ORGANS OF THE LOWER GASTROINTESTINAL TRACT

Most of the bacteria that normally live in the lower gastrointestinal (GI) tract live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine. The latter is arguably the most important organ of the digestive system. It is where most chemical digestion and virtually all absorption of nutrients take place.

The small intestine (also called the small bowel or gut) is the part of the GI tract between the stomach and large intestine. Its average length in adults is 4.6 m in females and 6.9 m in males. It is approximately 2.5 to 3.0 cm in diameter. It is called “small” because it is much smaller in diameter than the large intestine. The internal surface area of the small intestine totals an average of about 30 m². Structurally and functionally, the small intestine can be divided into three parts, called the duodenum, jejunum, and ileum, as shown in Figure 13.15 and described below.

The mucosa lining the small intestine is highly enfolded and covered with the finger-like projections called villi. In fact, each square inch (about 6.5 cm²) of mucosa contains around 20 thousand villi. The individual cells on the surface of the villi also have many finger-like projections — the microvilli, shown in Figure 13.16. There are thought to be well over 17 billion microvilli per square centimetre of intestinal mucosa! All of these folds, villi, and microvilli greatly increase the surface area for chyme to come into contact with digestive enzymes, which coat the microvilli, as well as forming a tremendous surface area for the absorption of nutrients. Inside each of the villi is a network of tiny blood and lymph vessels that receive the absorbed nutrients and carry them away in the blood or lymph circulation. The wrinkles and projections in the intestinal mucosa also slow down the passage of chyme so there is more time for digestion and absorption to take place.

DUODENUM

The duodenum is the first part of the small intestine, directly connected to the stomach. It is also the shortest part of the small intestine, averaging only about 25 cm (almost 10 in) in length in adults. Its main function is chemical digestion, and it is where most of the chemical digestion in the entire GI tract takes place.

The duodenum receives partially digested, semi-liquid chyme from the stomach. It receives digestive enzymes and alkaline bicarbonate from the pancreas through the pancreatic duct, and it receives bile from the liver via the gallbladder through the common bile duct (see Figure 13.17). In addition, the lining of the duodenum secretes digestive enzymes and contains glands — called Brunner’s glands — that secrete mucous and bicarbonate. The bicarbonate from the pancreas and Brunner’s glands — along with bile from the liver — neutralize the highly acidic chyme after it enters the duodenum from the stomach. This is necessary because the digestive enzymes in the duodenum require a nearly neutral environment in order to work. The three major classes of compounds that undergo chemical digestion in the duodenum are carbohydrates, proteins, and lipids.

Digestion of Carbohydrates in the Duodenum

Complex carbohydrates (such as starches) are broken down by the digestive enzyme amylase from the pancreas to short-chain molecules consisting of just a few saccharides (that is, simple sugars). Disaccharides, including sucrose and lactose, are broken down to simple sugars by duodenal enzymes. Sucrase breaks down sucrose, and lactase (if present) breaks down lactose. Some carbohydrates are not digested in the duodenum, and they ultimately pass undigested to the large intestine, where they may be digested by intestinal bacteria.

Digestion of Proteins in the Duodenum

In the duodenum, the pancreatic enzymes trypsin and chymotrypsin cleave proteins into peptides. Then, these smaller molecules are broken down into amino acids. Their digestion is catalyzed by pancreatic enzymes called peptidases.

Digestion of Lipids in the Duodenum

Pancreatic lipase breaks down triglycerides into fatty acids and glycerol. Lipase works with the help of bile secreted by the liver and stored in the gallbladder. Bile salts attach to triglycerides to help them emulsify, or form smaller particles (called micelles) that can disperse through the watery contents of the duodenum. This increases access to the molecules by pancreatic lipase.

JEJUNUM

The jejunum is the middle part of the small intestine, connecting the duodenum and the ileum. The jejunum is about 2.5 m (about 8 ft) long. Its main function is absorption of the products of digestion, including sugars, amino acids, and fatty acids. Absorption occurs by simple diffusion (water and fatty acids), facilitated diffusion (the simple sugar fructose), or active transport (amino acids, small peptides, water-soluble vitamins, and most glucose). All nutrients are absorbed into the blood, except for fatty acids and fat-soluble vitamins, which are absorbed into the lymph. Although most nutrients are absorbed in the jejunum, there are a few exceptions:

• Iron is absorbed in the duodenum.
• Vitamin B12 and bile salts are absorbed in the ileum.
• Water and lipids are absorbed throughout the small intestine, including the duodenum and ileum in addition to the jejunum.

