Urinary System

Elimination of waste products:
- Removal of nitrogenous wastes (products of protein metabolism).
- Elimination of toxins and drugs from the body.
Regulation of homeostasis:
- Water balance: Maintains proper hydration and fluid levels in the body.
- Electrolyte balance: Manages ion levels in the blood (e.g., Na+, K+, Cl-).
- Acid-base balance: Regulates blood pH through bicarbonate and hydrogen ion management.
- Blood pressure regulation: Through fluid regulation and action of hormones.
- Production of red blood cells: Stimulated by erythropoietin released by kidneys.
- Activation of vitamin D: Converts vitamin D to its active form for calcium absorption.

Kidneys: Pair of organs responsible for filtering blood and producing urine.
Ureters: Tubes that carry urine from kidneys to the bladder.
Urinary bladder: Storage organ for urine.
Urethra: Tube that carries urine from the bladder to the outside of the body.

The nephron is the functional unit of the kidney, made up of:
- Renal corpuscle (a filter)
- Renal tubule (site of reabsorption and secretion)

Glomerulus: A knot of capillaries surrounded by podocytes.
- Glomerular (Bowman's) capsule: Surrounds the glomerulus, collecting filtrate.
- Filtration occurs through podocyte slits allowing small molecules like water, glucose, vitamins, amino acids, ammonia, and urea to pass while blocking larger molecules like RBCs, WBCs, and large proteins.

Renal tubule extends from the glomerular capsule and empties into the collecting duct.
Subdivisions of the renal tubule:
1. Proximal convoluted tubule (PCT)
2. Nephron loop (loop of Henle)
3. Distal convoluted tubule (DCT)

Cortical nephrons: Located entirely in the cortex, constitute most nephrons.
Juxtamedullary nephrons: Positioned at the cortex-medulla boundary; their nephron loops dip into the medulla and play a critical role in urine concentration.
Both types drain urine into the collecting ducts which flow through the renal pyramids to the calyces and renal pelvis.

Glomerulus
- Specialized for filtration; higher blood pressure (60 mm Hg) than typical capillaries (10 mm Hg) due to the larger afferent arteriole.
- High pressure forces fluid into the glomerular capsule.
Peritubular capillaries
- Arise from the efferent arterioles of the glomerulus.
- Adapted for absorption with lower pressure, reabsorbing substances from the renal tubule.

Glomerular filtration: Occurs in glomerulus, forming a filtrate similar to plasma but lacking large proteins.
Tubular reabsorption: Takes place in renal tubules; both active and passive transport mechanisms involved to reclaim useful substances.
Tubular secretion: Moves additional wastes from peritubular blood into renal tubules, performed through active transport.

Glomerular filtration: Creates a plasmalike filtrate from blood.
Tubular reabsorption: Returns useful solutes to blood from the filtrate via peritubular capillaries.
Tubular secretion: Additional wastes added to filtrate from blood.
Water conservation: Concentrates urine by returning water to blood.

Blood pressure in glomerular capillaries drives filtration, creating a selective barrier.
Filtrate formed as long as systemic blood pressure is normal.
Oligouria: Abnormally low urine output (100-400 ml/day).
Anuria: Urine production less than 100 ml/day.
Hypertension can lead to glomerular damage (nephrosclerosis) and decreased renal blood supply, potentially causing renal failure.

Neural Control: Autonomic regulation of renal blood flow.
Hormonal Control: Involving substances like angiotensin II, aldosterone, and ADH.
Local Control: Intrinsic mechanisms acting within the kidney.

Renin: Enzyme released by juxtaglomerular apparatus in response to low blood pressure.
Angiotensin II: Causes vasoconstriction and stimulates aldosterone release, increasing blood volume and pressure.

Most reabsorption occurs in the PCT (about 65%).
Reabsorbed substances include:
- Water, glucose, amino acids, and ions (particularly Na+).
Reabsorption mechanisms:
- Passive transport: E.g., water flows by osmosis.
- Active transport: Utilizes ATP.

Proximal tubule:
- Reabsorbs: Water, glucose, bicarbonate, NaCl, amino acids.
- Secretes: Hydrogen ions and some drugs.
Descending loop:
- Reabsorbs: Water.
- Secretes: Urea.
Ascending loop:
- Reabsorbs: NaCl.
- Secretes: Potassium and urea.
Distal tubule:
- Reabsorbs: Sodium, water, potassium.
- Secretes: Urea and some drugs.
Collecting duct:
- Reabsorbs: Water and NaCl.
- Secretes: Potassium and urea.

ADH (Antidiuretic Hormone): Promotes water reabsorption in kidneys.
Aldosterone: Increases sodium and water reabsorption; released from adrenal cortex.
Natriuretic peptide: Decreases blood volume and pressure by opposing aldosterone and ADH effects.
Hormones critically regulate tubular reabsorption process and maintain blood pressure and homeostasis.

Average urine production: 1.0 to 1.8 liters per day.
Urine Composition vs. Filtrate:
- Filtrate includes water, glucose, ions, and nitrogenous wastes.
- Urine is the end product after reabsorption of water and nutrients.
Properties of Urine:
- Yellow color from urochrome.
- Normal pH: ~6.
- Specific gravity: 1.001 to 1.035 (low indicates diluted urine).
- Sterile and slightly aromatic.

Micturition: The act of emptying the urinary bladder.
Involuntary sphincter opens due to stretch receptor stimulation in the bladder; external sphincter can be consciously controlled.
Incontinence: Inability to control micturition.
Urinary retention: Inability to empty the bladder fully.

Kidneys develop early in gestation, functional by the third month.
Voluntary control over the urethral sphincter begins around 18 months.
Nighttime control may not fully develop until age 4.
UTIs are common before old age, predominantly caused by Escherichia coli.
Age-related decline in kidney function can lead to increased urgency and urinary retention in men.