CHAPTER 38 – DIGESTIVE AND EXCRETORY

SYSTEMS

38-1 – Food and Nutrition

Food and Energy

• Food is fuel for your body. It gives our body energy. Cells convert the chemical energy stored in the sugar glucose and other molecules into ATP.

Energy in food can be measured by burning it. When food is burned, the energy content of the food is converted to heat, which is measured in terms of calories. The amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius is 1 calorie. Scientists refer to the energy stored in food as dietary Calories with a capital C. One Calorie is equal to 1000 calories, or 1 kilocalorie (kcal).

An average sized teenager needs about 2200 Calories a day for females and 2800 Calories for males. More Calories are needed for more active people.

Nutrition is the study of food and its effects on the body. The science of nutrition tries to determine how food helps the body meet all of its various needs. Nutritionists recommend balanced diets that include many different types of food. Nutritionists also plan diets for people with particular needs, like diabetics.

Nutrients• Nutrients are substances in food that supply the energy and raw

materials your body uses for growth, repair, and maintenance. • The nutrients that the body needs are water, carbohydrates, fats,

proteins, vitamins, and minerals.

• The most important nutrient is water. Every cell in the human body needs water because many of the body’s processes, including chemical reactions, take place in water. Water makes up most of blood, lymph, and other body fluids. When we sweat, it is water. • People need at least 1 liter of water per day. If not enough water is

taken in, dehydration can result.

Carbohydrates – simple and complex carbohydrates are the main source of energy for the body. Sugars in fruits, honey, and sugar cane are simple carbohydrates (monosaccharides and disaccharides). Starches found in grains, potatoes, and vegetables are complex carbohydrates (polysaccharides).

Starches are broken down by the digestive system into simple sugars. These molecules are absorbed into the bloodstream and carried to cells throughout the body. Sugars that are not immediately used to supply energy are converted into the complex carbohydrate glycogen, which is stored in the liver and in skeletal muscles.

Fats – including fats (lipids) in your diet is very important. Fats are formed from fatty acids and glycerol. Your body needs essential fatty acids to produce cell membranes, myelin sheaths, and certain hormones. They also help the body absorb fat-soluble vitamins. When a person eats more food than is needed, the body stores the extra energy as fat. Deposits of fat protect body organs and insulate the body.

There are serious health consequences from a diet high in fat. This includes an increased risk of high blood pressure, heart disease, obesity, and diabetes.

Proteins – proteins supply raw materials for growth and repair of structures such as skin and muscle. Proteins have regulatory and transport functions (insulin and hemoglobin are proteins)Proteins are polymers of amino acids. The body can make 12 of the 20 amino acids. The other 8 we must get from food. Meat, fish, eggs, and milk contain all 8 esssential amino acids.

Vitamins – organic molecules that help regulate body processes, often working with enzymes. Most vitamins but be obtained from food. A diet with not enough vitamins can have serious consequences.

• There are two types of vitamins: fat-soluble and water-soluble• The fat-soluble vitamins A, D, E, and K can be stored in the fatty

tissues of the body. The body can store it for future use.• The water-soluble vitamins C and the B vitamins dissolve in water and

can’t be stored in the body. They should be included in the foods you eat each day. Eating a diet with a variety of foods is important to supply the daily vitamin needs of your body.

Taking extra doses of vitamin supplements doesn’t benefit the body and can sometimes be harmful. Too much vitamins A, D, E, and K can be toxic.

Minerals – minerals are inorganic nutrients that the body needs in small amounts. Some examples are calcium, iron, and magnesium.Calcium is needed for bones and teeth, iron makes hemoglobin. Calcium, sodium, and potassium are needed for normal functioning of nerves.

Nutrition and a Balanced Diet

The Food Guide Pyramid classifies foods into six groups. It shows how many servings from each group should be eaten every day.

38-2 The Process of Digestion

The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Several major accessory structures, including the salivary glands, the pancreas, and the liver, add secretions to the digestive system.

The Mouth• Mechanical digestion is the physical breakdown of large pieces of

food into smaller pieces. Chewing begins the process of mechanical digestion. • As you chew your food, digestive enzymes begin the breakdown of

carbohydrates into smaller molecules. This is called chemical digestion. During chemical digestion, large food molecules are broken down into smaller food molecules.

• The function of the digestive system is to help convert foods into simpler molecules that can be absorbed and used by the cells of the body. • Teeth are anchored in the bones of the jaw. The surfaces of the teeth

are protected by enamel. Teeth cut, tear, and crush food into small fragments.

• Salivary glands secrete saliva, which helps to moisten the food and make it easier to chew. The release of saliva can be triggered by the scent of food, especially when you are hungry!• Amylase – an enzyme in saliva that breaks the chemical bonds in

starches and releases sugars

The Esophagus

• The Esophagus is a food tube that leads to your stomach. • Smooth muscle contractions move the food down the esophagus to

the stomach. These contractions are called peristalsis.

• A thick ring of muscle, called the cardiac sphincter, closes the esophagus after food goes into the stomach to prevent the food from coming back up into the esophagus. • Heartburn is a painful, burning sensation just above the stomach.

This is caused by the backflow of stomach acid. This can be caused by overeating or drinking too much caffeine.

The Stomach

• Food from the esophagus empties into a large muscular sac call the stomach. The stomach continues the mechanical and chemical digestion of food.

