CH 22~The Digestion System
Introduction
• Digestive system – set of organs that breaks down food items into nutrients that can be delivered by bloodstream to cells in body
• Digestive system consists of two types of organs:
1. organs of alimentary canal (gastrointestinal (GI) tract or digestive tract)
organs are located from head to abdominopelvic cavity
2. accessory organs
Liver, pancreas, and gall bladder
Introduction
• Alimentary canal – continuous tube through which food passes directly
consists of oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine
• Accessory organs – not part of alimentary canal but assist in digestion in some way; located around alimentary canal and include teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Food generally does not come into direct contact with accessory organs, although teeth and tongue are exceptions
Basic Digestive Functions
• Most fundamental function of digestive system is to take food into body and break it down into its component nutrients so that they can be used by body cells
-Other functions include:
Playing a critical role in fluid, electrolyte, and acid-base homeostasis by taking in water and electrolytes via diet and delivering them to blood
Ingesting vitamins and minerals, producing hormones, and excreting metabolic wastes
Basic Digestive Processes
-Digestive system must perform following basic processes to carry out these functions:
Food and water are brought into digestive system by ingestion
Occurs via mouth under normal conditions
-Secretion – digestive organs contain both endocrine and exocrine glands that secrete a variety of substances—such as mucus, enzymes, acid, and hormones—to aid other digestive processes
-Ingested food and liquids pass from one digestive organ to next by process of propulsion
Propulsion is accomplished largely by rhythmic contractions of smooth muscle of alimentary canal called peristalsis
aided by mucus secreted by multiple organs
-Food breakdown occurs by process of digestion:
Mechanical digestion
Chemical digestion
-Once food particles are digested, they move through wall of alimentary canal into blood or lymphatic vessels by a process called absorption
Water, electrolytes, and vitamins are also absorbed into blood in same manner
-Certain ingested materials are not digestible or usable by body
such materials continue their transit through alimentary canal until they exit body as feces through defecation
Defecation also provides body with a way to eliminate certain metabolic wastes
note that defecation is simply a specialized form of propulsion
Regulation of Motility
-Motility, or movement, of alimentary canal is a key process in every region of the canal
In oral cavity, pharynx, superior portion of esophagus, and last portion of large intestine, motility is due to skeletal muscle
In rest of alimentary canal, motility is work of smooth muscle
-Each type of motility is regulated by nervous system and/or endocrine system
Nervous system regulation of motility consists of nerves of autonomicnervous system, or ANS
Enteric nervous system, or ENS is a self-contained branch of autonomic nervous system that extends from esophagus to anus (terminal portion of large intestine)
No innervation with the CNS
The Peritoneal Membranes
-Digestive organs that reside in abdominopelvic cavity share a common set of serous membranes, blood vessels, and nerves:
Peritoneal membranes – largest serous membrane in body
-Consists of following two layers:
Outer parietal peritoneum lines inner surface of body wall
Where parietal peritoneum meets abdominal organs, it folds in on itself to become inner visceral peritoneum, or serosa; forms outer tissue layer of such organs
Between these two peritoneal layers we find peritoneal cavity; contains serous fluid
Organization of Abdominopelvic Digestive Organs
• Organs located entirely within peritoneal cavity are intraperitoneal
• Other abdominal organs are retroperitoneal or partly or completely outside peritoneal cavity
-Visceral peritoneum folds over on itself around certain organs, particularly small intestine, to form structures called mesenteries
Mesenteries support and bind these organs together and keep small intestine in a particular shape that fits within abdominopelvic cavity
Mesenteries also house blood vessels, nerves, and lymphatic vessels, anchoring them in place; mesentery attached to much of large intestine is often called mesocolon
• Two mesenteries are especially prominent:
Greater omentum – unique among mesenteries in that it consists of four layers of folded visceral peritoneum; extends from base of stomach down into pelvis
Lesser omentum – smaller mesentery that extends from medial surface of stomach to liver
Organs of Digestion
-Digestive system begins at oral cavity, or mouth, where four different digestive processes occur:
ingestion
Secretion
chemical and mechanical digestion
propulsion
- Although oral cavity is technically part of alimentary canal, it houses two accessory organs: teeth and tongue
- In addition, three pairs of accessory organs, salivary glands, are located in and around oral cavity
These organs turn ingested food into a moist, chewed mass called a bolus; posterior oral cavity, pharynx, and esophagus then deliver bolus to stomach through process of swallowing or deglutition
The Teeth and Tongue
•-With assistance of masseter and temporalis muscles, teeth masticate, or chew, ingested food, grinding it into smaller pieces
Mastication aids digestion by increasing overall surface area of food, giving digestive enzymes more places to catalyze reactions of chemical digestion
-Tongue is involved during ingestion phase of digestion
Movement of tongue during chewing helps turn food into a bolus
-Tongue also pushes the food against hard palate during chewing, which assists in mechanical digestion, and also push bolus posteriorly during swallowing
Salivary Glands
-Three pairs of salivary glands secrete saliva, a fluid containing water, enzymes, mucus, and other solutes, through a duct into oral cavity
