Human Anatomy & Physiology - Weebly



Basic Anatomy & Physiology II Dr. L. Bacha

Chapter Outline (Marieb & Hoehn 6th ed)

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( two groups of organs form the digestive system (see Fig. 22.1):

1. alimentary canal

( what is it also called? gastrointestinal (GI) tract

( it is a continuous muscular tube that winds through the body, from the mouth to the anus

( list the organs that make up alimentary canal:

mouth, pharynx, esophagus, stomach, small intestine, large intestine

( why is food material in this tube technically outside the body?

because the canal is open to the external environment at both ends

2. accessory digestive organs

( list the accessory digestive organs:

teeth, tongue, gallbladder, and a number of large digestive glands - salivary glands, liver, & pancreas

22.1 What major processes occur during digestive system activity?

List and read about the 6 essential activities of the digestive system (shown next to the colored squares on p. 747 – 748; also see Figs. 22.2 and 22.3):

( Ingestion

( Propulsion

( peristalsis is the major means of propulsion and involves what?

alternating waves of contraction and relaxation of muscles in the organ walls

- what is its main effect?

to squeeze food along the tract, but some mixing occurs as well

( Mechanical breakdown

( what does mechanical breakdown increase and what does it prepare food for?

- increases the surface area of ingested food

- prepares food for digestion by enzymes

( what do mechanical processes include?

chewing, mixing food with saliva by the tongue, churning food in the stomach, and segmentation

- describe segmentation:

rhythmic local constrictions of the small intestine

( what is the function of segmentation?

mixes food with digestive juices and makes absorption more efficient by repeatedly moving different parts of the food mass over the intestinal wall

( Digestion

( digestion nvolves what?

- a series of catabolic steps in which enzymes secreted into the lumen (cavity) of the alimentary canal breakdown complex food molecules to their chemical building blocks

( Absorption

( absorption is the passage of digested end products (plus vitamins, minerals, and water) from the lumen of the GI tract into what?

the blood or lymph

( Defecation

( defecation eliminates indigestible substances from the body as feces

22.2 The GI tract has four layers and is usually surrounded by peritoneum

Relationship of the Digestive Organs to the Peritoneum

( most digestive organs reside in the abdominopelvic cavity

( the abdominopelvic cavity contains the peritoneal cavity, which is lined by serous membranes called the peritoneum

( what does the visceral peritoneum cover?

the external surfaces of most digestive organs

( what does the parietal peritoneum line?

the body wall

( name the slitlike potential space between the parietal and visceral peritoneum:

the peritoneal cavity

( the serous membranes lining the peritoneal cavity continuously produce a serous fluid called peritoneal fluid

- the peritoneal fluid in the peritoneal cavity lubricates the mobile digestive organs and allows them to do what?

glide easily across one another and along the body wall as they carry out their activities

( mesenteries are sheets of peritoneum that suspend organs of the abdominopelvic cavity to the body wall and attach some organs to others

( what are the functions of mesenteries?

- provide routes for blood vessels, lymphatics, and nerves to reach the digestive viscera

- hold organs in place

- store fat

Histology of the Alimentary Canal (see Fig. 22.5)

The walls of organs of the gastrointestinal tract from the esophagus to the anal canal have the same 4 basic layers:

1 The Mucosa

∙ the mucosa is a mucous membrane that lines the lumen of the alimentary canal from the mouth to the anus; read about the mucosa’s major functions

∙ there are 3 layers of the mucosa:

a . epithelium - lines the lumens of the organs; the type varies with the organ (stratified squamous or simple columnar)

b. lamina propria – loose areolar connective tissue

c. muscularis mucosae – thin layer of smooth muscle

2 The Submucosa

∙ the submucosa consists of connective tissue

∙ it contains the submucosal plexus, a nerve plexus that controls the activity of glands of the GI tract and smooth muscle of the muscularis mucosae

3 The Muscularis Externa

∙ what is this layer responsible for?

segmentation and peristalsis

∙ formed by either skeletal muscle (e.g. mouth, pharynx, part of the esophagus) OR smooth muscle (e.g., part of the esophagus, stomach, intestines)

∙ the muscularis externa is arranged in 2 layers (sometimes 3):

1) inner circular layer

2) outer longitudinal layer

∙ in several places along the gastrointestinal tract, the circular layer thickens, forming sphincters that act as what?

valves to control food passage from one organ to the next and prevent backflow

∙ the muscularis externa contains the myenteric plexus, a nerve plexus that controls motility of the GI tract

4 The Serosa

( serosa (= visceral peritoneum)

