Chapter 29 - Physiology of the Digestive System

Nervous mechanism of gastric emptying

1) "Enterogastric reflex" 2) Receptors in the duodenal mucosa are sensitive to presence of acid & to distension 3) Impulses over sensory & motor fibers in the vagus nerve cause a reflex inhibition of gastric persistalsis 4) IN addition to how this reflex inhibits gastric motility as food begins to fill the duodenum, we see that it may inhibit gastric secretion as well

Digestive Enzymes

1) "Extracellular", organic (protein) catalysts [Most act intracellularly in the body] 2) Operate in lumen of digestive tract, outside of any body cells 3) Properties of digestive enzymes: i) Specific in their action ii) Function optimally at a specific pH iii) Most enzymes catalyze a chemical reaction in both directions (forward & reverse reaction; rate law) iv) Enzymes are continually being destroyed or eliminated from the body & must continually be synthesized (even though they're not used up in the rxns they catalyze) v) Most digestive enzymes are synthesized as inactive "proenzymes", which are broken apart by kinases. 4) All digestive enzymes are classified as "hydrolases" because they catalyze the hydrolysis of food molecules Example: Lipase, Proteases like Pepsin and Trypsin

Fat Digestion

1) Fats must be emulsified by bile in small intestine before being digested 2) Pancreatic lipase is the main fat-digesting enzyme 3) Triglycerides are broken down into fatty acids, monoglycerides, and glycerol molecules

Saliva

1) Secreted by salivary glands 2) Mucus lubricates food and, with water, facilitates mixing 3) Amylase --> an enzyme that begins digestion of starches; "ptyalin" 4) A small amount of salivary lipase is released; function uncertain; it is practically worthless in the saliva because most ingested lipids must be emulsified before lipase can digest them easily 5) Sodium bicarbonate increases the pH for optimum amylase function

Carbohydrate Digestion

1) Carbohydrates are saccharide compounds 2) Polysaccharides are hydrolyzed by "amylases" found in saliva & pancreatic juice to form disaccharides 3) Final steps of carbohydrate digestion are catalyzed by the enzymes sucrase, lactase, & maltase, found in the cell membrane of epithelial cells covering the villi that line the intestinal lumen [substrates are disaccharides; turned into monosaccharides (glucose)] 4) Contact digestion*

Mastication

1) Chewing movements 2) Reduces size of food particles 3) Mixes food with saliva in preparation for swallowing

Segmentation

1) Described as a "mixing movement" 2) Digestive reflexes cause a back-and-forth movement with a single segment of the GI tract 3) Helps break apart chunks of food particles 4) Mixes food & digestive juices, & brings digested food in contact with intestinal mucosa to facilitate absorption

When rectum is FULL

1) External anal sphincter contracted 2) Internal anal sphincter relaxed Shifted control of the anal opening from an involuntary mode to a voluntary mode

Gastric inhibitory peptide

1) A hormone released by intestinal mucosa into bloodstream 2) Once it reaches stomach wall, it has an inhibitory effect on gastric muscle, decreasing its peristalsis and slowing passage of food into the duodenum

When rectum is EMPTY

1) External anal sphincter relaxed (open) 2) Internal anal sphincter contracted (closed)

Mechanisms of Absorption

1) For some substances such as water, absorption occurs by simple diffusion or osmosis 2) Other substances are absorbed through more complex mechanisms a) "Secondary active transport" --> how sodium is transported; pumped from GI lumen into internal environment b) "Sodium cotransport" (coupled transport) --> how glucose & amino acids are transported. This is ANOTHER CASE OF 2NDARY ACTIVE TRANSPORT because this movement does not occur without the Na+ [..] gradient maintained by the active transport of Na+ out of the cell's basal membrane. c) Fatty acids, monoglycerides & cholesterol are transported with the aid of bile salts from the lumen to absorbing cells of the villi d) Final step in lipid transport by intestine is formation of "chylomicrons", simply another type of micelle. They're water soluble and allows fats to be transported through lymph and into the bloodstream. e) Vitamins ADEK (fat-soluble) also depend on bile salts for their absorption 3) Transcellular absorption moves nutrient particle through cells before moving out and into blood or lymph (as described above) & paracellular absorption moves particles between cells, requires no energy expenditure by cells 4) After food is absorbed, it travels to the liver via the portal system 5) In summary, most absorption occurs in the small intestine

Esophageal stage (esophagus to stomach)

1) Involuntary movement 2) Contractions & gravity move bolus through esophagus & into stomach

Pharyngeal stage (oropharynx to esophagus)

1) Involuntary movement 2) To propel bolus from the pharynx to the esophagus, the mouth, nasopharynx, & larynx must be blocked 3) A combination of contractions & gravity move bolus into esophagus