• Two main types of acinar cells in salivary glands:
Serous cells secrete a water-based fluid with enzymes and other solutes; secretions are involved in digestive processes; generally released just before or during eating
Mucous cells secrete mucus; primarily involved in keeping oral mucosa moist, and so are released continually
Saliva
• Saliva consists primarily of:
Water
electrolytes such as sodium, chloride, and potassium ions
variable amounts of mucus, depending on type of salivary gland
• also contains following components:
Salivary amylase – first digestive enzyme that ingested food encounters; catalyzes beginning of carbohydrate digestion, breaking down large polysaccharides into smaller polysaccharides
Enzyme lysozyme catalyzes perforation of bacterial plasma membranes; allows bacteria-killing substances in saliva to enter and kill bacteria
Secretory IgA – antibody immunoglobulin A, or IgA, is found in body’s secretions, including saliva; IgA binds specific antigens on pathogens and mediates their destruction
Bicarbonate ions function is to neutralize (buffer) any acid from stomach that regurgitates into esophagus
Functions of Saliva
• Saliva performs several functions in oral cavity, including:
Moistening, lubricating, and cleansing oral mucosa
Lysozyme and IgA deter growth of pathogenic bacteria in oral cavity
Mechanical digestion, achieved by moistening and helping to mix ingested food into a bolus so it can be swallowed
Chemical digestion, achieved by actions of salivary amylase
Many food molecules dissolve in water of saliva, and these molecules then stimulate taste receptors on tongue
Pharynx
-Pharynx (throat) – only oropharynx and laryngopharynx are part of alimentary canal
• Oropharynx houses two sets of tonsils—
1. palatine tonsils
2. lingual tonsils
Tonsils perform defensive functions; protect remainder of alimentary canal from any pathogens that enter body via oral and nasal cavities
-Primary function of pharynx is propulsion in form of swallowing,
during which bolus passes through pharynx and into esophagus
The Esophagus
-Esophagus – muscular tube about 25 cm (10 in.) long found posterior to trachea; transports a bolus from pharynx to stomach
Esophageal mucosa, lined with stratified squamous nonkeratinized epithelium, contains esophageal glands, as does submucosa, which secrete mucus to lubricate bolus as it passes through esophagus
At junction of pharynx and esophagus, muscularis externa is modified into a sphincter called upper esophageal sphincter; controls passage of bolus into esophagus
At esophagus’ inferior end is another sphincter, gastroesophageal sphincter, also known as lower esophageal sphincter; regulates passage of bolus into stomach
-also prevents contents of stomach from re-entering esophagus
-Primary functions of esophagus are propulsion and a small amount of secretion of mostly mucus
During swallowing, skeletal muscle and smooth muscle of muscularis undergo peristalsis; pushes bolus inferiorly
Thick esophageal epithelium protects esophagus from abrasion by food but also prevents any significant absorption from taking place (example of Structure-Function Core Principle)
Swallowing or Deglutition
-Swallowing, or deglutition, is a specialized type of propulsion that pushes a bolus of food from oral cavity through pharynx and esophagus to stomach
-Swallowing relies on coordinated action of upper alimentary canal, including soft palate, pharynx, and esophagus; tongue also plays a role in this process, and is only accessory organ to directly participate in motility
•Consists of three phases:
voluntary phase: the tongue Pushes the bolus posteriorly towards the oropharynx.
pharyngeal phase: the bolus enters the oropharynx; the soft palate and epiglottis seal off the nasopharynx and larynx, respectively.
esophageal phase: peristaltic waves move the bolus down the esophagus to the stomach.
~Quick Checks~
1. Define alimentary canal and accessory organs and list organs that belong to each.
Alimentary canal: continuous tube through which food passes directly
Mouth
pharynx
esophagus
stomach
small intestine
large intestine.
Accessory organs: – not part of alimentary canal but assist in digestion in some way
Teeth
tongue
salivary glands
liver
gallbladder
pancreas.
2. Describe the major functions of the digestive system.
Ingestion of food
Digestion of food
Absorption of nutrients
Elimination of wastes
Helps maintain fluid, electrolyte, and acid-base balance
3. Describe the six basic processes of the digestive system.
Ingestion
Secretion
Propulsion
Digestion
Absorption
Defecation
Briefly explain regulation of motility by the endocrine and nervous systems.
Nervous system: ANS and ENS control digestive movements.
Endocrine system: Hormones regulate digestive activity and motility.
5. Describe the peritoneal membranes and the peritoneal cavity. Define intraperitoneal, retroperitoneal and mesenteries.
Parietal peritoneum: Lines abdominal wall.
Visceral peritoneum: Covers organs.
Peritoneal cavity: Space between layers containing serous fluid.
Intraperitoneal: Inside cavity.
Retroperitoneal: Outside/behind cavity.
Mesenteries: Peritoneal folds that support organs and carry blood vessels, nerves, and lymphatics.
6. Discuss the basic functions of the oral cavity, the teeth, and the tongue. Include definitions of bolus, deglutition and mastication.
Oral cavity: Ingestion, digestion, and swallowing.
Teeth: Chew food.
Tongue: Forms and moves food for swallowing.
Bolus: Chewed mass of food.
Mastication: Chewing.
Deglutition: Swallowing.
7. List the two types of cells in a salivary gland and explain what product each produces and when. List the three pairs of salivary glands.
Serous cells: Produce watery enzyme-rich secretions.
Mucous cells: Produce mucus.
Three pairs:
Parotid
submandibular
sublingual glands.
8. Describe the composition and functions of saliva.
Composition:
Water
electrolytes
mucus
salivary amylase
lysozyme
IgA
bicarbonate.