ILEUM

The ileum is the third and final part of the small intestine, directly connected at its distal end to the large intestine. The ileum is about 3 m (almost 10 ft) long. Some cells in the lining of the ileum secrete enzymes that catalyze the final stages of digestion of any undigested protein and carbohydrate molecules. However, the main function of the ileum is to absorb vitamin B12 and bile salts. It also absorbs any other remaining nutrients that were not absorbed in the jejunum. All substances in chyme that remain undigested or unabsorbed by the time they reach the distal end of the ileum pass into the large intestine.

The large intestine— also called the large bowel — is the last organ of the GI tract. In adults, it averages about 1.5 m (about 5ft) in length. It is shorter than the small intestine, but at least twice as wide, averaging about 6.5 cm (about 2.5 in) in diameter. Water is absorbed from chyme as it passes through the large intestine, turning the chyme into solid feces. Feces is stored in the large intestine until it leaves the body during defecation.

PARTS OF THE LARGE INTESTINE

Like the small intestine, the large intestine can be divided into several parts, as shown in Figure 13.18. The large intestine begins at the end of the small intestine, where a valve separates the small and large intestines and regulates the movement of chyme into the large intestine. Most of the large intestine is called the colon. The first part of the colon, where chyme enters from the small intestine, is called the cecum. From the cecum, the colon continues upward as the ascending colon, travels across the upper abdomen as the transverse colon, and then continues downward as the descending colon. It then becomes a V-shaped region called the sigmoid colon, which is attached to the rectum. The rectum stores feces until elimination occurs. It transitions to the final part of the large intestine, called the anus, which has an opening to the outside of the body for feces to pass through.

The diagram below (Figure 13.19) shows a projection from the cecum of the colon that is known as the appendix. The function of the appendix is somewhat uncertain, but it does not seem to be involved in digestion or absorption. It may play a role in immunity, and in the fetus, it seems to have an endocrine function, releasing hormones needed for homeostasis. Some biologists speculate that the appendix may also store a sample of the colon’s normal bacteria. If so, it may be able to repopulate the colon with the bacteria if illness or antibiotic medications deplete these microorganisms. Appendicitis, or infection and inflammation of the appendix, is a fairly common medical problem, typically resolved by surgical removal of the appendix (appendectomy). People who have had their appendix surgically removed do not seem to suffer any ill effects, so the organ is considered dispensable.

FUNCTIONS OF THE LARGE INTESTINE

The removal of water from chyme to form feces starts in the ascending colon and continues throughout much of the length of the organ. Salts (such as sodium) are also removed from food wastes in the large intestine before the wastes are eliminated from the body. This allows salts — as well as water — to be recycled in the body.

The large intestine is also the site where huge numbers of beneficial bacteria ferment many unabsorbed materials in food waste. The bacterial breakdown of undigested polysaccharides produces nitrogen, carbon dioxide, methane, and other gases responsible for intestinal gas, or flatulence. These bacteria are particularly prevalent in the descending colon. Some of the bacteria also produce vitamins that are absorbed from the colon. The vitamins include vitamins B1 (thiamine), B2 (riboflavin), B7 (biotin), B12, and K. Another role of bacteria in the colon is an immune function. The bacteria may stimulate the immune system to produce antibodies that are effective against similar, but pathogenic, bacteria, thereby preventing infections. Still other roles played by bacteria in the large intestine include breaking down toxins before they can poison the body, producing substances that help prevent colon cancer, and inhibiting the growth of harmful bacteria.

Serotonin is a neurotransmitter with a wide variety of functions in the body.  Sometimes called the “happy chemical,” it is used in the central nervous system to stabilize mood by contributing to a feeling of well being and happiness.  While this “happy chemical” function is important, serotonin is also important for critical brain functions, including support of learning, memory, and reward structures and regulating sleep.  Since serotonin’s main target is brain function, you may be surprised to learn that 90% of the human body’s supply of this neurotransmitter is located in the gastrointestinal tract, where it regulates intestinal movements.

