Digestive system - chapter 24

digestion and metabolism primary role

break down food we consume and absorb the digested (broken down) fragments into the blood

The GI tract or alimentary canal is a muscular tube that is open to the environment on both ends

accessory digestive organs produce secretions that help with digestion of food (saliva, bile, enzymes)

summary of digestion

Ingestion: taking food into the mouth

Propulsion: moving food along the GI tract, followed by peristalsis

Mechanical digestion: begins with checking, followed by churning in the stomach, and segmentation and emulsification by small intestine

Chemical digestion: breakdown of large molecules, into their building blocks by digestive enzymes

Absorption: digested food is transported into blood or lymph

Defecation: undigested material is eliminated from GI tract

teeth

or dentes, gomphosis joint

enamel: harder than bone = 96% CaPO4, no cells

dentin: calcified connective = 70% CaPO4, no cells

Pulp: CT containing blood & lymphatic vessels and nerves

two dentitions (sets of teeth)

• 20 deciduous or “baby teeth” which begin to erupt at 6 months and are lost between 6-12 years

• 32 permanent teeth, the last molars are the wisdom teeth

Tonsils

lymphoid containing WBC’s

Pharyngeal tonsil - behind nasal cavity

palatine tonsil - beside/behind tounge

lingual tonsil - bottom of tongue

3 pairs of salivary glands

1.5L of saliva/ day (mostly water 99.4% H2O + 0.6% ions, enzymes keep pH 7)

1. Parotid: produces salivary amylase: enzyme that breaks down starch (produces 25% of total saliva)

2. sublingual: secretes buffer & lubricant (produces 5% of saliva)

3. submandibular: Secretes buffers, mucins, salivary amylase & IgA (produces 70% of total saliva)

saliva production - stimulated by tactile and taste receptors & ANS activity in salivary reflex centres of medulla oblongata

Functions of the tongue

1. mechanical processing

2. assist chewing & swallowing

3. sensors to touch, temperature & taste

4. 4 secretes lubricating mucins, lingual lipase

(breakdown TGs @ pH 3-6)

pharynx

a common passageway for solid foods, liquids and air

regions of the pharynx

• nasopharynx - back of nasal cavity

• Oropharynx - back of mouth, behind tongue

• laryngopharynx - right before esophagus

deglutition steps

swallowing (the contraction and relaxation of esophageal sphincter)

2400x per day

step 1 - buccal phase (food is being swallowed) voluntary

step 2 - pharyngeal phase (epiglottis is covering trachea) reflex

step 3 - esophageal phase (parastalsis pushes food down esophagus)

step 4 - bolus enters stomach

Histology of the GI tract

from the esophagus to the anus the GI tracts consists of 3 layers:

mucosa - epithelial layer which produces mucus

• epithelium, laminate propria, muscularis mucosae (embedded with pacesetter cells)

Submucosa - connective tissue layer containing nerves, blood and lymph vessels

Muscularis Externa- smooth muscle layer

• longitudinal muscle & circular muscle

Serosa - outer connective tissue layer which forms the visceral peritoneum

** no serosa in oral cavity, pharynx, esophagus and rectum. Has collagen adventina instead

peritoneum

the body’s largest serous membrane (simple squamous epithelium + areolar CT) & it wraps around & stabilizes most abdominopelvic organs

• made up of visceral and parietal peritoneum, separated by peritoneal fluid

• infiltrated by blood and lymphatic vessels & nerves

• insulating layers of fat

5 major peritoneal folds

greater momentum- stomach

falciform ligament - liver- anterior wall of peritoneum

lesser omentum -liver-stomach

mesentery (proper) - small intestine

mesocolon - large intestine-dorsal wall

retroperitoneal organs: pancreas, duodenum, rectum, ascending & defending colon: kidneys

peristalsis

the coordinated contraction and relaxation of smooth muscle in the muscularis external

it results in the forward movemnet of chrome through the GI tract

segmentation

the mixing of chyme by alternating contraction and relaxation of the GI smooth muscle

the stomach

primary functions are:

• mechanically mix food

• begin protein digestion by secreting gastric juice

• control flow into the duodenum

food leaving the stomach is called chyme

stomach histology

chief cells and parietal cells

the stomach lining contains

• mucus secreting goblet cells

• gastrin secreting endocrine cells

• gastric pits that secret HCL and pepsinogen

when food enters the stomach, gastrin is released in to the blood

Gastrin

released into the blood when food enters the stomach

gastrin activates the gastric pits to secrete pepsinogen and HCL.

the lowered pH created by HCL converts pepsinogen to pepsin, which initiates protein catabolism (PSNS also activates gastric juice secretion)

HCL aslo: kills microbes, denatures proteins, breaks plant cell walls & connective tissue of meat

HCL secretion by parietal cells

pH 1.5-2

HCL is formed by moving H+ ion into the lumen separately from Cl- ion.