Functions:
Lubricates food
cleans mouth
begins carbohydrate digestion
helps taste.
9. Describe the role of the pharynx in regards to digestion.
Passageway that moves the bolus from the mouth to the esophagus during swallowing.
10. Describe the structure and functions of the esophagus including the roles of the upper esophageal sphincter and the gastroesophageal sphincter.
Muscular tube that carries food to the stomach.
Upper esophageal sphincter: Controls entry into esophagus.
Gastroesophageal (lower esophageal) sphincter: Controls entry into stomach and prevents reflux.
11. Explain the process of deglutition, including the three phases.
Voluntary phase: Tongue pushes bolus backward.
Pharyngeal phase: Bolus moves through pharynx; airway closes.
Esophageal phase: Peristalsis moves bolus to stomach.
Gross Anatomy of the Stomach
•Esophagus pierces diaphragm and passes through esophageal hiatus to empty into J-shaped organ known as stomach
• Lesser & greater curvatures
• Form longitudinal wrinkles called gastric rugae when empty
• 5 anatomical regions
• Region where esophagus empties into stomach is cardiac
• receives bolus when gastroesophageal sphincter relaxes
• Fundus is domed portion superior to esophageal opening
• Body is main portion of organ
• Pyloric region is narrow inferior end
• antrum & pyloric canal
• Terminal portion of stomach is pylorus, which abuts duodenum (first portion of small intestine)
• Pyloric sphincter- controls flow of ingested food between stomach and small intestine
Histology of the Stomach
• Stomach has same four tissue layers as rest of alimentary canal
with exceptions:
• Stomach has an additional oblique layer of smooth muscle allows stomach to perform churning, a motion that pummels food into a liquid called chyme
• Mucosa of stomach is heavily indented to form deep structures called gastric pits
• Contains columnar cells and numerous goblet cells, both of which secrete a thick mucus that lines and protects cells of stomach from its own secretions
• Gastric glands, found at base of gastric pits, are multiple branched glands that contain both endocrine cells that secrete hormones into bloodstream and exocrine cells that secrete an acidic, enzyme-containing fluid called gastric juice into lumen of stomach
-There are four main types of cells in gastric glands, each of which secretes a different product
1. Located at bottom of glands are enteroendocrine cells
secrete hormones that influence digestion
Ex: Gastrin which stimulates acid secretion
2. Chief cells secrete inactive precursor enzyme pepsinogen; when pepsinogen encounters an acidic pH, it becomes active enzyme pepsin, which begins protein digestion in stomach
3. Parietal cells secrete hydrochloric acid (HCl)
responsible for acidic pH of gastric juice
Acid is an important component of gastric juice because it activates pepsinogen, destroys many disease-causing organisms, and stimulates parietal cell production of chemical intrinsic factor; required for intestinal absorption of vitamin B12
4. Cells located near top, or “neck,” of gland are called mucous neck cells
secrete acidic mucus that prevents neutralization of acid produced by parietal cells
Functions of the Stomach
-Stomach performs three primary functions:
1. Secretion
2. Propulsion
3. Digestion
-Acid secretion from stomach – gastric glands of stomach secrete multiple exocrine products, including hydrochloric acid (HCl) released by parietal cells
Acid secretion occurs continuously throughout day between meals, at what is known as basal rate
-During eating, secretion can be divided into three phases:
1. cephalic phase
2. gastric phase
3. intestinal phase
Cephalic Phase
-Initial cephalic phase, mediated by sight, smell, taste, or even thought of food under direction of CNS
Prepares stomach to receive food by increasing release of hydrogen ions into it
Gastric Phase
• Gastric phase begins when food enters stomach and continues stimulation provided during cephalic phase
• Two stimuli trigger acid secretion during gastric phase:
• First, presence of food in stomach distends stomach wall, which stimulates neurons of ENS and sensory receptors involved with vagus nerve reflexes
• Second stimulus is presence of partially digested proteins in gastric juice
• stimulates enteroendocrine cells to produce and release gastrin-stimulates acid secretion
Intestinal Phase
• Intestinal phase – final phase of gastric acid secretion that is responsible for only about 10% of remaining acid secretion, after which it inhibits further acid secretion
• Triggered by presence of partially digested proteins in fluid entering duodenum; triggers enteroendocrine cells to release intestinal gastrin
This hormone has same effect as gastrin produced by stomach
To stimulate hydrogen ion secretion from parietal cells
• Stimulatory effect of intestinal phase is brief
• As chyme enters duodenum, declining pH and presence of lipids trigger enterogastric reflex that decreases vagal activity and acid secretion
• Low pH in duodenum also triggers production of hormones by cells of duodenal mucosa, including secretin and gastric inhibitory peptide (GIP); both hormones reduce acid secretion
Motility of the Stomach
• Stomach’s motility enables it to perform three actions:
1. receive food from esophagus
2. churn incoming bolus into chyme
3. control rate at which chyme empties into small intestine
-Receptive function – in its resting state, stomach has a volume of only about 50 ml (0.2 cup), but it can expand to about 1500 ml (6.3 cups) when filled with food and liquid
When food or liquid is swallowed, gastroesophageal sphincter and smooth muscle of fundus and body of stomach relax to allow stomach to fill, a process known as receptive relaxation
• Churning function – after a meal, smooth muscle layers of stomach begin to produce waves of peristalsis; initiated and controlled by a group of specialized cells collectively called gastric pacemaker
Peristalsis propels bolus to pylorus, where small amounts of chyme are propelled through pyloric sphincter into duodenum
Remainder of chyme is pushed backward into stomach where peristaltic waves churn and mix chyme, and process repeats
Pacemaker controls rate of peristaltic waves, which remains relatively constant at about three per minute
Emptying function – final function of gastric motility is to control movement of chyme into duodenum
-Different materials pass through pyloric sphincter at different rates
Liquids move rapidly from stomach to duodenum with essentially no delay
Solids must be converted to a nearly liquid state before they are able to enter small intestine
Key factor that determines rate of gastric emptying is amount and composition of chyme in duodenum
Emptying Function
• Control of gastric emptying is critical because duodenum must mix incoming chyme thoroughly before it moves to rest of small intestine
• Two reasons: first chyme is acidic, and duodenum must mix it with bicarbonate ions to avoid damaging intestinal mucosa
• Second, chyme is generally very concentrated and must be diluted with water from pancreatic juice to prevent chyme from drawing water into intestinal lumen by osmosis
• Duodenum can only process incoming chyme so quickly, and therefore must receive small amounts at a time
• About 3 ml at a time (injection engine)
Gastrocolic Reflex
• Triggers mass movements in the colon in response to the filling of the stomach with food.
