1. Urinary System
21 April 2017

2. Functions of Urinary System
The urinary system produces urine. The production of urine has three main functions:
Excretion of waste products of metabolism, especially urea
Osmoregulation – control of the water, salt, and acid-base balance of the body
Removal of drugs and toxins
Other functions
Regulation of blood pressure through production of enzyme renin
Stimulation of red blood cell production through the hormone erythropoeitin
Conversion of vitamin D to active form
Fun fact: Urea is a nitrogenous compound produced by the liver when we digest proteins
Fun fact: The two major places that remove or break down drugs and toxins are the liver and the kidney
Functions of Urinary System

3. Organs of the Urinary System

4. Urine is produced in the cortex by the nephrons
Urine is produced in the cortex by the nephrons. The nephrons are the functional unit of the kidney.
Urine is concentrated in the medulla
Urine collects in the calices, then flows into the renal pelvis then to the ureters
Kidney Structure
There are ~ 1 million nephrons in each kidney

5. Nephron Structure A nephron consists of 4 main parts:
Bowman’s capsule
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Each nephron empties into a collecting duct
The tubules of the nephron contain filtrate – the urine that is in process of being formed.
Capillaries twine all around the nephron … why?
Most nephrons are entirely within the cortex; some have loops of Henle that extend into the medulla
Nephron Structure

6. Nephrons & Urine Formation
There are three steps to urine formation
Filtration
Reabsorption
Secretion
Nephrons & Urine Formation

7. Nephrons & Urine Formation
Filtration
Blood from the glomerular capillaries filters into the Bowman’s capsule due to extremely high pressure in capillaries
Filtration is based on size only. Small items (everything in blood except the cells and most proteins) enter the Bowman’s capsule.
Passive process
Fun Fact: We produce ~180 L of filtrate a day … despite the fact that we have only about 5 L of blood. Nearly 99% of the water that is filtered out of the blood is reabsorbed the capillaries.
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Podocytes are cells that wrap around the capillaries of glomerulus, and form spaces where the small components of filter through.
Nephrons & Urine Formation

8. Nephrons & Urine Formation
Reabsorption
Reabsorption of important molecules (water, salts, amino acids, sugar, minerals) from tubules to capillaries
Some reabsorption is passive, but most relies on active transport
Different materials are reabsorbed in different sections (see diagram)
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
More on Loop of Henle!
Nephrons & Urine Formation

9. Nephrons & Urine Formation
Secretion
Active transport of unwanted materials from capillaries to filtrate
Useful for larger molecules (some drugs and toxins) and for regulation of blood pH
Occurs in convoluted tubules (esp. distal)
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Nephrons & Urine Formation

10. Responsible for most of the water and some salt reabsorption
Uses a countercurrent multiplier mechanism  by having the fluid flow in opposite direction, the exchange of materials can be maximized or minimized
The loop dips into increasingly salty medulla to extract as much water as possible from the filtrate
passive
active
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Loop of Henle

11. Loop of Henle Descending loop of Henle
Descending loop is permeable to water, but impermeable to salt
So, water passively diffuses from the loop of Henle into the interstitial fluid (and then into the peritubular capillaries)
passive
active
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Loop of Henle

12. Loop of Henle Ascending loop of Henle
Ascending loop is impermeable to water, but permeable to salt
So, water CANNOT re- enter the tubules, despite the fact that the filtrate is now more concentrated then the interstitial fluid
NaCl first moves passively out of the loop, then is actively transported out. This both retains our salts and creates the salty medulla environment
passive
active
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Loop of Henle

13. Collecting Duct Osmoregulation
The reabsorption of water in the collecting duct is controlled by hormones.
If blood solute concentration is high, the pituitary releases antidiuretic hormone (ADH) which opens aquaporins in the collecting duct, making it permeable to water.
If blood solute concentration is low, aquaporins are closed, making the duct impermeable to water
passive
active
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Collecting Duct Osmoregulation

14. Collecting Duct Osmoregulation
If you are dehydrated, will you produce ADH?
Yes!
If you produce ADH, will you produce a lot of urine, or a little urine?
Little! Your body will produce a small amount of concentrated urine, retaining precious water in your blood.
passive
active
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Collecting Duct Osmoregulation

15. Collecting Duct Osmoregulation
With ADH
Without ADH
Only capillary bed that starts and ends with an arteriole (instead of ending with venule). High pressure due to larger diameter of afferent than efferent artierole.
Collecting Duct Osmoregulation

16. Comparison of Fluid Composition
Think, Pair, Share: Explain these values
Solute
Plasma (mg/100 mL)
Filtrate (mg/100 mL)
Urine
(mg/100 mL)
Glucose
900
Urea 30
1200
proteins
740
Review: watch me!
Comparison of Fluid Composition

17. You Do Draw a nephron, labeling the parts and the major functions.
Draw a nephron, use different colored markers to show both the relative concentration (use dots!) of the each of the following chemicals within each region of the nephron and use arrows to show the flow of the substances in or out of the nephron.
water
salt
Sugar
Urea
You Do

18. Closure What were our objectives, and what did you learn?
What was our learner profile, and how did we demonstrate it?
How does what we did today relate to our unit question?
Closure