1. Go to Section: Interest Grabber Your Body’s Filter Have you ever seen a water-purification system attached to a faucet? This system removes impurities from the water such as arsenic or other chemicals that can be harmful to people. As water passes through the filters contained in the system, the impurities are trapped on the surface of the filters. Eventually, the water that comes out of this purifier is free of the impurities. Section 38-3 1. Your body has its own system for filtering blood. Why might the blood in your body need to be filtered? 2. What organ(s) do you think filters your blood? 3. How do you think the filtered materials leave your body?

2. Go to Section: Section Outline 38–3The Excretory System A.Functions of the Excretory System B.The Kidneys 1.Kidney Structure 2.Filtration 3.Reabsorption 4.Kidney Stones C.Control of Kidney Function D.Homeostasis by Machine Section 38-3

3. Go to Section: Functions of the Excretory System Every cell in the body produces metabolic wastes as a result of maintaining homeostasis –Wastes such as urea, excess salts and carbon dioxide must be removed from the body The skin, lungs and liver are three organs that assist in excretion –The skin excretes excess salts, water and a small amount of urea –The lungs excrete carbon dioxide –The liver takes excess amino acids from the blood stream and converts them to useful compounds In the process of conversion, poisonous nitrogen compounds are created, which are turned into urea Urea is then removed from the bloodstream by the body’s principal organs of excretion – the kidneys

4. Go to Section: Kidneys The kidneys have many roles –Remove waste products from the blood –Maintain blood pH –Regulate the water content of the blood and therefore blood volume As waste-laden blood enters the kidney from the renal artery, it removes urea, excess water and other wastes and passes them to the ureter The clean, filtered blood leaves the kidney through the renal vein and returns to circulation

5. Go to Section: Kidney (Cross Section) Cortex Medulla Vein Artery Kidney Ureter Urinary bladder Urethra The Urinary System Section 38-3

6. Go to Section: Kidney Nephron Cortex Medulla Renal artery Renal vein Ureter To the bladder Bowman’s capsule Glomerulus Capillaries Collecting duct To the ureter Loop of Henle Artery Vein Figure 38–17 Structure of the Kidneys Section 38-3

7. Go to Section: Filtration Most filtration occurs in the glomerulus. Blood pressure forces water, salt, glucose, amino acids, and urea into Bowman’s capsule. Proteins and blood cells are too large to cross the membrane; they remain in the blood. The fluid that enters the renal tubules is called the filtrate. Reabsorption As the filtrate flows through the renal tubule, most of the water and nutrients are reabsorbed into the blood. The concentrated fluid that remains is called urine. The Nephron Section 38-3

8. Go to Section: Control of Kidney Function The activity of the kidneys is controlled by the blood itself. Water is absorbed into the blood from the digestive system. –The concentration of water in the blood increases. Without your kidneys, the increase in water conc. would force water into cells and tissues, causing your body to swell –As the amount of water in the blood increases, the rate of water reabsorption in the kidneys decreases. Less water is returned to the blood and the excess water is sent to the bladder. With an increase in salt, your kidneys return less salt to the blood, and send the excess salt to the bladder.

9. Go to Section: Disease Kidney Stones –Substances such as calcium, magnesium or salts in the urine crystallize and form stones –These stones may block the ureter, causing great pain –Often treated using ultrasound waves The stones gets pulverized into smaller fragments, which are then excreted in the urine.

10. Go to Section: Homeostasis by Machine Dialysis –Blood is removed from the body through a tube and pumped through special tubing that acts like nephrons –Wastes diffuse out of the blood and into a fluid-filled chamber –The purified blood is then returned to the body

11. Go to Section: Vein Artery Shunt Air detector Dialysis machine Blood pump Blood in tubing flows through dialysis fluid Used dialysis fluid Compressed air Fresh dialysis fluid Figure 38–19 Kidney Dialysis Section 38-3

12. Video 2 Click the image to play the video segment. Video 2 Kidney Function