~Queck Check~
1. Describe the histology of the stomach and the cells of the gastric glands. Include the functions of the types of cells the gastric glands contain. Define chyme
Mucous cells: Secrete mucus to protect the stomach lining.
Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
Chief cells: Secrete pepsinogen (inactive protein-digesting enzyme).
Enteroendocrine cells: Secrete hormones such as gastrin.
Chyme: Semi-liquid mixture of partially digested food and gastric secretions.
Describe the acid secretion function of the stomach including the three regulatory phases. Define enterogastric reflex.
Cephalic phase: Food sight, smell, taste, or thought stimulates acid secretion.
Gastric phase: Food enters the stomach, causing increased acid secretion.
Intestinal phase: Food enters the small intestine, reducing stomach acid secretion.
Enterogastric Reflex
A reflex that slows stomach activity and gastric emptying when the small intestine becomes full or receives acidic chyme.
3. Explain the motility of the stomach in terms of its receptive function, churning function and emptying function.
Receptive function: Stomach relaxes and expands to store food.
Churning function: Muscular contractions mix food with gastric juices to form chyme.
Emptying function: Small amounts of chyme are released through the pyloric sphincter into the duodenum.
Introduction to Small Intestines
• Small intestine (small bowel) – 6 meters long (almost 20 feet)
longest portion of alimentary canal
• Four main processes occur in small intestine:
1. Secretion
2. Digestion
3. Absorption
4. Propulsion
• Cells of small intestine, known as enterocytes, produce multiple digestive enzymes, hormones, and mucus
• Enzymes, along with those released by pancreas, are responsible for bulk of chemical digestion
• No further nutrient digestion takes place by human enzymes once food leaves small intestine (bacteria in large intestine do digest certain nutrients)
• After nutrients are digested chemically, they are absorbed across enterocytes into either blood or lymph, along with water, vitamins, and other substances
• Small intestine also mixes and propels its contents along its length and into large intestine
Gross Anatomy of Small Intestine
• Small intestine has three divisions distinguished by histological differences:
1. Duodenum – initial segment; begins at pylorus; shortest of three divisions
2. Jejunum – middle; begins at duodenojejunal flexure and sits within peritoneal cavity
most active site for chemical digestion and absorption
3. Ileum – small intestine’s final segment
• terminates at portion of large intestine called cecum
• Ileocecal valve – sphincter that controls movement of materials from ileum into cecum
• Also prevents materials in large intestine from flowing backward into ileum
• Critical for preventing access to bacteria from large intestine
Structure of the Small Intestine
• Internal surface of small intestine contains three progressively smaller types of folds
• most absorption that occurs takes place in this region
• folds increase surface area available for absorption about 400 to 600 times
• Largest type of folds, visible with naked eye as ridges in wall, are called (1) circular folds
• Folds also slow down transit of chyme through small intestine
• gives nutrients more time to be digested, and small intestine cells (enterocytes) more time to absorb nutrients
• The smaller two types of folds are not visible to naked eye
• Mucosa folds into projections called (2) villi
• each villus consists of a layer of enterocytes surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal
• Between villi mucosa indents to form intestinal crypts
• house glands with both enteroendocrine cells and mucus-secreting goblet cells
• Smallest folds, (3) microvilli, are found in plasma membrane of enterocytes
• Each enterocyte has as many as 3000 microvilli
• gives cell appearance of a bristle brush, or brush border
• Digestive enzymes produced and secreted by enterocytes catalyze reactions that break down disaccharides and peptides
Motility of the Small Intestine
• Small intestinal motility differs during fasting and eating in following way:
• During fasting, small intestine exhibits slow, rhythmic contractions along its length in a pattern called migrating motor complex
• These contractions clear any remaining material, including leftover food and secretions
• Requires about 2 hours to push digesting food from duodenum to ileocecal valve
• Controlled by both ENS and a hormone called motilin; produced by cells in duodenal mucosa Motility of the Small Intestine
• Small intestine undergoes two types of movement during eating: 1-peristalsis and 2- segmentation:
• Peristalsis is accomplished by alternating contractions of longitudinal and circular layers of smooth muscle in muscularis externa
• Primary function is to propel chyme toward ileum and ultimately through ileocecal valve to cecum Motility of the Small Intestine
• Segmentation, also known as intestinal churning, involves contractions of only circular layer of smooth muscle
• Produces a “squeezing” motion
• Primary functions of segmentation are mechanical digestion and mixing chyme with intestinal and pancreatic enzymes as well as bile
~Quick Check~
1. Describe the functions of the small intestine and discuss the histology and functions of the small intestine’s circular folds, villi, and microvilli.
