The Urinary System

The Problem – Nitrogenous Waste As proteins and nucleic acids are catabolized, nitrogenous wastes, including ammonia, are produced in all animals. NH3 (ammonia) is water-soluble, a strong base, and can change the pH of blood. It must be eliminated! From Beatrice the Biologist’s blog

How is Waste Eliminated?

Organs Kidneys – produce urine Ureters – transports urine to bladder Urinary bladder – stores urine Urethra – transports urine to exterior

Functions Primary Functions: Secondary Functions: Eliminate metabolic wastes Regulate blood volume Regulate blood chemistry Secondary Functions: Metabolism of Vitamin D Production of renin Production of erythropoietin

Kidney Structure - External Location: superior lumbar Adrenal glands Renal artery Renal vein Renal hilus Renal capsule Adipose capsule

Kidney Structure – Internal Renal Cortex – superficial portion; urine formation Renal Columns – extensions of cortex Renal Medulla – deep to cortex Medullary pyramids – stripes are due to collecting ducts

Kidney Structure - Internal Renal Pelvis – funnel shaped tube; continuous with ureter Calyces – cup-shaped extensions of the pelvis that enclose the tips of the pyramids

Formed in the renal cortex Travels thru the collecting ducts in the renal medulla Drips out of collecting ducts into the minor calcyes Flows thru the major calyces Flows thru the renal pelvis Flows thru the ureter Flows thru and is stored in the urinary bladder Flows thru the urethra

Internal Kidneys – The Nephron Glomerulus – blood pressure forces smaller molecules into Bowman’s Capsule Proximal convoluted tubule Loop of Henle – concentrates urine Descending loop Ascending loop Distal convoluted tubule Collecting duct – transports urine to calyces Capillaries – recapture needed molecules

Bowman’s Capsule & Glomerulus High blood pressure in glomerulus causes nonselective, passive filtration Any product smaller than proteins are forced out of blood & into capsule: Water Glucose Salts Nitrogenous wastes Check out the size difference

Proximal Convoluted Tubule (PCT) Reabsorption – Mainly active transport is used to reabsorb some nutrients : salt, water, glucose, amino acids Secretion – Mainly active transport moves some unwanted materials into PCT: some poisons, hydrogen ions Has lots of mitochondria & microvilli. Why?

Loop of Henle Concentrates urine Descending loop – permeable to water Ascending loop – impermeable to water Concentration gradient

Distal Convoluted Tubule (DCT) & Collecting Duct DCT performs same role as PCT Collecting duct receives urine from several DCT’s Reabsorbs water, some urea (passive transport) Reabsorbs salt (active transport) Empties to calyx

Ureters & Bladder Urine flows through ureters due to: Gravity Smooth muscle Bladder stores urine Allows urine release to be discontinuous even though formation of urine is continuous.

Urethra & Control of Bladder Female urethra is ~3-4cm long Male urethra is ~20cm long Internal urethral sphincter – smooth muscle, involuntary External urethral sphincter – skeletal muscle, voluntary Disorder: urinary tract infection (UTI) More common in women than men

Micturition “Voiding,” or emptying the bladder Voluntary control of an involuntary reflex Steps Bladder collects urine until reach ~200ml. Stretch receptors activate, triggering bladder contraction reflexes Urine is forced past the internal urethral sphincter into the upper part of the urethra, causing urge to void If postponed, the reflex will occur again after another 2-300ml. are collected.

Urine Clear Pale to Deep yellow Slightly aromatic Can be affected by food Slightly more dense than water Specific gravity – comparison of weight to that of water pH is slightly acidic (~6)

Abnormal Urine Components

Homeostasis of Water & Electrolytes ADH (antidiuretic hormone) Prevents excessive water loss in kidney Regulated by hypothalamus & secreted by posterior pituitary Functions by making collecting duct more permeable to water

Homeostasis of Water & Electrolytes Aldosterone Normally, about 80% of sodium ions are reabsorbed in the PCT. When aldosterone is present, nearly all the rest is absorbed in the DCT & collecting duct. Increases water reabsorption, because water follows salt. Produced by adrenal cortex in response to renin-angiotensin mechanism triggered by low blood volume or pressure

Diuretics Interfere with normal reabsorption, causing increased urination Many drugs Alcohol

Urinary Disorders – Kidney Stones