Maintaining Water Balance

The Kidneys

Homeostasis

• Regulation of a stable internal environment no matter where we are or what we do…

• We put our bodies through A LOT on a daily basis– Adding food, liquid, chemicals– Changing temperature & activity levels

• BUT…. Our bodies roll with it!• How (examples)???

Excretory System

• Star QB of maintaining homeostasis in our bodies– Responsible for maintaining the right amount of

water and dissolved substances in your body (osmoregulation)

• Includes: – Kidneys– Bladder– Ureter– Urethra

Osmoregulation

• How our bodies get rid of the “stuff” we don’t need such as…– Byproducts of metabolized food

• Makes sure we don’t get dehydrated• Your body’s GREATEST balancing act

The Excretory System

• Not all animals excrete waste in the same way…

• Main byproduct of metabolizing food (mainly protein) is ammonia (NH3) TOXIC– Converted into either urea or uric acid depending

on the amount of water available to the animal

Urea:

Compound made from combining ammonia with carbon dioxide (in the liver)

Low toxicity: can hang out in circulatory system for a while with no ill effects

BUT….

You have to have extra water around to dissolve it and get rid of it (you pee)

Uric Acid:

Have to be light enough to fly and don’t have a lot of spare water

Excreted as paste (not a lot of water is needed)

Have you ever seen bird poop?

** The white part of it is actually the uric acid

How do humans excrete urea?

• Starts with the kidney– Maintains levels of water and dissolved materials– Controls blood pressure

• Very Inefficient Process– Filter the water and dissolved material out of blood

• 99% is reabsorbed back• 1% is sent on to make urine• Avg day: kidneys filter out 180L of fluid from your blood,

BUT only 1.5L gets peed out

How do your kidney’s do this?

• Use nephrons– Fun Fact: If you were to unravel all of your nephrons, they would

stretch ~180 km• Must follow blood flow to understand how they work…

– Heart – Kidney (enters via renal arteries)– Glomerulus– Bowman’s Capsule– Proximal Convoluted Tubule– Loop of Henle– Distal Tubule– Collecting Duct

Glomerulus & Bowmans Capsule

• Start point for single nephron

• Pressure is high enough that 20% of the fluid is squeezed out and enters the Bowmans Capsule

– Cup like sac (fluid is no longer blood, but filtrate: water, urea, small ions)

Promimal Convoluted Tubule

• Site of osmoregulation– Reabsorb water and other materials to create

whatever balance your body needs– Mainly reabsorbs organic solutes (ie: glucose);

recaptures Na, K and water

Loop of Henle

• Site where most of the reabsorption happens– Extracts most of the water that we need– Pumps salts – Creates a concentration gradient to cause more

water to be extracted

***By the time the filtrate gets to the bottom of the loop it is HIGHLY concentrated

Distal Tubule

• Regulates Na, K, Ca levels– Controlled by pumps and hormones that control

the reabsorption process• By the time its done, we have taken

everything we want OUT of the filtrate– Filtrate is now mainly excess water, urea and

excess waste

Collecting Duct reabsorbed

H2O excretion

urea passed through to bladder

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