HCL does not build up in the parietal cell which would mean low pH within that cell

a counter transport mechanism ejects the bicarbonate ions into the interstitial fluid and imports chloride ions into the cell

Bile

Bile : you produce 1 L/ day

bile is produced in the liver and stored in the gall bladder. When chyme enters the duodenum, the gall bladder contracts releasing bile into the duodenum through the bile duct

bile is composed of

• bile salts: made from cholesterol, these salts emulsify large drops of fat into smaller droplets → micelles = small lipid bile salt complexes

• bilirubin - a metabolite formed during heme degradation, this yellow pigment is converted into urobilinogen and stercobilin which gives faces it’s brown colour

• cholesterol

pancreatic juice

produced by the pancreas

consists of bicarbonate (HCO3) and digestive enzyme (trypsin, chymotrypsin, carboxypeptidase, amylase, lipase, and nuclease) produced by the pancreas and released by the pancreatic duct into the duodenum

6 Hormones involved in regulation of digestive enzymes

Gastrin

Secretin

Gastric Inhibitory peptide

Cholecystokinin

vasoactive Intestinal Peptide

enterocrinin

Garstrin

secreted by G cells in pyloric antrum when stimulated by th vagus nerve or when food arrives in the stomach

and enteroendocrine cells in the duodenum release gastrin when they are exposed to large quantities of undigested proteins

gastrin increases stomach motility and stimulating gastric acid and enzyme production

• stimulates gastric motility and juices

Secretin

released when chyme enters the duodenum

Secretin’s main effects is to increase the secretion of buffers by the pancreas, increasing the pH of the chyme

secretin also stimulates secretion of bile by the liver and reduces gastric motility and gastric sensory rates

• secretion of buffer and bile

Gastric Inhibitory Peptide

secreted when fats and carbohydrates (especially glucose) enters the small intestine

the inhibition of gastric activity stimulates insulin release at the pancreatic islets.

GIP has secondary effects, stimulating submucosal duodenal gland activity, stimulating lipid synthesis in adipose tissue, and increasing glucose use by skeletal muscles

• gastric inhibit - insulin stimulant

cholecystokinin

CCK

secreted when chyme arrives in the duodenum, especially when chyme contains lipids and partially digested proteins

the net effects of CCK are to increase the secretion of pancreatic enzymes and push pancreatic secretions and bile into the duodenum

The presence of CCK has two additional effects, it inhibits gastric activity and it appears to have CNS effects that reduce hunger

*gastric and hunger inhibit - enzyme & bile release

Vasoactive Intestinal Peptide

VIP

stimulates the secretion of intestinal glands, dilates regional cappilliares and inhibits acid production in the stomach

the dilation of capliallries in active areas of the intestinal tract, VIP provides an efficient mechanism for removing absorbed nutrients

• gastric inhibit - intestinal vasodilation

Enterocrinin

released by enteroendorine cells of the duodenum in response to stimulation by the vagus nerves before chyme reaches the pyloric sphincter and also when chyme enters the duodenum

It stimulates production of alkaline mucus by the duodenal submucosal glands, which protects the intestine from acidic chyme

• vagus - enterocrinin - alkiline mucus

regulation of bile production

when acid chyme enters the duodenum it stimulates the release of CCK and secretin into the blood stream

secretin - increase bile production in liver

CCK - causes contraction of the gallbladder and opens the hepatopancreatic sphincter so bile enters the duodenum

Vagal stimulation (PSNS) - further controls the gallbladder

regulation of pancreatic juice secretion

as acid chyme enters the duodenum, it stimulates the release of secretion and cholecystokinin into the blood stream

secretin - increase the production HCO3 buffer in the pancreas

CCK - increase the digestive enzyme production from the pancreas and open sphincter

vagal stimulation (PSNS) enhances pancreatic stimulation

3 phases of digestive activities

digestive activities of the gastrointestinal tract occur in 3 over lapping phases:

• the cephalic phase

• the gastric phase

• the intestinal phase

insulin

allows your cells to take up glucose & amino acids from the blood stream

two gastric reflexes

gastroenteritis reflex: stimulates motility and secretion along the entire small intestine

gastroileal reflex: triggers the opening of the ileocecal valve, allowing materials to pass from the small intestine into the large

both reflexes accelerate movement along the small intestine

chemical digestion of carbohydrates

salivary amylase begins break down in mouth

pancreatic amylase begins breaking oligosaccharides down in duodenum

polysaccharides → smaller saccharides (oligosaccharides) → pancreatic amylase (breaks down oligosaccharides) with enteric oligosaccharides, into disaccharides (lactose, maltose, sucrose) → enteric disaccharides break disaccharides into monosaccharides (glucose fructose, galactose)

Protein Digestion

protein digestion begins in the stomach with the enzyme pepsin

the release of pancreatic trypsin and chymotrypsin in the duodenum (converting proteins into large polypeptides)

enteric peptidases break large polypeptides into small polypeptides, then amino acids

Fat digestion

lingual lipase in the mouth and gastric lipase in the stomach break triglycerides into monoglycerides and fatty acids

bile acids in the small intestine begin to emulsify the fat droplets (unemulsified)

Pancreatic Lipase can then break emulsified triglycerides into monoglycerides and fatty acids

The small intestine

the small intestine has specialized features for absorption

• long length (2200 ft (squared))

