l39-l43: digestive system

ingestion

the act of taking food (or water) into the body

gastrointestinal tract (alimentary canal)

muscular tube that runs from the mouth to the anus lined by a mucus-secreting epithelium

mucosa

epithelium + areolar connective tissue (lamina propria)

submucosa

dense connective tissue, vessels, (and sometimes glands)

muscle layer

usually two layers of smooth muscle (muscularis)

serosa (or adventitia)

epithelium + areolar tissue (or dense connective tissue)

mesenteries

membranes that anchor abdominal organs to the peritoneum

peritoneum

double-layered serous membrane encasing most of the abdominal cavity

submucosal plexus

contains visceral sensory fibers, and parasympathetic and sympathetic postganglionic neurons (ANS)

myenteric plexus

contains ENS neurons (interneurons and motor neurons)

circular layer

smooth muscle arranged around the circumference of the tube

longitudinal layer

smooth muscle arranged along the length of the tube

visceral smooth muscle cells

connected by gap junctions and influenced by rhythmically active pacesetter cells

pacesetter cells

create regular rhythms of depolarization and repolarization (slow wave potentials) which can spread throughout the muscle layer

propulsion

refers to the movement of ingested material through the digestive tract

peristalsis

involves spreading waves of contraction in both muscle layers in a proximal > distal direction

physical digestion

fragmentation of food by purely physical forces

chemical digestion

breakdown of macromolecules into smaller components by chemicals or enzymes

segmentation

rhythmic cycles of circular muscle contraction that fragment the food bolus but do not produce net forward movement

absorption

uptake of nutrients, water, and vitamins from the digestive tract lumen into the bloodstream

mastication (chewing)

contributes to physical digestion by bringing two rows of teeth together

tongue

muscular organ which manipulates food and is also involved in gustation

• secretes lingual lipase which contributes to chemical digestion of fats

saliva

aqueous solution (>99% water) but also contains a digestive enzyme (salivary amylase), antibacterial enzymes and antibodies, and electrolytes that buffer the osmolarity and pH of the oral cavity

salivatory reflex

activation of chemoreceptors (taste cells) and mechanoreceptors in the oral cavity trigger salivation via long reflexes which are integrated in the medulla

swallowing (deglutition)

moves food from the oral cavity through the pharynx and esophagus

buccal phase

• tongue elevates, pushing the bolus against the hard palate

• tongue retracts, forcing the bolus into the oropharynx

• initiated voluntarily

pharyngeal phase

• presence of a bolus in the oropharynx triggers coordinated peristaltic-like contraction of pharyngeal muscless

• laryngeal and hyoid muscles contract and elevate the larynx, causing the epiglottis to close over the trachea

• muscles of the soft palate elevate the uvula, blocking the nasopharynx

• reflexing (triggered in response to mechanoreceptor activation)

esophageal phase

• neural activity in the myenteric plexus coordinates a contraction wave

• near the distal end of the esophagus, stretch receptors trigger the relaxation of the smooth muscle in the lower esophageal sphincter, allowing movement into the stomach

• reflexive (triggered in response to mechanoreceptor activation)

oblique layer

extra smooth muscle layer that enhances the strength and range of contractions of the stomach wall

bolus

compact mass of partially digested food mixed with oral secretions

chyme

soupy mix of food, saliva, and gastric (and later intestinal) secretions found within the lumen of the stomach and intestines

rugae

wrinkles or folds which allow the mucosal layer to stretch

gastric mucosa

columnar epithelium organized into pits, which replenish the epithelial cells, and glands, which secrete a variety of substances

parietal cells

secrete hydrochloric acid indirectly through a combination of types of transport

chief cells

secrete pepsinogen which is converted to the active enzyme pepsin within the acidified gastric lumen

G cells

secrete gastrin into the underlying tissue (and bloodstream) which enchances the activity of chief cells and parietal cells

cephalic phase

CNS stimulates and enhances gastric secretion in preparation for the reception of food

• chemo- or mechano-receptors in the oral cavity, pharynx and esophagus

• enhance vagal activity in the stomach

gastric phase

local neural and hormonal reflexes respond to stretch and luminal contents to enhance gastric secretion

• chemo- or mechano-receptors within the stomach wall

• enhance the secretions of the stomach and gastric muscle contractions to help deal with the material inside the gastric lumen

intestinal phase

gastric activity and the rate of gastric emptying is controlled by reflexes initiated in the duodenum

