Introducing DigestionDigestion: the process by which the food we eat is broken down into nutrients that can be absorbed into the blood and carried to the cells of the body

Digestive system: The collective unit of organs responsible for digestion

4 main steps: Ingestion: taking in nutrients Digestion: physical and chemical breakdown of complex food molecules Absorption: the transfer of digested nutrients from the digestive system to the bloodstream Egestion: the removal of waste food materials from the body

Specialized digestive systems:

1. Simple digestive systems: animals like flatworms or jellyfish have a digestive sac with a single opening into a gastrovascular cavity. Serves as an entry for and an exit for waste. Food is taken in and digested by chemicals released into the cavity and the nutrients are absorbed by cells that line the cavity, which further diffuse to other cells.

2. Complete Digestive system: Animals such as earthworms have a digestive tract with two openings (one for food intake and one for elimination). The tract is organized into specialized regions (mouth, pharynx, esophagus, crop, gizzard, intestine) that enable the breakdown and absorption of food as food moves along the tract in one direction.

Human Digestive System:

Complete digestive system The digestive tract in humans, unlike earth worms is much longer than our body Referred to as the gastrointestinal tract (GI tract) 7-9m in length The digestive system relies on other organ systems; muscle and bone( to catch and ingest),

circulatory (transport of oxygen and nutrients), the nervous system(regulate actions of digestive organs) and the endocrine system (delivery of hormones and enzymes like insulin for proper digestion)

Pharynx

Digestion in the mouth

Digestion begins in the mouth Food is broken down into smaller pieces by the teeth ( physical digestion)

o Teeth in the front of your mouth (incisor and canines), are specialized for cutting an grabbing your food.

o Molars ( back broad and flat teeth), are specialized for grinding and crushing foodo The type of teeth an animal has dictates the food it eats

Type of tooth Incisor Canine Premolar Molar*

Function Bite or cut food

Tear or shred food

Grind food Crush food

Number in Adult Mouth

8 4 8 12 (8 molars and 4 wisdom teeth)

Saliva

The presence of food in our mouth triggers the salivary glands. Which secrete a watery substance called saliva.

Saliva contains enzymes, chemicals that increase the rate of chemical reactions in living things.o Enzymes in saliva =Amylase: breaks down starch into simpler disaccharides (chemical

digestion) Saliva dissolves food particles, making it possible to taste food and forms it into a ball (bolus)

that can be swallowed Saliva contains mucus, a protective secretion that acts as a lubricant and aids in swallowing On average we produce 0.75 to 1.5 L of saliva per day.

Swallowing

Once the food has been chewed and mixed with saliva, the tongue pushes it to the back of the mouth where it is swallowed

Food is pushed into the pharynx (throat), the soft palate is raised to prevent food from entering the nasal passage

The larynx is raised against the epiglottis, a flap of cartilage at the root of the tongue, which is depressed during swallowing to cover the opening of the windpipe, this covers the trachea to prevent food from entering the lungs

Food moves from the pharynx to the stomach via the esophagus, a long muscular tube. Food moves down the esophagus in rhythmic, involuntary, wavelike contractions of the smooth

muscles of the gastrointestinal tract. This action is known as peristalsis. Peristalsis is responsible for the downward movement and it takes 8 seconds for food to move

down the esophagus. Food would still move downward even if you were to stand on your head!

The Stomach

J-shaped muscular organ that continues mechanical and chemical digestion Can store up to 2L of food

Sphincters: control the movement into and out of the stomacho Gastroesophageal: prevents stomach acid from entering when contractedo Pyloric: a ring of tightly closed muscle that allows chyme to enter intestines

4 layers:o Mucosa: innermost and extensively folded layer. Secretes gastric juices and mucus.

Epithelial cells divide rapidly to heal any damage from the gastric juices (lining replaced every 3 days)

o Submucosa: a layer of connective tissue that contains the nerves and blood vesselso Muscularis: consists of 3 layers of smooth muscle (longitudinal, circular, oblique)

Muscles contract frequently, churning and mixing food (physical digestion)o Serosa: smooth outer layer, holds the stomach in place and secretes a lubricating fluid

that eliminates friction with other organs

Chemical Digestion

esophagus

Gastroesophagealsphincter

serosa

longitudinal

circular

oblique

submucosa

Mucosa (gastric folds)

Pyloric sphincter Mus

cula

ris

Gastric juice breaks down the stomach contents into a semi-solid called Chyme (a mixture of gastric juices and food)

Gastric Juices:o about 2L per day is secretedo the secretion known as gastric juice contains a variety of substances that aid in the

digestion of foodo composed of acid, mucus and enzymes

Contents of gastric juice are:o Hydrochloric acid - loosens tough fibrous material kills bacteria that enters stomach with

food (pH of 2-3)o Pepsin – an enzyme that breaks down proteins into amino acids and polypeptideo Lipase – an enzyme that breaks down triglycerides into fatty acids and glycerol

Process:o Nerves in the lining of the stomach detect when food enters the stomach and stimulates

the secretion of gastric juice and stimulates muscular contractionso Nerves in the submucosa detect the presence of food and release the hormone gastrin

Gastrin: a hormone that releases hydrochloric acid and pepsinogen into the stomach

Pepsinogen when mixed with stomach acid is converted to its active form pepsin

Complications

Acid reflux: occurs if the gastroesophageal sphincter dose not close completely and acid from the stomach enters the esophagus

o Causes a burning sensation in the lower throat that we call heart burno Caused by an overfilled stomach, and excess acid production due to smoking

Stomach ulcers: a lesion or sore on the epithelium of the stomach. Can bleed and can be painful.o Occurs when the stomach lining is exposed to stomach acido Caused by the bacteria Heliobacter pylori burrowing into the mucosao Cured with antibiotics