Carry blood from the organs back to the heart. They have thinner walls than arteries and carry blood under lower pressure.
• Capillaries: Small diameter vessels that form dense networks in the organs. They have extremely thin walls to allow for the exchange of substances between blood and cells.