1. Urinary System and Excretion
Chapter 16 p

2. Functions 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

3. Urinary System Organs involved: Path of urine
Kidneys (Y)
Ureters (W)
Urinary bladder (Z)
Urethra (X)
Path of urine
Kidney  ureters  urinary bladder  urethra

4. Organs of the Urinary System
Kidney
Paired
Bean-shaped
Reddish-brown in colour
Connected to the renal artery/vein
Connected to ureter

5. The Kidney W – renal cortex (contains nehprons) X – renal medulla
(contains collecting ducts of nehprons)
Y - renal pelvis
Z – ureter

6. Organs 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

7. Organs of the Urinary System
Urethra
Remove urine from the body
Small tube that extends from the urinary bladder to an external opening

8. Urination 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

9. The Nephron A kidney is composed of over a million nephrons.
Renal cortex
Renal medulla
A kidney is composed of over a million nephrons.

10. The Nephron Proximal convoluted tubule Efferent arteriole
Distal convoluted tubule
Afferent arteriole
Bowman’s capsule
Peritubular
capillary network
Glomerulus
Renal vein
Collecting
Duct
Loop of Henle

11. Parts of a Nephron Nephron Glomerulus
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.

12. Parts of a Nephron Bowman’s capsule the bulbous unit of the nephron,
surrounds the glomerulus.
Blood is under hi- pressure

13. Parts of a Nephron Afferent (incoming) Arteriole
leads into Glomerulus
Efferent (outgoing) Arteriole
exits glomerulus
Tubular excretion (secretion)
Peritubular capillaries  venule  vein

14. Parts 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 increase surface area
Glucose (100%), a.a. (100%), bicarbonate (80-90%), water (65%), Na+ (65%)

15. Parts of a Nephron Distal Convoluted Tubule (DCT)
Tubular Secretion (move molecules from blood into the tubule)
Not specialized in reabsorption

16. Parts of a Nephron Loop of Henle Collecting Duct U-shaped
Connects the Proximal and Distal Convoluted Tubules
Collecting Duct
Receives secretion by several distal convoluted tubules
Carry urine to renal pelvis

17. Urine Formation – A 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
– when you REALLY have to pee, it feels as urgent as escaping from FIRE.

18. Urine Formation 1. 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

19. filtrate 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

20. 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

21. Urine Formation 2. 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

22. Tubular Reabsorption Involves 2 processes: Selective Reabsorption
Water Reabsorption

24. Selective 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

25. Selective 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

26. Water 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

27. So what’s in the proximal 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

28. Urine Formation 3. Tubular Secretion
Takes place at the Distal Convoluted Tubule
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

29. 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!

31. Defecation (or elimination) - the release of unabsorbed wastes (e. g
Defecation (or elimination) - the release of unabsorbed wastes (e.g., feces) from the digestive tract