The enteric nervous system (ENS), a division of the autonomic nervous system, is a mesh-like system of neurons that control GI function.  Interestingly, the ENS can act independently from both the sympathetic/parasympathetic nervous systems and the brain/spinal cord, which is why some scientists refer to the ENS as the “second brain”.  The ENS consists of over 500 million neurons (five times as many as in the spinal cord!) and lines the GI tract from the esophagus all the way to the anus. Serotonin is made in and secreted by the gut (small and large intestine) by enterochromaffin cells (also known as Kulchitsky cells) residing in the inner lining of the lower GI tract. While the main function of serotonin in the gut is to regulate digestion, it is also suspected to play a role in brain function — meaning that your gut may actually be partly dictating your mood!  Another piece of evidence for the gut-dictating-your-mood theory is the nature vagus nerve, a fibrous visceral nerve, in which 90% of the fibers are dedicated to sending information to the brain, and only 10% of the fibers receiving information from the brain. Some treatments for depression actually involved electrical stimulation of the vagus nerve!

In a 2015 study by researchers at Caltech, it was shown that gut bacteria promote serotonin production by enterochromaffin cells.  The study involved measuring the levels of serotonin levels in mice with normal gut bacteria, and then comparing that to the levels of serotonin in a population of mice with no gut bacteria.  The germ-free mice were found to produce only 40% of the serotonin that the mice with normal gut flora were producing.  Once the germ-free mice were re-colonized with normal gut flora, their production of serotonin returned to normal.  This led researchers to the conclusion that enterochromaffin cells depend on an interaction with gut bacterial flora to be able to produce necessary levels of serotonin.

Review

1. How is the mucosa of the small intestine specialized for digestion and absorption?
2. What digestive substances are secreted into the duodenum? What compounds in food do they help digest?
3. What is the main function of the jejunum?
4. What roles does the ileum play?
5. How do beneficial bacteria in the large intestine help the human organism?
6. When diarrhea occurs, feces leaves the body in a more liquid state than normal. What part of the digestive system do you think is involved in diarrhea? Explain your answer
7. What causes intestinal gas, or flatulence?

What causes constipation? – Heba Shaheed, TED-Ed, 2018.

Did you ever hear of a person looking at something or someone with a “jaundiced eye”? It means to take a negative view, such as envy, maliciousness, or ill will. The expression may be based on the antiquated idea that liver bile is associated with such negative emotions as these, as well as the fact that excessive liver bile causes jaundice, or yellowing of the eyes and skin. Jaundice is likely a sign of a liver disorder or blockage of the duct that carries bile away from the liver. Bile contains waste products, making the liver an organ of excretion. Bile has an important role in digestion, which makes the liver an accessory organ of digestion, too.

Accessory organs of digestion are organs that secrete substances needed for the chemical digestion of food, but through which food does not actually pass as it is digested. Besides the liver, the major accessory organs of digestion are the gallbladder and pancreas. These organs secrete or store substances that are needed for digestion in the first part of the small intestine — the duodenum — where most chemical digestion takes place. You can see the three organs and their locations in Figure 13.21.

Liver

The liver is a vital organ located in the upper right part of the abdomen. It lies just below the diaphragm, to the right of the stomach. The liver plays an important role in digestion by secreting bile, but the liver has a wide range of additional functions unrelated to digestion. In fact, some estimates put the number of functions of the liver at about 500! A few of them are described below.

STRUCTURE OF THE LIVER

The liver is a reddish brown, wedge-shaped structure. In adults, the liver normally weighs about 1.5 kg (about 3.3 lb.). It is both the heaviest internal organ and the largest gland in the human body. The liver is divided into four lobes of unequal size and shape. Each lobe, in turn, is made up of lobules, which are the functional units of the liver. Each lobule consists of millions of liver cells, called hepatic cells (or hepatocytes). They are the basic metabolic cells that carry out the various functions of the liver.

As shown in Figure 13.22, the liver is connected to two large blood vessels: the hepatic artery and the portal vein. The hepatic artery carries oxygen-rich blood from the aorta, whereas the portal vein carries blood that is rich in digested nutrients from the GI tract and wastes filtered from the blood by the spleen. The blood vessels subdivide into smaller arteries and capillaries, which lead into the liver lobules. The nutrients from the GI tract are used to build many vital biochemical compounds, and the wastes from the spleen are degraded and excreted.

FUNCTIONS OF THE LIVER

The main digestive function of the liver is the production of bile. Bile is a yellowish alkaline liquid that consists of water, electrolytes, bile salts, and cholesterol, among other substances, many of which are waste products. Some of the components of bile are synthesized by hepatocytes. The rest are extracted from the blood.

As shown in Figure 13.23, bile is secreted into small ducts that join together to form larger ducts, with just one large duct carrying bile out of the liver. If bile is needed to digest a meal, it goes directly to the duodenum through the common bile duct. In the duodenum, the bile neutralizes acidic chyme from the stomach and emulsifies fat globules into smaller particles (called micelles) that are easier to digest chemically by the enzyme lipase. Bile also aids with the absorption of vitamin K. Bile that is secreted when digestion is not taking place goes to the gallbladder for storage until the next meal. In either case, the bile enters the duodenum through the common bile duct.

