The Digestion and Absorption of Food

The Digestive System

Responsible for the intake, processing, and absorption of nutrients and water.
Critical for providing substrates, minerals, vitamins, and water.
Regulates and integrates metabolic processes.
Necessary for whole-body homeostasis and normal organ function.
Total-body balance: gain of a substance equals its loss.
Enteric nervous system: interacts with the nervous system to regulate gastrointestinal function.
Water and ion balance: achieved through regulation of excretion by the kidneys and absorption by the digestive system.
Endocrine, neural, and paracrine control illustrates information flow for homeostasis.
Relationship between absorptive capacity and circulatory/lymphatic systems for nutrient delivery.
Physiological functions controlled by multiple regulatory systems.
Acidity of stomach contents is influenced by hormones, paracrine factors, and neuronal inputs.
Epithelium regulates material transfer from environment to blood.
Digestion depends on basic chemistry and physics.
Form and function are related at all levels of structure.

Overview of the Digestive System

Includes the gastrointestinal (GI) tract (mouth, pharynx, esophagus, stomach, small intestine, large intestine) and accessory organs (salivary glands, liver, gallbladder, exocrine pancreas).
Accessory organs secrete substances into the GI tract via connecting ducts.
Overall function: process food into molecular forms for transfer to the body's internal environment.
GI tract is approximately 9 meters (30 feet) long in cadavers but shorter in life due to smooth muscle contractions.
Lumen of the tract is continuous with the external environment.
Large intestine is colonized by bacteria, which are harmless and beneficial in this location.
Food enters as large particles containing macromolecules unable to cross the intestinal epithelium.
Digestion: dissolving and breaking down food into small molecules by digestive enzymes and chemicals.
Polysaccharide digestion is initiated by amylase, triglyceride digestion by lipase, and proteins are digested by proteases.
Secretion: release of enzymes from exocrine cells by exocytosis into a duct connecting to the GI tract.
Some digestive enzymes are located on the apical membranes of the intestinal epithelium.
Absorption: movement of molecules produced by digestion, along with water and small nutrients, from the GI tract lumen across epithelial cells into the interstitial fluid.
Absorbed fats and fat-soluble nutrients are taken up by lymphatic vessels.
Other absorbed nutrients enter capillaries that drain into veins, merging to form the hepatic portal vein, which drains into the liver.
Motility: contractions of smooth muscles that mix luminal contents and move contents through the tract.
Peristalsis: muscular movements travel in a wavelike fashion in one direction.

Major Processes of the Gastrointestinal Tract

Secretion, digestion, absorption, and motility.
Digestion begins with chewing in the mouth, breaking food into smaller particles, mixing with salivary gland secretions, and forming a bolus.
Pharynx and esophagus: participate in the swallowing reflex that moves food to the stomach but do not contribute significantly to digestion.
Stomach: stores, dissolves, and partially digests macromolecules. It also regulates the rate at which it empties its contents into the small intestine.
Secretion of hydrochloric acid creates a highly acidic gastric environment.
Chyme: mixture of ingested food particles and gastric secretions.
Small intestine: divided into duodenum, jejunum, and ileum.
Surface area is vastly increased by villi.
Aided by secretions from the liver and pancreas, it digests all categories of macromolecules into molecules small enough to be transported through the epithelial lining.
Molecules, vitamins, minerals, and water enter blood and/or lymphatic vessels.
Water-soluble substances travel first to the liver.
Water-insoluble substances are absorbed into lymphatic vessels.
Large intestine: concentrates undigested material by absorbing ions and water and stores undigested material until expelled from the body in feces.
Contains bacteria that metabolize substances not absorbed by the small intestine.
Digestive system absorbs as much of any particular substance that is ingested.
Plasma concentration and distribution of absorbed nutrients are controlled by hormones and the kidneys.
Small amounts of metabolic end products are excreted via the liver.
Lungs and kidneys eliminate most of the body’s waste products.
Feces consist of bacteria and undigested material.
GI tract also has immune functions, producing antibodies, and fighting ingested microorganisms.
The small intestine has regions of lymphatic nodules that contain immune cells; these cells secrete factors that alter intestinal motility and kill microorganisms.
Average American adult consumes about $500-800$ g of food and $1200$ mL of water per day.
An additional $7000$ mL of fluid from salivary glands, gastric glands, pancreas, liver, and intestinal glands is secreted into the tract each day.
Approximately $8$ L of fluid entering the tract each day, as much as $99$ % is absorbed, only about $100-200$ mL is normally lost in the feces.
This small amount of fluid loss represents only $4$ % of the total fluids lost from the body each day.
Almost all the ions are also reabsorbed into the blood.
Secreted digestive enzymes are digested, and the resulting amino acids are absorbed into the blood.

