Digestion

• Breakdown of ingested food.

• Absorption of nutrients into the blood.

• Alimentary Canal• A continuous, coiled, hollow, muscular tube

that extends from the mouth to the anus.

• About 9 meters (30 ft) long.

Figure 14.1

Layers of Alimentary Canal Organs

Figure 14.3

Layers of the Alimentary Canal

• 1) Mucosa – innermost layer. In the esophagus friction-resisting Stratified Squamous Epithelium is found. Beyond the esophagus the epithelium is simple columnar.

• Specialized in different regions to: secrete mucous, secrete digestive enzymes, absorb digested materials.

• 2) Submucosa – Found just beneath the mucosa. Connective tissue containing blood vessels, nerve endings, lymph nodes.

• 3) Muscular Layer – Smooth muscle. Inner layer fibers encircle the canal, outer layer fibers run longitudinally the length of the canal. Contraction of inner layer narrows the tube. Wave-like contraction of the outer fibers push the narrowing and food down tube. (Peristalsis)

• 4) Serous Layer – (outermost) Visceral Peritoneum. Cells secrete serous fluid that lubricates the organs.

Mouth (Oral Cavity)• Entrance to the alimentary canal. Functions to

reduce the size of food and mix it with saliva.• Cheeks and lips check the size and

temperature of food.• Tongue aids in mixing food with saliva. Taste

buds determine the suitability of food.• Palate – Roof of the oral cavity. Anterior

portion is the hard palate. Posterior portion is the soft palate.

• Uvula – projection at the end of the soft palate. Causes a reflex that closes the nasal cavity during swallowing.

Teeth (2 sets)

• Primary or Deciduous Teeth – 20 total. Appear between 6 months and 2 ½ years of age. Shed in the same order as they appear. The roots are reabsorbed and pushed up by the next set.

• Secondary or Permanent Teeth - 32 total. Appear between the ages of 6 and 12 years. All but the 3rd molars. The “wisdom teeth” appear between 17 and 25 years of age.

Types of Teeth• Incisors – Front teeth. Chisel shaped.

Used for cutting. 4 in each jaw. (8 total)

• Canines – Cuspids or eye teeth. Used for tearing. 2 in each jaw. (4 total)

• Premolars – Bicuspids. Flat. Used for grinding. 4 in each jaw. (8 total)

• Molars – Also flat and used for grinding. 6 in each jaw. (12 total)

Parts of a Tooth• Crown – Portion above the gum. (gingiva)

• Root – Portion below the gum.

• Enamel – White covering of the crown. Hardest substance in the body. If lost it is not replaced.

• Dentin – beneath the enamel. Like bone.

• Pulp Cavity – Contains blood vessels, nerves and connective tissues.

• Periodontal Ligament – Attaches the tooth to the jawbone.

Salivary Glands – secrete saliva

• Saliva:• 1) Moistens food and dissolves food

chemicals so they can be tasted.

• 2) Binds food together. Contains Mucin. A protein that binds food together.

• 3) Begins carbohydrate digestion. Contains Salivary Amylase. Splits starches into disaccharides.

• 4) Saliva also contains lysozyme and antibodies that inhibit bacterial growth.

3 Pairs of Salivary glands

• Parotid Glands – Largest. Located in front of and below the ear. Secrete a clear watery fluid rich in Amylase.

• Submandibular Glands – Located in the floor of the mouth on the inside surface. Secretion is mostly mucous.

• Sublingual Glands – Located in the floor of the mouth under the tongue. Secretion is also primarily mucous.

Deglutition (Swallowing)

Figure 14.14

Swallowing

• Food, now in the bolus form, is transported to the stomach via the esophagus.

• Soft Palate prevents food from entering the nasal cavity.

• Epiglottis covers the entrance to the Trachea

• A Peristaltic wave moves food down the esophagus.

Esophagus• Runs from pharynx to stomach through the

diaphragm. About 25 cm long.• Conducts food by peristalsis • Muscles are thickened where they join the

stomach. When they are contracted it closes the entrance to the stomach.

• When the Peristaltic wave from swallowing reaches these muscles, they relax and open the stomach.

Stomach Anatomy

Figure 14.4a

Stomach AnatomyCapacity of about 1 Liter

Has inner folds called rugae

Regions of the stomach

-Cardiac region – near the esophageal opening– Fundus – stores food above cardiac region– Body – main portion– Pylorus – funnel-shaped terminal end

• Food empties into the small intestine at the pyloric sphincter

Stomach Functions

• Acts as a storage tank for food• Site of food breakdown. Mixes food with

Gastric Juice.• Chemical breakdown of protein begins• Delivers chyme (processed food) to the

small intestine• Limited amount of absorption. Alcohol,

glucose, water and some fat-soluble vitamins.

