Anatomy & Physiology: The Urinary System – Structure and Function of the Kidneys 1

The Urinary System

Overview of the Urinary System

The urinary system is essential for maintaining the body's internal environment by regulating fluid and electrolyte balance, removing wastes, and supporting homeostasis. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Kidneys: Major excretory organs that filter blood and produce urine.

• Ureters: Tubes that transport urine from the kidneys to the urinary bladder.

• Urinary bladder: Temporary storage reservoir for urine.

• Urethra: Tube that carries urine from the bladder out of the body.

Structure and Function of the Kidneys

Main Functions of the Kidneys

The kidneys play a vital role in maintaining homeostasis by performing several key functions:

• Regulating water volume and solute concentration: Controls the amount of water and dissolved substances in the body.

• Regulating ion concentrations in extracellular fluid (ECF): Maintains proper levels of ions such as sodium, potassium, and calcium.

• Ensuring long-term acid-base balance: Adjusts the pH of body fluids to keep them within a narrow range.

• Excreting metabolic wastes, toxins, and drugs: Removes harmful substances from the blood.

• Producing erythropoietin: Hormone that regulates red blood cell production.

• Producing renin: Enzyme that helps regulate blood pressure.

• Activating vitamin D: Converts vitamin D to its active form, which is important for calcium absorption.

• Carrying out gluconeogenesis: Generates glucose from non-carbohydrate sources during periods of fasting.

Location and External Anatomy of the Kidneys

The kidneys are retroperitoneal organs located in the superior lumbar region of the abdomen, between the levels of the T12 and L5 vertebrae.

• Right kidney: Positioned lower than the left due to the presence of the liver.

• Adrenal (suprarenal) gland: Sits atop each kidney and is involved in hormone production.

• Lateral surface: Convex in shape.

• Medial surface: Concave, featuring the renal hilum, a vertical slit that leads to the internal space called the renal sinus.

• Renal hilum: Entry and exit point for ureters, renal blood vessels, lymphatics, and nerves.

Diagram: The Urinary System

The urinary system includes the kidneys, ureters, bladder, and urethra. The renal artery supplies blood to the kidneys, while the renal vein drains blood away. The adrenal glands are located above the kidneys.

Key Terms and Definitions

• Homeostasis: The maintenance of a stable internal environment.

• Erythropoietin: A hormone produced by the kidneys that stimulates red blood cell production in the bone marrow.

• Renin: An enzyme released by the kidneys that plays a role in blood pressure regulation.

• Retroperitoneal: Located behind the peritoneum (the lining of the abdominal cavity).

• Renal hilum: The entry/exit site for structures servicing the kidney (blood vessels, nerves, ureter).

• Renal sinus: The internal cavity within the kidney.

Summary Table: Main Functions of the Kidneys

Function	Description	Example/Application
Water & Solute Regulation	Controls body fluid volume and solute concentration	Prevents dehydration or overhydration
Ion Balance	Regulates ions in extracellular fluid	Maintains sodium, potassium, calcium levels
Acid-Base Balance	Maintains pH of body fluids	Prevents acidosis/alkalosis
Excretion	Removes wastes, toxins, drugs	Eliminates urea, creatinine
Hormone Production	Produces erythropoietin and renin	Regulates RBC production and blood pressure
Vitamin D Activation	Converts vitamin D to active form	Supports calcium absorption
Gluconeogenesis	Produces glucose from non-carbohydrate sources	Helps maintain blood glucose during fasting

Additional info:

• The kidneys are protected by the rib cage and layers of fat and connective tissue, which help cushion and anchor them in place.

• Renal failure can result if the kidneys are unable to perform their regulatory and excretory functions.