Flashcards in Chaper 21 Deck (246):
The digestive system is also referred to as?
The gastrointestinal system
Long tube passing through the body. The tube has muscular walls lined with epithelium and is closed off by a skeletal muscle sphincter at each end.
The gastrointestinal tract
The primary function of the GI tract
To move nutrients, water, electrolytes from the external environment into the body's internal environment
When enzymes digest the cells of the GI tract itself
What must our digestive system do?
Proteins and complex carbohydrates, so our digestive systems must secrete powerful enzymes to digest food into molecules that are small enough to be absorbed into the body. At the same time these enzymes must not digest the cells of the GI tract itself
What happens if protective mechanisms against auto digestion fail?
We may develop raw patches known as peptic ulcers on the walls of the GI tract
Another challenge the digestive system faces daily is mass balance which is?
Matching fluid input with output
To maintain homeostasis, the volume of fluid entering the GI tract by intake or secretion must?
Equal the volume leaving the lumen
People ingest about 2 liters of fluid a day. In addition, the exocrine glands and cells secrete?
7 liters or so of enzymes, mucus, electrolytes, and water into the lumen of the GI tract. That volume of secreted fluid is the equivalent of one-sixth of the body's total body water (42 liters), or more than twice the plasma volume of 3 liters, and it must be reabsorbed or the body would rapidly dehydrate
Normally reabsorption is very efficient, and only about _____ mL of fluid is lost in the feces
Vomiting and diarrhea can become an emergency when?
GI secretions that would normally be reabsorbed are lost to the environment
A final challenge the digestive system faces is repelling foreign invaders. It is counterintuitive, but the largest area of contact between the internal environment and the outside world is?
In the lumen of the digestive system. And as a result, the GI tract with a total surface area about the size of a tennis court, faces daily conflict between the need to absorb water and nutrients, and the need to keep bacteria, viruses, and other pathogens from entering the body.
Because of the large surface area of the GI tract and the need to keep foreign invaders out, the epithelium of the GI tract is assisted by?
An array of physiological defense mechanisms, including mucus, digestive enzymes, acid, and the largest collection of lymphoid tissue in the body, the gut-associated lymphoid tissue (GALT)
By one estimate ____% of all lymphocytes in the body are found in the small intestine
How does the body meet the sometimes conflicting physiological challenges it faces?
By coordinating the four basic processes of the digestive system:
Functions of the Digestive System
5) Storage & elimination
What is digestion?
Chemical and mechanical breakdown of food into absorbable units
What is absorption?
Movement of material from GI lumen to ECF
What is motility?
Movement of material through the GI tract as a result of muscle contraction
What is secretion?
Movement of material from cells into lumen or ECF
Chemical and mechanical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body
Active process or passive transfer of substances from the lumen of the GI tract to the extracellular fluid
Refers to the transepithelial transfer of water & ions from the ECF to the digestive tract lumen as well as to the release of substances synthesized by GI epithelial cells
Motility and secretion are continuously regulated to maximized?
The availability of absorbable material
Motility is regulated because?
If food moves through the system too rapidly, there is not enough time for everything in the lumen to be digested and absorbed
Secretion is regulated because?
If digestive enzymes are not secreted in adequate amounts, food in the GI tract cannot be broken down into an absorbable form
When digested nutrients have been absorbed and have reached the body's cells what happens?
Cellular metabolism directs their use or storage
Some of the same chemical signal molecules that alter digestive motility and secretion also participate in?
The control of metabolism, providing an integrating link between the two steps
The digestive system begins with the?
Oral cavity (mouth and pharynx), which serves as a receptacle for food
In the oral cavity the first stages of digestion begin with?
Chewing & the secretion of saliva by three pairs of salivary glands: sublingual glands under the tongue, submandibular glands under the mandible (jawbone), and parotid glands lying near the hinge of the jaw
Once swallowed, food moves into the GI tract. At intervals along the tract there are?
Rings of muscle function as sphincters to separate the tube into segments with distinct functions
How does food move through the tract?
Propelled by waves of muscle contraction
Along the GI tract secretions are added to food by?
Secretory epithelium, the liver, and the pancreas, creating a soupy mixture known as chyme
Where does digestion take place?
Primarily in the lumen of the tube
The products of digestion are absorbed across the?
Epithelium and pass into the extracellular compartment. From there, they move into the blood or lymph for distribution throughout the body. Any waste remaining in the lumen at the end of the GI tract leaves the body through the opening known as the anus
What is the difference between digestion and metabolism?