Structure of the Gastrointestinal Tract Wall

From mid-esophagus to the anus, the wall of the GI tract has the mentioned general structure.
Apical surface is highly convoluted to increase surface area for absorption.
Epithelial cells are linked by tight junctions.
Invaginations of the epithelium form exocrine glands that secrete acid, $HCO_3$ , enzymes, water, ions, and mucus into the lumen.
Epithelium also secretes hormones into the blood.
Lamina propria: loose connective tissue with small blood vessels, neurons, and lymphatic vessels.
Muscularis mucosa: thin layer of smooth muscle involved in small movements of the mucosal surface.
Mucosa: epithelium, lamina propria, and muscularis mucosa.
Submucosa: connective-tissue layer with blood and lymphatic vessels and the submucosal plexus (network of neurons).
Muscularis externa: layers of smooth muscle that move and mix contents.
Two layers (except in stomach, which has three layers):
- Inner circular layer: contraction narrows the lumen
- Outer longitudinal layer: contraction shortens the tube
Myenteric plexus: network of neurons between muscle layers.
Neurons interconnect with the submucous plexus and project into smooth muscle layers.
Innervated by sympathetic and parasympathetic divisions of the autonomic nervous system.
Serosa: thin layer of connective tissue connecting the GI tract to the abdominal wall.
Epithelial surfaces are continuously replaced by new cells.
New cells differentiate as they migrate to the top of the villi, replacing older cells that die and are discharged into the intestinal lumen.
Dead cells release intracellular enzymes into the lumen, which then contribute to the digestive process.
About 17 billion epithelial cells are replaced each day, and the entire epithelium of the small intestine is replaced approximately every 5 days.
Enteroendocrine cells at the base of the villi secrete hormones that control gastrointestinal functions.

How Are Gastrointestinal Processes Regulated?

Control mechanisms of the digestive system primarily regulate conditions in the lumen and wall of the GI tract.
Control mechanisms are governed by the volume and composition of the luminal contents rather than by the nutritional state of the body.
Gastrointestinal reflexes are initiated by:
- Distension of the wall by the volume of luminal contents
- Chyme osmolarity (total solute concentration)
- Chyme acidity
- Chyme concentrations
Stimuli act on mechanoreceptors, osmoreceptors, and chemoreceptors in the wall of the GI tract and trigger reflexes that influence muscle layers and exocrine glands.

Neural Regulation

GI tract has its own local neural control, enteric nervous system, consisting of the myenteric and submucosal plexuses.
Neurons synapse with other neurons within a given plexus or end near smooth muscles, glands, and epithelial cells.
Many axons leave the myenteric plexus and synapse with neurons in the submucosal plexus, and vice versa.
Stimulation at one point in the plexus can lead to impulses conducted longitudinally up and down the tract.
Myenteric plexus influences smooth muscle activity and motility, whereas the submucosal plexus influences gland function and secretory activity.
Enteric nervous system contains adrenergic and cholinergic neurons and neurons that release other neurotransmitters
Interactions between neurons and effectors permit neural reflexes that occur entirely within the tract, independent of the CNS.
Efferent neurons from both the sympathetic and parasympathetic branches of the autonomic nervous system enter the intestinal tract and synapse with neurons in both plexuses.
CNS can influence the motility and secretory activity of the gastrointestinal tract.
Two types of neural-reflex arcs exist:
- Short reflexes: from receptors through nerve plexuses to effector cells within the GI tract
- Long reflexes: from receptors in the tract to the CNS by way of afferent nerves, and back to the nerve plexuses and effector cells by way of autonomic nerves.
Effects mediated by the CNS via autonomic neurons.

Hormonal Regulation

Hormones controlling the gastrointestinal system are secreted by enteroendocrine cells scattered throughout the epithelium of the stomach and small intestine.
Surface of enteroendocrine cell exposed to the lumen stimulates cell to secrete hormones into the blood.
Gastrointestinal hormones reach target cells via the circulation.
Four best-understood GI hormones are secretin, cholecystokinin (CCK), gastrin, and glucose-dependent insulinotropic peptide (GIP).

