Lesson 1: Excretory Organs

What goes in, must come out… This applies to living things. We started our study of the human body with the digestive system. The digestive system deals with the processes that involves taking in nutrients into the body and simplifying them so that they can enter the cells. Whatever is left over, would be waste and need to be eliminated, exhaled/expired, or excreted.

This unit deals with the excretion of the wastes from the circulatory system. The liquid wastes are excreted through the excretory organs.

Every metabolic process in the human body uses energy and generates wastes. If these waste products are not removed, they would quickly accumulate in harmful proportions. In fact, some wastes are poisonous and pose a serious threat to health if they are not removed promptly. The process of getting rid of metabolic wastes is called excretion.

There are many organs and systems responsible for excretion. For example, the skin removes salts and nitrogen-based wastes, the lungs remove carbon dioxide and the liver converts poisons and toxins to a form that can be eliminated by the kidneys.

In the last unit, we studied the body’s blood and immune system and the vital importance of these systems to your health. In this unit, we will study the structure and function of the excretory system.

Introduction

The human body is about 60% water. Two thirds of the water is in the cells while the rest is in the fluid around the cells. The chemical composition of this intercellular fluid must be carefully balanced. If it becomes too salty, the cells can become dehydrated. If it gets too watery the cells can bloat. If too much acid or base accumulates, the cells' enzymes can stop working. If metabolic wastes collect, the cells can die of poisoning. Homeostatic mechanisms within the body keep the chemical composition of the intercellular fluid constant when conditions change.

Excretory systems regulate the chemical composition of these body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. Components of the excretory system in humans include the kidneys, liver, lungs, and skin.

How does excretion differ from the elimination of wastes from the digestive system?

Outcomes

In this lesson, you will study the excretory organs and their importance in maintaining homeostasis in the body. By the end of this lesson, you should be able to:

Define the term excretion. Describe how excretion aids in maintaining homeostasis in the

body. List the primary metabolic wastes produced in human body (i.e.,

carbon dioxide, water, ammonia, mineral salts) and the source of each.

Compare the role of the major excretory organs (kidneys, lungs, skin, liver) in the excretory process.

Excretion and Homeostasis

Cells produce water and carbon dioxide as by-products of metabolic breakdown of sugars, fats, and proteins. Chemical groups such as nitrogen, sulfur, and phosphorous must be stripped, from the large molecules to which they were formerly attached, as part of preparing them for energy conversion. The continuous production of metabolic wastes establishes a steep concentration gradient across the plasma membrane, causing wastes to diffuse out of cells and into the extracellular fluid.

Multicellular organisms such as humans must have a specialized organ system to concentrate and remove wastes from the interstitial fluid and eventually from the body. The process of getting rid of metabolic wastes is called excretion.

Excretion aids homeostasis by removing metabolic wastes and regulating the salt and water balance. The blood picks up carbon dioxide, excess salts, nitrogen compounds, and any excess water that may be present in the interstitial fluid and carries them to the excretory organs, which channel the wastes out of the body. Therefore, the excretory system regulates volume of internal body fluids as well as eliminates metabolic wastes from the internal environment.

Fortunately, atoms and molecules do not wear out, but may be changed or rearranged and used over and over again. Many of the end products of various cell activities can be recycled and used in other processes. As a result, the amount of waste that actually needs to be discharged from the body is very small in relation to the amount of work done by all the cells of the human body.

Several organs take part in human excretion. The skin, lungs, liver, and the transport system play very important roles. However, the main excretory organs are the kidneys.

The metabolic wastes produced and the organs that excrete them are summarized in the table below.

Wastes Formed From Organ of Excretion

1. Carbon Dioxide Cellular Respiration lungs

2. Water Cellular Respiration & Dehydration Synthesis

kidneys, skin, lungs

3. Nitrogenous Wastes

Metabolism of Proteins (Amino Acids)

kidneys, liver, skin

a) Ammonia (extremely toxic)

deamination of amino acids by the liver

liver

b) Urea ammonia combined with carbon dioxide

kidneys, skin

c) Uric Acid product of the breakdown of nucleic acids, such as DNA

liver

4. Mineral Salts (NaCl, KSO4)

Metabolism of Various Food Substances

kidneys, skin

Table 7.1.1 Metabolic Wastes

Lesson 1 Overview

Following is a list of topics covered in this lesson.

Excretion and Homeostasis Skin and Homeostasis Excretion in the Lungs Role of Liver in Excretion Kidneys: Major Organs of Excretion

Skin and Homeostasis

The skin has many functions in the body. These include protection, regulation of body temperature, sensory reception, water balance, excretion, synthesis of vitamins and hormones, and absorption of materials. The skin's primary functions are to serve as a barrier to the entry of microbes and viruses, and to prevent water and extracellular fluid loss.

Your skin has approximately 2 million sweat glands. These tiny, coiled tubules are found in the dermis layer of skin, below the surface layer or epidermis. See the diagram below.

http://www.nigms.nih.gov/news/features/artificial_skin.html

These glands secrete sweat continuously, even when you are not exercising. Sweat consists primarily of water, along with salts, and some urea (nitrogen-based waste). These materials pass from the blood vessels in the skin into the sweat glands. The wastes travel up the tubules and onto the surface of the skin through tiny openings called pores. The wastes form perspiration on the skin, which eventually evaporates. The solid wastes in the perspiration remain on the surface of the skin or on clothing. Perspiration itself has no odour. However, bacteria living on the skin decompose the urea in perspiration, producing new compounds that do have odours.