Digestion takes place in the GI tract lumen, which is external to the body; metabolism takes place int he body's internal environment
What is the difference between absorption and secretion?
Absorption moves material from the GI lumen into the ECF; secretion moves substances from the cells or the ECF into the lumen
Narrow tube that travels through the thorax to the abdomen
When you swallow food it passes into the?
The esophageal walls are?
Skeletal muscle initially but transition to smooth muscle about two-thirds of the way down the length
Just below the diaphragm, the esophagus ends at the?
Stomach, a bag-like organ that can hold as much as 2 liters of food and fluid when fully (if uncomfortably) expanded
The stomach is divided into 3 sections
1) The upper fundus
2) The central body
3) The lower antrum
The stomach continues digestion that began in the mouth by?
Mixing food with acid and enzymes to create chyme
The pylorus (gatekeeper) or opening between the stomach and the small intestine is guarded by the?
Pyloric valve. This thickened band of smooth muscle relaxes to allow only small amounts of chyme into the small intestine at any one time
The stomach acts as an intermediary between the?
Behavioral act of eating and the physiological events of digestion and absorption int he intestine
Integrated signals and feedback loops between the intestine and stomach regulate?
The rate at which chyme enters the duodenum. This ensures that the intestine is not overwhelmed with more than it can digest and absorb
Most digestion takes place in the?
The small intestine is divided into 3 sections
1) The duodenum (the first 25 cm)
2) The jejunum
3) The ileum (the latter two together are about 260 cm long)
Digestion is carried out by?
Intestinal enzymes, aided by exocrine secretions from two accessory glandular organs: the pancreas and the liver
Secretions from the pancreas and liver enters the?
Initial section of the duodenum through ducts. A tonically contracted sphincter (the sphincter of Oddi) keeps pancreatic fluid and bile from entering the small intestine except during a meal
Digestion is essential completed in the?
Small intestine, and nearly all digested nutrients and secreted fluids are absorbed there, leaving about 1.5 liters of chyme per day to pass into the large intestine
The proximal section of the large intestine
What happens in the colon?
Watery chyme is converted into semisolid feces as water and electrolytes are absorbed out of the chyme and into the ECF
The terminal section of the large intestine
What happens when feces are propelled into the rectum?
Distension of the rectal wall triggers a defecation reflex. Feces leave the GI tract through the anus, with its external sphincter of skeletal muscle, which is under voluntary control
The portion of the GI tract running from the stomach to the anus is collectively called the?
In a living person, the digestive system from mouth to anus is about?
450 cm (nearly 15 feet) long. Of this length, 395 cm (about 13 feet) consists of the large and small intestines
The GI tract wall consists of 4 layers
1) An inner mucosa facing the lumen
3) Layers of smooth muscle known collectively as muscular is externa
4) A covering of connective tissue called serosa
The mucosa, the inner lining of the GI tract is created from?
1) A single layer of epithelial cells
2) The lamina propria, subepithelial connective tissue that holds the epithelium in place
3) The muscarlaris mucosae, a thin layer of smooth muscle
The entire wall is crumpled into folds called ______ in the stomach, and ______ in the small intestine
The intestinal mucosal also projects into the lumen in small fingerlike extensions known as?
Some of the deepest invaginations form?
The most variable feature of the GI tract that changes from section to section
The epithelial cells of the GI tract include?
1) Transporting epithelial cells (called enterocytes in the small intestine)
2) Endocrine and exocrine secretory cells
3) Stem cells
Transporting epithelial cells secrete?
Ions & water into the lumen, and absorb ions, water, and nutrients into the ECF
At the mucosal (apical) surface, secretory cells release?
Enzymes, mucus, and paracrine molecules into the lumen
At the serosal (basolateral) surface, secretory cells secrete?
Hormones into the blood or paracrine messengers into the interstitial fluid, where they act on neighboring cells
The cell-cell junctions that tie epithelial cells together?
In the stomach and colon, the cell-cell junctions form?
A tight barrier so that little can pass between the cells.
In the small intestine the junctions are?
Not as tight as they are in the stomach and colon.
The intestinal epithelium is considered leaky because?
Some water & solutes can be absorbed between the cells (paracellular pathway) instead of through them. These junctions have plasticity and that their "tightness" and selectivity can be regulated to some extent
The GI stem cells are?
Rapidly dividing, undifferentiated cells that continuously produce new epithelium in the crypts and gastric glands
As stem cells of the GI tract divide what happens?
The newly formed cells are pushed toward the luminal surface of the epithelium.