∙ it covers many organs in the abdominopelvic cavity

∙ it is the outermost layer; what is it formed of?

areolar connective tissue covered with mesothelium, a single layer of squamous epithelial cells

( adventitia

∙ in some cases, the outermost layer is an adventitia rather than a serosa

∙ an adventitia is formed by connective tissue only; not covered by a epithelium; the connective tissue blends with that of surrounding structures

∙ e.g., binds the esophagus to surrounding structures in the thoracic cavity

Here is a sketch to show the four layers (and sublayers) of the wall of a digestive organ:

Blood Supply: The Splanchnic Circulation

( read about the splanchnic circulation

22.3 The GI tract has its own nervous system called the enteric nervous system

Enteric Nervous System

( read about the enteric nervous system, which participates in both short and long reflex arcs (see Fig. 22.7)

⦁ Short reflexes – occur entirely within the GI wall

- are mediated entirely by enteric neurons in response to what?

stimuli within the GI tract

- the control of segmentation and peristalsis is largely due to short reflexes

⦁ Long reflexes – involve the central nervous system and extrinsic autonomic nerves

- generally speaking, what is the effect of parasympathetic inputs?

enhance digestive activities

- what is the effect of the sympathetic nervous system on digestive activities?

inhibits them

Basic Concepts of Regulating Digestive Activity

Read about the three key concepts that govern regulation of digestive activity!

22.4 Ingestion occurs only at the mouth

The Mouth

( what is the mouth also called?

the oral cavity, or buccal cavity

( what forms the superior boundary (the roof) of the mouth?

the palate

( posteriorly, the oral cavity is continuous with what?

the oropharynx

( read about the lips, cheeks, and the palate (

The Tongue

• the tongue is composed of interlacing bundles of what?

skeletal muscle fibers

• describe the functions of the tongue:

- grips the food and constantly repositions it between the teeth

- mixes food with saliva forming it into a compact mass called a bolus

- initiates swallowing by pushing the bolus posteriorly into the pharynx

- helps us form consonants when we speak

• intrinsic muscles of the tongue are skeletal muscles within the tongue; they are not attached to bone; intrinsic muscles allow the tongue to do what?

change its shape (but not its position), becoming thicker, thinner, longer, or shorter as needed for speech and swallowing

• extrinsic muscles of the tongue are skeletal muscles that originate on bones of the skull or the soft palate and insert onto the tongue

- they alter the position of the tongue (protrude it, retract it, move it side to side)

• define lingual frenulum and indicate its functions:

= a fold of mucosa secures the tongue to the floor of the mouth and limits its posterior movements

• the upper surface and sides of the tongue are covered with a variety of projections called lingual papillae (see Fig. 22.9), some of which contain microscopic taste buds

The Salivary Glands

• the salivary glands secrete components of saliva

• list the functions of saliva (p. 754):

- cleanses the mouth

- dissolves food chemicals so they can be tasted

- moistens food and helps compact it into a bolus

- contains the enzymes amylase that begins the digestion of starchy foods

• there are 3 pairs of extrinsic (major) salivary glands that secrete saliva into ducts that empty into the mouth (note their location in Fig. 22.10):

1. parotid glands

- describe the location of the parotid gland:

lies anterior to the ear between the masseter muscle and the skin

- each parotid duct opens into the oral cavity next to what tooth?

second upper molar

2. submandibular glands

- the submandibular gland lies along what?

the medial aspect of the mandibular body

- its ducts opens into the floor of the oral cavity at the base of what?

the lingual frenulum

3. sublingual glands

- the sublingual gland lies under what? the tongue

- it has numerous ducts that open into what? the floor of the mouth

Composition of Saliva

( mostly water; also contains electrolytes, mucin, lysozymes, IgA antibodies, and metabolic wastes

( name the two digestive enzymes in saliva:

salivary amylase and lingual lipase

Control of Salivation

( the parasympathetic nervous system stimulates the secretion of watery,

enzyme rich saliva

( the sympathetic nervous system stimulates the release of a thick, mucin-rich saliva;

strong activation of the sympathetic division almost completely inhibits release of saliva

The Teeth

• the roots of teeth are located in sockets (alveoli) of what two bones?

mandible and maxilla

• what is another word for chew? masticate

Dentition and Dental Formula

( 2 sets of teeth in humans (see Fig. 22.11):

( primary dentition - consists of deciduous teeth

( permanent dentition - consists of permanent teeth

( describe the features and functions of the four different types of teeth:

