The Big Picture
Your body produces waste as cells use nutrients for energy. Several body systems remove different types of waste. The urinary system removes waste products from the blood and helps keep water, electrolytes, and acid-base balance within the body.
The kidneys do much of this work. Tiny filtering units inside each kidney help separate substances in the blood and produce urine.
Urine then travels from the kidneys to the bladder, where it stays until urination.
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Start Here If You’re Overwhelmed
Focus on these core ideas:
- The urinary system removes waste products from blood and helps maintain water, electrolyte, and acid-base balance.
- The kidneys filter blood and perform six important functions.
- Each kidney has about a million nephrons, which act as filtering units.
- Urine formation, called diuresis, has three major steps.
- Urine follows this path: kidneys, ureters, bladder, urethra, then meatus.
- Average urine output is 1 500 mL per 24 hours.
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Course Notes: Key Ideas You Need to Know
How the Body Removes Waste
The urinary system is one of several body systems involved in removing waste.
- The digestive system removes solid waste.
- The lungs remove carbon dioxide.
- The skin releases water and other substances in sweat.
- The urinary system removes waste products from the blood.
Body cells also produce waste when they use nutrients for energy, especially proteins. Metabolism refers to chemical processes in body cells that create and use energy and produce cellular waste.
The urinary system also helps maintain:
- water balance
- electrolyte balance
- acid-base balance
The Kidneys
The kidneys are two bean-shaped organs. They sit just behind the abdominal cavity, against the back muscles on each side of the spine.
The lower edge of the rib cage helps protect them.
Six Main Kidney Functions
The kidneys perform six major tasks.
- Filter waste from the blood. They remove waste products such as urea, uric acid, ammonia, and creatinine. These substances result from protein metabolism.
- Regulate blood volume. The kidneys control how much water leaves the body. Excretion means eliminating a waste substance from the body.
- Help regulate electrolytes. They help control the electrolyte content of blood and body fluids.
- Help regulate acid-base balance. The kidneys help control blood pH by controlling the excretion of hydrogen ions.
- Help regulate blood pressure. They take part in long-term blood pressure regulation.
- Produce erythropoietin. Erythropoietin is a hormone that stimulates the production and regulation of red blood cells.
Nephrons: The Kidney’s Filtering Units
Each kidney contains about one million tiny nephrons.
Nephrons are the filtering units of the kidneys. They regulate the concentration of water and soluble, or dissolvable, substances by filtering the blood.
During this process, nephrons separate useful substances from toxins and waste products.
Useful substances involved include:
- water
- sodium
- amino acids
- glucose
The filtering process eventually produces urine as a waste product.
Diuresis: How Urine Forms
The production of urine is called diuresis.
Diuresis has three major steps:
- Glomular filtration
- Tubular reabsorption
- Tubular secretion
The order matters because each step describes a different part of urine formation.
Step 1: Glomular Filtration
Each nephron contains a renal (convoluted) tubule that ends in a Bowman’s capsule.
Inside the Bowman’s capsule is a cluster of tiny blood vessels called the glomerulus.
Blood travels through the capillaries of the glomerulus. The liquid part of the blood, which is mostly water, filters into the Bowman’s capsule.
The filtered material includes:
- waste products
- valuable nutrients
- minerals
- other substances small enough to cross the membrane
Step 2: Tubular Reabsorption
The filtered fluid then moves through a series of tubules.
Most of the water and other needed substances return to the blood. These substances then recirculate through the body.
This step is called tubular reabsorption.
Step 3: Tubular Secretion
During tubular secretion, additional substances move from the blood into the urine.
Secretion means the release of a substance into or out of the body after normal body functions.
This step makes final small adjustments to the composition of the blood and urine.
Toxins and waste products remain in the tubules and form urine.
The urine then moves through the tubule into a collecting tubule. The collecting tubule drains into the renal pelvis inside the kidney.
Ureters: From the Kidneys to the Bladder
Urine moves from the kidneys to the bladder through two ureters.
Each ureter is a narrow tube about:
- 25 to 30 cm long
- 10 to 12 in. long
Each ureter runs from the renal pelvis of one kidney to the urinary bladder.
Muscular peristalsis moves urine through the ureters toward the bladder.
The Bladder
The bladder is a hollow, muscular sac.
It sits toward the front of the lower part of the abdominal cavity.
The bladder stores urine until the urge to urinate occurs. This urge usually appears when the bladder contains about:
- 250 mL
- 8 oz
- a half-pint
Urethra and Meatus
Urine leaves the bladder through the urethra.
The opening at the end of the urethra is the meatus. Urine leaves the body through this opening.
The urethra differs in adults with male and female genitalia.
Adult With Male Genitalia
The urethra is about:
- 20 cm long
- 8 in. long
It provides an exit for urine. It also provides an exit for semen during ejaculation.
Adult With Female Genitalia
The urethra is about:
- 3 cm long
- approximately 1.2 in. long
Its purpose is to provide a route for urine from the bladder to the urinary meatus.
Differences in urethra length and anatomical position explain why adults with female genitalia are more prone to urinary tract infections than adults with male genitalia.
The Path Urine Follows
Keep this order clear:
- Urine forms in the kidneys.
- Urine moves from each kidney into a ureter.
- The ureters carry urine to the bladder.
- The bladder stores urine.
- Urine leaves the bladder through the urethra.
- Urine leaves the body through the meatus.
Normal Urine Appearance and Amount
Urine is normally a clear, yellowish fluid.
Over a 24-hour period, the amount of urine eliminated should be close to the amount of fluid consumed.
The average urine output is:
1 500 mL per 24 hours
Fast Review
Use these points for a quick review before a quiz or exam. Focus on the structures, functions, three urine-formation steps, and important numbers.
- The urinary system removes waste from blood and helps maintain water, electrolyte, and acid-base balance.
- The kidneys are two bean-shaped organs located behind the abdominal cavity, on each side of the spine.
- The kidneys filter waste, regulate blood volume and electrolytes, help regulate acid-base balance and blood pressure, and produce erythropoietin.
- Each kidney contains about one million nephrons, which act as filtering units.
- Diuresis means urine production and has three major steps: glomular filtration, tubular reabsorption, and tubular secretion.
- Urine travels from the kidneys through the ureters to the bladder, then leaves through the urethra and meatus.
- Each ureter is about 25 to 30 cm, or 10 to 12 in., long.
- The urge to urinate usually occurs when the bladder holds about 250 mL, or 8 oz, of urine.
- The adult urethra is about 20 cm, or 8 in., with male genitalia and about 3 cm, or approximately 1.2 in., with female genitalia.
- Average urine output is 1 500 mL per 24 hours, and the amount eliminated should be close to the amount of fluid consumed.
Final Pep Talk
There are several structures, functions, steps, and numbers in this topic, so review them in small groups. Focus first on the urine pathway and the three steps of diuresis, then return to the kidney functions and measurements.