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