The average life span of the GI epithelial cells is?
Only a few days, a good indicator of the rough life such cells lead
As with other types of epithelium, the rapid turnover & cell division rate in the GI tract make these organs susceptible to?
The lamina proporia is?
Subepithelial connective tissue that contains nerve fibers and small blood and lymph vessels. Also contains wandering immune cells, such as macrophages and lymphocytes, patrolling for invaders that enter through breaks in the epithelium
Absorbed nutrients pass into the blood and lymph here
In the intestine, collections of lymphoid tissue adjoining the epithelium form?
Small nodules and larger Peyer's patches that create visible bumps in the mucosa. These lymphoid aggregations are a major part of the gut-assoiciated lymphoid tissue (GALT)
The third region of the mucosa, the muscularis mucosae, separates the?
Mucosa from the submucosa
The muscularis mucosa is a?
Thin layer of smooth muscle, and contraction of this layer alters the effective surface area for absorption by moving the villi back and forth
The submucosa is
The layer of the gut wall adjacent to the mucosa. Composed of connective tissue with larger blood and lymph vessels
The submucosa also contains the submucosal plexus which is?
On of the two major nerve networks of the enteric nervous system. Innervates cells in the epithelial layer as well as smooth muscle of the muscularis mucosae
The enteric nervous system helps coordinate?
The outer layer of the GI tract, the muscularis externa, consists of?
Primarily two layers of smooth muscle: an inner circular layer and an outer longitudinal layer
Contraction of the muscularis externa inner circular layer does what?
Decreases the diameter of the lumen
Contraction of the muscularis externa outer longitudianl layer does what?
Shortens the tube
The second nerve network of the enteric nervous system, the myenteric plexus lies?
Between the longitudinal and circular layers. Controls & coordinates the motor activity of the muscularis externa
The outer covering of the entire digestive tract is?
The serosa, a connective tissue membrane that is continuation of the peritoneal membrane (peritoneum) lining the abdominal cavity
The peritoneum also forms sheets of?
Mesentery that hold the intestines in place so that they do not become tangled as they move
Is the lumen of the digestive tract on the apical or basolateral side of the intestinal epithelium? On the serosal or mucosal side?
The lumen of the digestive tract is on the apical or mucosal side of the intestinal epithelium
Name the 4 layers of the GI tract wall, starting at the lumen and moving out.
3) Muscularis externa
Name the structures a piece of food passes through as it travels from mouth to anus
Mouth > Pharynx > Esophagus > Stomach (fundus, body, antrum) > Small intestine (duodenum, jejunum, ileum) > Large intestine (colon, rectum) > Anus
Why is the digestive system associated with the largest collection of lymphoid tissue in the body?
Because the GI tract has a large, vulnerable surface area facing the external environment, it needs the immune cells of lymphoid tissue to combat potential invaders
Motility in the gastrointestinal tract serves two purposes
Moving food from the mouth to the anus and mechanically mixing food to break it into uniformly small particles. This mixing maximizes exposure of the particles to digestive enzymes by increasing particle surface area
Gastrointestinal motility is determined by?
The properties of the tract's smooth muscle and modified by chemical input from nerves, hormones, & paracrine signals
A series of contractions that begin in the empty stomach and end in the large intestine
The migrating motor complex
Responsible for forward movement
Responsible for mixing
Spontaneous depolarizations in GI smooth muscle
Muscle contraction in the gastrointestinal tract occur in three general patterns
1) Peristaltic contractions
2) Segmental contractions
3) The migrating motor complex
The migrating motor complex is what type of function?
A housekeeping function that sweeps food remnants and bacteria out of the upper GI tract and into the large intestine
Progressive waves of contraction that move from one section of the GI tract to the next
In peristalsis, circular muscles contract just behind a mass, or bolus, of food. This contraction does what?
Pushes the bolus forward into a receiving segment, where the circular muscles are relaxed. The receiving segment then contracts, continuing the forward movement
Peristalsis in the esophagus does what?
Propels material from pharynx to the stomach
Peristalsis in the stomach does what?
Contributes to food mixing in the stomach, but in normal digestion intestinal peristaltic waves are limited to short distances.
What influences peristalsis in all regions of the GI tract?
Hormones, paracrine signals, and the autonomic nervous system
What happens in segmental contractions
Short (1-5 cm) segments of intestine alternately contract and relax. In the contracting segments, circular muscles contract while longitudinal muscles relax. These contractions may occur randomly along the intestine or at regular intervals
What is the purpose of segmental contractions alternating contractions?
