Bring Science Alive! Cells and Genetics L3

LESSON 3

44

Interacting Body Systems How do your body systems work together?

Introduction Suppose you are on a team that is building a robot. You can think of the robot as a system and each part of the robot as a subsystem. The pieces that make up the structure of the robot are one subsystem. The wires that send signals to make the robot move are another subsystem.

The remote control that you use to control the robot’s movements is another subsystem. No single subsystem can make the robot perform all its functions, but it can function if all the subsystems work together.

How is the robot, and all its interacting parts, like a human being?

Like the robot, the human body is a system made up of interacting subsystems that allow it to function. Each part of the human body is part of subsystem called a body system. Each of these body systems performs a certain function for the body. Look at the runner’s heart, blood vessels, and bones. His heart pumps blood through blood vessels and delivers oxygen and nutrients to all parts of his body. His lungs take in oxygen that helps to supply energy to his muscles. Bones that connect to muscles enable him to move. To what body systems do these parts belong? How do you think they interact? What do you think would happen if one of these parts was not working properly?

In this lesson, you will learn how parts of the body work together to allow a person to function and move using the example of a person running. You will begin to learn about the subsystems that make up an organism’s body. These body systems exist in humans and many other animals and plants. Finally, you will learn how scientists can use other organisms’ body systems to model human body systems.

Vocabulary skeletal system a body system made up of the framework of bones that supports an organism’s body, protects its internal structures, and allows the body to move muscular system a body system made up of all the muscles of the body that cause movement when they contract, or shorten organ a typically self-contained structure that carries out a particular function for the body digestive system the body system made up of organs that break down food into substances that can be absorbed and used for energy and gets rid of the remaining solid waste that cannot be absorbed respiratory system the body system made up of the organs that bring oxygen into the body and remove carbon dioxide waste circulatory system the body system made up of the heart and blood vessels that circulate blood through the body excretory system the body system made up of the organs that get rid of waste reproductive system the body system made up of the organs that allow adults to produce offspring

Next Generation Science Standards Performance Expectations MS-LS1-3. Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells.

Science and Engineering Practices Engaging in Argument from Evidence Use an oral and written argument supported by evidence

to support or refute an explanation or a model for a phenomenon.

Crosscutting Concepts Systems and System Models Systems may interact with other systems; they may have subsystems and be a part of larger complex systems.

Science is a Human Endeavor

Disciplinary Core Ideas LS1.A. In multicellular organisms, the body is a system of multiple interacting subsystems. These subsystems are groups of cells that work together to form tissues and organs that are specialized for particular body functions.

Interacting Body Systems

45

1. The Skeletal and Muscular Systems

Figure 3.1A The skeletal system is made up of all the bones in the body. The muscular system is made up of all the muscles in the body. The two systems work together to move the body.

For most of human history, people have had to hunt for meat and search for edible plants to meet their needs for food. Hunting often required running after prey or hiding in trees and carefully throwing weapons at prey as they try to run away. How does the human body allow a person to run, stand, hide, and throw?

The skeletal system is an important body system for allowing a person to move. The skeletal system is the framework of bones that supports an organism’s body, protects its internal structures, and allows the body to move. This framework gives the body its overall structure and general shape. The structures that connect bones with other bones are called ligaments. Some bones, such as the skull and ribs, protect soft internal organs. The skull protects the brain, and the ribs form a cage that protects the heart and lungs. Other bones interact with muscles to cause movement.

The muscular system is also important for allowing a person to move. All the muscles of the body make up the muscular system.

Muscles cause movement when they contract, or shorten. Muscles connect to bones with structures called tendons. Not all muscles are for moving the skeleton, though. The heart is part of the muscular system because it, too, is made of muscle. The heart muscle contracting is what causes the heart to beat. Other muscles in the muscular system help move food or blood through the body.