∙ incisors -

- chisel-shaped

- adapted for cutting or nipping off pieces of food

∙ canines (cuspids) -

- conical or fanglike

- tear and pierce

∙ premolars (bicuspids) and molars -

- have broad crowns with rounded cusps (tips)

- best suited for grinding or crushing

( the dental formula is a shorthand way of indicating what?

the numbers and relative positions of the different types of teeth

- write the dental formula for the primary and permanent dentition:

Tooth Structure

( locate the following parts on Fig. 22.12:

∙ crown

- define the crown of a tooth:

enamel covered; is the exposed part of the tooth above the gingiva

∙ gingiva – gum

∙ enamel

- covers the crown of a tooth; what makes enamel the hardest substance in the body?

it is heavily mineralized with calcium salts, and its densely packed hydroxyapatite (mineral) crystals are oriented in force-resisting columns perpendicular to the tooth’s surface

∙ root

- define the root of a tooth:

the portion of the tooth embedded in the jawbone

∙ neck

- define the neck of a tooth:

constricted region that connects the crown and root

∙ cementum (cement) - a calcified connective tissue that covers the dentin of the root

∙ periodontal ligament

- fibrous connective tissue attached to the cementum

- anchors the tooth in the alveolus and acts as a shock absorber

∙ dentin

- describe dentin:

a protein-rich bonelike material, underlies the enamel cap and forms the bulk of the tooth

- dentin also acts as a shock absorber

∙ pulp cavity - a central cavity, surrounded by dentin, that contains pulp

(soft tissue with connective tissue, blood vessels and nerves)

∙ root canal - the extension of the pulp cavity through the root of the tooth

∙ apical foramen - an opening at the proximal end of each root

- what does the apical foramen allow?

allows blood vessels, nerves, and other structures to

enter the pulp cavity

Digestive Processes of the Mouth

( read about the 4 digestive processes that the mouth and its accessory organs are involved in

Mastication (Chewing)

( the mechanical breakdown of food begins with chewing in the mouth

22.5 The pharynx and esophagus move food from the mouth to the stomach

The Pharynx

( the oropharynx and laryngopharynx are both common passageways for what?

food, fluids, and air

( the pharynx is lined by mucous membrane; it has skeletal muscles that are involved in swallowing

The Esophagus

( describe the esophagus:

a muscular tube about 25 cm long and is collapsed when not involved in food propulsion

( name the opening in the diaphragm through which the esophagus pierces to enter the abdomen:

esophageal hiatus

( some features of interest:

∙ the mucosa of the esophagus has a nonkeratinized stratified squamous epithelium

∙ the circular layers of muscle in the muscularis externa in the superior and inferior parts of the esophagus are normally thickened and contracted, functioning as sphincters:

( upper esophageal sphincter - is between the pharynx and esophagus

( gastroesophageal sphincter (lower esophageal sphincter or cardiac sphincter) - is between the esophagus and the stomach

Digestive Processes: Swallowing

• the pharynx and esophagus merely serve as what?

conduits to pass food from the mouth to the stomach

∙ deglutition (swallowing) is a complicated process that involves the coordinated activity of what?

coordinated activity of over 22 separate muscle groups

∙ there are two major phases of deglutition (Fig. 22.14): the buccal phase and the pharyngeal-esophageal phases

1. BUCCAL PHASE:

( the buccal phase is voluntary and occurs where?

in the mouth

( what does the tongue do (as indicated in blue print in step 1 of Fig. 22.14 on p. 761)?

the tongue presses against the hard palate, forcing the food bolus into the oropharynx

2. PHARYNGEAL-ESOPHAGEAL PHASE:

a. Pharyngeal Phase:

( involuntary; begins when the bolus passes into oropharynx

( various cranial nerves, most importantly the vagus nerves, transmit what?

motor impulses from the swallowing center to the muscles of the pharynx and esophagus

( when food enters the pharynx, respiration is momentarily inhibited

( during the pharyngeal phase:

- the tongue blocks the mouth

- the soft palate and uvula rise to close off the nasopharynx

- the larynx rises so that the epiglottis covers the opening into the larynx

- the upper esophageal sphincter relaxes

- this allows food to enter the esophagus as muscles of the pharynx contract

b. Esophageal Phase:

( the bolus is moved along the esophagus toward the stomach by

wavelike peristaltic contractions (see Fig. 22. 3 (a) on p. 749)

