Digestion

Digestive Functions & Processes

Functions

ingestion = intake of food

digestion = breakdown of molecules

absorption = uptake of nutrients into blood or lymph

defecation = elimination of undigested material

Processes

motility = muscular contractions that break up food, mix it with enzymes & move it along

secretion = digestive enzymes & hormones

membrane transport = absorption of nutrients

Stages of Digestion

Mechanical digestion is physical breakdown of food into smaller particles

teeth & churning action of stomach & intestines

Chemical digestion is series of hydrolysis reactions that break macromolecules into their monomers

enzymes from saliva, stomach, pancreas & intestines

results

polysaccharides into monosaccharides

proteins into amino acids

fats into glycerol and fatty acids

Saliva

Functions of saliva

moisten, begin starch & fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus

Hypotonic solutions of 99.5% water and solutes:

amylase = begins starch digestion

lingual lipase = digests fat after reaches the stomach

mucus = aids in swallowing

lysozyme = enzyme that kills bacteria

immunoglobulin A = inhibits bacterial growth

electrolytes = Na+, K+, Cl-, phosphate & bicarbonate

pH of 6.8 to 7.0

Swallowing

The esophagus

Saliva contains salivary amylase. It hydrolyzes the glucose polymers starch and

glycogen. The epiglottis blocks the top of the windpipe when we swallow. Peristalsis

moves the bolus along the epiglottis.

The stomach

The stomach can hold 2L of food and water. The lining of the stomach secretes

gastric acid. Gastric acid contains HCl and pepsin. Pepsin is an enzyme that

hydrolyzes proteins. But it breaks proteins into smaller polypeptides.

Functions of Hydrochloric Acid

Activates enzymes pepsin & lingual lipase

Breaks up connective tissues & plant cell walls

liquifying food to form chyme

Converts ingested ferric ions (Fe+3) to ferrous ions (Fe+2) that can be absorbed

& utilized for hemoglobin synthesis

Destroys ingested bacteria & pathogens

Small Intestine

Nearly all chemical digestion and nutrient absorption occurs in the small intestine

Oral Cavity Stomach Small Intestine

Lumen Brush border

Carbohydrates

Salivary Amylase

Polysaccharides ==> smaller Polysaccharides

(starch,glycogen) (maltose)

Pancreatic Amylase

Polysaccharides ==> disaccharides (maltose)

Disaccharidases ==> monosaccharides

Proteins

Pepsin

Proteins ==> small polypeptide

Trypin, Chymotrypin

Polypeptides ==> smaller polypeptides ==>

aminopeptidase, carboxypeptidase ==> Amino Acids

Dipeptidases

small polypeptides ==> Amino Acids

Nucleic Acids

Nucleases

DNA, RNA ==> Nucleotides ==>

Nucelotidases ==> nuclesides ==> Nucelosidases

==> Nitrogenous bases, sugars, phosphates

Carbohydrates

Proteins

Fats

The digestive enzymes in the table below are summarized

according to type of food that they digest.

FOOD TYPE

ENZYME SOURCE PRODUCTS

CARBOHYDRATES Salivary amylase

Pancreatic amylase

Maltase

Salivary glands

Pancreas

Small intestine

Maltose

Maltose

Glucose

PROTEINS Pepsin

Trypsin

Peptidases

Stomach mucosa

Pancreas

Intestinal mucosa

Peptides

Peptides

Amino acids

FATS Lipase Pancreas Fatty acids

and glycerol

The table below shows digestive enzymes grouped by source of the enzyme.

SOURCE

ENZYME FOOD PRODUCT

MOUTH (salivary glands) Salivary amylase Polysaccharides Maltose

STOMACH Pepsin Proteins Peptides

PANCREAS Pancreatic amylase

Trypsin

Lipase

Polysaccharides

Proteins

Fats

Maltose

Peptides

Fatty acids

and glycerol

SMALL INTESTINE Maltase

Peptidases

Maltose

Peptides

Glucose

Amino acids

Digestive function is affected by hormones produced in many endocrine glands, but the most profound control is exerted by hormones produced within the gastrointestinal tract. The gastrointestinal tract is the largest endocrine organ in the body and the endocrine cells within it are referred to collectively as the enteric endocrine system. Three of the best-studied enteric hormones are:

Gastrin: Secreted from the stomach and plays an important role in control of gastric acid secretion.