Main functions: digestion and absorption of nutrients.
Circular folds (plicae circulares): increase surface area and slow movement of chyme.
Villi: fingerlike projections that increase surface area for absorption.
Microvilli: tiny projections on epithelial cells that form the brush border and further increase surface area for absorption and digestion.
2. Describe motility in the small intestine during fasting and eating.
During fasting: migrating motor complexes move residual materials through the intestine.
During eating: segmentation mixes chyme with digestive secretions, and peristalsis moves chyme toward the large intestine.
Introduction to Large Intestines
• Large intestine (large bowel) runs along border of abdominal cavity, surrounding small intestine and other abdominal organs like a frame
• Measuring about 1.5 meters (5 feet) long
• named because it has a larger diameter than small intestine
• Receives material from small intestine that was not digested or absorbed, and is a passageway for feces or fecal matter to exit body
• Active in absorbing water and electrolytes
• critical for maintaining fluid, electrolyte, and acid-base homeostasis
• Tasks of large intestine also include secretion (in form of mucus), propulsion, and defecation
• Houses numerous bacteria that perform important functions such as synthesizing vitamins
Anatomy of the Large Intestine
• Anatomical features of different regions of large intestines are as follows:
• Cecum, first portion of large intestine, is a blind pouch
• located in right lower quadrant of abdomen; intraperitoneal
• Features a smaller blind-ended pouch extended from its posteroinferior end, called vermiform appendix
• generally shortened to simply appendix
• Next and longest portion of large intestine is colon
• Divided into four portions:
1. Retroperitoneal ascending colon travels superiorly along right side of abdomen from right lower quadrant to right upper quadrant when it reaches liver, it makes a sharp left-hand turn at a junction called hepatic flexure (right colic flexure)
2. At hepatic flexure, ascending colon becomes intraperitoneal transverse colon, because it passes transversely across superior abdominal cavity
• at spleen, it takes a sharp turn inferiorly at a junction called splenic flexure (left colic flexure)
• Splenic flexure gives rise to retroperitoneal descending colon; passes along left side of abdominal cavity
• In left lower quadrant, descending colon becomes S-shaped sigmoid colon; intraperitoneal and passes toward sacrum
• Muscularis externa of large intestine is unique in that its longitudinal layer is not continuous throughout most of its length
• This layer is gathered into three bands or ribbons of muscle called taeniae coli
• Their constant tension bunches colon into pockets referred to as haustra
• Serosa, or visceral peritoneum, of large intestine contains fat-filled pouches known as epiploic appendages
After sigmoid colon, large intestine continues as rectum
• this portion of large intestine runs anterior to sacrum and is retroperitoneal
• Walls of rectum feature horizontal folds called rectal valves that allow passage of flatus (gas) without risking simultaneous passage of feces
• Rectum ends at anal canal, last portion of large intestine
Histology of the Large Intestine
• Large intestine exhibits following histological features:
• Mucosa of large intestine lacks villi and its cells lack microvilli
• These structural adaptations reflect fact that nutrient absorption is not large intestine’s primary function
• Mucosa is rich with goblet cells that secrete protective and lubricating mucus
Bacteria in the Large Intestine
• Large intestine hosts a staggering number of bacteria
•about 10 times more bacteria in large intestine than cells in human body
•bacteria make up as much as 60% of dry mass of feces
• These bacteria, normal flora (gut flora) consists of about 500 different bacterial species that coexist with humans in a symbiotic (mutually beneficial) relationship
-Functions:
• Produce vitamins – bacteria produce vitamins such as vitamin K, which is necessary for blood clotting
• Metabolize undigested materials – bacteria metabolize carbohydrates such as soluble fibers that small intestine is unable to digest, converting them into fatty acids and other molecules body can absorb and use
• Deter growth of harmful bacteria – normal flora prevents growth of pathogenic (disease-causing) microorganisms by competing for nutrients and producing chemicals that kill certain harmful bacterial species
• Stimulate immune system – during infancy, normal flora induces immune tolerance to their own antigens
• at same time, they stimulate development of mucosa-associated lymphoid tissue (MALT) and production of antibodies to pathogens
Motility of the Large Intestine
• Large intestine has two functional segments:
• proximal large intestine consists of ascending and transverse colon
•distal large intestine consists of descending and sigmoid colon, rectum, and anal canal
• Proximal large intestine is primary site of water and electrolyte absorption and bacterial activity, and exhibits two main types of motilities
• Distal large intestine performs a small amount of absorption, primarily of water, but its main role is to store fecal material until it is ready to be expelled during defecation
• Two main types of motility of proximal segment:
1. First is a type of segmentation (churning)
• Circular muscle of each haustrum contracts repeatedly; swirls material around in haustrum
• Aids in water and electrolyte absorption primarily controlled by local neurons of ENS; triggered by stretch
2. Other type of motility in proximal large intestine is a propulsive motion known as a mass movement (mass peristalsis)
• During a mass movement, multiple haustra undergo peristalsis; propelstheir contents toward distal large intestine
• Mass movements occur three to four times per day, and appear to be triggered by food consumption; initiates reflexes controlled by the enteric nervous system (ENS)
• Distal large intestine is much less motile than proximal large intestine
• when mass movements force fecal material into normally empty rectum, it initiates parasympathetic-mediated defecation reflex