It churns the intestinal contents, mixing them and keeping them in contact sequentially, in an oral-to aboral direction, digested material is propelled short distances
Motility disorders are among the?
More common gastrointestinal problems. They range from esophageal spasms and delayed gastric (stomach) emptying to constipation and diarrhea
A chronic functional disorder characterized by altered bowel habits and abdominal pain
Irritable bowel syndrome
Most of the gastrointestinal tract is composed of?
Single-unit smooth muscle, with groups of cells electrically connected by gap-junctions to create contracting segments
Difference between tonic contractions and phasic contractions
Tonic contractions that are sustained for minutes or hours occur in some smooth muscle sphincters and int he anterior portion of the stomach.
Phasic contractions with contraction-relaxation cycles lasting only a few seconds, occur in the posterior region of the stomach and in the small intestine
What is slow wave potentials?
Cycles of smooth muscle contraction and relaxation are associated with spontaneous cycles of depolarization and depolarization
How do slow wave potentials differ from myocardial pacemaker potentials?
The GI waves have a much slower rate and do not reach threshold with each cycle
The likelihood of a slow wave firing an action potential depends on what?
Input from the enteric nervous system.
When a slow wave does reach threshold what happens?
Voltage-gated calcium channels in the muscle fiber open, calcium enters, and the cell fires one or more action potentials
Contraction of smooth muscle, like that of cardiac muscle, is graded according to what?
The amount of calcium that enters the fiber
The longer the duration of the slow wave, the longer the?
1) The more action potentials fire
2) The greater the contraction force in the muscle
3) The longer the duration of contraction
Both amplitude and duration of a slow wave can be modified how?
By neurotransmitters, hormones, or paracrine molecules
Slow wave frequency varies by region of the digestive tract, ranging from ___waves/min in the stomach to ___waves/min in the duodenum
Slow waves originate in a network of cells called the?
Interstitial cells of Cajal
These modified smooth muscle cells lie between smooth muscle layers and the intrinsic nerve plexuses, and they may act as an intermediary between the neurons and smooth muscle
Interstitial cells of Cajal
These cells function as the pacemakers for slow wave activity in different regions of the GI tract
Interstitial cells of Cajal
In a typical day, ___ liters of fluid pass through the lumen of an adult's GI tract-equal to the contents of three 3-liter soft drink bottles! Only about ___ liters of that volume enter the GI system through the mouth. The remaining ___ liters of fluid come from body water secreted along the enzymes and mucus. About half of the secreted fluid comes from?
Accessory organs and glands such as the salivary glands, pancreas, and liver. The remaining 3.5 liters are secreted by epithelial cells of the digestive tract itself
A large portion of the 7 liters of fluid secreted by the digestive system each day is composed of?
Water and ions, particularly sodium, potassium, chloride, bicarbonate, and hydrogen
The ions of the digestive system are first secreted into what?
What about water?
Lumen of the tract, then reabsorbed. Water follows osmotic gradients created by the transfer of solutes from one side of the epithelium to the other.
How does water move in the digestive tract?
Through the epithelial cells via membrane channels or between cells (the paracellular pathway)
Gastrointestinal epithelial cells have distinct apical and basolateral membranes. Each cell surface contains?
Proteins for active transport, facilitated diffusion, and ion movement through channels. The arrangement of channels and transporters on the apical and basolateral membranes determines the direction of movement of solutes and water across the epithelium
Many of the membrane transporters of the GI tract are similar to those of the?
The basolateral membrane of the GI tract contains?
The ubiquitous sodium-potassium pump
What does motility include?
- putting food into the mouth
- Chewing, mixing with saliva
4) Peristalsis & Segmentation
- Squeezing by muscular contraction along and through the GI tract
Cotransporters in the GI tract include?
1) Sodium-potassium-2 chloride symporter (NKCC)
2) Chloride-bicarbonate exchangers (Cl- - HCO3)
3) Sodium-hydrogen exchanger (NHE)
4) Hydrogen-potassium pump
Ion channels of the GI tract include?
1) The apical sodium channel (ENaC) epithelial sodium channel
2) Potassium channels
3) Chloride channels, such as the gated chloride channel known as the cystic fibrosis transmembrane conductance regulator, or CFTR chloride channel
Defects in this cause the disease cystic fibrosis
Cystic fibrosis transmembrane conductance regulator (gated chloride channel)
1) Acid secretion by?
2) Bicarbonate secretion by?
3) Isotonic sodium chloride secretion by?
