Excretory System - Transporting Waste

Science, Grade 6


Table Of Contents: Excretory System - Transporting Waste

1. Functions of the Excretory System
The organs of the excretory system help to keep blood volume, body fluids and salts in balance while also helping to eliminate wastes.
2. Organs of the Excretory System
The kidneys and the bladder are the main organs of the excretory system. Our kidneys lie in the back of our abdomen and have a rich supply of blood vessels and filtration units. Once urine is formed in the kidneys, it travels down tubes known as ureters to the urinary bladder. Here, urine may be stored until it is released under voluntary control from the body through the tubular urethra.
3. Kidney Structure
The kidney is made up of an outer region known as the cortex and an inner region known as the medulla. The kidney is designed to filter the blood and remove waste materials. Therefore it has a very rich supply of blood. Within each kidney, there are over one million mini-filtration and tubular structures called nephrons. The nephrons filter blood, conserve water and form the urine.
4. Formation of Urine
Blood enters each of the nephron structures within a specialized capillary bed known as the glomerulus. Here, fluid and other very small particles are forced across the wall of the capillary into the elaborate system of nephron tubules. Nearly 85% of the fluid in the nephrons is reabsorbed back into the bloodstream. What is left in the kidney is waste liquids that become concentrated into urine. Although 180 liters of fluid per day is filtered across both kidneys, 99% of it is reabsorbed. So we only excrete about 1 to 2 liters of urine each day.
5. Urinalysis and Kidney Disease
The kidneys are an integral part of several organ systems. Medical testing of urine, called urinalysis, can detect traces of abnormal or illegal substances. The presence of substances in the urine, indicates that they were within the bloodstream, which is important medical information.
6. Excretion by Skin, Lungs and Digestive Tract
In addition to the kidneys, fluid may be excreted by the skin, lungs and digestive tract. Water is released as humidified air when we exhale from our lungs. Fluid may also be excreted with the feces from the digestive tract, especially in conditions of infection when diarrhea can lead to excessive loss of body fluids.