Phases of Gastrointestinal Control

Neural and hormonal control is divisible into three phases, cephalic, gastric, and intestinal, according to where the stimulus is perceived.
The cephalic phase is initiated when sensory receptors in the head are stimulated by sight, smell, taste, and chewing.
Efferent pathways are primarily mediated by parasympathetic neurons carried in the vagus nerves.
Four stimuli in the stomach initiate the gastric phase of regulation: distension, acidity, amino acids, and peptides.
Responses are mediated by short and long neural reflexes and by release of the hormone gastrin.
The intestinal phase is initiated by stimuli in the small intestine including distension, acidity, osmolarity, and various digestive products.
Mediated by short and long neural reflexes and by the hormones secretin, CCK, and GIP. Not in temporal sequence except at the very beginning of a meal.
During ingestion and the absorptive period, reflexes characteristic of all three phases may be occurring simultaneously.

Mouth, Pharynx, and Esophagus

Food is chopped into smaller bits, mixed with salivary secretions, molded into a slippery bolus, and swallowed.
Little digestion or absorption.
Stimulation of sensory and mechanoreceptors triggers the cephalic phase signals that activate subsequent regions of the tract.

Saliva

Secreted through short ducts from three pairs of salivary glands—the parotid, sublingual, and submandibular glands.
Contains mucus, water, $HCO_3^-$ , and several enzymes.
Moistens and lubricates food particles for swallowing.
Buffers the acidity of ingested foods and of metabolites from bacteria living on and around the teeth.
Viscous, slippery glycoprotein-rich protective secretion.
Protection from potentially harmful bacteria is provided by salivary lysozyme, an antibacterial enzyme.
Contains amylase and lipase, which begin the digestion of polysaccharides and triglycerides.
Salivary enzymes play a minor role because time to act on food is limited before inactivation occurs by the environment in the stomach.
Dissolves food molecules.
Secretion is controlled by sympathetic and parasympathetic neurons.
Both systems stimulate salivary secretion.
No hormonal regulation.
Absence of ingested material, low rate of salivary secretion keeps the mouth moist.
Smell or sight of food induces a cephalic phase of salivary secretion.
Increases markedly in response to a meal.
Increased secretion is accomplished by a large increase in blood flow to the salivary glands, which is mediated primarily by an increase in parasympathetic neural activity.
Sjögren’s syndrome: exocrine glands are rendered nonfunctional by infiltration of white blood cells and immune complexes.

Chewing

Controlled by somatic nerves to the skeletal muscles of the mouth and jaw.
Coordinated by pattern-generator circuits in the brainstem and reflexes activated by the pressure of food against the gums, hard palate at the roof of the mouth, and tongue.
Prolongs the subjective pleasure of taste.
Breaks up food particles, creating a bolus that is easier to swallow and, possibly, digest.

Swallowing

Complex reflex initiated when pressure receptors in the walls of the pharynx are stimulated by food or drink forced into the rear of the mouth by the tongue.
Receptors send afferent impulses to the swallowing center in the medulla oblongata of the brainstem.
As the ingested material moves into the pharynx, the soft palate elevates and lodges against the back wall of the pharynx, preventing food from entering the nasal cavity.
Impulses from the swallowing center inhibit respiration, raise the larynx, and close the glottis, keeping food from moving into the trachea.
As the tongue forces food farther back into the pharynx, the food tilts a flap of tissue, the epiglottis, backward to cover the glottis.
Skeletal muscle surrounds the upper third of the esophagus, and smooth muscle surrounds the lower two-thirds.
Each esophageal peristaltic wave takes about 9 seconds to reach the stomach.
Swallowing can occur even when a person is upside down or in zero gravity (outer space) because it is not primarily gravity but the peristaltic wave that moves the food to the stomach.
The lower esophageal sphincter opens and remains relaxed throughout the period of swallowing, allowing the arriving food to enter the stomach.
After the food passes, the lower esophageal sphincter closes, resealing the junction between the esophagus and the stomach.
The act of swallowing is a neural and muscular reflex coordinated by the swallowing center.
Both skeletal and smooth muscles are involved.
Afferent fibers from receptors in the esophageal wall send information to the swallowing center: this can alter the efferent activity.
Ability of the lower esophageal sphincter to maintain a barrier between the stomach and the esophagus is aided because the last portion of the esophagus lies below the diaphragm and is subject to the same abdominal pressures as the stomach.
These periods of relaxation, small amounts of acid contents from the stomach normally reflux into the esophagus.
The acid in the esophagus triggers a secondary peristaltic wave and also stimulates increased salivary secretion, which helps to neutralize the acid.
Smoking and ingestion of alcohol and caffeine can contribute to acid reflux.
Hydrochloric acid from the stomach irritates the esophageal walls, producing pain known as heartburn.

