Urinary System Structure and Function

Structures, Functions, and Disorders of the Urinary System

• The urinary system is responsible for removing wastes and excess fluids from the body.
• Kidneys: bean-shaped organs located between the peritoneum and back muscles. They filter blood and produce urine.
• Ureters: tubes that carry urine from the kidneys to the bladder.
• Bladder: hollow, muscular organ that collects and stores urine.
• Urethra: tube that connects the bladder to the outside of the body. Urinary meatus is the external opening where urine exits.
• Renal capsule: tough fibrous layer surrounding the kidney.
• Renal hilum: indentation where blood vessels and nerves enter/leave the kidney.
• Renal pelvis: funnel-shaped structure that collects urine from the collecting ducts for passage into the ureter.
• Renal cortex: outer portion of the kidney.
• Renal medulla: innermost part of the kidney.

Nephron: Functional Unit of the Kidney

• Bowman's capsule: double-walled capsule surrounding the glomerulus.
• Glomerulus: cluster of capillaries where filtration takes place.
• Proximal convoluted tubule: reabsorbs ions, water, and nutrients.
• Loop of Henle: portion of the nephron that leads from the proximal to the distal convoluted tubule.
• Distal convoluted tubule: portion of nephron between the loop of Henle and the collecting tubule.
• Collecting tubule: collects urine from the distal convoluted tubule and discharges it into the renal pelvis.

Functions of the Urinary System

• Excretion of wastes
• Formation of urine
• Fluid and electrolyte balance
• Elimination of urine

Formation of Urine

• Filtration: blood from the renal artery enters the glomerulus and high pressure forces some substances through the capillary walls, which act like a filter.
• Reabsorption: water and useful substances are reabsorbed. If blood levels of certain substances are high (e.g., glucose, amino acids, vitamins, sodium), they are not reabsorbed.
• Secretion: transports substances (e.g., creatinine, potassium, hydrogen, some drugs) into the collecting tubule.

Fluid and Electrolyte Balance

• Kidneys help maintain fluid and electrolyte balance.
• Chemical control through ADH (antidiuretic hormone) and aldosterone.
• If levels of substances in the blood are too low, substances are returned to the blood.
• If levels are too high, substances are excreted.
• Diuretics are substances that inhibit reabsorption of H₂O.

Elimination of Urine

• Average urinary output is 1500 ml per day.
• Urinalysis: examination of urine to determine presence of blood cells, bacteria, acidity level, specific gravity, and physical characteristics.

Disorders of the Urinary System

• Cystitis: inflammation of the urinary bladder. Most common cause is E. coli. Symptoms are dysuria (painful urination) and frequency. More common in females due to shorter urethra. Treatment: antibiotics.
• Enuresis: bedwetting. Can occur up to the age of 5 as part of normal childhood development. Most children outgrow bed-wetting on their own.
• Glomerulonephritis: inflammation of the kidney. May be acute (sudden) or chronic (slow development). Acute may occur after strep throat or other infections. Chronic develops slowly over years and often leads to complete kidney failure.
• Incontinence: involuntary urination; inability to control the release of urine.
• Renal calculi (kidney stones): made of crystals of calcium phosphate and uric acid. Symptoms include severe pain, nausea, vomiting, frequency, chills, fever, hematuria. Diagnosis by symptoms, ultrasound, and X-ray. Treatment: increasing fluids, medications, possible lithotripsy (shock waves).
• Renal failure: occurs when kidneys lose the ability to sufficiently filter waste from blood. May be acute or chronic. Oliguria (scanty amount of urine) is an early sign of stages 1-5. Stage 5 requires dialysis or kidney transplant.

Dialysis (Hemodialysis)

• Used for kidney failure.
• Involves the passage of blood through a device with a semi-permeable membrane.
• Dialysis serves as a substitute kidney.
• Blood from the patient flows through the machine and is filtered.
• Can be done at home or clinics.
• Takes 2-4 hours, 2-3 times per week.