Besides its roles in digestion, the liver has many other vital functions:

• The liver synthesizes glycogen from glucose and stores the glycogen as required to help regulate blood sugar levels. It also breaks down the stored glycogen to glucose and releases it back into the blood as needed.
• The liver stores many substances in addition to glycogen, including vitamins A, D, B12, and K. It also stores the minerals iron and copper.
• The liver synthesizes numerous proteins and many of the amino acids needed to make them. These proteins have a wide range of functions. They include fibrinogen, which is needed for blood clotting; insulin-like growth factor (IGF-1), which is important for childhood growth; and albumen, which is the most abundant protein in blood serum and functions to transport fatty acids and steroid hormones in the blood.
• The liver synthesizes many important lipids, including cholesterol, triglycerides, and lipoproteins.
• The liver is responsible for the breakdown of many waste products and toxic substances. The wastes are excreted in bile or travel to the kidneys, which excrete them in urine.

The liver is clearly a vital organ that supports almost every other organ in the body. Because of its strategic location and diversity of functions, the liver is also prone to many diseases, some of which cause loss of liver function. There is currently no way to compensate for the absence of liver function in the long term, although liver dialysis techniques can be used in the short term. An artificial liver has not yet been developed, so liver transplantation may be the only option for people with liver failure.

The gallbladder is a small, hollow, pouch-like organ that lies just under the right side of the liver (see Figure 13.24). It is about 8 cm (about 3 in) long and shaped like a tapered sac, with the open end continuous with the cystic duct. The gallbladder stores and concentrates bile from the liver until it is needed in the duodenum to help digest lipids. After the bile leaves the liver, it reaches the gallbladder through the cystic duct. At any given time, the gallbladder may store between 30 to 60 mL (1 to 2 oz) of bile. A hormone stimulated by the presence of fat in the duodenum signals the gallbladder to contract and force its contents back through the cystic duct and into the common bile duct to drain into the duodenum.

PANCREAS

The pancreas is a glandular organ that is part of both the digestive system and the endocrine system. As shown in Figure 13.25, it is located in the abdomen behind the stomach, with the head of the pancreas surrounded by the duodenum of the small intestine. The pancreas is about 15 cm (almost 6 in) long, and it has two major ducts: the main pancreatic duct and the accessory pancreatic duct. Both of these ducts drain into the duodenum.

As an endocrine gland, the pancreas secretes several hormones, including insulin and glucagon, which circulate in the blood. The endocrine hormones are secreted by clusters of cells called pancreatic islets (or islets of Langerhans). As a digestive organ, the pancreas secretes many digestive enzymes and also bicarbonate, which helps neutralize acidic chyme after it enters the duodenum. The pancreas is stimulated to secrete its digestive substances when food in the stomach and duodenum triggers the release of endocrine hormones into the blood that reach the pancreas via the bloodstream. The pancreatic digestive enzymes are secreted by clusters of cells called acini, and they travel through the pancreatic ducts to the duodenum. In the duodenum, they help to chemically break down carbohydrates, proteins, lipids, and nucleic acids in chyme. The pancreatic digestive enzymes include:

• Amylase, which helps digest starch and other carbohydrates.
• Trypsin and chymotrypsin, which help digest proteins.
• Lipase, which helps digest lipids.
• Deoxyribonucleases and ribonucleases, which help digest nucleic acids.

Review

1. Name three accessory organs of digestion. How do these organs differ from digestive organs that are part of the GI tract?

2. Fill in the blanks:

   The ____________ has many functions, some of which are producing bile, converting glucose to glycogen, and breakdown of toxic substances.

   The ____________ stores bile and then contracts to force its contents into the duodenum when needed.

   The ____________ is both an endocrine organ and a digestive organ.

   The ____________ exits the gallbladder, the ____________ exit the liver, and the ____________ exits the pancreas.  The first two ducts merge into the ______________ and then deliver their contents to the ____________ .

3. Describe the liver and its blood supply.
4. Explain the main digestive function of the liver and describe the components of bile and it’s importance in the digestive process.
5. What type of secretions does the pancreas release as part of each body system?
6. List pancreatic enzymes that work in the duodenum, along with the substances they help digest.
7. What are two substances produced by accessory organs of digestion that help neutralize chyme in the small intestine? Where are they produced?
8. People who have their gallbladder removed sometimes have digestive problems after eating high-fat meals. Why do you think this happens?
9. Which accessory organ of digestion synthesizes cholesterol?