The Skeletal and Muscular Systems Work Together

Skeletal System

46

Lesson 3

Muscular System

Bones Move by Muscle Contraction Contracting Biceps

Contracting Triceps

Figure 3.1B Muscles and bones work together to allow for movement, like when picking up an object. When the biceps muscle contracts, the triceps muscle relaxes and the arm bends at the elbow. When the triceps muscle contracts and shortens, the biceps relaxes and the arm straightens at the elbow.

The skeletal and muscular systems are examples of body systems.

A body system is a group of organs that works together to perform a particular function in the body. An organ is a typically self-contained structure that carries out a particular function for the body. A bone is an organ of the skeletal system. Tendons and ligaments are other organs in the skeletal system. The brain, heart, and stomach are examples of organs in other body systems. No single organ can do the job of an entire body system.

The skeletal and muscular systems work together to help a person move. As an example, Figure 3.1B shows how the biceps and triceps muscles in the upper arm move bones in the arm to bend at the elbow, like when you want to pick up an object. The biceps and triceps are attached to bones in the arm and shoulder by tendons. Starting with a straight arm, when the biceps muscle contracts, it pulls on the bones it is connected to. At the same time, the triceps relax so that the bone is not pulled in two directions at once. Since the lower part of the arm and the upper part of the arm are connected by a hinge, like a door, the lower part of the arm swings up. To straighten the arm at the elbow, the triceps will contract and the biceps relax. Many muscles attached to bones work in pairs like this to move the bones one direction or another.

Interacting Body Systems

47

2. The Digestive System

Food, like this cheeseburger, provides the body with energy for growth, repair, and life processes. But first, the hamburger must be broken down by the organs in the digestive system into nutrients that the body can use. In the mouth, the large carbohydrates in the bun will be broken down into sugars by chemical digestion.

Most people no longer hunt for their food, but humans still need to eat multiple times a day. All animals need food to survive. Food gives a person energy to run, throw, move, and even sleep. An organism’s body runs on the energy stored in food. How does the body get energy from food?

The body cannot directly use the energy found in food. To be used by the body, food must first be broken down, or digested, into simpler substances. The digestive system is the body system that breaks down food into substances that can be absorbed and used for energy and gets rid of the remaining solid waste that cannot be absorbed. Food is mostly made up of substances called carbohydrates, proteins, and fats.

These are broken up into simpler substances that can be absorbed and used throughout the body for energy.

The digestive system forms a long tube from the mouth to the rectum. Hollow, muscular organs make up different parts of the tube.

The main organs of the digestive system are the mouth, esophagus, stomach, small intestine, large intestine, and rectum. Each one has an important role in digesting food.

The Mouth and Esophagus  Digestion occurs in the mouth in two different ways. Large food particles are split into smaller pieces by chewing, grinding, and mashing with the teeth.

Food is also broken down chemically by digestive juices in the saliva.

Chemical digestion changes the food into different substances through chemical reactions. Saliva specifically breaks down large carbohydrates into simpler sugars.

Say, for example, you are eating a hamburger. Your teeth break up the pieces of the bun, meat, lettuce, and tomato. Your saliva breaks down the carbohydrates in the bun into sugars.

Your tongue helps by moving the food around in the mouth and then pushing it into the throat. Swallowing moves the food into the next organ, the esophagus.

The esophagus is the part of the digestive tube that connects the mouth to the stomach. Muscles in the esophagus push the food into the stomach.

48

Lesson 3

The Stomach  The stomach is a bag-like organ made of strong muscles. As the muscles in the stomach contract and relax, food is mixed and broken down.

When the food is in smaller pieces, it is easier to digest. Here, all the pieces of your hamburger are being broken apart.

Additionally, several digestive juices made in the stomach chemically digest proteins in the hamburger. From the stomach, the partially digested food moves into the small intestine.

Parts of the Digestive System

Mouth Esophagus

The Intestines and Solid Organs  The small intestine is the longest organ of Liver the digestive system, measuring more than six meters long in humans. Most of the digestion of proteins and carbohydrates takes place here. The small intestine Small Intestine absorbs most of the digested nutrients Large Intestine that have been broken down and transfers those nutrients to the blood stream. Blood transports the nutrients to all parts of the body, where the energy stored in the nutrients is released.

