Chapter 15: The Digestion and Absorption of Food

Average intake of carbohydrates

250 to 300 g per day in America, half of average daily intake of calories. 2/3 of this intake is starch, and the rest is sucrose and lactose. Little monosaccharides are present

Surface area of small intestine

250/300 square meters

Length of adult GI tract

30 feet in length, running from the mouth to anus

Protein intake

40 to 50 g per day to supply essential amino acids and replace the nitrogen contained in amino acids that are metabolized to urea. Americans eat more, 60 to 90g, 1/6 of the average daily caloric intake. Protein in the form of enzymes, mucus are secreted too.

Average consumption

500-800g and 1200mL of water per day, but this is only a fraction of the material entering the lumen of the GI tract, since an additional 7000mL of fluid from glands are secreted into the lumen each day.

Fat intake

70 to 100g per day in a typical American diet. 1/3 of the average daily calori intake.

Absorption vs elimination

99% is absorbed. Only 100 to 200mL of fluid is lost in the fece, representing 4% of total fluids lost from the body each day.

Circular muscle

A relatively thick inner layer of circular muscle, the fibers of which are oriented in a circular pattern around the tube so that contraction produces a narrowing of the lumen

Gallbladder

A small sac underneath the liver that branches from the common hepatic duct. It stores bile between meals and concentrates the organic molecules in bile by absrbing some ions and water.

Serosa

A thin layer of connective tissue surrounding the outer surface of the tube. Thin sheets of connective tissue connect the serosa to tha bdominal wall and support the GI tract in the abdominal cavity

Longitudinal muscle

A thinner outer layer, the contract of which shortens the tube

What happens to dead epithelial cells in the GI tract?

After being discharged into the intestinal lumen, they contribute to the digestive process.

How many different peptidases are there on the apical membrane of the epithelial cells?

At least 20, with various specificities for the peptide bonds they attack

Carboxypeptidases/aminopeptidases

Break peptides down into free amino acids. Carboxypeptidases are additional proteases secreted by the pancreas, and aminopeptidases are located on the apical membranes of the small-intestine epithelial cells. They split off amino acids from their respective sides of the fragments.

Pancreatic lipase

Catalyzes the splitting of bonds linking fatty acids to the first and third carbon atoms of glycerol, producing two free fatty acids and a monoglyceride Triglyceride -> monoglyceride + 2 fatty acids

Motility of small intestine

Caused by the smooth muscles in its walls 1) Mixes the luminal contents with the various secretions 2) Brings the contents into contact with the epithelial surface where absorption takes place 3) Slowly advances the luminal material toward the large intestine, the next segment of the alimentary canal

Dietary fiber

Cellulose and certain other complex polysaccharides found in vegetable matter that are not broken down by the enzymes in the small intestine and pass on to the large intestine, where they are partially metabolized by bacteria

How are intact proteins able to cross the intestinal epithelium and gain access to the interstitial fluid?

Combination of endocytosis and exocytosis. Greater in infants than in adults, and antibodies secreted into the mother's milk can be absorbed intact by the infant, providing immunity until their own matures

Bile

Contains HCO3-, cholesterol, phospholipids, bile pigments, organic wastes, and bile salts. The HCO3- helps neutralize acid, while the bile salts solubilize dietary fat, increasing the rate of digestion and absorption

Motility

Contractions of smooth muscle in the GI tract wall

Pharynx and esophagus

Do not contribute to digestion but provide the pathway for ingested materials to reach the stomach. The muscles in the walls control swallowing

Common bile duct

During meals, the smooth muscles in the gallbladder wall are stimulated to contract, causing a concentrated bile solution to be injected into the duodenum through here, which is an extension of the hepatic duct.

Pancreas

Elongated gland located behind the stomach, which has both endocrine and exocrine functions. Only the latter are directly involved in gastrointestinal function, and it excretes digestive enzymes and a fluid rich in HCO3-. The HCO3- neutralizes the acidic chyme, which would otherwise inactivate the pancreatic enzymes in the small intestine.

Epithelial cell replacement

Epithelial surfaces in the GI tract are continuously being replaced by new epithelial cells. In the small intestine, for example, new cells arise by cell division from cells at the base of the villi, which differentiate as they migrate to the top of the villus, replacing older cells that die and are discharged into the intestinal lumen.