What does the pancreas do? – Emma Bryce, TED-Ed. 2015.

What does the liver do? – Emma Bryce, TED-Ed, 2014.

13.6 DIGESTION AND ABSORPTION

DIGESTION

Digestion of food is a form of catabolism, in which the food is broken down into small molecules that the body can absorb and use for energy, growth, and repair. Digestion occurs when food is moved through the digestive system. This process begins in the mouth and ends in the small intestine. The final products of digestion are absorbed from the digestive tract, primarily in the small intestine. There are two different types of digestion that occur in the digestive system: mechanical digestion and chemical digestion. Figure 13.26 summarizes the roles played by different digestive organs in mechanical and chemical digestion, both of which are described in detail below.

MECHANICAL DIGESTION

Mechanical digestion is a physical process in which food is broken into smaller pieces without becoming changed chemically. It begins with your first bite of food (see Figure 13.27) and continues as you chew food with your teeth into smaller pieces. The process of mechanical digestion continues in the stomach. This muscular organ churns and mixes the food it contains, an action that breaks any solid food into still smaller pieces.

Although some mechanical digestion also occurs in the small intestine, it is mostly completed by the time food leaves the stomach. At that stage, food in the GI tract has been changed to the thick semi-fluid called chyme. Mechanical digestion is necessary so that chemical digestion can be effective. Mechanical digestion tremendously increases the surface area of food particles so they can be acted upon more effectively by digestive enzymes.

CHEMICAL DIGESTION

Chemical digestion is the biochemical process in which macromolecules in food are changed into smaller molecules that can be absorbed into body fluids and transported to cells throughout the body. Substances in food that must be chemically digested include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates must be broken down into simple sugars, proteins into amino acids, lipids into fatty acids and glycerol, and nucleic acids into nitrogen bases and sugars. Some chemical digestion takes place in the mouth and stomach, but most of it occurs in the first part of the small intestine (duodenum).

Digestive Enzymes

Chemical digestion could not occur without the help of many different digestive enzymes. Enzymes are proteins that catalyze, or speed up, biochemical reactions. Digestive enzymes are secreted by exocrine glands or by the mucosal layer of epithelium lining the gastrointestinal tract. In the mouth, digestive enzymes are secreted by salivary glands. The lining of the stomach secretes enzymes, as does the lining of the small intestine. Many more digestive enzymes are secreted by exocrine cells in the pancreas and carried by ducts to the small intestine. The following table lists several important digestive enzymes, the organs and/or glands that secrete them, the compounds they digest, and the pH necessary for optimal functioning. You can read more about them below.

Table 13.1 Digestive Enzymes

Digestive Enzyme	Source Organ	Site of Action	Reactant and Product	Optimal pH
Salivary Amylase	Salivary Glands	Mouth	starch + water ⇒ maltose	Neutral
Pepsin	Stomach	Stomach	protein + water ⇒ peptides	Acidic
Pancreatic Amylase	Pancreas	Duodenum	starch + water ⇒ maltose	Basic
Maltase	Small intestine	Small intestine	maltose + water ⇒ glucose	Basic
Sucrase	Small intestine	Small intestine	sucrose + water ⇒ glucose + fructose	Basic
Lactase	Small intestine	Small intestine	lactose + water ⇒  glucose + galactose	Basic
Lipase	Pancreas	Duodenum	fat droplet and water ⇒  glycerol and fatty acids	Basic
Trypsin	Pancreas	Duodenum	protein + water ⇒ peptides	Basic
Chymotrypsin	Pancreas	Duodenum	protein + water ⇒ peptides	Basic
Peptidases	Small intestine	Small intestine	peptides + water ⇒	Basic
Deoxyribonuclease	Pancreas	Duodenum	DNA + water ⇒ nucleotide fragments	Basic
Ribonuclease	Pancreas	Duodenum	RNA + water ⇒ nucleotide fragments	Basic
Nuclease	Small intestine	Small intestine	nucleic acids + water ⇒ nucleotide fragments	Basic
Nucleosidases	Small intestine	Small intestine	nucleotides + water ⇒ nitrogen base + phosphate sugar	Basic

Chemical Digestion of Carbohydrates

About 80% of digestible carbohydrates in a typical Western diet are in the form of the plant polysaccharide amylose, which consists mainly of long chains of glucose and is one of two major components of starch. Additional dietary carbohydrates include the animal polysaccharide glycogen, along with some sugars, which are mainly disaccharides.