There are two organs that are not part of the hollow tube that makes up most of the digestive system, but they help in digestion by making digestive juices flow into the small intestine.

The liver produces digestive juices that break down fats, like the fat in the hamburger meat. The pancreas produces digestive juices that continue to break down fats, carbohydrates, and proteins into even smaller substances.

Material that has not been absorbed in the small intestine passes into the large intestine. Here, water is removed. Most of what remains is waste. Waste is stored in the lower part of the large intestine, called the rectum, before it is eliminated from the body.

The trip your hamburger has made through your digestive system has lasted many hours and involved the interaction of many organs and systems. The digestive system breaks down food and the other organ systems transport this digested food to be used for energy all over the body. The muscles in the digestive system move food all the way through the tube, from the mouth to the rectum. Without the aid of muscles, blood, and other parts of the body, the digestive system would not be able to provide energy to the body.

Stomach

Rectum

Figure 3.2 The digestive system forms a long hollow tube from the mouth to the rectum. Different organs with different functions make up the parts of the tube, but they all work together to create a functional digestive system.

Interacting Body Systems

49

3. The Respiratory and Circulatory Systems You probably don’t pay much attention to the breaths you take every day or whether your heart is beating. If you are a runner, however, you may have noticed that your heart beats faster and you begin to breathe more rapidly when you start to run. Why does this happen?

The Respiratory System When you are running, you start breathing rapidly to take in more oxygen, and push out more carbon dioxide. Your body requires oxygen for the process of releasing energy from broken down food. Running uses a lot of energy, so more oxygen Figure 3.3 is needed. During these chemical reactions, carbon dioxide waste is The main organs of the respiratory formed. Too much carbon dioxide is toxic, so it must be removed from system are the lungs. The circulatory the body before it can cause harm. This causes you to breathe out more system is made up of the heart and to release that carbon dioxide. the blood vessels. Together these two The respiratory system is the body system that brings oxygen into systems transport blood, oxygen, and the body and removes carbon dioxide waste. The main organs of the nutrients throughout the body. Oxygen respiratory system are a pair of lungs. When you inhale, air containing helps to release energy from broken oxygen passes from the environment through your nose and mouth down food, which is essential for and into your lungs through tubes. At the end of the tubes, deep within running or even just surviving. the lungs, are many tiny air sacs that have very thin walls. Oxygen entering the lungs The Respiratory and Circulatory Systems passes from the tubes and through the walls of the air sacs. Here is where the respiratory Heart system interacts with the circulatory system.

Trachea

Lungs Blood Vessels

50

Lesson 3

The Circulatory System As you run, your heart begins to beat faster. The oxygen that was in your lungs moves into your blood, where it is carried all over the body.

The circulatory system, consisting of the heart and blood vessels, circulates blood throughout the body. The pumping action of the heart muscle is what moves the blood all around the body. Oxygenated blood near the lungs gets to the heart through a vein, a type of blood vessel. Veins carry blood to the heart from the rest of the body. The heart then pumps that blood all over the body through arteries, another type of blood vessel. Arteries carry blood away from the heart. As you run, your heart beats faster to pick up more oxygen from the lungs and transport it to where it is needed. In addition to oxygen, blood can transport nutrients and waste from all over the body to where it can be used or removed.

4. The Excretory System

Parts of the Urinary System If you have ever breathed out on a cold Kidney Kidney day, you have observed evidence that your breath contains water vapor. When you exhale you are breathing out wastes, like excess water and carbon dioxide. If you were running, what other ways do you think you might be getting rid of waste from your body?

There are different ways that the body gets rid of waste. In addition to breathing out, you might get rid of some waste by sweating, especially when you’re doing a rigorous activity like running. Most of the body’s liquid waste is removed in the form of urine.

Interestingly, you are less likely to need to urinate when running because much of the excess water in your body will be released through sweat instead of urine.