( the gastroesophageal sphincter relaxes to allow the bolus to enter the stomach

22.6 The stomach temporarily stores food and begins protein digestion

( the chemical breakdown of what begins in the stomach? proteins

( in the stomach, food is converted to a paste called what? chyme

( the stomach lies in the upper left quadrant of the abdominopelvic cavity

Gross Anatomy of the Stomach

( an empty stomach has a volume of 50 ml, but fully distended, it can hold about 4 liters

( when empty, the mucosa and submucosa of the stomach form large, longitudinal folds called what? rugae

( list the four main regions of the stomach and note their location in Fig. 22.15 (a):

the cardiac part, or cardia, the fundus, the body

the pyloric part

( the pyloric sphincter (pyloric valve) is between the pyloric region of the stomach and the duodenum

- it controls what?

stomach emptying

Microscopic Anatomy of the Stomach (see Fig. 22.16)

( features of the mucosa of the stomach:

∙ the lining epithelium of the stomach mucosa is a simple columnar epithelium composed entirely of cells that produce alkaline mucus

∙ gastric pits

- invaginations of the surface epithelium

( cells that line the gastric pits undergo cell division to continuously replenish the epithelium of the stomach

∙ gastric glands

- gastric glands are continuations of the gastric pits lined by epithelial cells that secrete gastric juice

Types of Gland Cells that form the gastric glands varies with the region of the stomach:

( parietal cells and chief cells are found in the gastric glands in the fundus and body regions:

( Parietal Cells

- what two substances do they secrete?

hydrochloric acid and intrinsic factor

- indicate the various functions of the extremely acidic contents of the stomach:

- necessary for activation and optimal activity of protein-digesting enzyme pepsin

- also helps digest food by denaturing proteins and breaking down cell walls of plant foods

- is harsh enough to kill many of the bacteria

ingested with foods

- intrinsic factor is required for what?

vitamin B12 absorption in the small intestine

( Chief Cells

- what substance do chief cells produce?

pepsinogen, the inactive form of pepsin

- chief cells also secrete what?

lipases (fat-digesting enzymes)

( gastric glands in the cardiac and pyloric regions

( are mainly formed by cells that produce mucus

( enteroendocrine cells are also present in gastric glands

- these are cells that produce hormones, such as histamine, serotonin, and gastrin

The Mucosal Barrier

( Definitely read about how the mucosa of the stomach acts as a barrier!

( from Table 22.2 on page 767, list the three major functions of the stomach:

1 mechanical breakdown and propulsion; peristaltic waves mix food with gastric juice and propel it into the duodenum

2 digestion: pepsin begins the digestion of proteins

3 absorption: absorbs a few fat soluble substances (aspirin, alcohol, some drugs)

Digestive Processes the Stomach (p. 764)

( chemical digestion in the stomach

( proteins begin to be broken down by the enzyme pepsin (formed from pepsinogen, which is inactive)

- remember what cells of the gastric glands produce pepsinogen? chief cells

( gastric lipases secreted by chief cells of the stomach and lingual lipase from salivary glands both have a minor role in digesting lipids in the stomach

( what is the stomach’s only function that is essential to life?

the secretion of intrinsic factor

( what is intrinsic factor required for?

intestinal absorption of vitamin B12

( what is vitamin B12 needed for?

to produce erythrocytes

Regulation of Gastric Secretion

The production of acid and enzymes by the gastric mucosa is controlled by the nervous system and hormones:

( neural controls

- consist of both long reflexes (parasympathetic vagus nerve mediated) and short (local enteric) reflexes

- in each case, what is released to stimulate output of gastric juice?

acetylcholine (Ach)

- in contrast, activation of sympathetic nerves does what?

depresses secretory activity

( hormonal control of gastric secretion is mainly due to the hormone gastrin

THERE ARE THREE OVERLAPPING PHASES OF GASTRIC SECRETION THAT OCCUR DURING DIGESTION:

• CEPHALIC PHASE

( begins before food enters the stomach

( triggered by the aroma, taste, sight, or thought of food

( during this phase, parasympathetic nerves stimulate the salivary glands to

secrete saliva and stimulate gastric glands (via the vagus nerve) to secrete gastric juice

• GASTRIC PHASE

( begins when food enters the stomach

( distension of the stomach by food, partially digested proteins in the stomach and an increase in pH (due to food entering the stomach) activate the parasympathetic nervous system, enteric neurons, and the release of the hormone gastrin from the stomach, which collectively:

∙ increase gastric secretion by the gastric glands

∙ increase motility of the stomach

∙ relax the pyloric sphincter to promote gastric emptying

• INTESTINAL PHASE

( begins when chyme enters the duodenum

( stimulation: initial distension of the duodenum stimulates cells of the intestine to release intestinal gastrin, a hormone which stimulates gastric glands to continue secretion of gastric juice for a brief period