2) Pancreas and duodenum
3) Intestines and salivary glands
What secretes hydrochloric acid into the lumen of the stomach?
Parietal cells deep in the gastric glands
Acid secretion in the stomach averages ____ liters per day and can create a luminal pH as low as __. The cytoplasmic pH of the parietal cells is about ____, which means the cells are pumping ____ against a gradient that is 2.5 million times more concentrated in the lumen
1-3 liters per day
low as 1
Describe the parietal cell pathway for acid secretion
The process begins when hydrogen from water inside the partial cell is pumped into the stomach lumen by an hydrogen potassium pump in exchange for potassium entering the cell. Chloride then follows hydrogen through open chloride channels, resulting in net secretion of hydrochloric acid by the cell. While acid is being secreted into the lumen, bicarbonate made from carbon dioxide and the hydroxide (OH-) from water is absorbed into the blood. The buffering action of bicarbonate makes blood leaving the stomach less acidic, creating an alkaline tide
Bicarbonate secretion into the duodenum does what?
Neutralizes acid entering from the stomach
A small amount of bicarbonate is secreted by?
But most comes from the?
Small amount- duodenal cells
Most- pancreas, which secretes a watery solution of sodium bicarbonate (NaHCO3). They also secrete hormones from islet cells tucked among the acinar cells
The exocrine portion of the pancreas consists of?
Lobules called acini that open into ducts whose lumens are part of the body's external environment
The acinar cells secrete?
Digestive enzymes, and the duct cells secrete the sodium bicarbonate (NaHCO3) solution
Bicarbonate production requires?
High levels of the enzyme carbonic anhydrase
In the pancreas and/or duodenum, bicarbonate produced from carbon dioxide and water is secreted by?
An apical chloride bicarbonate exchanger. Chloride enters the cell on a basolateral NKCC cotransporter and leaves via an apical CFTR (Cystic fibrosis transmembrane conductance regulator) channel. Luminal chloride then re-enters the cell in exchange for bicarbonate entering the lumen. Hydrogen ions produced along with bicarbonate leave the cell on basolateral sodium hydrogen exchangers. The hydrogen thus reabsorbed into the intestinal circulation helps balance bicarbonate put into the blood when parietal cells secrete hydrogen into the stomach
In the pancreatic duct cell and duodenal cell sodium and water movement in these tissues is?
A passive process, driven by electrochemical and osmotic gradients
For intestinal and colonic crypt cells and salivary gland acini, and pancreatic duct cell or duodenal cell, the net movement of negative ions from the ECF to the lumen attracts what?
Sodium which moves down its electrochemical gradient through leaky junctions between the cells. The secretion of sodium and bicarbonate into the lumen creates an osmotic gradient, & water follows by osmosis. The net result is secretion of a watery sodium bicarbonate solution
Is the movement of hydrogen into the stomach active or passive?
Crypt cells in the small intestine and colon secrete?
An isotonic sodium chloride solution that mixes with mucus secreted by goblet cells to help lubricate the contents of the gut
In the crypt cells in the small intestine and colon, the active chloride secretion is similar to?
The pancreatic cells
In the crypt cells in the small intestine and colon, chloride from the ECF enters cells via?
NKCC transporters, then exits into the lumen via apical CFTR channels. Sodium & water follow along the paracellular pathway, with the end result being secretion of isotonic saline solution.
Digestive enzymes are secreted by?
Either by exocrine glands (salivary glands and the pancreas) or by epithelial cells in the mucosa of the stomach and small intestine
Proteins which means that they are synthesized on the rough ER, packaged by the golgi complex into secretory vesicles, and then stored in the cell until needed. On demand they are released by exocytosis
Many intestinal enzymes are?
Not released free into the lumen but remain bound to the apical membranes of intestinal cells, anchored by transmembrane protein stalks or lipid anchors
Some digestive enzymes are secreted in an inactive proenzyme form known collectively as?
Zymogens must be activated in the?
GI lumen before they can carry out digestion. This late activation allows enzymes to be stockpiled in the cells that make them without damaging those cells.
Zymogen names often have?
The suffix-ogen added to the enzyme name such as pepsinogen
The control pathways for enzyme release vary but include.
A variety of neural, hormonal, and paracrine signals. Usually stimulation of parasympathetic neurons in the vagus nerve enhances enzyme secretion
A viscous secretion composed primarily of glycoproteins called mucins
The primary functions of mucus are?
To form a protective coating over the GI mucosa and to lubricate the contents of the gut.
Where is mucus made?
