THE URINARY SYSTEM   The body must rid itself of the waste products of cellular activity.  The process of removing metabolic waste, called EXCRETION, is just as vital as digestion in maintaining the body's internal environment.  The Urinary System not only excretes waste but also helps maintain Homeostasis by returning the content of water and other substances in the blood.   OBJECTIVES:  Define the term excretion, and list the functions of each major excretory organs.  Identify the major parts of the kidney.  Relate the structure of a nephron to its function.  Explain how the process of filtration, reabsorption, and tubular secretion help maintain homeostasis. Name the main parts of the urinary system.   1. THE PROCESS BY WHICH METABOLIC WASTES ARE REMOVED FROM THE BODY IS CALLED EXCRETION.   2. METABOLIC WASTES INCLUDE EXCESS WATER AND SALTS, CARBON DIOXIDE FROM CELLULAR RESPIRATION, NITROGENOUS COMPOUNDS FROM THE BREAKDOWN OF PROTEINS, AND UREA.   3. THE SKIN, LUNGS, AND KIDNEYS-ALONG WITH THEIR ASSOCIATED ORGANS-MAKE UP THE EXCRETORY SYSTEM.   4. The SKIN excretes excess water and salts, and a small amount of urea.   5. The KIDNEYS excrete the Nitrogenous Wastes, the excretion of Water is necessary to dissolve wastes and is closely regulated by the Kidneys, the Main Organ of the Urinary system.   6. The LUNGS excrete most of the carbon dioxide.   THE KIDNEYS   1. THE MAIN ORGANS OF THE EXCRETORY SYSTEM ARE THE KIDNEYS.  We have Two BEAN-SHAPPED Kidneys, one on each side of the spinal cord near the lower back, one behind the Stomach the other behind the Liver.  Together they Regulate the Chemical Composition of Blood.   2. Two Blood Vessels Enter and Leave each Kidney.  The Renal Artery Enters each Kidney and the Renal Vein Exists each Kidney.   3. A Third Vessel, the URETER, leaves each Kidney carrying fluid to the URINARY BLADDER.   4. Waste-laden Blood Enters the Kidney through the Renal Artery.   Excess water, Urea, and other waste products are removed from the blood and are collected in the URETER.  The most common mammalian metabolic waste is UREA.  The Filtered Blood exits through the Renal Vein.   5. UREA is a Nitrogenous product made by the Liver.  Nitrogenous Wastes are initially brought to the Liver as AMMONIA, a Chemical Compound of Nitrogen so Toxic that it could not remain in the body without harming cells.   6. The Liver Removes Ammonia from the blood and converts it to the less harmful substance Urea.  The Urea enters the Bloodstream and is then removed by the Kidneys.   KIDNEY STRUCTURE   1. Each Kidney is a Bean-Shaped organ, about the size of a Fist.   2. The Kidney has THREE Regions; the Inner part called the RENAL MEDULLA; the Outer part called the RENAL CORTEX and the RENAL PELVIS, a Funnel Shaped Structure in the Center of the Kidney.   3. The Renal Cortex contains the NEPHRONS, THE BASIC FUNCTIONAL UNIT OF THE KIDNEYS.   4. EACH NEPHRON IS A SMALL INDEPENDENT FILTERING UNIT. IN EACH KIDNEY THERE ARE ABOUT 1 MILLION NEPHRONS.   5. Nephrons filters water and solutes from blood.  Most of the Filtrate is reclaimed from them.  The rest, form an Amber Colored Liquid called URINE enters tubelike collecting ducts.  These lead to the Kidneys Central Cavity and the entrance to a Ureter.   6. Each Nephron has its own blood supply and its own collecting tubule, which leads to the Ureter.   7. As blood enters a Nephron through an Arteriole, impurities are filtered out and emptied into he collecting tubule.  Purified blood leaves the nephron through a Venule.   8. The process of Blood Purification involves Two Separate Processes-FILTRATION AND REABSORPTION.   FILTRATION   1. When blood enters a Nephron, it flows into a network of 50 Capillaries known as a GLOMERULUS.   