The process of chemical digestion for some carbohydrates is illustrated Figure 13.28. To chemically digest amylose and glycogen, the enzyme amylase is required. The chemical digestion of these polysaccharides begins in the mouth, aided by amylase in saliva. Saliva also contains mucus — which lubricates the food — and hydrogen carbonate, which provides the ideal alkaline conditions for amylase to work. Carbohydrate digestion is completed in the small intestine, with the help of amylase secreted by the pancreas. In the digestive process, polysaccharides are reduced in length by the breaking of bonds between glucose monomers. The macromolecules are broken down to shorter polysaccharides and disaccharides, resulting in progressively shorter chains of glucose. The end result is molecules of the simple sugars glucose and maltose (which consists of two glucose molecules), both of which can be absorbed by the small intestine.

Other sugars are digested with the help of different enzymes produced by the small intestine. Sucrose (or table sugar), for example, is a disaccharide that is broken down by the enzyme sucrase to form glucose and fructose, which are readily absorbed by the small intestine. Digestion of the sugar lactose, which is found in milk, requires the enzyme lactase, which breaks down lactose into glucose and galactose. Glucose and galactose are then absorbed by the small intestine. Fewer than half of all adults produce sufficient lactase to be able to digest lactose. Those who cannot are said to be lactose intolerant.

Chemical Digestion of Proteins

Proteins consist of polypeptides, which must be broken down into their constituent amino acids before they can be absorbed. An overview of this process is shown in Figure 13.29. Protein digestion occurs in the stomach and small intestine through the action of three primary enzymes: pepsin (secreted by the stomach), and trypsin and chymotrypsin (secreted by the pancreas). The stomach also secretes hydrochloric acid (HCl), making the contents highly acidic, which is a required condition for pepsin to work. Trypsin and chymotrypsin in the small intestine require an alkaline (basic) environment to work. Bile from the liver and bicarbonate from the pancreas neutralize the acidic chyme as it empties into the small intestine. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, these are further broken down into amino acids by other enzymes called peptidases, also secreted by the pancreas.

Chemical Digestion of Lipids

The chemical digestion of lipids begins in the mouth. The salivary glands secrete the digestive enzyme lipase, which breaks down short-chain lipids into molecules consisting of two fatty acids. A tiny amount of lipid digestion may take place in the stomach, but most lipid digestion occurs in the small intestine.

Digestion of lipids in the small intestine occurs with the help of another lipase enzyme from the pancreas, as well as bile secreted by the liver. As shown in the diagram below (Figure 13.30), bile is required for the digestion of lipids, because lipids are oily and do not dissolve in the watery chyme. Bile emulsifies (or breaks up) large globules of food lipids into much smaller ones, called micelles, much as dish detergent breaks up grease. The micelles provide a great deal more surface area to be acted upon by lipase, and also point the hydrophilic (“water-loving”) heads of the fatty acids outward into the watery chyme. Lipase can then access and break down the micelles into individual fatty acid molecules.

Chemical Digestion of Nucleic Acids

Nucleic acids (DNA and RNA) in foods are digested in the small intestine with the help of both pancreatic enzymes and enzymes produced by the small intestine itself. Pancreatic enzymes called ribonuclease and deoxyribonuclease break down RNA and DNA, respectively, into smaller nucleic acids. These, in turn, are further broken down into nitrogen bases and sugars by small intestine enzymes called nucleases.

Bacteria in the Digestive System

Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria known as the “gut flora” (Figure 13.30). But don’t worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth.

Gut bacteria have several roles in the body. For example, intestinal bacteria:

• Produce vitamin B12 and vitamin 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 fibre and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach.

A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food.

It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet — there are over 7 billion people on the planet!

The bacteria in your digestive system are from anywhere between 300 and 1,000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the body’s immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.

As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship.

Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weigh just about two pounds.

CONTROL OF THE DIGESTIVE PROCESS

The process of digestion is controlled by both hormones and nerves. Hormonal control is mainly by endocrine hormones secreted by cells in the lining of the stomach and small intestine. These hormones stimulate the production of digestive enzymes, bicarbonate, and bile. The hormone secretin, for example, is produced by endocrine cells lining the duodenum of the small intestine. Acidic chyme entering the duodenum from the stomach triggers the release of secretin into the bloodstream. When the secretin returns via the circulation to the digestive system, it signals the release of bicarbonate from the pancreas. The bicarbonate neutralizes the acidic chyme.  See Table 13.2 for a summary of the major hormones governing the process of chemical digestion.

