Urinary System

28 April 2014

Functions of Urinary System

» The urinary system produces urine. The production of urine has three main functions:1) Excretion of waste products of metabolism, especially urea2) Osmoregulation – control of the water, salt, and acid-base

balance of the body3) Removal of drugs and toxins

» Other functions˃ Regulation of blood pressure through production of enzyme

renin˃ Stimulation of red blood cell production of 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

Organs of the Urinary System

Kidney Structure

» 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

There are ~ 1 million nephrons in each kidney

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

» Capillaries twine all around the nephron … why?

» Most nephrons are entirely within the cortex; some have loops of Henle that extend into the medulla

The tubules of the nephron contain filtrate – the urine that is in process of being formed.

Nephrons & Urine Formation

There are three steps to urine formation

» Filtration

» Reabsorption

» Secretion

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.

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) More on

Loop of Henle!

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)

Loop of Henle

» 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

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

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

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

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

Collecting Duct Osmoregulation

With ADH

Without ADH

Comparison of Fluid Composition

Think, Pair, Share: Explain these values

Solute Plasma (mg/100 mL)

Filtrate (mg/100 mL)

Urine(mg/100 mL)

Glucose 900 900 0

Urea 30 30 1200

proteins 740 0 0

Review: watch me!

You Do

1) Draw a nephron, labeling the parts and the major functions.

2) 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

Closure

1) What were our objectives, and what did you learn?

2) What was our learner profile, and how did we demonstrate it?

3) How does what we did today relate to our unit question?