In specialized exocrine cell called mucous cells in the stomach, serous cells in salivary glands, and goblet cells in the intestine
These cells make up between 10-24 percent of the intestinal cell population
The signals for mucus release include?
Parasympathetic innervation, a variety of neuropeptides found in the enteric nervous system, & cytokines from immunocytes
Parasitic infections and inflammatory processes in the gut also cause substantial increase in?
Mucus secretion as the body attempts to fortify its protective barrier
Saliva is a complex hyposmotic fluid secreted by the?
Salivary glands of the oral cavity.
The salivary glands, like the exocrine pancreas, are organized into?
AAcini and ducts
The components of saliva include?
Water, ions, mucus, and proteins such as enzymes and innumoglobulins
The salivary glands are?
Exocrine glands, with a secretory epithelium that opens to the outside environment through a duct
Saliva is controlled by the?
Autonomic nervous system
Primary stimulus for secretion of saliva
Parasympathetic innervation, but there is also some sympathetic innervation to the glands.
Bile is a?
Nonenzymatic solution secreted from hepatocytes or liver cells
The key components of bile are?
1) Bile salts, which facilitate enzymatic fat digestion
2) Bile pigments, such a bilirubin, which are the waste products of hemoglobin degradation
3) Cholesterol, which is excreted in the feces
Bile salts, which act as detergent to solubilize fats during digestion, are made from?
Steroide Bile acids combined with amino acids
Bile is secreted into?
Hepatic ducts that lead to the gallbladder which stores and concentrates the bile solution
During a meal, contraction of the gallbladder sends bile into the?
Duodenum through the common bile duct, along with a watery solution of bicarbonate and digestive enzymes from the pancreas
The gallbladder is an organ that is not essential for normal digestion, and if the duct becomes blocked by gallstones, the gallbladder can be?
Removed without creating long term problems
Chewing and churning create what?
Smaller pieces of food with more surface area exposed to digestive enzymes
The pH at which different digestive enzymes function best reflects?
The location where they are most active
Enzymes that act in the stomach work well at?
Those enzymes secreted in the small intestine work best in?
Most absorption takes place in the?
Small intestine, with additional absorption of water & ions in the large intestine
The surface area for absorption is greatly increased by?
The presence of fingerlike villi and by the brush border on the luminal surface of enterocytes, created from numerous microvilli on each cell
The crypts also add to surface area, but the crypt cells are specialized for?
Fluid and hormone secretion
Once absorption of nutrients & ions across the GI epithelium, most of the nutrients enters?
The capillaries within the villi. The exception is fats, which mostly enter lymph vessels called lacteals
Digestion and absorption are not directly regulated except in a few instances. Instead they are influenced primarily by?
Motility and secretion in the digestive tract, the two processes that in turn are regulated by hormones, the nervous system, and local control mechanisms
About half the calories the average American ingests are in the form of?
Carbohydrates, mainly starch & sucrose (table sugar)
Intestinal carbohydrate transport is redistricted to?
Monosaccharides, which means that all complex carbohydrates & disaccharides must be digested if they are absorbed
We are unable to digest cellulose because?
We lack the necessary enzymes.
The complex carbohydrates we can digest are? How?
Starch & glycogen. The enzyme amylase breaks long glucose polymers into smaller glucose chains & into disaccharide maltose. Maltose & other disaccharides are broken down by intestinal brush border enzymes known as disaccharidases (maltase, sucrase, lactase)
The end products of carbohydrate digestion are?
Glucose, galactose, and fructose
Intestinal glucose and galactose absorption uses
Transporters identical to those found in the renal proximal tubule
What are the transporters for glucose and galactose?
The apical sodium glucose symporter (SGLT) & the basolateral GLUT-2 transporter. These transporters move galactose as well as glucose
Fructose absorption is not?
Sodium dependent. Fructose moves across the apical membrane by facilitated diffusion on the GLUT-5 transporter & across the basolateral membrane on GLUT-2
Unlike carbohydrates, which are ingested in forms ranging from simple to complex, most ingested proteins are?
Polypepties or larger. Not all proteins are equally digested by humans
The enzymes for protein digestion are classified into two broad groups
Endopeptidases and exopeptidases
What are endopeptidases, more commonly called proteases
They attack peptide bonds in the interior of the amino acid chain and break a long peptide chain into smaller fragments
Proteases (endopeptidases) are secreted as?