Rectum

Final segment of the large intestine. Its contractions, along with relaxation of associated sphincter muscles, expel the feces through defecation.

Villi

Fingerlike projections, where the surface of each one is covered with a layer of epithelial cells whose surface membranes form microvilli

Structure of GI tract wall

From mid esophagus to the anus, the apical surface is highly convoluted, which greatly increases the surface area available for absorption.

Epithelium

From the stomach on, covered by a single layer of epithelial cells linked together along the edges of their apical surfaces by tight junctions. Invaginations form exocrine glands that secrete acid, enzymes, water, ions, and mucus into the lumen. Other cells secrete hormones into the blood, important in regulating digestion and appetite. Some ducts also travel from the abdominal cavitiy to this layer for external exocrine glands

Phospholipids

In food and secreted in bile, providing the emulsifying agents. Amphipathic molecules consisting of two nonpolar fatty acid chains attached to a glycerol, with a charged phosphate group located on glycerol's third carbon.

Where is digestion/absorption complete usually?

In the jejunum, a fourth of the way through the small intestine. Because of the large remainder, the removal of portions of the small intestine as a treatment for disease do not necessarily result in nutritional deficiencies, depending on which part is removed. The remaining tissue can often increase its digestive and absorptive capacities to compensate for the removal

Goblet cells

Interspersed between the absorptive epithelial cells with microvilli are these cells that secrete mucus that lubricates and protects the inner surface of the wall of the small intestine.

GI trat immune functions

It can produce antibodies and fight infectious microorganisms that are not destroyed by the acidity of the stomach

What happens to most of the fat absorbed in the small intestine?

It enters the lacteals. Material absorbed by the lacteals reaches the general circulation by eventually emptying from the lymphatic system into large veins through the thoracic duct

Where does most fluid loss occur?

Kidneys and respiratory system

Villi components

Lacteal, and a capillary network.

Liver

Large organ located in the upper-right portion of the abdomen. Its primary purpose in digestion is to secrete bile

Starting form of ingested foods

Large particles containing macromolecules, such as proteins and polysaccharides, which are unable to cross the intestinal epithelium. Vitamins and minerals, however, can cross the epithelium intact

Feces

Leaves the digestive system via the anus at the end of the GI tract. Consist almost entirely of bacteria and ingested material that was neither digested nor absorbed, and therefore was never actually absorbed into the internal environment

Lamina propria

Lies below the epithelium, a layer of loos connective tissue through which pass small blood vessels, nerve fibers, and lymphatic vessels.

Lipid state in digestion

Lipids are insoluble in water and aggregate into large lipid droplets in the upper portion of the stomach. Pancreatic lipase is a water-soluble enzyme, and can only be active on the surface of a lipid droplet.

Digestive system regulation

Local neural control of the enteric nervous system and CNS

Enteroendocrine cells

Located at the base of the villi, secreting hormones that control a wide variety of GI functions, including motility and exocrine pancreatic secretions

Circular folds

Located in the mucosa and submucosa, they are covered with villi

Trypsin/chymotrypsin

Major proteases in the small intestine secreted by the pancreas that breakdown proteins further, which can be absorbed if small enough.

Elimination

Minor function of the GI tracts in healthy individuals, where small amounts of certain metabolic end products are excreted via the GI tract, primarily by way of bile. Most elimination occurs through the lungs and kidneys of waste products

Small intestine

Most absorption and digestion occur here. Hydrolytic enzymes (some on the apical membranes of the intestinal lining cells, others secreted by pancreas) break down molecules of intact or partially digested carbohydrates, fats, proteins, and nucleic acids into monosaccharides, fatty acids, amino acids, and nucleotides. Vitamins, minerals, and water, which do not require digestion, are also absorbed in the small intestine.

Peristalsis

Muscular movements that travel in a wavelike fashion in one direction along the length of a part of the tract

Submucosal plexus

Network of neurons. They cross to interact with the epithelium exocrine cells and the nerves in the myenteric plexus. Generally influences secretory activity

Does the digestive system regulate concentrations?

No, these are primarily controlled by hormones from endocrine glands and by kidneys. The digestive system merely gets it in.