( inhibition: continued presence of chyme in the duodenum stimulates sympathetic nerves and the production of hormones by the small intestine (secretin, cholecystokinin, and gastric inhibitory peptide), which collectively:

∙ decrease production of gastric juice by the gastric glands

∙ inhibit gastric motility and gastric emptying

∙ stimulate secretion of pancreatic juice and release of bile from the gallbladder

Regulation of Gastric Motility and Emptying (p. 768)

( the muscularis externa of the stomach has 3 layers of smooth muscle tissue

( stomach contractions of the muscularis externa not only accommodate its filling and cause its emptying, but they also do what?

compress, knead and mix the food with gastric juice to produce chyme

( each wave of peristalsis causes a small amount of chyme to be forced through the pyloric sphincter into the duodenum (first part of the small intestine) to cause gastric emptying, so that gradually the stomach empties chyme into the duodenum

23.7 The liver secretes bile; the pancreas secretes digestive enzymes

The Liver

Gross Anatomy of the Liver

( the liver is the second largest organ of the body

( it is wedge-shaped and is located just inferior to the diaphragm

( the liver has how many primary lobes? four

( blood supply of the liver:

( the liver receives blood from two sources:

1. hepatic artery – carries oxygenated blood

2. hepatic portal vein - carries deoxygenated, nutrient-rich blood from systemic capillaries of digestive organs (stomach, intestines, etc) to sinusoids of the liver

Microscopic Anatomy of the Liver (see Fig. 22.24)

( microscopic lobules of the liver are roughly hexagonal (six-sided) structures consisting of plates of liver cells called hepatocytes, which are organized like bricks in a garden wall

- the hepatocyte plates radiate outward from a what?

a central vein running in the longitudinal axis of the lobule

( portal triads are located in the connective tissue where three or more lobules meet

∙ list the three basic structures that a portal triad contains:

1. a branch of the hepatic artery proper (supplying oxygen-rich arterial blood to the liver)

2. a branch of the hepatic portal vein (carrying venous blood laden with nutrients from the digestive viscera)

3. a bile duct

( other components of a lobule:

- liver sinusoids alternate with the plates of hepatocytes and radiate from the central vein in the center of a lobule

(blood from branches of the hepatic artery and hepatic portal vein flows through the sinusoids and empties into the central vein

- hepatic macrophages (Kupffer cells) occupy the sinusoids

- what do these phagocytic cells remove?

debris such as bacteria and worn-out blood cells from the blood as it flows past

( functions of hepatocytes:

- hepatocytes produce most plasma proteins (albumin, clotting factors, complement, etc.)

- indicate the four things hepatocytes can do (( listed next to the four blue bullets in the top right column on p. 773):

⦁ Secrete some 900 ml of bile daily

⦁ Process bloodbourne nutrients in various ways (e.g., they store glucose as glycogen and use amino acids to make plasma proteins

⦁ Store fat-soluble vitamins

⦁ Play important roles in detoxification, such as ridding the blood of ammonia by converting it to urea

Bile: Composition and Enterohepatic Circulation

( features of bile:

1. what color is bile? yellow-green

2. it is alkaline (pH 7.6 to 8.6)

3. contains water, ions, cholesterol, etc. (NO digestive enzymes!)

4. contains bile salts (which are the only substances in bile with a digestive function)

a. emulsify fat globules into fat droplets

- bile salts disperse (emulsify) large clumps of fat molecules called fat globules into smaller clumps called fat droplets so they can more readily be chemically digested by enzymes

b. play a role in the absorption of long chain fatty acids, glycerol, cholesterol, and fat soluble vitamins by coating them, forming micelles

5. contains bile pigments, such as bilirubin, which is a normal product of RBC destruction and is excreted into bile by hepatocytes

( duct system that carries bile (( see Fig. 22.27 on p. 777):

∙ bile secreted by hepatocytes enters into bile canaliculi, which are tiny intercellular canals formed by the cell membranes of adjacent hepatocytes

( bile canaliculi lead into bile ducts at the periphery of lobules

( bile ducts merge and eventually exit the liver as the common hepatic duct

( joins with the cystic duct from the gallbladder to form the common bile duct

( the common bile duct merges with the pancreatic duct and leads into the hepatopancreatic ampulla that opens into the duodenum; the opening is controlled by the hepatopancreatic sphincter

The Gallbladder

• a thin-walled muscular sac that may appear green when filled with bile

• about the size of a kiwi fruit (about 10 cm (4 in) long, it snuggles in a shallow fossa on the inferior surface of the liver from which its rounded fundus protrudes

• the gallbladder stores and concentrates bile until it is needed in the small intestine;

- when the small intestine is empty, a valve around the common bile duct closes and bile backs up into the cystic duct to the gallbladder for storage

The Pancreas

• why is the pancreas important to digestive processes?