The excretory system is the body system that gets rid of wastes. Not everything can be organized into clear distinct units in science, and biologists have to accept some amount of Bladder ambiguity, or inexactness, in making a distinction between body systems. For example, some of the organs of the excretory system also belong to other body systems. As you have already learned, the Figure 3.4 digestive system also eliminates waste after the nutrients from digestion The urinary system is part of the are absorbed, so the large intestine, including the rectum, is part of both excretory system. Kidneys filter and the digestive and excretory systems. The lungs are part of the excretory remove waste and excess water from system as well as the respiratory system. They function to take in oxygen, blood. The bladder stores this excess but they also get rid of gas wastes such as water vapor and carbon dioxide. water as urine until it is excreted.

In addition to getting rid of waste, one of the important functions Other parts of the excretory system of the excretory system is to regulate the amount of water in the body. remove water vapor, carbon dioxide, The urinary system, which is part of the excretory system, helps to and solid wastes from foods. get rid of liquid waste by filtering the blood, as well as regulating the amount of water in the body. The major organs of the urinary system are the kidneys and the bladder. As blood flows through the kidneys, they filter out most of the materials that are dissolved in the blood.

Some of the materials, such as nutrients and water, are returned to the blood, but wastes are not. The kidneys keep the amount of water in the body constant by controlling how much water is returned to the blood.

The cleansed blood returns to the heart through veins. The excess water and wastes travel to the bladder, a hollow bag made of muscle that stores the wastes as urine. A tube at the bottom of the bladder carries the urine out of the body.

Interacting Body Systems

51

Interaction Between the Respiratory and Circulatory Systems To body

To body

From body From body

To lungs To lungs

From lungs

From lungs

Heart

Lung

Lung From body

To body

5. Systems Work Together Figure 3.5 Body systems, like the respiratory and circulatory system, interact to fulfill the needs of the body. In this example, oxygen-poor blood is pumped by the heart into the lungs, where oxygen is added. The blood then travels back to the heart, which pumps the oxygenrich blood to the rest of the body.

52

Lesson 3

Each body system plays an important role when you run, but no one system on its own can make you run. What makes it possible for your body to run, jump, throw, and even survive?

All the body systems have to interact to make a functional body.

You have learned that to run, the body uses muscles to move bones.

Because muscles need energy, a runner needs to eat food, which is broken up into nutrients by the digestive system. The nutrients are carried from the digestive systems to the muscles using the circulatory system. In the muscles, energy is released from the nutrients in chemical reactions that use oxygen. Oxygen is delivered to the muscles by the respiratory and circulatory systems. The lungs take in oxygen from the air, where it is transferred to the blood. The heart muscle contracts to pump that oxygen-rich blood to the whole body. While traveling through the body, the blood picks up waste, like carbon dioxide, and brings it to the organs of the excretory system, like the lungs, where it can be removed from the body.

Even for a runner to just breathe in and out requires the muscular and skeletal systems working with the respiratory system. The diaphragm is a large flat muscle that stretches below the lungs. When the diaphragm contracts, the ribs move outward, enlarging the chest cavity. This motion pulls air into the lungs. To exhale, the diaphragm relaxes and the ribs move inward. The smaller chest cavity forces the lungs to push air out of the body. These are just some examples of how the body functions through the interactions of the body systems.

Key Science Concept

Interactions Among Body Systems Many interactions between the body systems take place in a person’s body. To run, people must move their legs and arms to push their body forward. As they do this, their heart starts to speed up and they breathe more quickly. They are using up energy and losing water as they begin to sweat. Here are just a few examples of how their body systems are working together to make all these things occur.

Circulatory and Excretory The runner’s excretory system removes wastes through the runner’s skin, large intestine, and lungs. Many of these waste products are transported to the excretory system through the circulatory system.

Muscular and Skeletal The muscular system works with the skeletal system. Muscles are attached to bones. When the muscles in the runner’s leg contract, the bones in the leg move to a new position and the leg bends.

Respiratory and Circulatory The respiratory system works with the circulatory system to take in oxygen from the environment and carry it to all parts of the runner’s body through a network of blood vessels.