Inactive proenzymes from epithelial cells in the stomach, intestine, and pancreas and are activated in the GI tract lumen
Examples of proteases include
Pepsin secreted in the stomach, & trypsin & chymotrypsin secreted in the pancreas
Single amino acid from peptides by chopping them off the ends, one at a time
The most important digestive exopeptidases are two isozymes of carboxypeptidase secreted by the?
The primary products of protein digestion are?
Free amino acids, dipeptides, all of which can be absorbed
Most free amino acids are carried by?
Sodium dependant cotransport proteins similar to those in the proximal tubule of the kidney
Dipeptides and tripeptides are carried into the mucosal cell on the oligopeptide transporter PepT1 that uses hydrogen dependent cotransport. Once inside the cell the oligopeptides have two possible fates
Most are digested by cytoplasmic peptidases into amino acids, which are then transported across the basolateral membrane & into the circulation.
Those that are not digested are transported intact across the basolateral membrane on a hydrogen dependent exchanger
The transport system that moves oligopeptides also is responsible for?
Intestinal uptake of certain drugs, including beta-lactam antibiotics, angiotensin-converting enzyme inhibitors, and thrombin inhibitors
Some peptides larger than three amino acids are absorbed by?
Transcytosis after binding to membrane receptors on the luminal surface of the intestine
In newborns, peptide absorption takes place primarily in?
Intestinal crypt cells
One of the most common antigens responsible for food allergies is?
Gluten, a component of wheat
Fats and related molecules in the Western diet include.
Triglycerides, cholesterol, phospholipids, long-chain fatty acids, and the fat-soluble vitamins
Nearly ___% of our fat calories come from triglycerides because they are the primary form of lipid in both plants and animals
Fat digestion is complicated by the fact that most lipids are not particularly water soluble. As a result, the aqueous chyme leaving the stomach contains a coarse emulsion of large fat droplets, which have less surface area than smaller peptides. To increase the surface area available for enzymatic fat digestion, the liver secretes?
Bile salts into the small intestine. Bile salts help break down the coarse emulsion into smaller, more stable particles
Bile salts, like phospholipids of cell membranes, are amphipathic meaning that they have both a hydrophobic region and a hydrophilic region. The hydrophobic region of bile salts associate with the?
The polar side?
Surface of lipid droplets, while the polar side chains interact with water, creating a stable emulsion of small, water-soluble fat droplets
Enzymatic fat digestion is carried out by?
Lipases, enzymes that remove two fatty acids from act triglyceride molecule. Result is one monoglyceride & two fatty acids
The bile salt coating of the intestinal emulsion complicates digestion because?
Lipase is unable to penetrate the bile salts. For this reason fat digestion also requires colipase, a protein cofactor secreted by the pancreas
Fat digestion requires a protein cofactor secreted by the pancreas called co-lipase, what is the function?
Co-lipase displaces some bile salts, allow lipase access to fats inside the bile salt coating
Phospholipids are digested by?
As enzymatic and mechanical digestion proceed, fatty acids, bile salts, monoglycerides, phospholipids, & cholesterol from?
Small disk-shaped micelles
Because fats are lipophilic many are absorbed by?
Fatty acids and monoglycerides move out of their micelles and diffuse where?
Across apical membrane into the epithelial cells
What happens once monoglycerides and fatty acids are inside the enterocytes?
They move to the smooth endoplasmic reticulum where they recombine into triglycerides
Macromolecules broken down: Complex form of protein?
Building blocks of protein?
Macromolecules broken down: Complex form of
Polysaccharides: Glycogen, starch
Building blocks: Monosaccharides: glucose, fructose, galactose
Macromolecules broken down: Complex form of fat?
Complex form of fat: Triglyceride
Building blocks: 3 fatty acids + glycerol
When food comes into the mouth, the feedforward cephalic vagal reflex begins?
Secretion in the stomach
In the stomach parietal cells deep in the gastric glands secrete?
Gastric acid (HCl)
What is the stomachs gastric acid (HCl)? (4 things)
It has multiple functions:
1) Activates pepsin, the enzyme that digests proteins
2) It denatures the proteins by breaking disulfide & hydrogen bonds that hold the protein in its tertiary structure. This is important because unfolding the protein chains makes the peptide bonds between amino acids accessible to pepsin
3) Helps kill bacteria & other ingested microorganisms
4) Inactivates salivary amylase, which stops carbohydrate digestion that began in the mouth
What are the enzymes secreted by the stomach?
1) Chief cells in the gastric gland secrete the inactive enzyme pepsinogen, which is cleaved to activate pepsin in the lumen of the stomach by the action of hydrogen
2) Gastric lipase is co-secreted with pepsin
What is pepsin?