- because it produces enzymes that break down all categories of foodstuffs

• the pancreas extends from the right side of the body between the C-shaped duodenum, across the abdomen, to the left side near the spleen

• the endocrine tissues, called pancreatic islets, are scattered among the acini and contain cells that release two important hormones: insulin and glucagon

• the exocrine part of the pancreas is formed by clusters of epithelial cells called pancreatic acini (singular is “acinus”) that secrete pancreatic juice (see Fig. 22.25)

- pancreatic juice is carried by the pancreatic duct and the accessory pancreatic duct from the pancreas to what specific organ (see Fig. 22.27)?

duodenum

Composition of Pancreatic Juice

( pancreatic juice consists of:

∙ water

∙ bicarbonate ions (sodium bicarbonate), which makes pancreatic juice slightly alkaline

- the slightly alkaline secretion is important because the enzymes that function in the small intestine do not function in an acid environment

∙ digestive enzymes:

- the cells of the pancreatic acini produce a variety of digestive enzymes that function in the small intestine!

Bile and Pancreatic Secretion into the Small Intestine

Anatomy of Duct Systems

( review the duct system in Fig. 22.27

Regulation of Bile and Pancreatic Secretion

( read about how hormones and neural stimuli regulate both the secretion of bile and pancreatic juice

22.8 The small intestine is the major site for digestion and absorption

( most chemical digestion and absorption of nutrients takes place in the small intestine!

Therefore, it has structural adaptations for these functions:

1. great length - in the living person, the small intestine is 2 to 4 meters (7-13 ft) long

2. circular folds

3. villi

4. microvilli

Gross Anatomy (of the Small Intestine)

( list the three subdivisions of the small intestine:

duodenum; jejunum; ileum

- the ileocecal valve (sphincter) is between the ileum and the cecum of the large intestine

Microscopic Anatomy

Modifications of the Small Intestine for Absorption

( remember, most chemical digestion and absorption of nutrients takes place in the small intestine, so there are special features of the small intestine that facilitate chemical digestion and absorption of nutrients: its great length, circular folds, villi, and microvilli

• Circular folds - permanent folds of the mucosa and submucosa; about 1 cm tall (Fig. 22.29 a)

- these circular folds greatly increase surface area for chemical digestion and absorption

- they cause chyme to spiral slowly through the small intestine, to increase time for digestion and absorption

• Villi (singular is “villus”)

- villi are finger-like projections of the mucosa (Fig. 22.29)

- how high are the villi?

over 1 mm high

- consists of a core of lamina propria covered by simple columnar epithelium with goblet cells

- the lamina propria of villi contains blood capillaries and a lacteal (a lymphatic capillary) for absorption of nutrients

• Microvilli

( the simple columnar epithelium of the mucosa of the small intestine contains simple columnar absorptive epithelial cells (called enterocytes) that have microvilli

- all together the microvilli form the brush border

( microvilli are fingerlike projections of the cell membrane and cytoplasm of the epithelial cells

∙ the cell membrane of the microvilli bears enzymes referred to a what?

brush border enzymes

∙ the microvilli increase surface area for chemical digestion and increase surface area for absorption of nutrients into the epithelial cells

Histology of the Small Intestine Wall

The mucosa and submucosa are modified for the function of the small intestine. Along with having villi covered by an epithelium with microvilli, the mucosa of the small intestine has intestinal crypts (intestinal glands; crypts of Lieberkűhn):

- intestinal crypts are deep crevices between the bases of the villi

- they are lined with glandular epithelial cells that secrete intestinal juice

There are five major types of cells in the epithelium of the villi and crypts:

1. Enterocytes

- most numerous; they are simple columnar absorptive cells with microvilli

- what are they bound by?

tight junctions

- in the villi, the primary responsibility of these enterocytes is what?

absorbing nutrients and electrolytes

- in the crypts, enterocytes are primarily secretory cells that secrete what?

intestinal juice

∙ intestinal juice is a watery mixture that contains what?

mucus

∙ intestinal juice serves as what?

a carrier fluid for absorbing nutrients from chyme

2. Goblet cells

- what do goblet cells secrete?

mucus

3. Enteroendocrine cells - are scattered amongst the enterocytes; they secrete hormones such as secretin and cholecystokinin

4. Paneth cells – found deep in the intestinal crypts

- paneth cells fortify the small intestine’s defenses by releasing what?

antimicrobial agents such as defensins and lysozyme, an antibacterial enzyme

- their secretions do what?

destroy certain bacteria and help to determine which bacteria colonize the intestinal lumen

5. Stem cells

- at the bases of the crypts, the stem cells continuously divide to replenish the epithelium!