• Chemical digestion – the stomach releases substances, including enzymes, that break down the food. Pepsin and hydrochloric acid are enzymes that breaks down protein. Chemical digestion of carbohydrates does not happen in the stomach. It resumes in the small intestines.• Mechanical digestion – stomach muscles contract to churn and mix

stomach fluids and food, which creates a mixture called chyme. After an hour or two the chyme enters the small intestine.

The Small Intestine• Small intestine – digestive organ where most chemical digestion takes

place• Food enters the duodenus first. This is the first of three parts of the

small intestine and is where almost all of the digestive enzymes enter the intestine. Most of the chemical digestion and absorption of food happens in the small intestine. Chyme mixes with enzymes and digestive fluids from the pancreas, the liver, and the lining of the duodenum.

• Pancreas – a gland behind the stomach.• The pancreas produces hormones to regulate blood sugar levels. It

also produces enzymes to break down carbohydrates, proteins, lipids, and nucleic acids. The pancreas also produces sodium bicarbonate, which neutralizes stomach acid so the enzymes can be effective. (acid can made enzymes not work well)

Liver – a large organ above and to the right of the stomach that produces bile. Bile is a fluid loaded with lipids and salts. It dissolves and disperses fat found in fatty foods so enzymes can reach the smaller fat molecules and break them down. Bile is stored in the gallbladder.

Absorption in the Small Intestine• The three parts of the small intestine are the duodenum, the jejunum,

and the ileum. All together the small intestine is about 6 meters long. • The small intestine absorbs nutrients. It is covered with finger-like

projections called villi.• Villi provide a lot of surface area for the absorption of nutrient

molecules. Slow, wavelike contractions of smooth muscle s move the chyme along this surface.

By the time food leaves the small intestines, there are no more nutrients in it. Only water, cellulose, and other indigestible substances are left.

The Large Intestine• When chyme leaves the small intestine, it enters the large intestine,

or colon. • The large intestine removes water from the undigested material that

is left. It then passes through the rectum and is eliminated from the body.

Digestive System Disorders• Acids from the stomach can sometimes damage the stomach’s lining,

producing a hole in the stomach wall. This is a peptic ulcer. Doctors used to this that peptic ulcers were caused by too much acid in the stomach. They now know that they are caused by an infectious disease that ca be cured with antibiotics. The cure rate is 90%.• Other disorders include diarrhea and constipation. If not enough

water is absorbed, diarrhea occurs. If too much water is absorbed, constipation occurs.

38-3 The Excretory System

Functions of the Excretory System• The role of the excretory system is to get rid of waste products. • Excretion is the process in which metabolic wastes are eliminated.• The skin and the lungs are two organs of excretion. The skin excretes

excess water, salts, and urea in the form of sweat. Lungs excrete carbon dioxide.

• The liver also plays an important role in excretion. It converts amino acids into useful compounds, leaving waste. The liver converts the waste into urea. Urea and other wastes are removed from the bloodstream through the kidneys.• The kidneys play an important role in maintaining homeostasis. They

remove waste products from the blood, maintain blood pH and regulate the water content of the blood and blood volume.

The Kidneys• The kidneys are located on both sides of the spinal column new the

lower back. • Ureter – a tube that leaves the kidneys and carries urine to the

urinary bladder.• Urinary bladder – a saclike organ where urine is stored before being

excreted.

• There are two regions in the kidneys. The inner part is called the renal medulla. The outer part is called the renal cortex. The functional parts of the kidney are called nephrons. • Each nephron has its own blood supply: an arteriole, a venule, and a

network of capillaries connecting them. As blood enters a nephron through the arteriole, impurities are filtered out and emptied into the collecting duct. The purified blood exits the nephron through the venule.

Blood purification involves filtrations and reabsorption.

• Filtration – passing a liquid or gas through a filter to remove wastes• The filtration of blood takes place in the glomerulus.• Glomerulus – small network of capillaries encased in the upper end of

the nephron by a hollow, cup-shaped structure called Bowman’s capsule• Fluid from blood flows into Bowman’s capsule. The materials that are

filtered from the blood are called the filtrate. The filtrate contains water, urea, glucose, salts, amino acids, and some vitamins.

• Reabsorption – the process in which liquid is taken back into a vessel.• Most of the material removed from the blood at Bowman’s capsule

makes its way back into the blood. • The material that remains is called urine. • Urine is concentrated in the loop of Henle.• Loop of Henle – a section of the nephron tubule in which water is

conserved and the volume of urine is minimized.

Urine is stored in the bladder until it is released from the body through a tube called the urethra.

Urine Testing• Drugs remain in the filtrate and come out in the urine. Testing the

urine is a good way to see if a person has done drugs.

Kidney Stones• Calcium, magnesium, or uric acid salts in the urine can crystallize and

form kidney stones. The stones can block the ureter and cause a lot of pain. Kidney stones can be treated with ultrasound waves. The sound waves pulverize the stones into smaller fragments, and then they can come out with urine.

Homeostasis by Machine• If anything goes wrong with the kidneys, serious medical problems

can develop. Humans have two kidneys and can live with just one, but if both are damaged there are two things that can keep a person alive: a kidney transplant or dialysis.

A kidney dialysis machine removes blood from the body through a tube inserted in the arm and pumps it through special tubing that acts like nephrons. Tiny pores in the tubing allow salts and small molecules, including nitrogen wastes to pass through. Wastes diffuse out of the blood into the fluid-filled chamber which allows purified blood to be returned to the body.

• Dialysis is expensive and time-consuming. It takes several hours a day and about three times a week. • Medical science is working on making an artificial kidney.