The Stomach

Food particles that enter the stomach are thoroughly mixed with gastric secretions to form chyme.
Contains molecular fragments of proteins and polysaccharides; droplets of fat; and ions, water, and various other molecules ingested in the food.
Alter ionization of polar molecules, leading to denaturation of protein.
Alters the ionization of polar molecules, leading to the denaturation of proteins, disrupts the extracellular network of connective-tissue proteins that form the structural framework of tissues and kills bacteria.
Major food components are polysaccharides and fat.
Acts as a storage vessel that periodically empties some chyme into the small intestine at a rate that favors the complete digestion and absorption of a meal.

Anatomy

Esophagus opens into the body of the stomach, the uppermost part of which is referred to as the fundus.
The lower portion of the stomach, the antrum, has a thicker layer of smooth muscle and is responsible for mixing and grinding the stomach contents.
At the junction between the antrum and the small intestine is a ring of contractile smooth muscle called the pyloric sphincter.
Epithelial layer lining the stomach invaginates into the mucosa, forming many tubular glands.

Secretions of the Stomach

Cells at the opening of the gastric glands secrete a protective coating of mucus and $HCO_3^-$ .
Lining the walls of the glands are parietal cells, which secrete acid and intrinsic factor, and chief cells, which secrete pepsinogen.
Intrinsic factor is a protein that binds and allows the absorption of vitamin B12.
Gastric glands in the antrum contain enteroendocrine cells called G cells, which secrete gastrin.
Enterochromaffin-like (ECL) cells release histamine.
D cells secrete the polypeptide somatostatin.
Acidic environment in the stomach alters the ionization of polar molecules.

HCl Production and Secretion

Stomach secretes about 2 L of hydrochloric acid per day.
$[H^+]$ in the lumen of the stomach may reach > $150$ mM, which is 1 to 3 million times greater than the concentration in the blood.
Requires an efficient production mechanism to generate large
numbers of hydrogen ions.
The origin of the hydrogen ions is $CO_2$ in the parietal cell, which contains the enzyme carbonic anhydrase.
Carbonic anhydrase catalyzes the reaction between $CO<em>2$ with water to produce carbonic acid, which dissociates to $H^+$ and $HCO</em>3^-$ .
Primary active $H^+/K^+$ -ATPase pumps in the apical membrane pump hydrogen ions into the lumen of the stomach. This primary active transporter also pumps $K^+$ into the cell, which then leaks back into the lumen through $K^+$ channels.
As $H^+$ is secreted into the lumen, $HCO_3^-$ is moved across the basolateral membrane and into the capillaries in exchange for $Cl^-$ , which maintains electroneutrality.
Removal of the end products ( $H^+$ and $HCO_3^-$ ) of this reaction enhances the rate of the reaction by the law of mass action.
Coupled production and secretion of $H^+$ .
Increased acid secretion results from the transfer of $H^+/K^+$ -ATPase proteins from the membranes of intracellular vesicles to the plasma membrane by fusion of these vesicles with the apical membrane.
Transfer of water channels (aquaporins) to the apical plasma membrane of kidney collecting-duct cells in response to ADH.
Three chemical messengers stimulate the insertion of $H^+/K^+$ -ATPases into the plasma membrane thereby increasing acid secretion:
- Gastrin (a gastric hormone)
- Acetylcholine (ACh, a neurotransmitter)
- Histamine (a paracrine substance).
Somatostatin inhibits acid secretion.
Parietal cell membranes contain receptors for all four of these molecules.
Chemical messengers not only act directly on the parietal cells but also influence each other’s secretion.
During a meal, the rate of acid secretion increases markedly as stimuli arising from the cephalic and gastric phases alter the release of the four chemical messengers described in the previous paragraph.
These stimuli use some of the same neural pathways used during the cephalic phase.