An endopeptidase that carries out the initial digestion of proteins. Particularly effective on collagen and therefore plays an important role in digesting meat (protein)
Less than ___% of fat digestion takes place in the stomach
What are the paracrines secreted by the stomach?
1) Enterochromaffin-like (ECL) cells secrete histamine, a paracrine signal that promotes acid secretion by parietal cells.
2) D-cells secrete somatostatin (SS), known as growth hormone-inhibiting hormone. This is the primary negative feedback signal for gastric phase secretion. It shuts down acid secretion directly & indirectly & also inhibits pepsinogen secretion
Hormone secretion in the stomach
G-cells found deep in the gastric glands, secrete the hormone gastrin into the blood. Gastrin release is stimulated by the presence of amino acids and peptides in the stomach, by dissension of the stomach, and by neural reflexes mediated by gastrin-releasing peptide
Acid in the stomach lumen stimulates?
Acid also triggers?
1) Pepsinogen release from chief cells through a short reflex. In the lumen acid converts pepsinogen into pepsin, & protein digestion begins
2) Triggers somatostatin release from D cells which acts via negative feedback to inhibit secretion of gastric acid, gastrin, histamine, & pepsinogen
The stomach has 3 general functions
1) Storage. The stomach stores food and regulates its passage into the small intestine, where most digestion & absorption takes place
2) Digestion- The stomach chemically and mechanically digests food into the soupy mixture of uniformly small particles called chyme
3) Protection- Protects the body by destroying many of the bacteria & other pathogens that are swallowed with food or trapped in airway mucus. At the same time the stomach must protect itself from being damaged by its own secretions
Before food even arrives, digestive activity in the stomach begins with the?
Long vagal reflex of the cephalic phase. Then once food enters the stomach, stimuli in the gastric lumen initiate a series of short reflexes that constitute the gastric phase of digestion.
In gastric phase reflexes, distension of the stomach and the presence of peptides air amino acids in the lumen activate endocrine cells & enteric neurons. Hormones, neurocrine secretions, & paracrine molecules then influence motility and secretion
What happens when food arrives from the esophagus to the stomach?
The stomach relaxes & expands to hold the increased volume. This neurally mediated reflex is called receptive relaxation
What happens if we ingest more than we need form a nutritional standpoint? Why is this important?
The stomach must regulate the rate at which food enters the small intestine.
Without this regulation the small intestine would not be able to digest & absorb the load presented to it, & significant amounts of unabsorbed chyme would pass into the large intestine. The epithelium of the large intestine is not designed for large-scale nutrient absorption, so most of the chyme would pass out in the feces, resulting in diarrhea.
While the upper stomach is quietly holding food, the lower stomach is busy with digestion. In the distal half of the stomach, a series of?
Peristaltic waves pushes the food down towards the pylorus, mixing food with acid and digestive enzymes. As large food particles are digested to the more uniform texture of chyme, each contractile wave squirts a small amount of chyme through the pylorus into the duodenum. Enhanced gastric motility during a meal is primarily under neural control & is stimulated by dissension of the stomach
The secretory cell of the gastric mucosa, mucous neck cells, secrete what?
1) Mucus-physical barrier between lumen & epithelium
2) Bicarbonate- Buffers gastric acid to prevent damage to epithelium
The secretory cell of the gastric mucosa, Parietal cells, secrete what?
1) Gastric acid (HCl)- Activates pepsin; kills bacteria
2) Intrinsic factor- Complexes with vitamin B12 to permit absorption
The secretory cell of the gastric mucosa, enterochromaffin-like cell (paracrine), secretes what?
Histamine- Stimulates gastric acid secretion
The secretory cell of the gastric mucosa, chief cells, secretes what?
1) Pepsin(ogen) (enzyme)- Digests proteins
2) Gastric lipase (enzyme)- Digests fat
The secretory cell of the gastric mucosa, D-cells, secretes what?
Somatostatin- Inhibits gastric acid secretion
The secretory cell of the gastric mucosa, G-cells, secretes what?
Gastrin- Stimulates gastric acid secretion
Under normal conditions, the stomachs gastric mucosa protects itself from acid & enzymes by a?
Mucus-bicarbonate barrier. Mucus cells in the neck of gastric glands secrete both substances
Describe how the stomachs gastric mucosa layer protects itself?
The mucus forms a physical barrier, & the bicarbonate creates a chemical buffer barrier underlying the mucus
What is the pH of bicarbonate in the stomach?
Close to 7, even when the pH in the lumen is 2.