The Digestive SystemLizzie Tillo

Caroline Gerhardt

Functions: ingests food, breaks it down, absorbs nutrients, eliminates undigestible remains

Not ALL animals have a digestive tract Ex) sponges- food vacuoles

Most animals have a gut or digestive tract Food is digested into small particles that can

cross the plasma membranes

Digestive Tracts

Incomplete tracts have a single opening- entrance for food/exit for wastes Ex) planarians

Complete tracts have a mouth and an anus, and often many specialized parts Ex) earthworms

Typhlosole: internal fold that increases the surface area of digestive tract in order to absorb more nutrient molecules

Incomplete/Complete Tracts

Filter Feeders Ex) clams

Water is always moving into the mantel cavity through the incurrent siphon (slitlike opening), depositing small particles on the gills

Larger particles moved by ciliary action to the labial palps, which force them through the mouth & into the stomach

Ex) baleen whale Baleen hangs from the roof of the mouth and filters krill

(small shrimp) from the water

Continuous Feeders

Require a storage area for food, such as a crop or a stomach

Can be carnivores (ex. blue herons) or herbivores (ex. elephants)

Ex) Squid (mollusk) Has 2 long tentacles with toothed rings that seize prey Pieces pulled into mouth by radula, a tonguelike

structure Stomach (with cecum) retains food until digestion is

complete

Discontinuous Feeders

Filter feeders- omnivores Land snails- herbivores Spiders, sea stars- carnivores Horses- sharp incisors for clipping off blades of grass;

molars for crushing the grass, disrupting the cell wall Bacteria called cecum is able to digest cellulose

Cattle, sheep, deer- graze quickly & swallow partially chewed grasses into a rumen Cud is formed and regurgitated; chewed and swallowed

again for complete digestion

Diet Adaptations

Extracellular 2 major stages:

Mechanical digestion: physical breakdown of food into smaller particles

Chemical digestion: requires enzymes secreted by the digestive tract or by accessory glands to break down macromolecules

Human Digestive Tract

Beginning of digestive tract Palate (roof of the mouth)- separates the oral cavity from the

nasal cavity Cheeks and lips retain food while it is chewed by teeth and

mixed with saliva 3 major pairs of salivary glands send their juices to the mouth

through ducts Salivary amylase- enzyme in saliva that begins the process of

starch digestion Disaccharide maltose is typical end product

Bolus- food and saliva mixture formed in preparation for swallowing

Mouth

Pharynx- common passageway for both food intake and air movement; located between the mouth and the esophagus

Esophagus- tubular structure (25 cm) that takes food to the stomach

Sphincters- muscles that encircle tubes and act as valves (contract- close, relax-open) Lower gastroesophageal sphincter- where esophagus enters

stomach When food enters the esophagus, peristalsis occurs-

rhythmical contraction that serves to move the contents along the tubular organs

Pharynx and Esophagus

Lies on left side of body beneath the diaphragm Deep folds (rugae) disappear as the stomach fills to about 1 liter Epithelial lining contains millions of gastric pits, which have

gastric glands that produce gastric juice pH of about 2; kills bacteria and stops the activity of salivary

amylase Thick layer of mucus protects wall Ulcers- open sores caused by the gradual destruction of tissues Food + gastric juice- chyme

Slow manner in which it enters the small intestine allows for thorough digestion

Stomach

Small in diameter, but about 6 m in length Duodenum- first 25 cm Duct brings bile from the liver and gallbladder, and pancreatic juice

from the pancreas Bile- emulsifies fat, causing fat droplets to disperse in water Slightly basic pH due to sodium bicarbonate from pancreatic juice to

neutralize chyme Food digestion is completed by the enzymes in pancreatic juice and

enzymes produced by the intestinal wall Fingerlike projections called villi- composed of microvilli; greatly

increase surface area for absorption of nutrition Lacteal- vessels of a villus where nutrients are absorbed Then carried to all cells of the body by the boodstream

Small Intestine

Includes cecum, colon, rectum, and anus Larger in diameter but shorter in length (1.5 m) Absorbs water, salts, and some vitamins Stores undigestible material until it is eliminated at the

anus (**NO digestion occurs here**) Cecum- blind end of large intestine; has small projection

called the appendix (plays a role in fighting infections) Colon joins the rectum (last 20 cm of intestine)

About 95% of water is absorbed by the small intestine, and much of the rest is absorbed by the colon

Diarrhea- occurs when water is not absorbed; can lead to serious dehydration

Large amounts of bacteria (esp E Coli) ; break down undigestible material and produce some vitamins

Wastes leave through anus (feces: 25% sold material, 75% water)

Colon is subject to polyps (small growths arising from mucosa) Dietary fat increases likelihood of colon cancer; causes increase in bile secretion, and

intestinal bacteria convert bile salts to substances that promote the development of cancer

Large Intestine

Deep in abdominal cavity Endocrine function- secretes insulin and

glucagon, which keep blood glucose level within normal limits

Exocrine function- produce pancreatic juice with sodium bicarbonate and digestive enzymes for food Neutralizes acid chyme from stomach Amylase digests starch, trypsin digests protein,

and lipase digests fat

Pancreas

Largest gland in body; upper right section of abdominal cavity Triads (between the lobules-structural and functional units)

consist of bile duct, a branch of the hepatic artery, and a branch of the hepatic portal vein

“gatekeeper to blood”; as blood passes through the liver removes poisonous substances and detoxifies them; also removes and stores iron and vitamins

Regulates quantities of cholesterol Maintains blood glucose level Produces bile stored in the gallbladder

Bile slats emulsify fat in the small intestine, providing a larger surface area

Liver

Life-threatening: hepatitis and cirrhosis Affect the entire liver and hinder its ability to repair itself

Jaundice- skin and whites of eyes have a yellowish tinge (bilirubin is deposited in skin due to its large amount in the blood)

Hepatitis: inflammation of the liver A: sewage-contaminated drinking water B: sexual contact or contaminated needles C: contact with infected blood- can lead to chronic hepatitis, liver

cancer, and death Cirrhosis: liver tissue replaced by scar tissue; often in alcoholics,

as the liver is forced to break down excessive amounts Can recover if rate of regeneration exceeds rate of damage

Liver Disorders

Pear-shaped, muscular sac attached to the surface of the liver

Stores excess bile Reabsorbs water so that bile becomes a thick,

mucuslike material can leave and proceed to the duodenum Cholesterol content can come out of solution and

form crystals, whose growth can lead to gallstones Could block common bile duct and cause obstructive

jaundice

Gallbladder