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: 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 ureterFlows 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