Duodenal glands in the submucosa of the duodenum produce alkaline mucus:

- what does the mucus help to do?

that helps neutralize acidic chime moving in from the stomach

Intestinal Juice

( the intestinal glands normally secrete what?

1 to 2 L of intestinal juice daily

( is intestinal juice normally slightly alkaline? yes

( intestinal juice is enzyme-poor because intestinal

enzymes are bound to the microvilli of the brush border

Motility of the Small Intestine (page 781); (see Fig. 22.3 b on p. 749)

( segmentation

- localized mixing contractions that mix chyme with the digestive juices and bring the particles of food into contact with the mucosa for chemical digestion and absorption

( peristalsis - weak, wave-like contractions that propel chyme toward the large intestine

Digestive Processes in the Small Intestine

(chemical digestion in the small intestine is a collective effort of:

∙ enzymes in pancreatic juice produced by the pancreas

∙ enzymes in intestinal juice

∙ brush border enzymes on the microvilli produced by the epithelial cells in the small intestine

( the end products of digestion listed in the box on the previous page are then absorbed

Absorption in the Small Intestine (this begins on page 787, but I have summarized it here!)

90% of absorption of nutrients takes place in the small intestine

10% takes place in the stomach and large intestine

Here is a summary of absorption:

nutrients in the lumen of the small intestine

(

into the simple columnar absorptive epithelial cells(enterocytes)

(

into the underlying lamina propria

LACTEALS BLOOD CAPILLARIES

( (

23.9 The large intestine absorbs water and eliminates feces (page 906!)

( the large intestine extends the ileocecal valve to the anus; it is ~5 ft. long, 2.5 in. in diameter

( what are the major digestive functions of the large intestine?

- to absorb most of the remaining water from indigestible food residues

- store the residues temporarily

- then eliminate them from the body as semisolid feces

- absorbs metabolites produced by resident bacteria as they ferment carbohydrates not absorbed in the small intestine

Gross Anatomy (of the Large Intestine)

( the colon has three bands of smooth muscle called teniae coli

- their tone puckers the wall of the large intestine into what?

pocketlike sacs called haustra

Subdivisions of the Large Intestine

( List subdivisions of the large intestine (and locate them in Fig. 23.33):

cecum, appendix, colon, rectum, and anal canal

( cecum – saclike; the first part of the large intestine

- the ileocecal valve controls the passage of chyme from the ileum to the large intestine

( appendix

- what does the appendix contain and what is its role?

masses of lymphoid tissue, and as part of MALT it plays an important role in body immunity

- additionally, the appendix serves as what?

serves as a storehouse of bacteria and recolonizes the gut when needed

- the twisted structure of the appendix makes it susceptible to what?

blockage

( colon

- list the four distinct regions of the colon (not counting the flexures) and note their locations in Fig. 23.33 a:

ascending colon; transverse colon; descending colon; sigmoid colon

( rectum

- the sigmoid colon joins the rectum in the pelvic canal at the level of the third sacral vertebra

( anal canal

- the last segment of the large intestine

- lies in the perineum, entirely external to the abdominopelvic cavity

- the anal canal is about how long? 3 cm

- the anal canal opens to the body exterior at what opening? anus

- name the two sphincters of the anal canal, indicate the type of muscles that form each, and whether each is voluntary or involuntary:

1) Internal anal sphincter

involuntary; composed of smooth muscle

2) External anal sphincter

voluntary; composed of skeletal muscle

Microscopic Anatomy (of the Large Intestine)

( the epithelium of the large intestine forms intestinal crypts, as in the small intestine, but there are not villi and no circular folds

( the mucosa of the large intestine does NOT produce digestive enzymes

( the simple columnar epithelium contains numerous goblet cells that produce mucus:

- what are the functions of the mucus produced by the goblet cells?

- eases the passage of feces

- protects the intestinal wall from irritating acids and gases released by resident bacteria

Bacterial Microbiota (Bacterial Flora)

( the bacterial flora of the large intestine consists of about over a thousand different types of bacteria!