Accessory organs

Not part of the GI tract but secrete substances into it via connecting ducts

Large intestine

Only small quantities of water, ions, and undigested material pass on to here. It temporariliy stores the undigested material (some of which is metabolized by bacteria) and concentrates it by absorbing ions and water.

Digestive function

Overall function is to process ingested foods into molecular forms that are then transferred, along with small molecules, ions, and water, to the body's internal environment, where the circulatory system can distribute them to cells.

Amylase

Partially digests polysaccharides

Emulsification

Process of dividing the droplets of lipids into smaller ones, increasing surface area for digestion

Pepsin

Produced from an inactive precursor pepsinogen, it is first partially broken down to peptide fragments. Secrete by chief cells as pepsinogen, and activated by low pH in the lumen of the stomach through a rapid, autocattalytic process. It is inactivated when it enters the small intestine and is neutralized by HCO3-.

CNS

Receives information from the GI tract (afferent input) and has a vital influence on GI function (efferent output)

Stomach

Saclike organ located between the esophagus and small intestine.

How do we taste?

Saliva dissolves food molecules, and only in the dissolved state can the molecules react with chemoreceptors in the mouth.

Myenteric plexus

Second network of neurons between the two muscle layers. Neurons project from the submucosal plexus to here, and this layer is innervated by nerves from the sympathetic and parasympathetic divisions of the autonomic nervous system. it has neurons that project to the submucosal plexus too. It generally influences smooth muscle activity

Saliva

Secreted by three pairs of exocrine salivary glands located in the head, drains into the mouth through a series of short ducts.

Why does the nutrients absorbed enter the liver first before the rest of the circulatory system?

The liver contains enzymes that can metabolize (detoxify) harmful compounds that may have been ingested, thereby preventing them from entering the circulation

Absorption

The molecules produced by digestion, along with water and small nutrients that do not require digestion, move from the lumen of the GI tract across a layer of epithelial cells and enter the blood/lymph

Secretion

The process by which substances for digestion are released into the lumen of the GI tract

Lymphatic nodules

The small intestine has regions of this immune tissue that contain immune cells which secrete factors that alter intestinal motility and kill microorganisms

What happens to digestive enzymes after being used?

They are themselves digested and the resulting amino acids are absorbed into the blood.

What do the microvilli, villi, and circular folds do?

They increase the small intestine's surface area about 600-fold, increasing absorptive capacity.

GI tract and anti-division drugs

Treatments that inhibit cell division, such as with cancer drugs/radiation therapy, make the GI tract susceptible to damage

Digestion of starch locations

1) 5% mouth through salivary amylase 2) Briefly in upper part of stomach before gastric acid inactivates the amylase 3) 95% small intestine through pancreatic amylase

Carbohydrate digestion and absorption steps

1) Amylases break polysaccharides down into maltose and chains of glucose 2) All the disaccharides (lactose, sucrose, maltose) are broken down into monosaccharides by brush border enzymes on the apical membrane 3) Fructose enters the epithelial cells by facilitated diffusion through GLUT 4) Glucose and galactose enter the cell through secondary active transport coupled to sodium gradient, all into the cell, in the sodium-glucose cotransporter (SGLT) 5) Monosaccharides leave the epithelial cells and enter the interstiail fluid by facilitated diffusion through various GLUT in the basolateral membranes, most absorbed in the first 20% of the small intestine 6) The energy required for absorption is provided by the sodium-potassium ATPase pumps on the basolateral membrane, sodium excess being created on the outside and potassium excess on the inside, although potassium channels allows it to flow down the gradient.

Muscularis externa components

1) Circular muscle 2) Myenteric plexus 3) Longitudinal muscle There is a third muscular layer in the stomach

Four major processes of digestive system

1) Digestion 2) Secretion 3) Absorption 4) Motility

Parts of small intestine

1) Duodenum 2) Jejunum 3) Ileum

Mucosa components

1) Epithelium 2) Lamina Propria 3) Muscularis mucosa

Digestive system components

1) GI tract 2) Accessory organs/tissues

Purposes of saliva

1) It moistens and lubricates the food particles, facilitating swallowing. 2) It also contains amylase to digest, 3) Dissolves food molecules 4) Antipathogenic properties

Submucosa components

1) Major blood and lymphatic vessels, which travels between all layers 2) Submucosal plexus

Emulsification requirements

1) Mechanical disruption of the large lipid droplets into smaller droplets (motility of GI tract, in the stomach and small intestine) 2) An emulsifying agent, which acts to prevent the smaller droplets from reaggregating back into large droplets (bile salts and phospholipids).