Table 13.2: Major Hormones Governing Chemical Digestion

Hormone	Source Organ	Target Organ	Trigger	Result
Gastrin	Stomach walls	Stomach	High protein intake	HCL and pepsin release, stomach churning
Secretin	Duodenum	Pancreas Gallbladder	Acidic chyme entering the duodenum	Release sodium bicarbonate, release bile
Cholecystokinin (CCK)	Duodenum	Pancreas Gallbladder	Partially digested fat and protein in duodenum	Release lipase, trypsin, release bile

Nerves involved in digestion include those that connect digestive organs to the central nervous system, as well as nerves inside the walls of the digestive organs. Nerves connecting the digestive organs to the central nervous system cause smooth muscles in the walls of digestive organs to contract or relax as needed, depending on whether or not there is food to be digested. Nerves within digestive organs are stimulated when food enters the organs and stretches their walls. These nerves trigger the release of substances that speed up or slow down the movement of food through the GI tract and the secretion of digestive enzymes.

ABSORPTION

When digestion is finished, it results in many simple nutrient molecules that must go through the process of absorption from the lumen of the GI tract to blood or lymph vessels, so they can be transported to and used by cells throughout the body. A few substances are absorbed in the stomach and large intestine. Water is absorbed in both of these organs, and some minerals and vitamins are also absorbed in the large intestine, but about 95% of nutrient molecules are absorbed in the small intestine. Absorption of the majority of these molecules takes place in the second part of the small intestine, called the jejunum. There are, however, a few exceptions — for example, iron is absorbed in the duodenum, and vitamin B12 is absorbed in the last part of the small intestine, called the ileum. After being absorbed in the small intestine, nutrient molecules are transported to other parts of the body for storage or further chemical modification. Amino acids, for instance, are transported to the liver to be used for protein synthesis.

The epithelial tissue lining the small intestine is specialized for absorption. It is highly enfolded and is covered with villi and microvilli, creating an enormous surface area for absorption. As shown in Figure 13.32, each villus also has a network of blood capillaries and fine lymphatic vessels called lacteals close to its surface. The thin surface layer of epithelial cells of the villi transports nutrients from the lumen of the small intestine into these capillaries and lacteals. Blood in the capillaries absorbs most of the molecules, including simple sugars, amino acids, glycerol, salts, and water-soluble vitamins (vitamin C and the many B vitamins). Lymph in the lacteals absorbs fatty acids and fat-soluble vitamins (vitamins A, D, E, and K).

Feature: My Human Body

The process of digestion does not always go as it should. Many people suffer from indigestion, or dyspepsia, a condition of impaired digestion. Symptoms may include upper abdominal fullness or pain, heartburn, nausea, belching, or some combination of these symptoms. The majority of cases of indigestion occur without evidence of an organic disease that is likely to explain the symptoms. Anxiety or certain foods or medications (such as aspirin) may be contributing factors in these cases. In other cases, indigestion is a symptom of an organic disease, most often gastroesophageal reflux disease (GERD) or gastritis. In a small minority of cases, indigestion is a symptom of a peptic ulcer of the stomach or duodenum, usually caused by a bacterial infection. Very rarely, indigestion is a sign of cancer.

An occasional bout of indigestion is usually nothing to worry about, especially in people less than 55 years of age. However, if you suffer frequent or chronic indigestion, it’s a good idea to see a doctor. If an underlying disorder such as GERD or an ulcer is causing the indigestion, this can and should be treated. If no organic disease is discovered, the doctor can recommend lifestyle changes or treatments to help prevent or soothe the symptoms of acute indigestion. Lifestyle changes might include modifications in eating habits, such as eating more slowly, eating smaller meals, or avoiding fatty foods. You also might be advised to refrain from taking certain medications, especially on an empty stomach. The use of antacids or other medications to relieve symptoms may also be recommended.

Review

1. Define digestion. Where does it occur?
2. What is the function of pepsin?
3. Identify two organ systems that control the process of digestion by the digestive system.
4. What is mechanical digestion? Where does it occur?
5. Describe chemical digestion.
6. What is the role of enzymes in chemical digestion?
7. What is absorption? When does it occur?
8. Where does most absorption occur in the digestive system? Why does most of the absorption occur in this organ, and not earlier in the GI tract?

