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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

100 mL


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


Functions of the Digestive System

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

Small intestine


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

The colon


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

The rectum


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
2) Submucosa
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?

Gastric glands


The most variable feature of the GI tract that changes from section to section

Epithelial cells


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?

Developing cancer


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

Lamina proporia


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?

Digestive function


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.

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


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

Peristaltic contractions


Responsible for mixing

Segmental contractions


Spontaneous depolarizations in GI smooth muscle

Slow waves


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


Peristalsis is?

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?

9 liters
2 liters
7 liters
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?

Renal tubule


The basolateral membrane of the GI tract contains?

The ubiquitous sodium-potassium pump


What does motility include?

1) Ingestion
- putting food into the mouth
2) Mastication
- Chewing, mixing with saliva
3) Deglutition
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?

1) Stomach
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

(FIGURE 21.5A)
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


Enzymes are?

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


Mucus is?

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

Goblet cells


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?

Acidic pH


Those enzymes secreted in the small intestine work best in?

Alkaline pH


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


Exopeptidases release?

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?

Pancreatic phopholipase


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?

Simple diffusion


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?

Amino acids


Macromolecules broken down: Complex form of
Building blocks?

Polysaccharides: Glycogen, starch
Building blocks: Monosaccharides: glucose, fructose, galactose


Macromolecules broken down: Complex form of fat?
Building blocks?

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.


When does mucus secretion increase?

When the stomach is irritated, such as by the ingestion of aspirin or alcohol