Did you know that the skin and kidneys excrete the similar waste products? The composition of sweat and urine are very similar (except urine is much more concentrated)!

Heat and cold receptors are also located in the skin. When the body temperature rises, the hypothalamus, the region of the brain responsible for coordinating many nerve and hormone functions, sends a nerve signal to the sweat-producing skin glands, causing them to release about 1-2 liters of water per hour, cooling the body through evaporation. The hypothalamus also causes dilation of the blood vessels of the skin, allowing more blood to flow into those areas, causing heat to be dissipated from the skin surface.

When body temperature falls, the sweat glands constrict and sweat production decreases. If the body temperature continues to fall, the hypothalamus will trigger the body to engage in thermiogenesis, or heat generation, by raising the body's metabolic rate and by shivering.

This is an excellent example of negative feedback mechanisms in action.

Excretion in the Lungs

Carbon dioxide and water are produced by all cells during cellular respiration. The blood carries these to the lungs. Carbon dioxide diffuses into the alveoli and is removed from the body when we exhale. Some water also leaves the lungs as water vapour.

The lungs can also eliminate alcohol, which, since it is made up of small molecules, can be rapidly passed from the bloodstream into the alveoli of the lungs and exhaled. This is why the breathalyzer test is an effective and simple way of determining the amount of alcohol that people have taken into their systems.

Role of Liver in Excretion

Although, we obtain most of our energy from carbohydrates and fats, we sometimes use proteins for energy. Some proteins and other nitrogenous compounds are broken down in the liver by a process called deamination. The extraction of energy from proteins produces ammonia (NH3), a highly toxic substance. The liver converts ammonia to a non-toxic substance called urea by combining it with carbon dioxide. The blood then delivers urea to the kidneys, where it is excreted.

The liver also breaks down red blood cells. About 120 days after they are formed, erythrocytes are destroyed by special cells in the liver. These cells engulf the red blood cells and break down their hemoglobin. Iron released from the hemoglobin is carried by the blood to various parts of the body. In the bone marrow the iron is used in the manufacture of new hemoglobin. Another part of the broken down hemoglobin is the yellow pigment bilirubin. This pigment enters the bile, passes into the small intestine, and is eventually removed from the body with the feces.

The liver is also responsible for removing potentially hazardous chemicals from the blood. It "detoxifies" the blood. For this reason, alcoholics and other types of drug addicts have a higher incidence of liver disease. The liver breaks down harmful and foreign substances, such as many ingested poisons and drugs after they are absorbed by the digestive system and before they reach the rest of the body. Since the liver has the enzymes to break down the natural materials, it is also able to destroy certain drugs. This function can be life-saving, because it makes destructive substances harmless, and prevents hormones, medications, and other chemicals from accumulating to harmful levels. Cortisone and sulfur-containing medicines are examples of drugs that the liver breaks down.

The liver's ability to make substances harmless has limits. Materials can accumulate in the liver and destroy its tissues. For example, alcohol taken in excess over a period of time can deteriorate the liver. Pollutants, such as PCBs (polychlorinated biphenyls) and pesticides, such as DDT, can also harm the liver.

Kidneys: Major Organs of Excretion

The kidneys are the major organs of excretion in the human body. They form part of the urinary system and function primarily in filtering wastes from the blood. The kidneys also play a key role in fluid balance in the body.

In the next lesson, you will study the urinary system and the function of the kidneys in detail.

Lesson 1 Questions

Please answer the following six questions.

1. Define the term excretion.

2. Describe two ways that excretion contributes to homeostasis?

3. Complete the following table.

Wastes Formed From Organ of Excretion

1. Cellular Respiration lungs

2. Water Cellular Respiration & Dehydration Synthesis

3. Nitrogenous Wastes

Metabolism of Proteins (Amino Acids)

kidneys, liver, skin

a) Ammonia (extremely toxic)

liver

b) ammonia combined with carbon dioxide

kidneys, skin

c) Uric Acid product of the breakdown of nucleic acids, such as DNA

4. Mineral Salts (NaCl, KSO4)

Metabolism of Various Food Substances

4. Describe the role of the skin in temperature regulation of the human body.

5. How does the formation of urea prevent poisoning?

6. Why is it said that the liver "detoxifies" the body?

Activity - Mammalian Diving Reflex

What happens to the heart rate when a person falls into cold water? Test the mammalian diving reflex with this activity and see how your body responds. Materials: pan filled with about 8 cm of cold water, stopwatch or clock with second hand, towel, partner

1. Have your partner measure your resting pulse for 15 seconds and calculate your beats per minute.

2. Hold your breath and submerge your face in the pan of cold water for 15 seconds. Before you start, predict what will happen to your pulse rate. Have your partner measure your pulse rate for the 15 seconds that your face is submerged. Calculate your beats per minute.

3. Switch roles and repeat the procedure.

Answer the following questions:

1. What happens to your pulse rate when your face is submerged in water?

2. Why do you think this happens?

3. How might this help someone who has fallen into very cold water?

Lesson 1 Summary

In this lesson, you have studied that excretory systems regulate the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients. You have also learned about the role of the liver, lungs, and skin in excretion. In the next lesson, you learn about the role of the urinary system in maintaining homeostasis.