Pepsin Secretion

Secreted by chief cells in the form of an inactive precursor called pepsinogen.
Pepsin is produced from pepsinogen by exposure to low pH in the lumen of the stomach.
Zymogens: synthesized and stored intracellularly in inactive forms, referred to collectively, does not act on proteins inside the cells that produce them, thereby protecting the cell from proteolytic damage.
Enzymes are synthesized and stored intracellularly in inactive forms
Pepsin is active only in the presence of a high $H^+$ concentration (low pH).
Inactivated when it enters the small intestine, where $HCO_3^-$ secreted by the small intestine and pancreas neutralizes the $H^+$ .
The primary pathway for stimulating pepsinogen secretion is input to the chief cells from the enteric nervous system.
Secretion parallels acid secretion.
Not essential for protein digestion, adequate digestion by enzymes in the small intestine and digestion of collagen contained in the connective-tissue matrix of meat.

Gastric Motility

An empty stomach has a volume of only about 50 mL and a luminal diameter of only slightly larger than that of the small intestine.
When a meal is swallowed, the smooth muscle in the stomach wall relaxes before the arrival of food, allowing the stomach’s volume to increase to as much as 1.5 L with little increase in pressure.
Receptive relaxation is mediated by parasympathetic nerves innervating the stomach’s enteric nerve plexuses, with coordination provided by afferent vagal input from the stomach and by efferent input from the swallowing center in the brain.
Nitric oxide and serotonin released by enteric neurons mediate this relaxation.
Each wave begins in the body of the stomach and produces only a ripple as it proceeds toward the antrum
Powerful contraction, both mixes the luminal contents and closes the pyloric sphincter.
Only a small amount of chyme is expelled into the duodenum with each wave.
Backward motion of chyme, called retropulsion, generates strong shear forces.
Pyloric sphincter prevents regurgitation of stomach contents from entering the esophagus.
The rate (approximately three per minute) is generated by pacemaker cells in the longitudinal smooth muscle layer.
Membrane depolarization further depolarizes the membrane, thereby bringing it closer to threshold. Action potentials may be generated at the peak of the slow-wave cycle if threshold is reached, causing larger contractions.
Frequency is determined by the intrinsic basic electrical rhythm and remains essentially constant, the force of contraction is determined by neural and hormonal input to the antral smooth muscle.
Depends upon the contents of both the stomach and small intestine.
Gastrin increases the force of antral smooth muscle contractions.
Distension of the stomach also increases the force of antral contractions through long and short reflexes triggered by mechanoreceptors in the stomach wall.
Gastric emptying is inhibited by distension of the duodenum, the presence of fat, high acidity (low pH), or hypertonic solutions in the lumen of the duodenum.
Referred to as the enterogastric reflex.
This prevents overfilling of the duodenum
Rate of gastric emptying has significant clinical implications particularly when considering what food type is eaten with oral medications.
Meal rich in fat content tends to slow oral drug absorption due to a delay of the drug entering the small intestine through the pyloric sphincter.
Autonomic neurons can be activated by the CNS independently of the reflexes originating in the stomach and duodenum and can influence gastric motility.
Increase in parasympathetic activity increases gastric motility, whereas increase in sympathetic activity decreases motility.
Pain and emotions can alter motility

The Small Intestine

Macro- and microscopic structure of the wall of the small intestine is particularly elaborate.
Circular folds are covered with fingerlike projections called villi.
Surface of each villus is covered with a layer of epithelial cells whose surface membranes form small projections referred to as microvilli.
Interspersed between cells are goblet cells that secrete mucus.
Combination of circular folds, villi, and microvilli increases the small intestine’s surface area about 600-fold over that of a flat-surfaced tube.
Total surface area is about 250 to 300 square meters, roughly the area of a tennis court.
The center of each intestinal villus is occupied by both a single, blind-ended lymphatic vessel—a lacteal—and a capillary network.
Small intestine is divided into three segments: duodenum, jejunum, and ileum.
Duodenum and part of the jejunum, where normal chyme from the stomach is fully digested and absorbed.
Small intestine has a very large reserve for the absorption of most nutrients.

Secretions

Approximately 1500 mL of fluid is secreted by the cells of the small intestine from the blood into the lumen each day.
Water movement driven out by secreted mineral ions.
Secretion, along with mucus, lubricate the surface of the intestinal tract and help protect the epithelial cells.
Na+ ions secreted into the tract drive secondary active transporters that absorb monosaccharides and amino acids from the lumen into epithelial cells.

