Lesson Explainer: Removing Waste Biology • Second Year of Secondary School

In this explainer, we will learn how to describe the processes by which waste products are removed from the human body.

The cells of the human body are constantly carrying out a huge number of chemical reactions. These reactions serve a huge variety of purposes from fighting infection to extracting energy from food.

We refer to the chemical reactions that occur in the body as metabolism. Many of these metabolic reactions create by-products that are unnecessary or even harmful to the body. We call these by-products “metabolic waste.” The process of removing metabolic waste from the body is called excretion.

Excretion is an essential part of maintaining homeostasis. Homeostasis is what scientists call the maintenance of a constant, normal internal environment or a state of balance within the body. It is what allows all of our systems to function normally, and disruptions to homeostasis can cause illness and sometimes death.

The group of organs that work together to remove waste from the body are called the excretory system. The primary organs of the excretory system are the lungs, the skin, the kidneys, and the liver as you can see in Figure 1. You may notice that these organs also belong to other organ systems. The lungs are a part of the respiratory system, and the large intestine is in the digestive system. Because an organ system is a group of organs that work together to perform a function, organs can be classified in more than one system since they may have more than one function!

The waste products generated are either nonmetabolic or metabolic. Nonmetabolic waste is solid and insoluble and can be removed from the body as feces. Metabolic waste can be either liquid, or soluble waste, or gaseous waste. Solid or insoluble waste is removed from the body as feces. Liquid or soluble waste is removed from the body as urine and sweat. Gaseous waste is removed from the body when we exhale.

It is important to note that excretion refers only to the metabolic waste materials that leave the individual cells through their cell membranes. Some of the products of excretion are a part of the feces the large intestines produce, but the act of passing feces out of the body is not itself considered to be excretion. This is instead referred to as egestion.

The large intestine, also called the colon, is the organ responsible for the production of feces. The production of feces is the primary method of removing solid and insoluble waste from the body. Feces consist of water, undigested foods, bacteria, dead cells from the digestive system, and other waste chemicals that are not able to be dissolved in water.

In order to produce feces, the large intestine (as shown in Figure 2) receives the remnants of undigested food from the small intestine and reabsorbs all the products that may still be useful to the body. This includes most of the water used in digestion and many of the ions, or charged molecules, also sometimes called electrolytes. What is left at the end of this process is called feces, and they are then transferred to the rectum where they are stored.

We may think of feces as mostly undigested food. In fact, by some estimates, more than half of the mass of human feces is made up of bacteria that populate the digestive system.

Maintaining the perfect balance of ions in the body is an important aspect of homeostasis. An example of a typical ionic compound is sodium chloride, also known as common table salt. When dissolved in water, the sodium and chloride ions separate. The sodium has a positive charge and the chloride has a negative charge. This is what makes sodium chloride an ionic compound. You can see this in Figure 3.

Ions like sodium and chloride are commonly used by cells for many different functions. For this reason, they need to be maintained within a specific concentration at all times to allow the cells to function properly. Too much or too little of ions like sodium and potassium in the body can interfere with the functions of our nerves and muscles!

The kidneys are excretory organs responsible for the production of urine as shown in Figure 4. Urine is the primary method for removing liquid and soluble wastes, such as urea, from the body. Urine contains water, any excess ions that the human body does not need, urea, and other waste chemicals that are filtered from the blood. Urine made by the kidneys is stored in the urinary bladder.

Urea is a by-product of metabolism related to proteins. Proteins are made of smaller molecular subunits called amino acids. When the amino acids are metabolized, they produce a chemical called ammonia that is highly toxic to our cells. In some animals, like fish, ammonia can be safely produced as a by-product because it is released into the surrounding water where it becomes diluted. However, due to its toxicity in humans, the ammonia that is produced is transported to the liver where it is converted into urea, a chemical that is much safer for the body.

Another by-product of protein metabolism in some animals is uric acid, which is solid and water insoluble. This can help preserve water in lizards that live in the desert. Water preservation can also be important for humans in hot weather, and the kidney can respond by conserving more water and reducing the amount of water in urine.