13.7 NUTRITION

Nutrition is defined as the interaction of an organism and its food.  There are several ways to get the recommended nutrition you need: a Nutritionist, your private physician, the United State Department of Agriculture (USDA) and the Department of Health and Human Services.  It is important that you get the proper nutrition to maintain homeostasis in the body as you will read in the rest of this chapter.

The foods and beverages that people consume have a profound impact on their health. The scientific connection between food and health has been well documented for many decades, with substantial evidence showing that healthy dietary patterns can help people achieve and maintain good health and reduce the risk of chronic diseases throughout all stages of the lifespan. Yet, Federal data show that from the first edition of the Dietary Guidelines for Americans in 1980 through today, Americans have fallen far short of meeting its recommendations, and diet-related chronic disease rates have risen to pervasive levels and continue to be a major public health concern.  The U.S. Departments of Agriculture (USDA) and of Health and Human Services (HHS) update the Dietary Guidelines at least every 5 years, based on the current science. A fundamental premise of the Dietary Guidelines is that everyone, no matter their age, race, or ethnicity, economic circumstances, or health status, can benefit from shifting food and beverage choices to better support healthy dietary patterns.

Major Themes of the Dietary Guidelines

Consume a healthy eating pattern that accounts for all foods and beverages within an appropriate calorie level. A healthy eating pattern includes:

• A variety of vegetables from all of the subgroups—dark green, red and orange, legumes (beans and peas), starchy, and other
• Fruits, especially whole fruits
• Grains, at least half of which are whole grains
• Fat-free or low-fat dairy, including milk, yogurt, cheese, and/or fortified soy beverages
• A variety of protein foods, including seafood, lean meats and poultry, eggs, legumes (beans and peas), and nuts, seeds, and soy products
  Oils

A healthy eating pattern limits:

• Saturated fats and trans fats, added sugars, and sodium
• Cholesterol, in order to limit saturated fats.

Key Recommendations that are quantitative are provided for several components of the diet that should be limited. These components are of particular public health concern in the United States, and the specified limits can help individuals achieve healthy eating patterns within calorie limits:

• Consume less than 10 percent of calories per day from added sugars
• Consume less than 10 percent of calories per day from saturated fats
• Consume less than 2,300 milligrams (mg) per day of sodium
• Limit alcohol consumption —up to one drink per day for women and up to two drinks per day for men..

High consumptions of certain foods, such as those high in saturated or trans-fat, sodium, added sugars, and refined grains may contribute to the increased incidence of chronic disease. Additionally, excessive consumption of these foods replaces the intake of more nutrient-dense foods.

My Plate

Estimating portions can be done using the MyPlate Planner. Recall that the MyPlate symbol is divided according to how much of each food group should be included with each meal. Note the MyPlate Planner Methods of Use:

• Fill half of your plate with vegetables such as carrots, broccoli, salad, and fruit.
• Fill one-quarter of your plate with lean meat, chicken, or fish (about 3 ounces)
• Fill one-quarter of your plate with a whole grain such as ⅓ cup rice
• Choose one serving of dairy
• Add margarine or oil for preparation or addition at the table

For information about on My Plate go to the USDA’s site:  MyPlate | U.S. Department of Agriculture

Figure 13.33 Choose My Plate

What are Nutrients?

The foods we eat contain nutrients. Nutrients are substances required by the body to perform its basic functions. Nutrients must be obtained from our diet, since the human body does not synthesize or produce them. Nutrients have one or more of three basic functions: they provide energy, contribute to body structure, and/or regulate chemical processes in the body. These basic functions allow us to detect and respond to environmental surroundings, move, excrete wastes, respire (breathe), grow, and reproduce. There are six classes of nutrients required for the body to function and maintain overall health. These are carbohydrates, lipids, proteins, water, vitamins, and minerals. Foods also contain non-nutrients that may be harmful (such as natural toxins common in plant foods and additives like some dyes and preservatives) or beneficial (such as antioxidants).

Macronutrients

Nutrients that are needed in large amounts are called macronutrients. There are three classes of macronutrients: carbohydrates, lipids, and proteins. These can be metabolically processed into cellular energy. The energy from macronutrients comes from their chemical bonds. This chemical energy is converted into cellular energy that is then utilized to perform work, allowing our bodies to conduct their basic functions. A unit of measurement of food energy is the calorie. On nutrition food labels the amount given for “calories” is actually equivalent to each calorie multiplied by one thousand. A kilocalorie (one thousand calories, denoted with a small “c”) is synonymous with the “Calorie” (with a capital “C”) on nutrition food labels. Water is also a macronutrient in the sense that you require a large amount of it, but unlike the other macronutrients, it does not yield calories.