The Digestive System

pH lecture

Functions of Digestive System

Ingestion – active, conscious process of taking materials into mouth

Mechanical Processing – crushing process that increases the surface area of the food, exposing it to more enzymatic activity (occurs in mouth and stomach)

Digestion – break down of food into small fragments suitable for absorption

Functions of Digestive System (Continued) Secretion – release

of water, acids, enzymes and salts by epithelium of digestive tract

Absorption – movement of ions, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract

Excretion – removal of waste from body fluids

Digestive Pathway

Oral cavity (teeth and tongue)

Pharynx Esophagus Stomach Small Intestine

– Duodenum, Jejunum, Ileum

Large Intestine– Cecum or

(ascending Colon), Colon or (Transverse Colon), Rectum or (Descending Colon)

Digestive Pathway CONT…..

Don’t forget to label the 3 parts of each type of intestine, the pancreas and the appendix

Histology of Digestive Tract

Two major types of tissue that constantly produce a lubricating liquid, allowing all components of digestive tract to slide without friction

Visceral peritoneum – covers organs

Parietal peritoneum – lines inner surfaces of body wall

Oral Cavity Also referred to as the

buccal cavity Functions to:

– Perform sensory analysis of material before swallowing

– Mechanically process material with teeth, tongue, and palatal surfaces

– Lubricate material by mixing it with mucus and saliva

– Perform limited digestion of carbohydrates and lipids

Anatomy of Oral Cavity (Continued)

The tongue functions to:

Mechanically process food

Manipulate food to assist in chewing

Sensory analysis through touch, temp, and taste

Secretion substances including the enzyme lingual lipase (which initiates lipid digestion)

Saliva and Salivary Glands 3 pairs of salivary glands:

– Parotid salivary gland – produce saliva that has high concentration of salivary amylase (begins to break down starches, though no absorption occurs in oral cavity)

• 25% of Saliva

– Sublingual salivary gland –produce saliva that acts as buffer and lubricant

• 5%

– Submandibular salivary gland – produce saliva rich in buffer, libricating proteins called mucins, and salivary amylase

• 70%

Bad Breath

Clinically termed Halitosis

Typically caused by breakdown of epithelial cells, blood cells, and bacteria combined with saliva on tongue

Low Salivary flow – dehydration and morning breath due to breathing through mouth during sleep

Garlic and Onion Breath – compounds taken in blood to lungs

Teeth

Perform mastication (chewing) Mastication breaks down tough

connective tissues in meat and plant fibers and helps to saturate food with saliva

Pharynx and Epiglottis

Pharynx: common passageway for food, liquid, and air

Epiglottis: prevents food from going into the trachea and instead directs it to the esophagus

Esophagus A hollow muscle tube

approximately 25 cm long and 2 cm in diameter

Empties into the stomach through the cardiac sphincter, which prevents backflow from the stomach into the esophagus

Peristalsis= muscle movement of the esophagus. Food at this area is called bolus

Stomach

Performs 4 major functions:– Stores ingested food– Mechanically breaks

down ingested food– Chemically breaks

down ingested food Ingested substances

combine with digestive juices of stomach to form acidic soupy substance called chyme

Anatomy of Stomach Shaped like a J Lesser curvature (1st

part) Greater curvature (2nd

part) Can be divided into four

regions:– Cardia– Fundus– Body– Pylorus

• Pyloric sphincter regulates the release of chyme into the small intestine

Stomach’s Volume

Volume of stomach increases while you eat and decreases as it releases chyme

When full it can contain between 1-1.5 liters of material

Digestion—breaking food down both physically & chemically Food breakdown as mechanical digestion

Mixing food in the mouth by tongue & teeth, churning food in stomach & segmentation in small intestine

Mechanical digestion prepares food for further degradation by enzymes

Digestive System FunctionsMechanical & Chemical Digestion

Digestion—breaking food down both physically & chemically Food breakdown as chemical digestion

Enzymes break down food molecules into their building blocks

Each major food group uses different enzymes

Carbohydrates are broken to simple sugars

Mouth & small intestines

Proteins are broken to amino acids

Stomach & small intestines

Fats are broken to fatty acids & alcohols Small intestines

Digestive System Functions

The Small Intestine Responsible for 90% of

nutrient absorption (most of rest occurs in large intestine)

Averages 6 m (19.7 ft) in length

Averages diameter of 2.5-4.0 cm

Duodenum – “mixing bowl” – combines chyme with digestive secretions from pancreas and liver

Jejunum – bulk of chemical digestion and nutrient absorption occurs here

Ileum – the longest segment which ends in ileocecal valve (separating ileum from cecum of large intestine)

Histology of Small Intestine Plicae circulares

are folds that line the small intestine, greatly increasing surface area for absorption

Plicae are lined by tiny fingerlike projections called intestinal villi, which are covered by simple columnar epithelium

Pancreas Lies posterior to the stomach

and is bound posteriorly to abdominal cavity

Produces 1000 ml of digestive enzymes/day (pancreatic juice) and delivers them to the duodenum through pancreatic duct

When chyme enters the small intestine, it triggers the duodenum to release the hormone secretin, which in turn stimulates the pancreas to release these enzymes

Liver The largest visceral

organ, weighing about 1.5 kg

Primary function is to regulate the composition of circulating blood

Blood from absorptive surfaces of digestive tract enters liver

Liver cells extract nutrients or toxins from blood and then allows filtered blood to reenter systemic circulation

It stores excess nutrients, and releases these to correct nutrient deficiencies

Gallbladder Stores and

concentrate bile immediately before it is released in the the small intestine

It only releases bile when stimulated by the intestinal hormone cholecytstokinin CCK, which is released when chyme enters the duodenum

The Large Intestine Begins where it

meets the Ilium and ends at the anus

Major functions: Reabsorption of

water and the compaction of intenstinal contents into feces

Absorption of vitamins liberated by bacterial action

The storage of fecal material prior to defecation

More About the Large Intestine

Approx. 1.5 m long 7.5 cm wide Divided into three

parts: Cecum Colon (ascending,

transverse and descending)

Rectum

Regions of Colon

The Rectum Expandable Temporarily stores feces, as movement of feces

into rectum triggers urge to defecate Anal canal is last portion of rectum Internal anal sphincter – involuntary (smooth) External anal sphincter – voluntary (skeletal)

Constipation Usually results due to an

infrequent urge to defecate Infrequent bowel movements

(fewer than 3 per week) Difficulty in at least 25% of

the movement Feeling of incomplete

evacuation Type 1 and 2 qualify as

constipated Potential Causes

– Incomplete chewing of food– Insufficient fiber in diet– Dehydration– Quitting smoking (nicotine is a

laxative)

THE END

Vestigial Structures

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