Circulatory and Digestive The circulatory system works with the digestive system to transport nutrients to all parts of the runner’s body. Nutrients are produced when food is digested.

Circulatory and Muscular In this runner’s legs, the circulatory system is interacting with the muscular system.

Energy-rich nutrients and oxygen dissolved in the blood travel through arteries to the muscles.

Energy and oxygen allow the muscles to work.

53

6. The Reproductive System You have learned that organisms produce offspring. What body systems are needed to produce offspring?

Humans, as well as all other organisms, use a reproductive system to produce offspring. The reproductive system is the body system that allows adults to produce offspring. Unlike most other body systems, the reproductive system is not fully functional as soon as you are born. It must first mature, a process that usually occurs in your teen years. Also unlike other body systems, the male and female reproductive systems are different. However, both make small specialized reproductive structures. The male system produces sperm. The female reproductive system produces eggs. Both kinds of structures contain information needed to create a new organism.

The Male Reproductive System  The main organs of the male reproductive system are the two testes and the penis. The testes produce sperm, and the penis releases the sperm out of the body. The sperm are produced in tubules within the testes and then mixed with fluid and stored until they are released.

Figure 3.6 The reproductive organs in males and females are different. The male testes make sperm that are released from the body through the penis. The female ovaries make eggs. An egg that is fertilized by sperm develops into a fetus in the uterus, where it is protected and nourished.

The Female Reproductive System  The main organs of the female reproductive system are two ovaries, a uterus, and a vagina.

The female system has two functions, to produce eggs and to protect a developing fetus. Eggs are produced in the ovaries. If a sperm and an egg unite, the fertilized egg can develop into a fetus. The fertilized egg travels through a tube from the ovaries to the uterus. It attaches to the inner wall of the uterus and begins to develop into a fetus. The uterus provides protection and food for the developing fetus. After about nine months of development and growth, the fetus is ready to be born.

The muscles of the uterus contract, and the fetus is pushed out of the mother’s body through the vagina during birth.

Parts of the Male and Female Reproductive Systems Uterus

Testes

Testes

Penis

54

Lesson 3

Ovary

Vagina

Ovary

Comparing Animal Body Systems Comparing Skeletal Systems

Shell

Exoskeleton

Skeleton

Comparing Circulatory Systems

Heart

Heart

7. Body Systems in Other Animal Species You may not look like a grasshopper or a snail, but there are many ways that your body is similar to theirs. How do human body systems compare to those of other types of animals?

Because other types of animals have similar needs for energy and resources as humans, their body systems can have similar structures and functions to human body systems. For example, even snails, insects, and fish have circulatory systems made up of a heart and blood vessels. This fish’s circulatory system works like a human circulatory system. Other animals, like snails and grasshoppers have an open circulatory system, meaning oxygen and nutrients dissolved in fluid do not have to stay in the tubes of the blood vessels until they reach their destination. Instead, the body is soaked in this fluid. Even though this system is not exactly like the human system, it has the same function of pumping oxygen and nutrients throughout the body.

Although neither a snail nor a grasshopper has bones, it has a form of a skeletal system that gives the body structure, support, and protection. A grasshopper’s exoskeleton is a version of a skeleton on the outside of its body. A grasshopper’s muscles are attached to its exoskeleton in the same way that a human’s muscles are attached to his or her bones. Even a snail’s shell provides similar functions that a skeletal system does in the human body, especially protection.

Heart

Figure 3.7 All of these animals have ways to support the body like a skeletal system does in humans. Some have shells, while others have skeletons or exoskeletons. The animals all have a circulatory system that transports oxygen and/or nutrients throughout the body.

Interacting Body Systems

55

8. Plant Body Systems A Venus flytrap is a plant that catches insects and digests them, using the digested food for growth. Does this plant have a digestive system, like in humans or other animals? Although these plants don’t have a stomach or a small intestine, they have systems that have similar functions to the body systems you have learned about. Even plant species that don’t catch animals for food are in many ways similar to humans and other animals. What systems in plants have similar functions to those in humans?