~Quick Checks~
1. Describe the functions of the large intestine and discuss the histology of the large intestines.
Functions: Absorption of water and electrolytes, formation and storage of feces, and defecation.
Histology: Contains simple columnar epithelium with many goblet cells that secrete mucus; lacks villi.
2. Describe the functions of the bacteria within the large intestines.
Ferment undigested carbohydrates.
Produce vitamins, especially vitamin K and some B vitamins.
Help prevent growth of harmful bacteria.
3. Describe roles of the proximal and distal portions of the large intestines and explain the motility of the large intestine.
Proximal large intestine: Absorbs water and electrolytes and forms feces.
Distal large intestine: Stores feces until elimination.
Motility: Includes haustral churning, peristalsis, and mass movements that move feces toward the rectum for defecation.
The Pancreas
•Pancreas – gland with both endocrine and exocrine functions
• Its endocrine secretions, hormones insulin and glucagon affect most cells in body
• Its exocrine secretions are digestive enzymes secreted primarily by clusters of acinar cells
• Enzymes are released into ducts of pancreas that empty into alimentary canal
• Main pancreatic duct travels down middle of pancreas where it which receives secretions from acinar cells
• Near duodenum, main pancreatic duct merges with duct from liver and gallbladder, after which it empties into duodenum
• Smaller accessory pancreatic duct also used
• Pancreatic juice – collective secretions of pancreatic acinar and duct cells
• consists of water and multiple digestive enzymes and other proteins
• Duct cells also secrete bicarbonate ions, a base, which make pancreatic juice alkaline
• Alkaline juice helps neutralize acidic chyme that enters duodenum from stomach and protects duodenum from damage by acid
• Its digestive enzymes, secreted by acinar cells, are crucial in chemical digestion
• Enzymes catalyze reactions that digest carbohydrates, proteins, lipids, and nucleic acids
• Pancreatic secretion occurs at a basal rate between meals; during eating, pancreatic secretion rises due to parasympathetic and hormonal stimulation
The Liver and Gallbladder
• Pyramid-shaped liver is located in right upper quadrant just inferior to diaphragm surface
•Gallbladder is a small sac found on posterior side of liver
•Both organs have close anatomical and functional relationships:
• Anatomy of liver– covered by a thin connective tissue capsule and visceral peritoneum; composed of four lobes
• Large right lobe and left lobe are located on right and left sides of liver, respectively; separated by a fold of visceral peritoneum called falciform ligament; also anchors liver to anterior abdomen
• Round ligament – on liver’s inferior surface; remnant of umbilical vein that was present in fetus
• Smaller caudate lobe and quadrate lobe are located on posterior side of liver’s right lobe
• Porta hepatis – indentation found on liver’s posterior side where numerous blood vessels enter and exit liver, including hepatic artery, hepatic portal vein, and hepatic veins
• Nerves, lymphatic vessels, and common hepatic duct also enter and exit via porta hepatis
Histology of the Liver
• Liver lobule – basic unit of liver, has following histological features
• Composed of flattened plates of cells, called hepatocytes arranged in shape of a hexagon and stacked on top of one another with a small central vein in each lobule’s center
• Portal triad – at each of six corners of lobule
• composed of following three structures: a hepatic arteriole, a portal venule, and a small bile duct that carries bile
• Hepatic arterioles and portal venules both drain into large, leaky capillaries, called hepatic sinusoids that pass between rows of hepatocytes
• Sinusoids have walls that are not continuous
• allows large substances to enter and exit from bloodstream
The Liver and Gallbladder
• Blood flows slowly through sinusoids as materials are exchanged between blood and hepatocytes, eventually draining into central vein
• Central veins merge and drain into hepatic veins, which in turn feed into inferior vena cava
• Bile flows through liver lobule in opposite direction, from hepatocytes into tiny ducts (bile canaliculi) which eventually drain into a bile duct
Liver Functions
• Functions of liver – liver has one of most diverse sets of functions of any organ in body
• Releases both endocrine and exocrine secretions, and converts harmful chemicals into nontoxic substances that body can eliminate
• Main digestive function is to produce bile
• Bile- a liquid that contains multiple components, including water, electrolytes, and organic compounds
• Bile serves two critical functions – required for digestion and absorption of lipids, and mechanism by which liver excretes wastes and other substances that kidneys cannot excrete
• Liver produces bile continually but generally does not secrete it at a basal rate
• Liver excretes bilirubin; a waste product that results from breakdown of hemoglobin by spleen
• Some remains in blood and is excreted by kidneys
• (note that yellow urobilinogen is largely responsible for color of urine)
• Liver performs a host of other functions, including:
• Nutrient metabolism – processes nutrients obtained from diet
• Detoxification – detoxifies substances produced by body, and substances that we eat or drink, some of which are harmful to body
• Excretion – directly excretes bilirubin in bile along with other substances liver processes, particularly certain antibiotics; also modifies substances so that they can be excreted by kidneys
Bile
• One of bile’s main organic compounds is bile salts (derived from cholesterol)
• Bile salts are amphiphilic, meaning they have both polar and nonpolar parts; allows them to interact with both lipids and watery environment of small intestine
• Coat lipids in duodenum and physically break them apart into smaller pieces, a process known as emulsification; mechanical digestion necessary for chemical digestion and absorption of lipids
• Bile also contains varying amounts of materials that liver excretes, including cholesterol, waste products, and toxins such as heavy metals
Gallbladder
• Gallbladder – small sac that sits on posterior liver
• Receives most of bile from common hepatic duct
• Stores bile, concentrates it (removing water), and releases it when stimulated
• Bile release is stimulated by CCK
• triggers contraction of smooth muscle in wall of gallbladder, causing gallbladder to release bile into cystic duct
• Cystic duct joins common hepatic duct to form common bile duct and joins main pancreatic duct near duodenum to form hepatopancreatic ampulla
• Ampulla is surrounded by a ring of smooth muscle called hepatopancreatic sphincter; controls emptying of bile and pancreatic fluids into duodenum
