Urinary SystemUrinary Systemandand
ExcretionExcretion
Chapter 16
p. 303-316
Functions of the Urinary SystemFunctions of the Urinary System
• Maintain homeostasis – excretes urine, which contains metabolic
wastes products, out of the body– Maintains salt and water balance– Maintains blood acid-base balance– Secretes hormones
Urinary SystemUrinary System
• Organs involved:– Kidneys (Y)– Ureters (W)– Urinary bladder (Z)– Urethra (X)
• Path of urineKidney ureters
urinary bladder urethra
Organs of the Urinary SystemOrgans of the Urinary System
• Kidney– Paired– Bean-shaped– Reddish-brown in colour– Connected to the renal artery/vein– Connected to ureter
The KidneyThe Kidney
• W – renal cortex (contains nehprons)
• X – renal medulla(contains collecting ducts of nehprons)
• Y - renal pelvis
• Z – ureter
Organs of the Urinary SystemOrgans of the Urinary System
• Ureter– Conduct urine from
kidney to bladder– Small – 25cm long
5mm in diameter
• Urinary bladder– Stores urine until
expulsion – Bladder is expandable– Sphincters control the
backflow of urine
Organs of the Urinary SystemOrgans of the Urinary System
• Urethra– Remove urine from the body – Small tube that extends from the urinary
bladder to anexternal opening
UrinationUrination
• Bladder fills up to 250ml with urine• Stretch receptors send sensory nerve impulses
to the spinal cord.• Motor neurons cause the bladder to contract,
sphincters relax• Urination occurs• Brain controls this
reflex until a suitable time
The NephronThe Nephron
A kidney is composed of over a million nephrons.
Renal cortex
Renal medulla
The NephronThe Nephron
Bowman’scapsule
Efferent arteriole
Proximal convoluted tubule
Distal convoluted tubule
CollectingDuctLoop of Henle
Glomerulus
Afferent arteriole
Renal vein
Peritubular capillary network
Parts of a NephronParts of a Nephron
• Nephron– the functional unit of the kidney– involved in filtration and selective reabsorption
of blood
• Glomerulus– a cluster of capillaries enclosed
by the Bowman’s capsule.
Parts of a NephronParts of a Nephron
• Bowman’s capsule– the bulbous unit of the nephron, – surrounds the glomerulus.
Blood isBlood isunderunderhi- pressurehi- pressure
Parts of a NephronParts of a Nephron
• Afferent (incoming) Arteriole– leads into Glomerulus
• Efferent (outgoing) Arteriole– exits glomerulus– Tubular excretion (secretion)– Peritubular
capillaries venule vein
Parts of a NephronParts of a Nephron
• Peritubular capillary network– takes up reabsorbed material, and drains into the
Renal vein– helps maintain concentration gradient in the medulla
for reabsorption
• Proximal Convoluted Tubule (PCT)– Tubular (Selective) Reabsorption
– Lined by microvilli to increasesurface area
– Glucose (100%), a.a. (100%), bicarbonate (80-90%), water (65%), Na+ (65%)
Parts of a NephronParts of a Nephron
• Distal Convoluted Tubule (DCT)– Tubular Secretion
(move molecules from blood into the tubule)
– Not specialized in reabsorption
Parts of a NephronParts of a Nephron
• Loop of Henle– U-shaped– Connects the Proximal and Distal Convoluted
Tubules
• Collecting Duct– Receives secretion by several
distal convoluted tubules– Carry urine to renal pelvis
Urine Formation – Urine Formation – A 3 Step ProcessA 3 Step Process
• Excretion - release of metabolic wastes and excess water
• General picture:1. Filter by pressure
2. Reabsorb by active and passive transport
3. Secrete Urine
FiREs FiREs – when you REALLY have to pee, it feels
as urgent as escaping from FIRE.
Urine FormationUrine Formation1. Filtration1. Filtration
• Pressure Filtration / Glomerular Filtration– Takes place at the glomerulus and Bowman’s
capsule– Blood enters from the renal afferent arteriole
and slows down as it enters capillaries– High BP in the glomerulus
forces molecules to move from blood into the Bowman’s Capsule
• Movement of molecules is drive by a pressure gradient
• Only molecules that are small enough to fit through the walls of the glomerular capillaries move into the nephron– H2O, urea, glucose, AA, uric acids, salts
filtrate
What if BP is too low?What if BP is too low?
• If BP is too low:– A hormone-like substance called renin is
released from a specialized tissue in the glomerulus called juxtaglomerulus apparatus
– Renin• Ensures adequate pressure filtration
is happening by constricting the glomerulus, therefore increase BP
Urine FormationUrine Formation2. Tubular Reabsorption2. Tubular Reabsorption
• Takes place in the proximal Convoluted Tubule
• Involves transferring molecules from filtrate in nephron back to peritubular capillaries
• Occurs by active and passive transport
Tubular ReabsorptionTubular Reabsorption
• Involves 2 processes:– Selective Reabsorption– Water Reabsorption
Selective ReabsorptionSelective Reabsorption
• Molecules are actively transported acorss the nephron membrane into the peritubular capillaries (Eg. glucose, AA, Na+, vitamins)– Note: Cl- passively follows Na+ by charge
attraction; therefore NaCl actually leaves the nephron in this step
– This increases the salt content of blood compared to the filtrate
Selective ReabsorptionSelective Reabsorption
• Active transport requires carrier protein (thus, ATP and Mitochondria)– Once all carrier proteins are used, excess
molecules in the filtrate will appear in urine– Diabetes Mellitus: when excess glucose
occurs in the blood
Water ReabsorptionWater Reabsorption
• Water is reabsorbed into the peritubular capillaries
• Solutes (glucose, AA, salt, vitamins) that are actively pumped out of the nephron create an Osmotic Gradient– Blood in peritubular capillaries is hypertonic to
the interstitial fluid, water is passively drawn from the nephron by osmosis
So what’s in the proximal So what’s in the proximal convoluted tubule?convoluted tubule?
• Materials NOT reabsorbed– Some water – A lot of nitrogenous / metabolic waste– Excess salt ions
• And what is in the peritubular capillaries? (reabsorbed materials)– Most water– Nutrients– Required salt ions for the blood
Urine FormationUrine Formation3. Tubular Secretion3. Tubular Secretion
• Secretion– Movement of materials
from nephron back to the blood via active transport
• H+ is excreted from the nephron if blood pH is high (too basic). H+ acts as a buffer to neutralize pH.
• Excretion– Movement of excess
wastes from blood into nephron via active transport
– Materials are added to urine (Eg. Drugs, ions (H+, NH3)
• If blood pH is low (too acidic), excreting excess H+ ions to the nephron will help raise pH back to homeostasis
– Takes place at the Distal Convoluted Tubule
Urine Made!Urine Made!
• Once filtrate reaches the collecting duct, 99% of water has been reabsorbed into the blood
• Collecting duct is also permeable to urea (diffuses out)
• Water continues to reabsorb in the collecting duct (when ADH (hormone) is present)
• Filtrate is now called urine!
• Defecation (or elimination) - the release of unabsorbed wastes (e.g., feces) from the digestive tract