Structures Like the Human Skeletal System Plants’ stems have a function similar to that of an animal’s skeletal system. Plants don’t have skeletons or shells, but their rigid stems hold a plant upright and give it support. Stems also hold the leaves up to sunlight so that they can make food. In addition, a plant’s root system anchors the plant in soil so that it doesn’t blow over in wind.

A plant’s circulatory system carries food, water, and nutrients between its roots and leaves. The human circulatory system does essentially the same thing. If you look closely at a leaf, you can see these tubes branching off and traveling to the different parts of the leaf.

Structures Like the Human Circulatory System Many species of plants have a circulatory system in their stems that consists of two sets of long hollow tubes. Xylem tubes carry water and dissolved nutrients upward from the roots to the leaves. Phloem tubes carry food from the leaves, where it is made, to all parts of the plant. If you cut a stalk of celery crosswise, you can see tiny bundles of these tubes.

You can also see these branching tubes on the top and bottom surfaces of some leaves. Like blood vessels in humans, these tubes carry food, water, and nutrients all over the body of the plant.

Xylem and Phloem

56

Lesson 3

Stomata

Structures Like the Human Respiratory System Plants also have a respiratory system, which is responsible for taking in air and releasing waste. However, unlike in animals, plants take in carbon dioxide and get rid of oxygen. Plants need carbon dioxide to make food by a process called photosynthesis. They also must get rid of excess water vapor as well as oxygen, a waste material from photosynthesis. Like the lungs, plants have structures that help regulate this exchange of gases in the air. Plants have stomata, which are tiny openings in leaves that act like a plant’s respiratory system. Stomata are generally too small to be seen by eye, so you need a microscope to take a picture of the stomata on a plant leaf. Stomata open and close to control how much gas passes through. Carbon dioxide enters the plant from the atmosphere, and oxygen and water vapor exit.

This is a very close-up photo of opened and closed stomata on the underside of a plant leaf. Stomata are part of the plant’s respiratory system. They regulate how much carbon dioxide is taken in and how much oxygen and water vapor is released.

Structures Like the Human Reproductive System The function of a plant’s reproductive system is to produce offspring, just like an animal’s reproductive system. Also, like animals, plants make sperm and eggs. Often, sperm can be found within pollen and eggs can be found within flowers. In many plants, a fertilized egg will develop into a seed, in the same way that a fetus can form from a fertilized egg in humans.

Interestingly, some plants are able to make sperm and eggs within the same plant, so not all plants can be distinguished as male or female.

Interacting Body Systems

57

9. Model Systems for Research

In 1632, Rembrandt attended a dissection for training doctors. Soon after, he painted The Anatomy Lesson of Dr. Nicolaes Tulp. Before model systems were readily used, body systems were learned primarily by dissecting dead bodies.

58

Lesson 3

How do people know so much about body systems? How is it possible to know how the heart and lungs interact when you cannot see through a person’s skin into their body? And, how can a scientist learn to treat a person that has problems with the interaction between his or her heart and lungs as the result of a disease?

Today, there are lots of different technologies that help doctors see into someone’s body, but people have known about the heart and lungs before these technologies were made. One way that people have learned about body systems is through dissections of the bodies of people who have died. Although body dissections have occurred since around 200 b.c.e., in the 1500s, scientists began to dissect bodies as a way to answer questions about the body and train medical doctors.

Studying the body after death is helpful for learning about structures but not always helpful for developing treatments to diseases that affect body systems.

Another way scientists have learned about body systems and diseases is by using model organisms. As you learned, many organisms have similar body systems to humans. Diseases in humans can take many years to develop. But scientists can learn about the disease faster by studying organisms, like mice, that develop a disease or its symptoms in only a few weeks. Similarly, fruit flies have been used to learn about heart failure, problems with body development, and even sleep needs in humans. Other model organisms, even plants, yeasts, or bacteria, are used for different kinds of studies, like understanding cancer.

Using model organisms is a controversial issue in many societies.

Decisions about which species can or cannot be used in scientific research are affected by the values and the needs of the society. In general, scientists are open to new ideas, including good alternatives to living model systems, when they can be used to answer a question.