Pancreatic Secretions

Secrete $HCO_3^-$ and a number of digestive enzymes into the duodenum.
Secreted from lobules called acini at the end of the pancreatic duct system.
Epithelial cells lining the pancreatic ducts secrete $HCO_3^-$ .
High acidity of the chyme coming from the stomach would inactivate the pancreatic enzymes in the small intestine if the acid were not neutralized by the $HCO_3^-$ in the pancreatic fluid.
Pancreatic duct cells secrete $HCO<em>3^-$ into the duct lumen via an apical membrane $Cl^-/HCO</em>3^-$ exchanger, while the $H^+$ produced is exchanged for extracellular $Na^+$ on the basolateral side of the cell.
Mutations in the CFTR that cause cystic fibrosis result in decreased pancreatic $HCO_3^-$ secretion.
Water movement into the lumen leads to a thickening of pancreatic secretions; this can lead to clogging of the pancreatic ducts and pancreatic damage.
Enzymes secreted by the pancreas digest fat, polysaccharides, proteins, and nucleic acids to fatty acids and monoglycerides, sugars, amino acids, and nucleotides, respectively.
Proteolytic enzymes are secreted in inactive forms (zymogens) to protect pancreatic cells from autodigestion.
Enterokinase is a proteolytic enzyme that splits off a peptide from pancreatic trypsinogen, forming the active enzyme trypsin.
Nonproteolytic enzymes (e.g., amylase and lipase) are released in fully active form.
Pancreatic secretion increases during a meal, mainly stimulated by secretin and CCK released from enteroendocrine cells small intestine.
Secretin is stimulant for $HCO_3^-$ secretion, whereas CCK mainly stimulates acinar cell secretion.

Bile Formation and Secretion

Exocrine secretions from the liver enter the small intestine, and are essential for normal digestion.
Primary concern is the liver’s exocrine functions related to the secretion of bile.
Bile contains $HCO_3^-$ , cholesterol, phospholipids, bile pigments, a number of organic wastes, and a group of substances collectively termed bile salts.
Major components are:
1. bile salts
2. phospholipids
3. $HCO_3^-$ and other ions
4. cholesterol
5. bile pigments and small amounts of other metabolic end products
6. trace metals
Bile salts and phospholipids synthesized in the liver help solubilize fat in the small intestine.
Liver secretes cholesterol extracted from the blood into the bile.
Bile salts are formed from cholesterol in the liver and are also amphipathic.
Bile pigments also have small amounts of fat-soluble vitamins and cholesterol.
Are substances formed from the heme portion of hemoglobin when old or damaged erythrocytes are broken down in the spleen and liver.
The predominant bile pigment is bilirubin, which is extracted from the blood by hepatocytes and actively secreted into the bile.
Secretion is stimulated by secretin in response to the presence of acid in the duodenum.
Between meals, the sphincter of Oddi remains closed and dilute bile is shunted into the gallbladder, where the organic components of bile become concentrated, as some NaCl and water are absorbed into the blood.
The signal for gallbladder contraction and sphincter relaxation is the intestinal hormone CCK, appropriately.

Digestion and Absorption in the Small Intestine

Most of the digestion and absorption of nutrients occurs in the small intestine, which exemplifies the general principle of physiology that controlled exchange of materials.

Carbohydrate

Average daily intake of carbohydrates is about $250$ to $300$ g per day in a typical American diet.
This represents about half the average daily intake of calories.
Plant polysaccharide starch, along with disaccharides sucrose and lactose.
Fructose consumption is fairly low when eating whole foods, but can be significant when diet includes sweetened processed food.
Cellulose and other complex polysaccharides, is dietary fiber, are not broken down by the enzymes in the small intestine.
Digestion of starch by salivary amylase begins in the mouth but accounts for only a small fraction of total starch digestion.
$~95$ % or more starch digestion is completed in small intestine by pancreatic amylase.
Salivary and pancreatic amylase are the disaccharide maltose and a mixture of short, branched chains of glucose molecules.
Digested to monosaccharides: glucose, galactose, and fructose through the apical membranes of the small-intestine epithelial cells (brush-border).
Fructose enters the epithelial cells by facilitated diffusion via a glucose transporter (GLUT), whereas glucose and galactose undergo secondary active transport coupled to $Na^+$ via (SGLT).
Most ingested carbohydrates are digested and absorbed within the first 20% of the small intestine