Waste products like urea are then filtered from the blood by the kidneys. The kidneys work by filtering almost everything out of the bloodstream, then putting back just the right mixture of nutrients, water, and ions to maintain homeostasis. The final mixture of materials removed from the blood by the kidneys is called urine.

Even though urea is less toxic than ammonia, it becomes toxic if it is allowed to accumulate in the body, which is why urea must be constantly removed from the blood by the kidneys.

Students often conclude that the urea in urine must be what gives it its characteristic yellow color. However, urea is a colorless chemical. The pigment in urine actually comes from the breakdown of old red blood cells by the spleen!

The skin removes waste from the body in the form of sweat. Sweat is secreted from special glands found in the middle layer of the skin (called the dermis). The primary function of sweat is to regulate body temperature. However, along with urine, sweat is one of the ways our bodies excrete excess water, ions, and small amounts of urea.

The skin is composed of three layers, the epidermis, the dermis, and the subcutaneous layer as shown in Figure 5. In this explainer, we will just describe the epidermis and the dermis.

The epidermis is the waterproof outer covering and the cells beneath it that replace the outer layer as it wears off. This layer contains melanocytes that produce melanin that gives the skin its pigment.

The layer underneath is called the dermis, and it houses most of the organs of the skin such as the hair follicles, sweat glands, and sebaceous gland (involved in producing oils to lubricate the hair and skin). The sweat gland contains a coiled secretory tubule where sweat is produced and secreted. The hair follicle, supplied with an erector muscle, is surrounded by many blood vessels that can dilate in order to supply more blood to the area. This dilation can help divert heat to the skin and together with any produced sweat can help cool the body.

One of the main ionic components of sweat is sodium chloride, which we mentioned is the same compound as salt we commonly use to flavor food. This is why your sweat has a salty taste! Surprisingly, we are almost always sweating, even when we do not notice it. The sweat just usually evaporates as quickly as it is produced. When sweat evaporates, it leaves behind this salt and other wastes that can be processed by bacteria on the skin to give an unpleasant odor unless washed away. If unwashed, these accumulations can lead to the sweat glands becoming blocked, which can form painful bumps on the skin.

We may not think of water as a waste product since we need it to survive, but excess water in the body can disrupt homeostasis, increase blood pressure, and damage cells. Do not worry about it too much, though. The skin and the kidneys work together very efficiently to maintain the perfect balance of water in our bodies at all times. One of the best ways to keep your excretory system functioning smoothly is to make sure you drink enough fluids!

The lungs are the site of the removal of gaseous waste from the body. Gaseous waste in humans almost completely consists of carbon dioxide. This carbon dioxide is a by-product of an important metabolic process known as cellular respiration.

Cellular respiration is occurring constantly in almost all of the human body’s cells. Cellular respiration breaks down oxygen and glucose, producing water and carbon dioxide. This process releases energy that is stored in a molecule called ATP. ATP is what our cells use to power almost all of our life functions.

When our bodies are more active, our cells need more ATP, so cellular respiration increases. This means that we need more oxygen and make and release more carbon dioxide. This explains why our breathing rates increase when we are physically active!

The excess carbon dioxide made during cellular respiration diffuses out of our cells and into the blood, which carries it to the lungs where it is excreted when we exhale. Some water is also removed from our bodies when we exhale as it evaporates from the moist surfaces within our lungs and nasal passages, which is why you may notice condensation when you breathe onto a cool piece of glass.

An increase in carbon dioxide concentration in the blood can decrease the pH of the blood, making it more acidic. You will recall that many of the metabolic processes are facilitated by enzymes, which operate in a narrow range of pH and temperature. A decrease in blood pH can cause the enzymes to denature, disrupting many of the chemical reactions that keep us alive.

A buildup of carbon dioxide in the bloodstream can have rapid negative effects. This, more than a lack of oxygen, is why it is so hard to hold your breath for a long time!

You can see the process of cellular respiration in Figure 6.

Excretion is an essential life function and an important aspect of the maintenance of homeostasis. The buildup of wastes in the human body can quickly lead to illness or death. The organs of the excretory system work together to remove solid and insoluble waste, liquid and soluble waste, and gaseous waste from the body.

Let’s summarize what we have learned about excretion from this explainer.