Cholecystokinin: A small intestinal hormone that stimulates secretion of pancreatic enzymes and bile.

Secretin: Another hormone secreted from small intestinal epithelial cells; stimulates secretion of a bicarbonate-rich fluids from the pancreas and liver.

HORMON

ES Major Activities Stimuli for Release

Gastrin Stimulates gastric acid secretion and proliferation of gastric

epithelium Presence of peptides and amino acids in gastric lumen

Cholecyst

okin

in

Stimulates secretion of pancreatic enzymes, and contraction and

emptying of the gall bladder

Presence of fatty acids and amino acids in the small

intestine

Secretin Stimulates secretion of water and bicarbonate from the pancreas

and bile ducts Acidic pH in the lumen of the small intestine

Ghrelin Appears to be a strong stimulant for appetite and feeding; also a

potent stimulator of growth hormone secretion.

Not clear, but secretion peaks prior to feeding and

diminishes with gastric filling

Motilin Apparently involved in stimulating housekeeping patterns of motility

in the stomach and small intestine Not clear, but secretion is associated with fasting

Gastric

inhi

bitor

y

poly

pept

ide

Inhibits gastric secretion and motility and potentiates release of

insulin from beta cells in response to elevated blood glucose

concentration

Presence of fat and glucose in the small intestine

Hormones and Digestion:Feedback loops.The hormone Gastrin stimulates the

production of gastric acid.The pyloric sphincter regulates the release of chyme (nutrient

broth) into the small intestine.Bile contains bile salts that aid digestion and absorption of

fats.

Secretin hormone production is stimulated by acid chyme entering the duodenum. This

hormone stimulates the pancreas to release

bicarbonate to neutralize the acid.

CCK stimulates the gall bladder to release bile and the panceas to release pancreatic juices.

If the chyme is rich in fats causes the release of enterogastrone. This hormone inhibits

peristalsis and the release of secretion of acid.

The small intestine

Essential Amino Acids

The human liver can synthesize 9 of the 20 amino acids used in proteins. Those that cannot be synthesized are

called the essential amino acids. They must be supplied

by diet.

Bile Ducts and Gallbladder

Definition:

Bile is produced by the liver cells and collects in the small bile ducts between

the microscopic liver lobules which lead to the larger ducts. Outside the liver,

two outflowing hepatic ducts unite to form the common hepatic duct. From here

the bile enters the cystic duct and the common bile duct . The common bile duct

joins the pancreatic duct to enter the duodenum. The opening is controlled by a

valve which regulates the flow of gastric juices (bile, pancreatic enzymes) into

the duodenum.

Function:

Bile is produced in the liver and aids the digestion of fatty food substances. Between 250 and 1000 millilitres of bile are produced each day. The most important constituents of bile are: water, bile salts, cholesterol , phospholipids . Between meals, the bile is stored in the gallbladder, which contracts once or twice as food is taken. This causes the bile to enter the duodenum via the common bile duct. 80 to 90% of the bile acids are later reabsorbed by the small intestine and re-enter the liver via the bloodstream (portal system).

Fat Digestion & Absorption

Absorption and Motility

Transit time is 12 to 24 hours

reabsorbs water and electrolytes

Feces consist of water & solids (bacteria, mucus, undigested fiber, fat & sloughed epithelial cells

Haustral contractions occur every 30 minutes

distension of a haustrum stimulates it to contract

Mass movements occur 1 to 3 times a day

triggered by gastrocolic and duodenocolic reflexes

filling of the stomach & duodenum stimulates motility

moves residue for several centimeters with each contraction

Defecation

Stretching of the rectum stimulates defecation

intrinsic defecation reflex via the myenteric plexus

causes muscularis to contract & internal sphincter to relax

relatively weak contractions

defecation occurs only if external anal sphincter is voluntarily relaxed

parasympathetic defecation reflex involves spinal cord

stretching of rectum sends sensory signals to spinal cord

splanchnic nerves return signals intensifying peristalsis

Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards

feces will fall away