Protein

A healthy adult requires a minimum of about $40$ to $50$ g of protein per day to supply essential amino acids and replace the nitrogen contained in amino acids that are metabolized to urea.
American diet contains about 60 to 90 g of protein per day.
Protein first partially broken down to peptide fragments in the stomach by pepsin.
Further breakdown is completed in the small intestine by the enzymes trypsin and chymotrypsin.
Breakdown to free amino acids by carboxypeptidases and aminopeptidases.
Short chains of two or three amino acids, by secondary active transport coupled to the $H^+$ gradient.
Amino acids enter the epithelial cells by secondary active transport coupled to $Na^+$ .
Small amounts of intact proteins are able to cross the intestinal epithelium and gain access to the interstitial fluid by endocytosis and exocytosis.
The absorptive capacity for intact proteins is much greater in infants than in adults

Fat

Daily intake of lipids is 70 to 100 g per day in a typical American diet.
Most of this in the form of fat (triglycerides).
Represents about one-third of the average daily caloric intake.
Occurs to a very limited extent in the mouth and stomach, but it predominantly occurs in the small intestine.
pancreatic lipase
Reacts at the surface of a lipid droplets.
Rate of digestion is substantially increased by division the large lipid droplets into many 1 nm in diameter.

Small Intestine Water and Mineral Absorption

Water is the most abundant substance in chyme.
Approximately 8000 mL of ingested and secreted water enters the small intestine each day, but only 1500 mL passes on to the large intestine because 80% of the fluid is absorbed in the small intestine
Epithelial membranes of the small intestine are permeable to water.
Membranes are permeable to water and net water diffusion occurs across the epithelium by the active absorption of solutes.
Na+ accounts for much of the actively transported solute
Cl− and $HCO_3^-$ are absorbed with the $Na^+$ and contribute, as well, to large absorption
Potassium, magnesium, phosphate, and calcium ions are absorbed,
Trace elements such as iron, zing, and iodine

Iron Absorption

Necessary for O2 and an important component of enzymes
Actively transport into the intestinal epithelial cells, ferritin is then incorporated, this protein–iron complex store.
Released in the blood side, this binds with the plasma membrane
Absorbed that is released at the lumen which are not bond to then is expelled in the fecal matter.
In the body the stores are ample and then transcription of gene encoding and ferritin occur, then binding absorption occur increase in the intestine and cellular decreases.
Decreases in store leads to low concentrations and release
This leads to a accumulate of bodily function in tissues

The Small Intestine Absorption Pathways

* fat- soluble 
* all from the circulation to the lymphatic
* Lymph vessels in from the intestine, as they empty the liver
* This creates a portal circulation where materials from blood get processed

The Small Intestine Motility

Functions:

o mixes luminal content
o contents contact with epithelial surface
o advances lumbar material toward the Large intestine.

Stationary contractions where the contents move but contraction does not.

Each segmented contraction is only a few centimeters in size
Contraction cycle known as segmented

Pacemaker cells initiate, but they also use
the basic electrical rhythm
Where threshold is reached, action potential occurs, this then contributes to contraction.

The temporal changes, from stomach and intestines have basic a few contraction levels, more contractions and less contractions and different times periods

a meal is digested this action stops the segmented action ceases and the starts action travels out to the Large intestine

The Large Intestine

Main functions include stool storage, reabsorbing of water and
some ions, there are some metabolic products
This is about 6.5 in diameter, small epithelial 1.5 m in length small in length, lack the folding and mucosa layers .

Cecum acts ad the first part, ileocecal valve
Then 3 portion exist the ascending, transverse, and descending is
the rectum is the anal opening

The Large Intestine secretion, digestion

Secrete scant fluids, little digestive activity, it all functions to absorb
some H2O and ions some product

Fluid absorptions by active
transport (NA) and osmosis

This is a net balance of blood in the lumen. Then the bacterial processes are involved in the synthesis of vitamin K. With air containing carbon gas

Large Intestine Motility and defection

Contraction occur at a slower rate than in the SI (once every 30)
Intake last around 1-2 days or more (bacterial growing) for that time

With mass movement in the transverse portion
The innervation occurs at sympathic and parasympathic
The muscle is the smooth muscles which the opening is closed
Sudden movements and and volume occurs when the contractions force.
The conscious urges are activated, and signals for muscle to relax are sent which lead the defecation or excretion
In reverse movement can occur if too long. Spinal cord can impact, you will lose the voluntary movement overthe defecation.