Model organisms, like fruit flies, can and have been used to learn about diseases that affect human body systems.

LESSON SUMMARY

Interacting Body Systems The Skeletal and Muscular Systems The skeletal and muscular systems work together to provide structure for the body and allow for body movement.

The Digestive System The body gets nutrients and energy from food. The digestive system breaks down food into nutrients from which energy can be released. It also gets rid of solid waste.

The Respiratory and Circulatory Systems The respiratory and circulatory systems work together to get oxygen all over the body and to remove carbon dioxide waste.

The Excretory System The excretory system gets rid of waste. The urinary system is part of the excretory system. It removes liquid waste from the blood and regulates the amount of water in the body.

Body Systems Work Together Body systems work together. Without this cooperation, the body would not function.

The Reproductive System The organs of the male reproductive system produce sperm. The organs of the female reproductive system produce eggs. An egg fertilized by a sperm can form a fetus.

Body Systems in Other Animal Species Non-human animals have needs that are fulfilled by their body systems, like in humans.

Plant Body Systems Plants have body systems with similar functions to those of animals. They help a plant meet its needs.

Model Systems for Research Scientists often use model organisms to understand functions of the body that cannot be understood from direct observation of the human body.

Interacting Body Systems

59

READING FURTHER

The Human Body in Outer Space A rocket ship has blasted off from Earth and is heading to the International Space Station—the ISS. Although blast-off was the most dangerous part of the mission, the astronauts onboard still face plenty of risks. Their bodies could be in trouble while they are in space and even after they return home. What will they do in space, and what will put their bodies at risk? Let’s follow them on their trip and find out.

Life Aboard the International Space Station After a one to three day trip, our astronauts arrive at the International Space Station, which orbits 400 km above Earth. While our newcomers climb aboard the ISS, the other astronauts living there will head for home. They have been living on the ISS for up to a year, near the limit that humans have tested being exposed to the extraordinary conditions of space.

The International Space Station is huge and like no home on

Astronauts from many countries live and work at the International Space Station, where they conduct research and adjust to doing everyday things differently. Although the ISS is large, it is full of equipment and living space is tight.

Earth. The ISS is about the size of a football field, and it gets bigger as astronauts build onto it. It has two bathrooms, a gym, and a large bay window that offers a panoramic view of space.

While the astronauts conduct research, they must learn to live life weightlessly in space. Astronauts brush their teeth with toothbrushes and toothpaste, but there is no running water, so they must spit into a washcloth or swallow the toothpaste. To get clean, astronauts use special soap and shampoo that don’t need to be rinsed off. Space toilets, too, are more complex than those on Earth. They work like a vacuum cleaner with fans that suck air and wastes into the toilet. Astronauts have to hold onto bars with their hands and feet so that they don’t float off the seat.

Eating in space is also different. There are no refrigerators, so most food is sealed in individual airtight packages so that it doesn’t spoil.

Salt and pepper are available only as liquids because dry seasonings could float into air ducts and block them. Our astronauts seem to be able to adjust to doing everyday things a bit differently in space. However, their bodies may suffer the risks that space presents.

60

Lesson 3

These two astronauts living on the International Space Station are trying to catch a fresh snack that is floating freely because of microgravity.

Microgravity: Bodies Break Down in Space One of the hazardous conditions in space is microgravity, the low gravity condition where everything seems to be weightless.

You experience microgravity on a roller coaster when you float up from your seat on the first drop and have to be held down by a safety belt. You seem to float because both you and the roller coaster car are falling at the same rate. This is why astronauts and objects on the ISS float. Microgravity may look like fun, but it can damage an astronaut’s health. Over millions of years, the human body has adapted to gravity on Earth. When the body spends a long time in microgravity, it suffers in several ways.

