CK-12 Life Science
Page 45
Liver
The liver has a wide range of functions, a few of which are blood detoxification, maintaining glucose balance, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function.
The liver is one of the most important organs in the body when it comes to detoxifying or getting rid of foreign substances or toxins, especially from the gut. The liver filters blood from the intestine. This filtering process can remove a wide range of microorganisms such as bacteria, fungi, viruses and parasites from the blood. Almost 2 quarts of blood pass through the liver every minute.
The liver also has several roles in maintaining glucose levels, including gluconeogenesis (the synthesis of glucose from certain amino acids, lactate or glycerol), glycogenolysis (the breakdown of glycogen into glucose), and glycogenesis (the formation of glycogen from glucose).
Bacteria in the Digestive System
The large intestine provides a home for trillions of bacteria. Most of these bacteria are helpful. They have several roles in the body. For example, intestinal bacteria:
Produce vitamins B12 and K.
Control the growth of harmful bacteria.
Break down poisons in the large intestine.
Break down some substances in food that cannot be digested, such as fiber and some starches and sugars.
Keeping Your Digestive System Healthy
Most of the time, you probably aren’t aware of your digestive system. It works well without causing any problems. However, most people have problems with their digestive system at least once in awhile. Did you ever eat something that didn’t “agree” with you? Maybe you had a stomachache or felt sick to your stomach. Maybe you had diarrhea. These could be symptoms of foodborne illness.
Foodborne Illness
Harmful bacteria can enter your digestive system in food and make you sick. This is called foodborne illness. The bacteria, or the toxins they produce, may cause vomiting or cramping, in addition to the symptoms mentioned above. You can help prevent foodborne illness by following a few simple rules:
Keep hot foods hot and cold foods cold. This helps prevent any bacteria in the foods from multiplying.
Wash your hands before you prepare or eat food. This helps prevent bacteria on your hands from getting on the food.
Wash your hands after you touch raw foods such as meats, poultry, fish, or eggs. These foods often contain bacteria that your hands could transfer to your mouth.
Cook meats, poultry, fish, and eggs thoroughly before eating them. The heat of cooking kills any bacteria the foods may contain so they cannot make you sick.
Food Allergies
Food allergies are like other allergies. They occur when the immune system reacts to harmless substances as though they were harmful. Almost 10 percent of children have food allergies. Some of the foods most likely to cause allergies are shown in Figure below. Eating foods you are allergic to may cause vomiting, diarrhea, or skin rashes. Some people are very allergic to certain foods. Eating even tiny amounts of the foods causes them to have serious symptoms, such as difficulty breathing. If they eat the foods by accident, they may need emergency medical treatment.
Figure 17.15
Some of the foods that commonly cause allergies are shown here. They include nuts, eggs, fish, milk, and shellfish. Are you allergic to any of these foods?
If you think you may have food allergies, a doctor can test you to find out for sure. The tests will identify which foods you are allergic to. Then you can avoid eating these foods. This is the best way to prevent the symptoms of food allergies. To avoid the foods you are allergic to, you may have to read food labels carefully. This is especially likely if you are allergic to common food ingredients, such as soybeans, wheat, or peanuts.
A food intolerance, or food sensitivity, is different to a food allergy. A food intolerance happens when the digestive system is unable to break down a certain type of food. This can result in stomach cramping, diarrhea, tiredness, and weight loss. Food intolerances are often mistakenly called allergies. Lactose intolerance is a food intolerance. A person who is lactose intolerant does not make enough lactase, the enzyme that breaks down the milk sugar lactose. About 75 percent of the world’s population is lactose intolerant.
Constipation
Constipation means that a person has three bowel movement or less each week. The stools may also be hard and dry. Sometimes the stools are difficult or painful to pass. The person may feel “draggy” and full.
Some people think they should have a bowel movement every day. This is not necessarily true. There is no “right” number of bowel movements. What is normal for one person may not be normal for another. It depends on the foods they eat, how much they exercise, and other factors.
At one time or another, almost everyone has constipation. In most cases, it lasts for a short time and isn’t serious. However, constipation can be very uncomfortable. You can follow these tips to help prevent it:
Eat enough high-fiber foods, including vegetables, fruits, beans, and whole grains.
Drink plenty of water and other liquids.
Exercise regularly.
Don’t ignore the urge to have a bowel movement.
Following these tips will help keep your digestive system working properly. It will help you feel good and stay healthy.
Lesson Summary
The digestive system breaks down food, absorbs nutrients, and gets rid of food wastes.
Digestive enzymes speed up the reactions of chemical digestion.