Structure of the Stomach Mucosa

Figure 14.4b–c

Stomach Secretions• Mucous lining is thick and made up of several

types of cells• 1) Mucous Cells – produce a sticky alkaline

mucous• 2) Parietal Cells – Produce hydrochloric acid• 3) Chief Cells – Produce protein digesting

enzymes. (pepsinogens) Released in an inactive form, they become active (Pepsin) when mixed with HCl.

• 4) Endocrine Cells – Produce Gastrin. A hormone that increases the protein digesting enzymes produced by the stomach

• Food in the stomach mixed with gastric juice eventually forms a semi-fluid paste called chyme.

• Acidic environment of stomach is very hostile to microorganisms

• Babies also produce Rennin. An enzyme that digests milk protein (Caesin)

• Stomach continually churns and empties into the small intestines about 30 ml at a time. Takes 4 to 6 hours to empty.

Chemical Digestion in the Small Intestine

Figure 14.6

Pancreas

• Located near the entrance to the small intestines.

• Produce pancreatic juice. Contains a wide spectrum of digestive enzymes that breakdown all categories of food.

• Pancreatic juice enters the small intestines (duodenum) through the Pancreatic Ducts

Pancreatic Enzymes• 1) Pancreatic Amylase – Splits starches

into disaccharides.

• 2) Pancreatic Lipase – Breaks down fat molecules into fatty acids and glycerol.

• 3) Proteinases (Trypsin, Chymotrypsin, Carboxypeptidases) – Break down long chains of amino acids (Polypeptides) into small chains of amino acids.

• 4) Nucleases – Breakdown Nucleic acids into nucleotides..

Liver and Gall Bladder• Liver is the largest gland in the body

• Only digestive function is the production of Bile

• Bile is a yellow-green solution containing bile pigments (mainly bilirubin, a breakdown product of hemoglobin) and bile salts

• Bile does not contain any digestive enzymes. Bile salts emulsify fats by breaking large fat globules into smaller ones

• Bile is stored and concentrated in the Gall Bladder. Bile is released into the duodenum through the bile duct

Small Intestines• Longest section of the alimentary canal.

(8-18 ft.) Extends from the Pyloric Sphincter to the Ileocecal valve.

• 3 subdivisions• 1) Duodenum – 1st 10 inches.

• 2) Jejenum – next 40% of the length.

• 3) Ileum – Last 60% of the length.

• Suspended from the abdominal wall by the a double layered Mesentery which contains nerves and blood vessels.

Functions of the Small Intestines• 1) Receive secretions from the liver and

pancreas

• 2) Completes the digestion of chyme

• 3) Site of nutrient absorption into the blood

• 4) Transports residue to the large intestines

• The inner wall of the small intestines has innumerable tiny projections called villi that greatly increases the surface area of the intestinal wall

Villi of the Small Intestines

Each villus consists of a layer of simple columnar epithelium surrounding a blood capillary and a

lacteal

Each villus consists of a layer of simple columnar epithelium surrounding a blood

capillary and a lacteal

Absorption in the Small Intestines• Water is absorbed along the length of the small

intestines• Enzymes located on the cell membrane of the

absorptive cells of the small intestines breakdown the food to its final absorbable form

• Carbohydrates – in the form of disaccharides are broken down into glucose and absorbed by active transport

• Proteins – in the form of peptides are broken down into amino acids and absorbed by active transport

• Nucleic Acids are broken down by nucleases into nucleotides and absorbed by active transport.

• Lipids are absorbed by diffusion

Large Intestine

Figure 14.8

Large Intestines

• Larger in diameter but shorter than the small intestines (1.5 meters long)

• Attaches to the small intestines at the ileocecal valve and extends to the anus

• Regions:– Cecum – Dilated pouch-like structure that

hangs below the ileocecal valve. Veriform Appendix hangs from the cecum. The appendix has no digestive function

• Colon – 4 distinct regions1) Ascending Colon

2) Transverse Colon

3) Descending Colon

4) Sigmoid Colon

Rectum

Anal Canal

Functions of the Large Intestines

• Absorbs water and electrolytes

• Resident bacteria metabolize remaining nutrients making vitamins (K and some B vitamins) and releasing gases (flatus) that contribute to the odor of feces (methane and hydrogen sulfide)

• Secretes Mucous that lubricates and holds feces together