• Bile contains calcium salts and cholesterol
• both can precipitate and form hard lumps called gallstones
~Quick Checks~
1. Describe the pancreas and its digestive functions.
The pancreas is an accessory digestive organ.
It secretes pancreatic juice into the duodenum.
Pancreatic juice contains:
Bicarbonate ions to neutralize acidic chyme.
Digestive enzymes that break down carbohydrates, proteins, fats, and nucleic acids.
2. Describe liver functions that pertain to digestion. Explain the role of bile. Define emulsification.
The liver's major digestive function is the production of bile.
Bile helps digest fats by breaking large fat globules into smaller droplets.
Emulsification is the process of breaking large fat droplets into many small droplets, increasing surface area for lipase action.
3. Describe the other functions of the liver.
Processes and stores nutrients.
Stores glycogen, vitamins, and minerals.
Removes toxins and drugs from the blood.
Destroys old red blood cells.
Produces plasma proteins.
Helps regulate blood glucose levels.
4. Describe the function of the gallbladder and trace the pathway of bile to the small intestine.
The gallbladder stores and concentrates bile.
Pathway of bile:
Liver produces bile.
Bile enters the right and left hepatic ducts.
Hepatic ducts join to form the common hepatic duct.
Bile travels through the cystic duct to the gallbladder for storage.
When needed, bile leaves the gallbladder through the cystic duct.
It enters the common bile duct.
It empties into the duodenum of the small intestine.
Overview of Digestion and Absorption
• Process of digestion breaks food down into smaller and smaller pieces until only individual nutrient molecules remain
• Two types of digestion:
1. Mechanical digestion, in which food is physically broken into smaller parts
• All nutrients go through same mechanisms of mechanical digestion—mastication in mouth, churning in stomach, and segmentation in small intestine
2. Chemical digestion, in which chemical bonds between food molecules are broken
• Chemical digestion varies for each nutrient because digestive enzymes are specific for a single type of bond in a single type of nutrient
• Most digestive enzymes catalyze hydrolysis reactions that use a water molecule to break a bond between two molecules
• This explains why so much water is secreted with fluids like gastric and pancreatic juices
• water molecules are key components of reactions that chemically break down food molecules
• Digestive enzymes speed up these reactions by a process known as enzymatic hydrolysis
• Once nutrients are digested into component molecules, they must enter body
Digestion and Absorption of Carbohydrates
Digestion begins in mouth with help of salivary amylase from salivary glands; catalyzes reactions that break long polysaccharides into shorter oligosaccharides:
• Generally, accomplishes little actual chemical digestion because food simply is not in mouth long enough for it to have much effect
• Salivary amylase digestion continues in stomach, but stomach acid generally inactivates this enzyme before it can complete task
• Chemical digestion of carbohydrates resumes in small intestine, where polysaccharides and oligosaccharides encounter pancreatic amylase
• In step 2: the end result of activity of all these enzymes is production of monosaccharides glucose, fructose, and galactose
• Brush border-*including enzymes lactase, maltase, and sucrase
-Fructose is unable to bind Na+/glucose
cotransporter…Absorbed by facilitated diffusion
Once in blood, they are delivered to liver via hepatic portal vein for processing
Step by step
polysaccharides are broken into oligoand disaccharides in reaction catalyzed by pancreatic amylase
brush border enzymes catalyzed the breakdown of disaccharides into monosaccharides
the na+/k+ creates a gradient for na+ absorption from the fluid in the lumen
this gradient drives the secondary active transport of glucose and galactose via the na+/glucose cotransporter
fructose is absorbed by facilitated diffusion.