Microgravity has a negative effect on an astronaut’s muscles and bones. When astronauts float, they don’t need to use their bones and muscles to walk. When these body parts are not used, they start to break down. So, astronauts will lose muscle, especially in their legs. Bones also lose calcium, which is deposited in the kidneys and blocks the flow of urine. Without gravity, the bones in the spine spread apart, causing back problems. Loss of muscle causes further problems. It makes our astronauts less coordinated and weaker. It also decreases their ability to safely operate the

Even hair floats in microgravity.

Astronaut Sunita Williams must be careful that her floating long hair doesn’t get tangled in equipment in the ISS.

spacecraft and its equipment and increases their risk of injury.

To reduce the damage to their bones and muscles, our astronauts spend more than two hours a day using exercise equipment such as treadmills. After returning to Earth, they must further exercise to repair some the damage done to their bones and muscles.

Microgravity also affects the circulatory system. The heart doesn’t have to beat as hard against the pull of earth’s gravity in space, so it gets weaker and can’t pump as much oxygen-rich blood to the muscles, which places additional stress on their functioning. Decreased gravity also causes the blood to shift upward from the legs and lower body to the head and upper body.

The body responds by making less blood, decreasing the overall blood pressure. Then, when our astronauts return to Earth, their blood pressure may be so low that they faint.

Interacting Body Systems

61

Radiation: Solar and Cosmic Rays Damage the Body Our astronauts face another danger while on their mission, this one from radiation, waves and high-energy particles that travel through space. Space radiation comes from the sun and other stars. These types of radiation are called solar and cosmic rays. The sun constantly emits solar radiation such as ultraviolet rays. During sudden solar flares, even larger amounts of radiation are thrown into space. Other cosmic rays come from exploding stars in outer space.

At Earth’s surface, the thick atmosphere shields people from most of the radiation. But the atmosphere where the ISS is in orbit is much thinner. Damage from exposure to high levels of radiation

The sun emits large amounts of radiation, especially during solar flares like those shown in yellow. Damage from solar radiation is harmful because it builds up in an astronaut’s body.

builds up over time and can cause many health problems, such as nausea, muscle weakness, and the inability to fight infections. It can cause some body organs to stop functioning properly. Most seriously, it can cause cancer.

The ISS is made of materials that protect our astronauts against radiation, but they aren’t very protected when they are outside during spacewalks. Nor can they be completely protected from cosmic rays inside. The best protection for our astronauts is to keep track of the amount of radiation they are exposed to and

Radiation from the sun and exploding stars is harmful to astronauts’ health.

It is especially dangerous when astronauts are outside the ISS during spacewalks.

62

Lesson 3

try to limit it. A control station on Earth monitors the amount of radiation near the ISS at all times. Spacewalks are not scheduled for times when solar flares are most intense. Also, astronauts wear devices that measure the total amount of radiation they are exposed to while in space.

How Long Is Too Long in Space?

Many astronauts would love to live on Mars or cruise through Saturn’s rings. Unfortunately, these missions are not possible right now because they might require astronauts to be in space for very long periods of time. One of the goals of current ISS research is to

This image shows an artist’s concept of a future Mars mission. Many challenges will have to be solved before this mission will be realistic for astronauts.

prepare for longer-term space exploration.

A one-year stay in the ISS has already been shown to have negative effects on astronauts’ health, so a longer trip looks even riskier. With current technology, a trip to Mars would take about eight months—and that’s only one way. A round trip with a short visit would take at least two years. Such a trip is too dangerous until new technology to protect our astronauts is designed and tested.

American engineers are already designing and testing Orion, the next type of spacecraft that will take astronauts to faraway places. When Orion is ready to send astronauts to Mars, planned for 2021, it will need to address many challenges to long-term space travel. For example, the spacecraft will have special radiation shields to protect the astronauts. These shields were developed by a group of high school students as part of a science and engineering competition!

Even as our astronauts return to Earth and begin to recover from their trip to the ISS, chances are they are already dreaming of a trip to Mars one day. Like all astronauts, they are aware of the risks of their job. But they thrive on the challenges and otherworldly experience that space travel brings. Without a doubt, the things they have learned on the ISS will help them plan their next trip. ◆ Interacting Body Systems

63