Urinary System An overview

Functions of the Urinary System

Functions of the Urinary System Produces, excretes urine Essential for homeostasis Keeps volume of bodily fluids constant Keeps levels of essential chemicals constant Electrolytes, water, acid/base balance “Clears” (cleans) blood of waste products produced by cellular metabolism (e.g. nitrogenous wastes) Uremia- toxic buildup of metabolic wastes Blood pressure regulation

Characteristics of Urine Normal Characteristics Color: Transparent yellow, amber, straw-colored Compounds Mineral ions (Na+, Cl-, K+) Nitrogenous wastes (ammonia, creatinine, urea, uric acid) Suspended solids (sediment, bacteria, blood cells, casts) Urine pigments Odor Slight odor pH 4.6-8.0 Specific gravity 1.001-1.035

Structures of the urinary system (Macroscopic)

Major structures of the urinary system Kidney Retroperitoneal Ureters Urinary Bladder Urethra

The Kidney Renal cortex – Outer “bark” Renal medulla – Inner Renal pyramids Triangular divisions Renal columns Divide pyramids Renal papilla Narrow, innermost point of a pyramid Renal pelvis Expanded end of upper end of ureter Calyx Division of renal pelvis

Microscopic Structure of the Kidney

Microscopic Structure of the Kidney

Functional Unit: Nephron > 1 million in a kidney Can be cortical or juxtamedullary “Tiny funnel” with a very long stem Highly convoluted (many bends) 2 principal components: Renal corpuscle Renal tubule

Renal corpuscle components Bowman’s capsule Cup/sac-like-shaped top of a nephron Glomerulus Ball of capillaries, lined with podocytes Found in Bowman’s capsule Afferent, efferent arterioles

Renal Tubule Composed of: Proximal convoluted tubule (PCT) Loop of Henle (longer in Juxtamedullary) Distal convoluted tubule (DCT) Collecting tubule

Formation of Urine

Formation of Urine: Overview Filtration Secretion Reabsorption All occur within the functional unit of the kidney, the nephron

Filtration Of water, dissolved substances out of blood into Bowman’s capsule Occurs in “renal corpuscles” Contains larger afferent, smaller efferent arterioles Creates high pressure in glomerulus Filters blood across fenestrated capillaries, lined w/ podocytes Hold in proteins, cells Lose water, ions = ultrafiltration

Glomerular Filtration Rate 120 ml/minute = 180 liters/day! Must maintain a minimum blood pressure for kidneys to filter Hemorrhage, etc., can cause kidney function (and urine output) to cease

180 L of urine a day? No… Luckily, most of the fluid that leaves blood during filtration returns to the blood during reabsorption…

Reabsorption Movement of substances out of renal tubules into the blood (into peritubular capillaries) Water Glucose Other nutrients Sodium, other ions Begins in proximal convoluted tubule, continues in loop of Henle, distal convoluted tubule, and collecting tubule Reabsorb ~178 L/day (97-98% of water)

Mechanism? Osmosis! (Flow of water from High to Low) Glucose: Almost all glucose reabsorbed (340 mg/min) Exceptions exist! Diabetes mellitus – too much sugar in bloodstream, exceeds ability of kidney tubule cells to reabsorb glycosuria (glucose in blood) Water is lost along with glucose transport… most people with diabetes mellitus urinate excessively!

Ions (sodium, others) only partially reabsorbed Most are actively transported back into blood from tubular urine Amount varies from time to time… depends on salt intake Allows body to maintain homeostasis (cells can be damaged by too much/too little salt)

Loop of Henle Descending loop: Reabsorbs water, large ions and molecules Ascending loop: Final adjustment Hormones regulate the amount of sodium, water reabsorption

Water reabsorption 90% passive thru osmosis in PCT, descending Loop of Henle, DCT 10% occurs in last parts of DCT, collecting ducts

Secretion Substances move into urine in distal, collecting tubules from blood in capillaries around tubules “Reabsorption in reverse” H+, K+ ions, ammonia, drugs (Bold = active transport; non-bold = diffusion)

Regulation Regulated by hormones Antidiuretic hormone (ADH) – increases water reabsorption Angiotensin II – constricts efferent, systemic arterioles – alters filtration rate, increases pressures Aldosterone – Produced by adrenal cortex; retains sodium, excretes potassium – raises blood pressure Affected by other substances Diuretic (causes great outflow of urine) Caffeine –decreases Na+ absorption, causing diuresis Lasix – works on Loop of Henle, producing massive diuresis

Excretion Urine stored in bladder, leaves body through urethra A.k.a… Urination “Voiding” Micturation Urine stored in bladder, leaves body through urethra

Concentration Glomerulus “squeezes” blood out into the Bowman’s capsule Fenestrated inner lining Podocytes, w/ small holes, trap small proteins Filtrate runs across the capsule into the proximal convoluted tubule (PCT) PCT lined with BV’s that reabsorb nutrients from it, excrete wastes into it. PCT passes filtrate downward through loop of Henle, then back up Juxtamedullary nephrons extend deep into the medulla, have much longer loops of Henle Tubular Reabsorption – Urine in tubules must be reabsorbed to prevent dehydration Solutes are actively and passively reabsorbed (glucose, aa’s, sodium, chloride, potassium, phosphate, bicarbonate)