Human Body Systems Tour

Welcome to the Excretory System. I am Alan Appendix, and I will be your tour guide for the final section of your tour. The Excretory System is responsible for removing waste substances from the Body. This process is called excretion. There are four major tour destinations, also called organs, which we will visit in this section of the tour. They are skin, kidneys, lungs, and liver, and they all help in the excretory process. We will begin this tour in the kidneys.

The Kidneys
These are bean shaped organs which are a part of the excretion of urine. The inside of the kidneys is divided into two layers. The inner, darker section is known as the medulla. This is where the wastes are collected in the kidneys. The outer, lighter coloured section is the cortex, a thick skin on the outside of the kidneys. This is where the nephrons are located. It also protects the rest of the body from the harmful wastes filtered out by the kidneys. Our tour group also needs to be careful of these wastes because urea (a by-product of the digestion of protein), the main waste filtered out by the body system, is acidic and can be harmful.

The function of the kidneys is to filter wastes from the bloodstream and excrete them from the body with water as urine. The blood enters the kidneys through the renal artery. This is a large artery with blood that contains bodily wastes. These wastes are mainly urea, a compound made of carbon, nitrogen, and oxygen which is a by-product of the digestion of protein, and uric acid, a by-product of the metabolism of nucleic acid. The artery fans out into small capillaries which spread out inside the kidneys and distributes the blood to the nephrons. The nephrons are located in the cortex and are small cells made of a capillary with its winding tubes.  Each has a Loop of Henle coming out of it towards the medulla in the center of the kidney. The nephrons are the parts of the kidney which filter the wastes from the small capillaries inside the kidneys. The blood flows through the nephrons and leaves the kidneys through the renal vein. By this time, the blood is free of wastes and can continue its function through the body. The wastes are collected from the medulla into the ureter, a tube that comes out of the kidney and carries the urine to the bladder. From there, it can be released from the body through the urethra. We will exit the kidneys through the renal vein into the bloodstream.

The Skin
After the kidneys, we will follow the bloodstream to visit the skin, the layer of tissue which covers the entire body and all of its organs. It is an organ that protects the inside of the body, but it also removes of excess water, salt, urea and uric acid (a by-product of the metabolism of nucleic acid). These materials are taken from the capillaries in the skin into the cells. The wastes are soon excreted through pores in the skin as sweat. This sweat also lowers the body's overall temperature.

The Lungs
We will now follow the bloodstream further around the body to the lungs. You have seen the lungs before as a part of the respiratory system, but they also play a part in the excretory system. The lungs are a pair of organs which are a part of the respiratory system, but they also function in the excretory system. When the diaphragm contracts, it pulls air into the lungs. The oxygen in the air is absorbed and transported to the cells through the bloodstream. When the cells in the body receive and use the oxygen, as a part of the process of cellular respiration, carbon dioxide is created as a waste. This is transported by the bloodstream back to the lungs so that the carbon dioxide is released into the air in the lungs. When the diaphragm relaxes and forces the air out of the body system, the carbon dioxide is also transported out of the body system. This system is repeated many times so that oxygen continues to be provided for the body and, as it is used and wastes are created, the carbon dioxide waste can continue to be released out of the body system.

The Liver
We will now follow the bloodstream again into the liver. The liver is one of the most complex organs in the human body so it has many different functions. It mainly produces enzymes for digestion, but it functions in the excretory system by filtering poisons, wastes and harmful substances out of the bloodstream. Some wastes that are filtered out of the bloodstream by the liver are food additives, insecticides, drugs, alcohols and other harmful chemicals. The liver creates many different enzymes, some which help to convert harmful chemicals into materials which are not so harmful to the body. Some materials can be converted into urea, a common waste within the body which is harmful but not poisonous if removed quickly. When the chemicals are released into the liver, these enzymes are produced and mixed with the chemicals to convert them into less dangerous materials. These new chemicals are then released into the bloodstream so they can be filtered out of the body by the kidneys. Because the liver filters our all these harmful wastes from the body, it is important that you eat very few foods with chemical additives and try to choose foods with less food colorings, because this can help so keep your liver safe.

But before this tour ends, you should notice the two separate processes involved with blood purification. They are filtration and reabsorption. To witness these two processes, we have to return to the kidneys. Let's go!

Filtration
Now we will travel to the upper part of a nephron. You will see a network of fifty capillaries called a Glomerulus, which is the site of filtration. Look! A beautiful landmark is up ahead! It is called Bowman's Capsule, which surrounds the Glomerulus. Out of the plasma that gets transferred through the kidney, about 20% gets filtered into the nephron. Materials from the blood are pushed into the Bowman's Capsule from the Glomerulus. Filtrate refers to the materials that get filtered from the blood by osmosis. These materials include water, urea, glucose, salts, amino acids, and vitamins. Notice how these only contain small molecules? Well, other materials such as plasma proteins, cells, and platelets stay in the blood because they are too large too go through the membrane. The diagram on your right will explain this further. This whole process is called filtration.

Reabsorption
We will now look at a process called reabsorption. Reabsorption occurs when most of the materials eliminated from the blood at Bowman's Capsule return back to the blood. About 180 litres of filtrate travel from the blood into the collecting kidney tubules everyday. However, reabsorption allows 99% of the water that is collected by the Bowman's Capsule and about 53% of the urea, to be reabsorbed into the blood. Reabsorption continues as filtrate is passed along the nephron. At the Proximal Tubule (a section of the nephron), most of the reabsorption occurs, and about three quarters of the water in the filtrate goes back to the capillaries through osmosis. Through Active Transport, glucose and minerals also return to the blood. Some more reabsorption can occur in the last section of the nephron, the convoluted tubule. Here, the material that is left is called urine, which is made up of urea, water, and excess salts. As we touched on earlier, the Loop of Henle helps concentrate urine, conserve water, and minimize the volume of urine.

The urine is now flowing through the renal pelvis all the way to a narrow tube called the ureter. The ureter goes into the urinary bladder, where urine is stored until it is removed through the urethra. The purified blood will now return to the Circulatory System by going through the renal vein. All the systems in the Human Body connect in so many places, our tour could keep on going around and around!

But unfortunately, our Human Body Systems Tour is now coming to an end. The urethra is about to empty, and our tour group will go along too! From all of us here at the Soaring Sphincter Travel Agency (Janine White Blood-Cell, Monica Larynx, Tyler Dendrite, Jocelyn Alveoli and Alan Appendix), we hope that you have learned a lot about Systems in the Human Body. Feel free to come back and visit anytime you like!