Digestion

1) Mechanical Digestion 2) Chemical Digestion

Process of Absorption

1) Passage of substances (digested foods, water, salts, vitamins) through the intestinal mucosa into the blood or lymph 2) Most absorption occurs in the small intestine

The Big Picture: Digestion & the Whole Body

1) Primary contribution of the digestive system to overall homeostasis is to provide a constant nutrient concentration in the internal environment 2) Secondary roles of digestive system: i) Absorption of nutrients ii) Teeth & tongue, along with respiratory & nervous system, are important in producing spoken language iii) Gastric acids aid the immune system by destroying potentially harmful bacteria 3) To accomplish its functions, digestive system needs other systems to contribute: i) Regulation of digestive motility & secretion requires the nervous & endocrine systems ii) Oxygen for digestive activity is dependent on proper functioning of the respiratory & circulatory systems iii) Integumentary & skeletal systems support & protect the digestive organs iv) Muscular system is needed for ingestion, mastication, deglutition, & defecation to occur normally

Overview of Digestive Function

1) Primary function of the digestive system --> to bring essential nutrients into the internal environment so that they are available to each cell of the body 2) Mechanisms used to accomplish the primary function of the digestive system: i) Ingestion --> food is taken in (by mouth) ii) Digestion --> breakdown of complex nutrients into simple nutrients (physical vs. chemical) iii) Motility of the GI wall --> physically breaks down large chunks of food material & moves food along the tract; includes peristalsis & segmentation iv) Secretion --> digestive enzyme secretion facilitates chemical digestion (breakdown of large molecules into small molecules) v) Absorption --> movement of nutrients through the GI mucosa into the internal environment vi) Elimination --> excretion of material that is not absorbed (feces) from the rectum, through the anus; defecation vii) Regulation --> coordination of digestive activity (motility, secretion, etc) 3) The digestive tract is functionally an extension of the external environment --> material does not truly enter the body until it is absorbed into the internal environment

Absorption

1) Process of Absorption 2) Mechanisms of Absorption

Deglutition

1) Process of swallowing; complex process requiring coordinated & rapid movements. Involves 3 main stages: 2) Oral stage (mouth to oropharynx)* 3) Pharyngeal stage (oropharynx to esophagus)* 4) Esophageal stage (esophagus to stomach)*

Protein Digestion

1) Protein compounds are made up of twisted chains of amino acids 2) Proteases catalyze hydrolysis of proteins into intermediate compounds and, finally, into amino acids 3) Main proteases: "pepsin" in gastric juice, "trypsin" & "chymotrypsin" in pancreatic juice, "peptidases" in intestinal brush border

Secretion

1) Saliva 2) Gastric Juice 3) Pancreatic Juice 4) Bile 5) Intestinal Juice

Hydrochloric acid (HCl)

1) Secreted by "parietal cells" 2) HCl decreases the pH of chyme for activation & optimum function of pepsin 3) Released actively into the gastric juice by H-K pumps (proton pumps) 4) Vesicles in the resting parietal cell move to the apical surface when the cell becomes active -- thus increasing the surface area for the process of secretion

Pancreatic Juice

1) Secreted by acinar & duct cells of the pancreas It contains various digestive enzymes, all of which are secreted as zymogens -- inactive proenzymes. Ex: Trypsin is released as the zymogen trypsinogen which becomes activated by "enterokinase". 2) "Proteases" (ex. trypsin & chymotrypsin) --> enzymes that digest proteins & polypeptides; activates other enzymes such as the ones listed below 3) "Lipases" --> enzymes that digest emulsified fats 4) "Nucleases" --> enzymes that digest nucleic acids such as DNA & RNA 5) "Amylase" --> an enzyme that digests starches 6) Sodium bicarbonate increases the pH for optimum enzyme function; its manufacture also helps restore normal pH of blood

Intestinal Juice

1) Secreted by cells of intestinal exocrine cells 2) Mucus & water lubricate & aid in continued mixing of chyme [goblet cells produce mucus] 3) Sodium bicarbonate increases pH for optimum enzyme function

Gastric Juice

1) Secreted by gastric glands 2) Pepsin (secreted as inactive pepsinogen by "chief cells" aka "zymogenic cells" in the gastric glands) --> a protease that begins the digestion of proteins 3) Hydrochloric acid (HCl, secreted by parietal cells)*, which activate/convert pepsinogen to pepsin. 4) "Intrinsic factor" (secreted by parietal cells) --> protects vitamin B12 from acids of the stomach & later facilitates its absorption in small intestines 5) Mucus & water lubricate, protect, & facilitate mixing of chyme