2. The Glomerulus is encased in the upper end of the Nephron by a Cup-Shaped structure called BOWMAN'S CAPSULE.   3. The Blood is under pressure and the walls of the capillaries and Bowman's Capsule are permeable, much of the Fluid from the blood filters into Bowman's Capsule and the material Filtered from the blood flows through the RENAL TUBULE, a long tube with permeable walls.   4. The Renal Tube Consists of Three PARTS: THE PROXIMAL CONVOLUTED TUBULE, THE LOOP OF HENLE, AND THE DISTAL CONVOLUTED TUBULE.   5. MATERIALS FROM BLOOD ARE FORCED OUT OF THE GLOMERULUS AND INTO THE BOWMAN'S CAPSULE DURING A PROCESS CALLED FILTRATION.   6. THE MATERIALS THAT ARE FILTERED FROM THE BLOOD ARE KNOWN AS FILTRATE.   7. The Filtrate contains water, urea, glucose, salts, amino acids, and vitamins.   8. Plasma proteins, cells and platelets are too large to pass through the membrane; they remain in the blood.   REABSORPTION   1. Approx. 180 liters of filtrate pass from the blood into the collecting tubules each day.  Not all of this is Excreted. 2. Most of the materials removed from the blood at Bowman's Capsule makes its way back into the blood by a process known as REABSORPTION.   3. Approximately 99 percent of the water that is filtered into the Bowman's Capsule is Reabsorbed into the Blood.   4. Reabsorption proceeds along the Nephron's Tubular Parts.   5. Most Reabsorption occurs in the Proximal Tubule.  In this region, about 75 percent of the Water in the Filtrate returns to the Capillaries by Osmosis.   6. Glucose and minerals are returned to the blood by Active Transport.   7. Some additional reabsorption occurs in the distal Convoluted Tubule.   8. When the filtrate reaches the distal Convoluted Tubule, some substances pass from the blood into the filtrate through a process called SECRETION.  These substance include wastes and toxic materials.  The pH of the blood is adjusted by Hydrogen Ions that are secreted from the Blood into the Filtrate.   9. The material that remains in the distal convoluted tubule is called URINE, and consists of EXCESS salts, water, and urea.   10.  The Urine becomes concentrated in a section of the Nephron called the LOOP OF HENLE, this area helps to conserve water and minimize the volume of urine.   11.  Urine from the collecting ducts flows through the Renal Pelvis and into a narrow tube called a URETER.  A Ureter leads from each Kidney to the URINARY BLADDER. Urine is collected in the Urinary Bladder and stored until it can be released through the URETHRA.   12.  At least 500 mL (17 oz) of urine must be eliminated every day because this amount of fluid is needed to remove potential toxic materials from the body to maintain homeostasis.   13.   A normal adult eliminates from 1.5 L (1.6 qt) to 2.3 L (2.4 qt) of Urine a DAY, Depending on the amount of water taken in and the amount of water lost through Respiration and Perspiration.   14.  Purified Blood is returned to the Circulatory System through the Renal Vein.   CONTROL OF KIDNEY FUNCTION   1. The main proposes of our Kidneys is to maintain the CHEMICAL Composition of our Blood.   2. The Kidneys are the Master Chemist of the Blood Supply.   3. Two Important Things Controlled by the Kidneys are; CONCENTRATION OF WATER IN BLOOD; AND THE LEVEL OF SALT IN OUR BLOOD.   4. Drink too much liquid, and the Kidneys will decrease the rate of reabsorption, excess water is sent to the Urinary Bladder to be excreted.   5. Eat Salty Foods and the Kidneys will respond by returning less salt to the Blood by Reabsorption.  The excess is excreted in our Urine.   6. The Kidneys ensure that the composition of our Blood remains Constant.