• local sensory receptors (chemo-, mechano-) within the lumen of the duodenum

• inhibit the secretions and motility of the stomach to slow down the rate of gastric emptying and slow the influx of chyme into the intestine

cholecystokinin

peptide hormone released by the duodenum that controls the release of bile and pancreatic juice

• released in response to the presence of fatty or peptide rich chyme in the duodenum

• triggers contraction of the gallbladder and relaxation of the hepatopancreatic sphincter

• can also act on the endocrine cells within the pancreas to enhance the secretion of insulin

intestinal crypts

source of new epithelial cells and enteroendocrine cells

mass movements

powerful coordinated peristaltic contractions within the colon

• influenced by upstream portions of the digestive tract and are often triggered or enhanced by stretch receptor activation in the stomach and duodenum

defacation reflex

triggers relaxation of the internal anal sphincter, the external anal sphincter can be voluntarily controlled

internal anal sphincter short reflex

triggers a series of peristaltic contractions in the rectum that move the feces toward the anus

internal anal sphincter long reflex

spinal reflex coordinated by sacral parasympathetic motor neurons that stimulates mass movements and relaxes the sphincter

external anal sphincter

innervated by somatic motor neurons, whose activity can be controlled by descending axons that come from primary motor cortex

acinar cells

produce a diverse array of digestive enzymes within the pancreas

pancreatic and intestinal zymogens

activated in the duodenum, mostly by protein-protein interactions

trypsinogen

pancreatic zymogen converted to trypsin by enzymes present on the duodenal enterocytes

trypsin

catalyzes the conversion of most other pancreatic zymogens to their active forms

brush border enzymes

intestinal enzymes associated with the microvilli of enterocytes that typically catalyze the breakdown of small chains into dimers and monomers that are suitable for absorption

bile salts

can bind to fat globules, forming micelles which allow lipases to access the lipid molecules

lipids from micelles

diffuse directly through the apical membrane of enterocytes and are processed by intracellular organelles

chylomicrons

triglycerides reassembled inside enterocytes with phospholipids and specialized proteins

• extruded by the basolateral membrane and transported via the lymphatic system into the bloodstream

chyle

chylomicron rich lymphatic fluid that enters the circulation via the thoracic duct

monosaccharides

absorbed from the lumen via either facilitated diffusion or co-transport with sodium ions into intestinal capillaries that flow into the portal vein system

amino acids

transported via a combination of Na⁺ co-transporters on the apical surface and transporters on the basolateral surface

vitamins

organic molecules that cannot be made in sufficient amounts by the body’s own cells

minerals

both secreted and absorbed through combinations of bulk transport, ion channels, and co-transport which can be independently regulated through the action of different hormones

aldosterone

increases Na+ uptake

PTH & calcitriol

increase Ca²⁺ uptake

water

crosses the leaky intestinal epithelium through osmosis via paracellular transport

vitamin C

water soluble and small; uptake via facilitated diffusion

vitamin E

lipid soluble; transported via micelles and chylomicrons

vitamin B12

water soluble but large; requires endocytosis, mediated by a protein co-factor (intrinsic factor)

absorptive state

occurs when ingested food begins to be absorbed, and typically lasts around four hours

• primary hormone is insulin

• promotes the storage of lipids and use of blood glucose for ATP generation

post-absorptive state

occurs when internal energy stores must be used for ATP generation

• regulatory hormones include glucagon, epinephrine, and glucocorticoids

• promote gluconeogenesis, lipolysis, and the use of fatty acids for ATP generation

liver

anchored in the right superior portion of the abdominal cavity by the peritoneum and mesenteries

hepatic ligaments

folds of the mesentery that divide the liver into lobes

porta hepatis

where the majority of these vessels and ducts converge

hepatic portal vein system

collects blood from many organs within the abdominal cavity

hepatocytes

liver cells organized into lobules, sheets of cells with capillaries on either side

liver capillaries (sinusoids)

have large gaps between adjacent endothelial cells and the underlying basement membrane

primary functions of the liver

relate in some way to the processing of nutrients (and vitamins) from ingestion through to excretion

bilirubin

potentially toxic waste product from the breakdown of haem

stellate macrophages

phagocytose old RBCs and recycle their contents (especially iron)

somatomedins

secreted in response to GH release from the anterior pituitary

angiotensinogen

converted to the hormone angiotensin by renin when BP is low

thrombopoietin

stimulates platelet formation

hepcidin

important for iron homeostasis

class 3 (lipid derivative) hormones

often broken down (inactivated) by enzymes within the liver

cirrhosis

end-stage liver disease, when scar tissue has replaced functional liver tissue