Motility purposes

1) Mix the luminal contents with the various secretions 2) Move the contents through the tract from mouth to anus

GI tract components

1) Mouth 2) Pharynx 3) Esophagus 4) Stomach 5) Small intestine 6) Large intestine

Triglyceride digestion locations

1) Mouth 2) Stomach 3) Predominantly in the small intestine through pancreatic lipase

GI tract wall layers

1) Mucosa 2) Submucosa 3) Muscularis externa 4) Serosa

Saliva components

1) Mucus 2) HCO3-

Secretory organs for the duodenum

1) Pancreas 2) Liver

What foods are not dissolved much by acid?

1) Polysaccharides 2) Fat

Protein digestion and absorption steps

1) Proteases break protein down into small peptides 2) Small peptides are transported into the cell, through secondary active transport coupled to the H+ gradient inwards 3) Peptidases break down remaining peptides into amino acids, which are transported into the cell by secondary active transport coupled to the sodium gradient inwards. 4) The small peptides transported in are broken down by peptidases into amino acids 5) Amino acids leave the cell through the basolateral membrane, through facilitated-diffusion transporters 6) The same sodium-potassium pump in the basolateral membrane is used to provide energy for the processes.

Accessory organs examples

1) Salivary glands 2) Liver 3) Gallbladder 4) Exocrine pancreas

Protein digestion locations

1) Stomach with pepsin 2) Small intestine through trypsin and chymotrypsin (largely complete absorptionin the upper portion of the small intestine)

Stomach functions

1) Store macromolecules in food 2) Dissolve macromolecules in food 3) Partially digest macromolecules in food 4) Regulate rate at which its contents empty into the small intestine 5) Kill most bacteria that enters along with food, although not completely effective. Some survive to colonize in the rest of the GI tract

Rate of epithelial cell replacement

17 billion are replaced each day, and the entire epithelium of the small intestine is replaced approximately every 5 days.

Small intestine size

2.4cm in diameter, 3m in length

Muscularis mucosa

Separates lamina propria from underlying tissues. A thin layer of smooth muscle that may be involved in the movement of intestinal structures called villi

Where are the contents of the GI tract?

Since the lumen is continuous with the external environment, the contents are technically outside the body. This is beneficial, allowing the large intestine to be colonized by harmless/beneficial bacteria which would be harmful if inside the body

Lacteal

Single, blind-ended lymphatic vessel in the center of each intestinal villus

Microvilli

Small projections on villi, also known as the brush border.

Chyme

Solution, reduced through the digestive actions of the stomach from food particles. Contains molecular fragments of proteins and polysaccharides, droplets of fat, salt, water, and various other small molecules ingested in the food. None of these molecules, except water, can cross the epithelium of the gastric wall, so little absorption occurs

Absorption processes

Specific transporter-mediated processes absorb monosaccharides and amino acids into the intestinal epithelial cells. Fatty acids merely diffuse through. Almost all mineral ions are actively absorbed by transporters, and water diffuses passivley down gradients

How does the acidic environment in the stomach break down food?

The acidic environment alters the ionization of polar molecules, leading to denaturation of protein. This exposes more sites for digestive enzymes to break down the proteins, and disrupts the extracellular network of connective-tissue proteins that form the structural framework of the tissues in food

Degree of absorption

The digestive system will absorb as much of any particular substance that is ingested, within fairly wide limits. There are some important exceptions

Digestion

The dissolving/breaking-down process, accomplished by the action of HCl in the stomach, bile from the liver, and other digestive enzymes released by the system's exocrine glands. Some of the digestive enzymes are located on the apical membranes of the intestinal epithelium.

Hepatic portal vein

The first destination of absorbed nutrients that enter the blood capillaries from the small intestine, large intestine, pancreas, and portions of the stomach. This then leads to the liver, which leads to a second capillary network before leaving the liver to return to the heart.