The main organs of the digestive system are the mouth, esophagus, stomach, small intestine, and large intestine.
Bacteria in the large intestine produce vitamins and have other roles in the body.
You can follow simple tips to help keep your digestive system healthy.
Review Questions
What are three functions of the digestive system?
Describe the roles of the mouth in digestion.
In which organs of the digestive system does absorption of nutrients take place?
Identify two roles of helpful bacteria in the large intestine.
List two rules that can help prevent foodborne illness.
Explain the role of enzymes in digestion. Give examples to illustrate your answer.
Describe peristalsis, and explain why it is necessary for digestion.
How can the inner surface area of the small intestine be as big as a basketball court? How does this help the small intestine absorb nutrients?
Assume a person has an illness that prevents the large intestine from doing its normal job. Why might the person have diarrhea?
Explain why eating high-fiber foods can help prevent constipation.
Further Reading / Supplemental Links
CK–12, High School Biology, Chapter 38, Lesson 2.
Carol Ballard. The Digestive System. Heinemann Library, 2003.
Robert J. Sullivan. Digestion and Nutrition. Chelsea House Publications, 2004.
Sherri Mabry Gordon. Peanut Butter, Milk, and Other Deadly Threats: What You Should Know about Food Allergies. Enslow Publishers, 2006.
Steve Parker. Break It Down: The Digestive System. Raintree, 2006.
http://digestive.niddk.nih.gov/ddiseases/pubs/bacteria
http://digestive.niddk.nih.gov/ddiseases/pubs/constipation_ez
http://hypertextbook.com/facts/2001/AnneMarieThomasino.shtml
http://kalishresearch.com/a_gluten.html
http://physiwiki.wetpaint.com/page/Chapter+4:+Enzymes+and+Energy?t=anon
http://www.biologyinmotion.com/minilec/wrench.html
http://www.cfsan.fda.gov/~dms/a2z-b.html
http://www.fsis.usda.gov/Factsheets/Cleanliness_Helps_Prevent_Foodborne_Illness/index.asp
http://www.mayoclinic.com/health/food-allergies/AA00057
http://www.textbookofbacteriology.net/normalflora.html
http://en.wikiped
ia.org/wiki/Stomach
Vocabulary
absorption
Process in which substances are taken up by the blood; after food is broken down into small nutrient molecules, the molecules are absorbed by the blood.
chemical digestion
Digestion in which large food molecules are broken down into small nutrient molecules.
constipation
Having three or less bowel movements each week.
digestion
Process of breaking down food into nutrients.
digestive system
Body system that breaks down food, absorbs nutrients, and gets rid of solid food waste.
duodenum
The first part of the small intestine; where most chemical digestion takes place.
elimination
The process in which solid food waste passes out of the body.
enzyme
A substance, usually a protein, that speeds up chemical reactions in the body.
esophagus
The narrow tube that carries food from the throat to the stomach.
food allergies
A condition in which the immune system reacts to harmless substances in food as though they were harmful.
foodborne illness
An illness caused by harmful bacteria that enter the digestive system in food.
food intolerance
Occurs when the digestive system is unable to break down a certain type of food.
ileum
The third part of the small intestine; covered with villi; the few remaining nutrients are absorbed in the ileum.
jejunum
The second part of the small intestine; where most nutrients are absorbed into the blood; lined with tiny “fingers” called villi.
large intestine
The relatively wide tube between the small intestine and anus where excess water is absorbed from food waste.
mechanical digestion
Digestion in which large chunks of food are broken down into small pieces.
peristalsis
Involuntary muscle contractions which push food through the digestive system.
small intestine
The narrow tube between the stomach and large intestine where most chemical digestion and absorption of nutrients take place.
stomach
The sac-like organ at the end of the esophagus where proteins are digested.
villi
Contain microscopic blood vessels; nutrients are absorbed into the blood through these tiny vessels; located on the jejunum and the ileum.
Points to Consider
After nutrients are absorbed into the blood, think about how the blood could carry them to all the cells of the body. How does the blood travel? What keeps the blood moving?
Chapter 18: Cardiovascular System
Lesson 18.1: Introduction to the Cardiovascular System
Lesson Objectives
Identify the main structures of the cardiovascular system.
Identify three types of blood vessels.
Describe the differences between the pulmonary and the systemic circulations.
Identify the main structures of the lymphatic system.
Outline how the cardiovascular and the lymphatic systems work together.
Check Your Understanding
What is an organ system?
What are the three types of muscles found in the human body?
Introduction
Your cardiovascular system has many jobs. It acts as a message delivery service, a pump, a heating system, and a protector of the body against infection. Every cell in your body depends on your cardiovascular system. In this chapter, you will learn how your cardiovascular system works and how it helps to maintain homeostasis.