all three monosaccharides cross the basal side of the enterocyte membrane and the diffuses in to the blood
Digestion and Absorption of Proteins
• Chemical digestion of proteins does not begin until they reach stomach, where they encounter enzyme pepsin
• Chief cells of gastric glands produce inactive precursor pepsinogen
• Pepsinogen requires a pH of about 2 to become pepsin, and pepsin is inactivated completely at a pH of 7
• Activated pepsin catalyzes reactions that digest proteins into smaller polypeptides, oligopeptides, and some free amino acids
• Pepsin reactions catalyze only a small amount of proteins in ingested food
Majority of protein digestion takes place in small intestine with help of pancreatic enzymes and brush border enzymes
• First pancreatic enzyme to become activated is precursor trypsinogen
• It becomes active enzyme trypsin when it encounters enzymes on intestinal brush border
• Like carbohydrates, once amino acids are in the blood, they are delivered to liver via hepatic portal vein for processing
• Majority (90%) of lipids taken in by diet are triglycerides
• consist of three fatty acid molecules bound to a glycerol core molecule
• Nonpolar lipids tend to stick together, forming large globules rather than distributing evenly in a water-based solution
• Lipid globules can be broken up into smaller particles in two ways:
1. Mechanical digestion—mastication in mouth, churning in stomach, and segmentation in small intestine
2. Bile salts in small intestine emulsify lipids
• gives digestive enzymes enough surface area on which to work to efficiently digest lipids
Absorption of Lipids
• Lipids are not delivered directly to liver via hepatic portal vein after absorption
• Hepatic portal vein delivers leftover bile salts to liver, where they are used to make new bile
• Lipids travel within chylomicrons through lymphatic vessels and eventually to thoracic duct, where they join blood with rest of lymph
• As chylomicrons travel through blood capillaries, lipids are progressively removed and enter cells, where they can be used for a number of different purposes
Digestion and Absorption of Nucleic Acids
• Nucleic acids in food we eat begin chemical digestion in small intestine with help of pancreatic enzymes called nucleases
• Nucleases catalyze reactions that break nucleic acids into individual nucleotides
• Further digestion occurs via brush border enzymes
• remove phosphate group and sugar from nucleotide to leave a phosphate ion, ribose or deoxyribose, and a nitrogenous base
• These three molecules are absorbed via primary and secondary active transport mechanisms into capillaries in villi
Absorption of Water and Electrolytes
• Alimentary canal absorbs large quantities of water, electrolytes, and vitamins
• Much of this absorption occurs in small intestine, but a significant quantity occurs in large intestine too
• On average, over 9 liters of water enter small intestine each day
• About 2 liters of water are ingested, and remaining 7 liters are secreted into alimentary canal by alimentary canal itself and accessory organs
• Of this 9 liters, about 8 liters are absorbed into enterocytes of small intestine
• Most of remaining water is absorbed into enterocytes of large intestine, leaving only about 0.1 liter of water to be excreted in feces
Absorption of Vitamins
• Vitamins – chemicals generally provided by diet that are involved in a host of metabolic reactions
• Two types of vitamins:
• Water-soluble vitamins, which are polar molecules
• Most water-soluble vitamins are absorbed in small intestine by diffusing through enterocytes’ plasma membranes
• B complex and C
• Fat-soluble vitamins, which are lipid-based and mostly nonpolar
• Fat-soluble vitamins are packaged into micelles with fats and other lipids and are absorbed with them
• A, D, E, K
~Quick Check~
1. Explain the two types of digestion. Explain the process of enzymatic hydrolysis reactions of nutrient molecules.
Mechanical digestion: Physical breakdown of food into smaller pieces (chewing, churning, segmentation).
Chemical digestion: Breakdown of food molecules by enzymes.
Enzymatic hydrolysis: Water is added to nutrient molecules, breaking chemical bonds and producing smaller molecules that can be absorbed.
2. Describe the digestion and absorption of carbohydrates, proteins, lipids and nuclei acids. Include the names of all enzymes involved. Define micelle.
Carbohydrates
Digestion
Salivary amylase
Pancreatic amylase
Brush-border enzymes: maltase, sucrase, lactase
Absorption
Monosaccharides (glucose, fructose, galactose) are absorbed into blood capillaries.
Proteins
Digestion
Pepsin (stomach)
Trypsin
Chymotrypsin
Carboxypeptidase
Aminopeptidase
Dipeptidase
Absorption
Amino acids are absorbed into blood capillaries.
Lipids
Digestion
Lingual lipase
Gastric lipase
Pancreatic lipase
Bile aids digestion by emulsifying fats.
Absorption
Fatty acids and monoglycerides combine with bile salts to form micelles.
Lipids enter intestinal cells, become chylomicrons, and move into lymphatic vessels.
Micelle: A small spherical structure made of bile salts that transports fatty acids and monoglycerides to the intestinal epithelium.
Nucleic Acids
Digestion
Pancreatic nucleases
Nucleosidases
Phosphatases
Absorption
Nitrogenous bases, sugars, and phosphate groups are absorbed into the blood.
3. Describe the absorption of water, electrolytes and both types of vitamins.
Water
Absorbed by osmosis through the intestinal wall.
Electrolytes
Sodium, potassium, chloride, calcium, phosphate, and iron are absorbed by diffusion or active transport.
Fat-Soluble Vitamins
Vitamins A, D, E, and K are absorbed with lipids through micelles and chylomicrons.
Water-Soluble Vitamins
Vitamins B and C are absorbed by diffusion or active transport directly into the blood.
Quick Memory Aid
Carbs → Monosaccharides
Proteins → Amino acids
Lipids → Fatty acids + Monoglycerides
Nucleic acids → Bases + Sugars + Phosphates
ADEK = Fat-soluble vitamins