The Urinary System

Chapter 26: The Urinary System

Functions: The urinary system is responsible for removing waste products from body fluids, regulating blood composition, pH, volume, pressure, osmolarity, and producing hormones. It plays a vital role in maintaining homeostasis.
Main Components: The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra.

Kidneys are reddish, bean-shaped organs located retroperitoneally in the abdominal cavity, between the last thoracic and third lumbar vertebrae.
Size: An adult kidney measures approximately 10-12 cm in length, 5-7 cm wide, and has a mass of 135-150 g.
Surrounding Layers: Each kidney is surrounded by three layers: the renal capsule (inner), adipose capsule (middle), and renal fascia (outer).

Excretion of Wastes: The kidneys filter metabolic wastes such as urea, ammonia, creatinine, and uric acid, collectively known as nitrogenous wastes.
Regulation of Ionic Composition: The kidneys manage blood levels of ions like Na+, K+, Ca2+, and Cl− by regulating their excretion in urine.
pH Regulation: They help maintain blood pH by excreting excess hydrogen ions and conserving bicarbonate ions.
Volume and Pressure Control: The kidneys adjust blood volume and pressure by conserving or eliminating water, impacting blood pressure indirectly.
Osmolarity Maintenance: They ensure stable osmolarity close to 300 mOsm/liter by managing water and solute losses.
Hormone Production: The kidneys produce calcitriol (active form of vitamin D) and erythropoietin, which stimulates red blood cell production.
Blood Glucose Regulation: The kidneys can synthesize glucose during fasting through gluconeogenesis.

Nephrons are the functional units of the kidneys, with approximately 1 million in each kidney.
Parts of a Nephron:
- Renal Corpuscle: Comprises the glomerulus (capillary network) and Bowman's capsule (double-walled structure).
- Renal Tubule: Divided into proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT).
Types of Nephrons: 80-85% are cortical nephrons with short loops; 15-20% are juxtamedullary nephrons with long loops extending deep into the medulla.

Three Basic Processes: 1) Glomerular filtration, 2) Tubular reabsorption, 3) Tubular secretion.
Filtration: Fluid from blood plasma is filtered into the Bowman's capsule, forming glomerular filtrate.
Reabsorption: Most filtered substances (water, nutrients, and ions) are reabsorbed back into the bloodstream primarily in the proximal convoluted tubule.
Secretion: Additional substances (wastes, excess ions) are secreted from blood into the renal tubule.

The kidneys regulate urine production based on hydration levels. ADH (Antidiuretic Hormone) promotes water reabsorption; a lack leads to dilute urine.
Countercurrent Mechanisms: The nephron loop and vasa recta create an osmotic gradient crucial for concentrating urine.

Ureters: Muscular tubes that transport urine from the kidneys to the urinary bladder through peristaltic movements.
Urinary Bladder: Hollow muscular organ that stores urine until it is excreted. It can hold 700-800 mL of urine.
Urethra: Tube that carries urine (and semen in males) from the bladder to the exterior. In females, it is shorter (4 cm) than in males (20 cm).

With aging, kidney size and function decline, affecting blood flow, filtration capacity, and increasing susceptibility to infection and chronic conditions.
Common conditions include urinary tract infections, increased frequency of urination, and urinary incontinence.

Dialysis: A procedure to artificially clear wastes from the blood, necessary in severe kidney failure.
Urinalysis: A critical tool for assessing kidney function, identifying abnormalities such as albumin, glucose, or blood in urine, which indicate disease.
Renal Plasma Clearance: An important measure of kidney function that indicates how effectively the kidneys clear substances from the bloodstream.