Functions of the Cardiovascular System
The cardiovascular system shown in Figure below is the organ system that is made up of the heart, the blood vessels, and the blood. Your cardiovascular system has many important roles in maintaining homeostasis. It moves nutrients, hormones, gases (such as oxygen) and wastes (such as carbon dioxide) to and from your body cells. It also helps to keep you warm by moving warm blood around your body. To do these tasks, your cardiovascular system works with other body systems such as the respiratory, endocrine, and nervous systems.
Figure 18.1
The cardiovascular system moves nutrients and other substances throughout the body.
The Movement of Gases
It could be said that the movement of gases, especially oxygen and the waste product of cellular respiration, carbon dioxide, is one of the most important aspects of the cardiovascular system. But the cardiovascular system cannot do this alone. It must work with other organ systems, especially the respiratory system (discussed in a later chapter), to move these gases throughout your body.
Oxygen is needed by every cell in your body as it is the final electron acceptor during aerobic cellular respiration. You breath oxygen in and carbon dioxide out through your respiratory system. Once oxygen enters your lungs, it must diffuse into your blood stream for transport around your body.
Oxygen is transported in your blood by attaching to the hemoglobin protein. The oxygen diffuses from the blood into the tissues, while carbon dioxide diffuses in the opposite direction. Carbon dioxide is transported back to the lungs, where it diffuses out of the blood and into your lungs for release from your body.
Parts of the Cardiovascular System
Your heart pushes the blood around your body through the blood vessels. The heart, shown in Figure below, is made of cardiac muscle (refer to the Skin, Bones, and Muscles chapter). The heart is connected to many blood vessels that bring blood all around the body. The cardiac muscle contracts and pumps blood through the heart and blood vessels.
Figure 18.2
Blood is collected in the heart and pumped out to the lungs, where it releases carbon dioxide and picks up oxygen before it is pumped to the rest of the body.
Blood Vessels
The job of these blood vessels is to channel the blood around the body. There are three main types of blood vessels in the body; arteries, veins, and capillaries.
Arteries are blood vessels that carry blood away from the heart. Arteries have thick walls that have a layer of smooth muscle, as shown in Figure below. Arteries usually carry oxygen-rich blood around the body. The blood that is in arteries is under pressure. The contractions of the heart muscle causes blood to exert force on the walls of the arteries. This force is referred to as blood pressure. Blood pressure is highest in the arteries and decreases as the blood moves into smaller blood vessels. Thick walls help prevent arteries from bursting from the pressure of blood.
Figure 18.3
Arteries are thick-walled vessels with many layers, including a layer of smooth muscle.
Every cell in the body needs oxygen, but arteries are too large to bring oxygen and nutrients to single cells. Further from the heart, arteries form smaller arteries. These smaller arteries branch into smaller vessels. The smaller blood vessels help to bring nutrients and oxygen and take away waste from body tissues.
The tiniest blood vessels in the body are called capillaries. The walls of capillaries are only a single layer of cells thick. Capillaries connect arteries and veins together, as shown in Figure below. Capillaries also allow the delivery of water, oxygen and other substances to body cells. They also collect carbon dioxide and other wastes from cells and tissues. Capillaries are so narrow that blood cells must move in single file through them.
A capillary bed is the network of capillaries that supply an organ with blood. The more metabolically active a tissue or organ is, the more capillaries it needs to get nutrients and oxygen.
Figure 18.4
Capillaries connect arteries and veins.
Blood is carried back to the heart in blood vessels called veins. Veins have thinner walls than arteries do, as you can see in Figure below. The blood in veins is not under pressure. Veins have valves that stop blood from
moving backward. Blood is moved forward in veins when the surrounding skeletal muscles squeeze the veins. Blood that is carried by veins is usually low in oxygen. The exception is the pulmonary veins that return oxygen-rich blood to the heart from the lungs.
Figure 18.5
The walls of veins are not as thick as artery walls; veins have valves that stop blood from flowing backward.
Blood is a body fluid that is a type of connective tissue. Blood is made of blood cells, and a fluid called plasma. The main types of cells found in blood are red blood cells and white blood cells. Red blood cells are the cells that carry oxygen. Oxygen-rich blood is bright red and oxygen-poor blood is dark red. You will learn more about blood in a later lesson in this chapter.
The cardiovascular system of humans is closed. That means the blood never leaves the large loop of blood vessels in which it travels. Other animals such as invertebrates have open circulatory systems, in which their blood can leave the blood vessels.