Secretin

1) Secreted by intestinal mucosa in presence of acid, partially digested proteins & fats 2) Inhibits gastric secretion 3) Stimulates secretion of pancreatic juice low in enzymes and high in alkalinity (bicarbonate) 4) Enhances effects of CCK

Bile

1) Secreted by the liver; stored & concentrated in the gallbladder 2) "Lecithin" & "bile salts" emulsify fats by encasing them in shells to form tiny spheres called "micelles" 3) Sodium bicarbonate increases pH for optimum enzyme function 4) Contains excreted substances like cholesterol, products of detoxification, & bile pigments (ex. bilirubin) which are all waste products excreted by the liver & eventually eliminated in the feces. It is the bile pigments that give feces its brown shit colour. Gray feces would be a sign that bile seretion is abnormal.

Oral stage (mouth to oropharynx)

1) Voluntarily controlled; (by cerebral cortex) 2) Formation of a food bolus in the middle of the tongue; 3) Tongue presses bolus against the palate & food is then moved into the oropharynx.

Peristalsis

1) Wavelike ripple of the muscle layer of a hollow organ; 2) Progressive motility that produces forward movement of matter along the GI tract

Peristalsis & Segmentation

2 main types of motility produced by the smooth muscle of the GI tract, which can both occur together in an alternating fashion. When this happens, food is churned and mixed as it slowly progresses along GI tract. 1) Peristalsis* 2) Segmentation*

Regulation of Motility

A) Gastric motility i) Food in the stomach is churned (via propulsion & retropulsion processes = forward & backward movement) & mixed with gastric juices to form chyme ii) Chyme is ejected about every 20 seconds into the duodenum; emptying the stomach takes approximately 2-6 hours B) Gastric emptying controlled by hormonal & nervous mechanisms i) Hormonal Mechanism* ii) Nervous Mechanism*

Chemical Digestion

All changes in chemical composition of food as it travels through the digestive tract. Chemical changes result from hydrolysis -- process in which compound unites with H20 and breaks down further. 1) Digestive Enzymes 2) Carbohydrate Digestion 3) Protein Digestion 4) Fat Digestion 5) Residues of Digestion

Conscious decision to defecate

Causes the... 1) External anal sphincter to relax (relaxed) 2) Internal anal sphincter relaxed

Mechanical Digestion

Consists of all movement (motility) of the digestive tract that brings about the following: A) It changes ingested food from large particles into minute particles, facilitating chemical digestion. B) Digestive tract churns contents of the GI lumen to mix with digestive juices & ensure contact with the surface of the intestinal mucosa, facilitating absorption. C) Propel food along the alimentary tract, eliminating digestive waste from the body. 1) Mastication 2) Deglutition 3) Peristalsis & Segmentation 4) Regulation of Motility 5) Intestinal Motility

Control of Digestive Gland Secretion

Exocrine digestive glands secrete when food is present in digestive tract or what is SEEN, SMELLED, or IMAGINED. 1) Control of Salivary Secretion 2) Control of Gastric Secretion 3) Control of Pancreatic Secretion 4) Control of Bile Secretion 5) Control of Intestinal Secretion

Elimination

Expulsion of feces from the digestive tract; referred to as defecation 1) Defecation 2) Constipation 3) Diarrhea

Hormonal mechanism of gastric emptying

Fats in duodenum stimulate the release of gastric inhibitory peptide, which acts to decrease peristalsis of gastric muscle & slows passage of chyme into duodenum

Intestinal Motility

Includes peristalsis & segmentation 1) Segmentation in duodenum & upper jejunum mixes chyme with digestive juices from the pancreas, liver, & intestinal mucosa 2) Peristalsis rate picks up as chyme approaches end of jejunum, moving it through small intestine into the large intestine 3) After leaving stomach, passage of chyme all the way through the small intestine takes approximately 5 hours 4) Peristalsis regulated in part by intrinsic stretch reflexes; stimulated by "cholecystokinin (CCK)"

Constipation

contents of the lower part of the colon & rectum move at a slower than normal rate; extra water is absorbed from the feces, resulting in a hardened stool

Diarrhea

result of increased motility of the small intestine (chyme moves through small intestine too quickly), causing decreased absorption of water & electrolytes and producing a watery stool; may also result from bacterial toxins that damage the water reabsorption mechanisms of the intestinal mucosa

Defecation

results from a reflex brought about by stimulation of receptors in the rectal mucosa that is produced when the rectum is distended

Residues of Digestion

some compounds of food resist digestion & are eliminated as feces; humans lack the enzymes required to hydrolyze them Example: cellulose (dietary fiber)

Contact digestion

when substrates bind onto enzymes located on the surface of the brush border and complete carbohydrate digestion