• 800 circular folds (plicae circulares)

• villi - each villus has both blood and lymph supply for absorption

• microvilli

characteristics of the 3 segments of the duodenum

duodenum (10" inches) - duodenal glands secrete bicarbonate rich mucus

• neutralize chyme, get pH up to 7-8

Jejunum (8 feet)

• extensive villi and microvilli because of the large amounts of food absorption

Ileum (11 feet)

• contains aggregated lymphoid nodules (lymphocytes “hangout” and screen for pathogens)

How do the regional specializations of the small intestine change along it’s length

the duodenum has small villi & numerous mucous glands

the jejunum has numerous villi for absorption. Most chemical digestion & absorption takes place here

The ilium contains aggregated lymphoid nodules

*90% of nutrient absorption takes place in the small intestine

(10% in the large)

Absorption into the small intestine

most nutrients are absorbed actively into the mucosa of the GI tract eg. by a Na cotransporter (and H2O follows Na)

• glucose, galactose amino acids and most water soluble vitamins are absorbed this way (sodium dependent glucose transport)

• water and fructose are absorbed by facilitated diffusion

absorption of lipid soluble substances

fatty acids are absorbed by direst or facilitated diffusion & enter lymph

fat soluble vitamins A,D & E are absorbed similarly to FA

• vitamin B12 is absorbed with the help of intrinsic factor, which is secreted in th gastric juice (by parietal cells). Vitamin B12 is required for erythropoiesis

lacteals

lymphatic capillaries that absorb dietary fats through the villi of the small intestine. This fluid is called chyle

lacteals merge with larger lymphatic vessels to transport the chlme to the thoracic duct

once the fat (in chylomicrons) travels to the liver, it is converted to lipoproteins (HDL or LDL) and can enter the bloodstream to be carried to tissue

the absorption state

insulin stimulates:

• glucose uptake and glycogenesis

• amino acid intake and protein synthesis

• triglyceride synthesis

Androgens, estrogens and growth hormone also stimulate protein synthesis

Glycolysis and aerobic respiration provide the ATP needed to power cellular activities aswell as the synthesis of lipids and proteins

tenia coli

3 bands of longitudinal smooth muscle of the muscularis externa

Verniform appendix

vestigial organ or “bacterial safe house”

vermiform - “wormlike”

the appendix, when infected can cause death. Those who have had it are perfectly healthy afterwards

the appendix used to be seen as vestigial (no longer needed), but research has recently suggested that the appendix serves as a safe house for useful bacteria when illness flushes these bacteria from the rest of the intestines

this function is expected to be useful in a culture lacking modern sanitation, where diarrhea may be prevalent

this function would explain the health of individuals without appendixes in developed countries, and the worm like shape of the appendix, with copious amounts of immune tissue

regardless of the appendix’s function it is extremely important to have it removed immediately the event of appendicitis

large intestine (chyme mixes with bacteria)

the large intestine’s primary role is water absorption from the undigested material and aid in elimination of feces

also absorbs Na and other electrolytes, vitamin K, small amounts of vitamin B9 (folic acid), vitamin B7 (biotin), and vitamin B5 (pantothenic acid) produced by enteric bacteria

unlike the small intestine the large intestine does not continuously move the fecal bulk toward the anus. The large intestine uses “mass" movements” occurring only periodically push the fecal bulk toward the rectum (3-4 times a day)

defecation

when the mass movements of the large intestine move faces to the sigmoid colon, this activated the sensory neurons in the GI wall

the PSNS relaxes the internal sphincter of the anus & somatic impulses contract the external sphincter (skeletal muscle)

we have conscious control over the external sphincter

during dedication mass movements push faces toward the anus and is aided by conscious contraction of the abdominal wall and diaphragm (valsava maneuver)

portal stimulation

the superior and inferior mesenteric veins drain the blood from the GI tract that contains the absorbed nutrients

these veins drain hepatic portal vain leading into the liver

this allows the liver to control the amount of nutrients entering the general circulation

the hepatic portal vein delivers 2/3 of the blood entering the liver

The livers role in digestion and metabolism

in addition to it’s role in bile production, the liver aids in digestion by:

1) storage of glycogen, Lipid, fat soluble vitamins and iron resevoirs

2) destroying dissolved bacteria by filtering portal blood through sinusoids containing macrophages called Kupffer Cells

3) Maintaining glucose, amino acids & fatty acid levels in blood (at times through interconverting nutrient types e.g. conversion of carbohydrates to lipids)

4) catabolism of amino acids and producing urea

5)) production of fat acids, triglycerides, cholesterol, and lipoproteins (ie. HDL and LDL)

6) metabolism of drugs, hormones (E,NE, sex hormones, steroid hormones), and toxins in the ER (endoplasmic reticulum)

7) breakdown of hormones, antibodies and old RBC’s

8) synthesis of clotting factors an plasma proteins

Fluid Turnover in the GI tract

in order to mechanically and chemically digest food, the GI tract secretes about 7 litres of fluid, including bile, enzymes and mucus

therefor failure of absorption can rapidly cause dehydration through losses of ingestion and secreted fluid (up to 9 litres per day)