( the last stage of digestion occurs in the colon through the activity of bacteria in the colon

( the bacteria in the large intestine:

∙ ferment any remaining carbohydrates and release H+ ions, CO2 and methane gas

∙ convert remaining protein to amino acid and break down amino acids

∙ convert bilirubin to stercobilin, which gives feces their brown color

∙ ( synthesize B vitamins and some vitamin K

Digestive Processes in the Large Intestine

( some absorption occurs in the large intestine, including absorption of some water, electrolytes, some vitamins (B and K)

( as the chyme remains in the large intestine for 12 to 24 hours, water absorption converts it to a semisolid form called feces

( from Table 23.2 on p. 904, indicate the major functions of the large intestine?

( Digestion – some remaining food residues are digested by gut bacteria (which also produce some vitamin K and some B vitamins)

( Absorption – absorbs most remaining water, electrolytes (largely NaCl), and vitamins produced by bacteria

( Propulsion - propels feces toward rectum by mass movements

( Defecation – reflex triggered by rectal distention; eliminates feces from body

Motility of the Large Intestine

( haustral contractions:

∙ describe haustral contractions:

slow segmenting movements that last about one min that occur every 30 min or so

∙ as a haustrum fills with food residue, the distension stimulates what?

its muscle to contract

∙ these movements mix the residue, which aids in what?

this aids in water absorption

( mass movements (mass peristalsis)

= long slow-moving, strong waves of contraction that begin in the transverse colon and force the feces in the colon into the rectum

( The Defecation Reflex (see Fig. 23.35)

Mass movement drives the feces from the colon into the rectum ( distends the rectum ( initiates the urge to defecate ( internal anal sphincter relaxes involuntarily; external anal sphincter relaxes voluntarily ( defecation occurs (

The End!

-----------------------

PART I

PART 2

Remember “flight-or fight”…does your mouth become more moist or dry when you are scared?

I know this is silly, but I remember that PARIETAL cells produce HCl because parietal ends in an “L”!

My summary of stimulation of HCL secretion by parietal cells in the stomach:

1. Parasympathetic nerve impulses stimulate the release

of acetylcholine (ACh), which:

1. increases secretion of pepsinogen by chief cells

2. increases secretion of HCl by parietal cells

3. stimulates release of gastrin by enteroendocrine

cells (G cells)

2. Gastrin produced by G cells increases gastric secretion

3. Histamine produced by mast cells stimulates the parietal

cells to secrete HCl

(Note: the histamine receptors on the parietal cells are called

H2 receptors, and are different from the H1 receptors of allergic reactions.)

( When only one of the three chemicals binds to parietal cell receptors, HCl secretion is scanty; when all three chemicals bind to receptors, HCl production is significant

Duct System of the Liver,

Gallbladder, and Pancreas

2. common hepatic duct

3. cystic duct

4. common bile duct

5. opening of accessory pancreatic

duct

6. pancreatic duct

7. hepatopancreatic ampulla and sphincter

8. duodenal papilla

Is the pH of the chyme in the stomach acidic or alkaline? You may have been noticing that various secretions produced by the small intestine or that empty into the small intestine (bile and pancreatic juice) are ALKALINE. This is important to neutralize the acidic chyme that empties from the stomach into the small intestine to create a proper pH environment for the function of enzymes that act in the small intestine.

( First, a little review of chemical digestion so far:… due to the actin of enzymes in saliva produced by salivary glands and enzymes produced in the stomach, chyme entering the small intestine contains partially digested carbohydrates, proteins and lipids

A SUMMARY OF THE END PRODUCTS OF CHEMICAL DIGESTION in the GI Tract:

carbohydrates ( monosaccharides

proteins ( amino acids and peptides

lipids ( fatty acids and monoglycerides

( NOTE: before lipids are chemically digested, bile salts emulsify fat globules into fat droplets

DNA and RNA ( sugars and nucleotides

( absorption = passage of digested nutrients, water, electrolytes, etc., from the lumen of the gastrointestinal tract into the blood or lymph

monosaccharides, amino acids, short-chain fatty acids, water and ions, water-soluble vitamins, etc.

( long-chain fatty acids, monoglycerides,

fat soluble vitamins and cholesterol, etc.

to blood in subclavian vein via lymphatic vessels

to sinusoids of the liver via the hepatic portal vein

( the long-chain fatty acids, monoglycerides, fat soluble vitamins, and cholesterol are surrounded by bile salts to form micelles to get close to the microvilli of the epithelial cells; then they are released close to the cell membrane and diffuse into the cell.

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