Digestive System Anatomy and Function

• Nutrients are required by all body cells, but cells cannot move to a food source
• The food has to be converted into usable forms, then delivered to the cells
• The digestive system transforms food into nutrients with help from the circulatory system

Anatomy and Physiology

• The digestive system organs are separated into the alimentary canal and the accessory digestive organs

Alimentary Canal

• The alimentary canal is a continuous, hollow muscular tube, running through the ventral body cavity, open at both ends
• Organs of the alimentary canal:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Anal canal

Functions

• Ingestion: Food is placed into the mouth and acted on, active and voluntary
• Propulsion: Foods are processed and propelled from one organ to the next
  • Swallowing is food movement which depends on peristalsis
  • Peristalsis involves involuntary contraction and relaxation of muscles in the organ walls
• Food breakdown
  • Mechanical Digestion: Food is physically fragmented into smaller pieces, preparing it for further degradation
  • Examples: mixing of food by the tounge, churning of food in the stomach, segmentation in the small intestine
  • Chemical Digestion: Large food molecules are broken into building blocks by enzymes
• Absorption: Transport of digested end products from the GI tract lumen, to the blood or lymph
  • Digested foods enter mucosal cells through active or passive transport
• Defecation/Elimination: Elimination of indigestible residues from the GI tract through the anus, in the form of feces

Mouth

• Food enters the digestive tract here, also known as the oral cavity, which is lined with mucous membrane
• Structures:
  • Lips
  • Cheeks
  • Palate
  • Uvula
  • Vestibule
  • Oral cavity proper
  • Tongue
  • Lingual frenulum
  • Palatine tonsils
  • Lingual tonsil

Pharynx

• From the mouth, food passes posteriorly into the oropharynx and laryngopharynx

Esophagus

• Runs from the pharynx, through the diaphragm, to the stomach, also known as the gullet
• Tissues:
  • Mucosa
  • Submucosa
  • Muscularis Externa
  • Serosa
  • Intrinsic Nerve Plexuses
• Size and Function: is about 25 cm (10 inches) long, it is a passageway that conducts food by peristalsis to the stomach
• Structure: From the esophagus to the large intestine, the alimentary canal walls have the same four tissue layers or tunics

Mouth, Pharynx, & Esophagus

• Activities here include food ingestion, breakdown, and propulsion

Food Ingestion and Breakdown

• Physical and Chemical digestion begin here

Physical Breakdown

• Food is physically broken into smaller particles by chewing

Chemical Breakdown

• Salivary amylase begins starch digestion, breaking it down into maltose, when food mixes with saliva

Stimulation of Saliva

• Larger amounts of saliva are released when food enters the mouth or chewing

Passageways

• The pharynx and esophagus have no digestive function
• Simply provides passageways to carry food to the next processing site, the stomach

Food Propulsion

• Food is swallowed to be sent on its way to the mouth

Deglutition

• Swallowing, complex process of coordinated activity of structures such as tongue, soft palate, pharynx, and esophagus

Buccal Phase of Deglutition

• Voluntary phase occurs in the mouth
• Once food is chewed and mixed with saliva, a bolus is forced into the pharynx

Pharyngeal-Esophageal Phase

• The 2nd involuntary phase transports food through the pharynx and esophagus
• Parasympathetic nervous system controls this phase, promoting mobility of the digestive organs from this point on

Food Routes

• Undesired routes are blocked off
• The tongue blocks off the mouth
• The soft palate closes off the nasal passages
• The larynx rises so its opening is covered by the epiglottis

Stomach Entrance

• Food reaches the distal end of the esophagus, it presses against the cardioesophageal sphincter
• This causes it to open, for food to enter the stomach

Stomach (Alimentary Canal)

• Regions:
  • Cardiac Region
  • Fundus
  • Body
  • Pylorus
  • Rugae
  • Greater Curvature
  • Lesser Curvature
• Location: The C-shaped stomach is on the left side of the abdominal cavity, mostly hidden by the liver and the diaphragm
• Function: Temporary storage of food and a site for food breakdown, varies from 15 to 25 cm
• Size: The diameter and volume depend on food amount
  • It can hold about 4 liters (1 gallon) of food when full
  • It collapses inward on itself when empty

Stomach Mucosa

• A simple columnar epithelium of mucous cells produces a protective layer of bicarbonate-rich alkaline mucus
• This protects the stomach wall from acid damage

Gastric Glands

• The otherwise smooth lining is dotted with millions of deep gastric pits which lead into gastric glands
• These glands secrete a solution called gastric juice

Intrinsic Factor

• Some stomach cells produce this substance Needed for vitamin b12 absorption from the small intestine

Chief Cells

• Produce protein-digesting enzymes, mostly pepsinogens

Parietal Cells

• Produce corrosive hydrochloric acid, making the stomach contents acidic and activates enzymes

Enteroendocrine Cells

• Produce local hormones like gastrin, important to the digestive activities of the stomach

Chyme

• After food is processed, resembles heavy cream, and is called chyme

Stomach Activities

• Food breakdown and food propulsion

Food Breakdown

• Sight, smell, and taste of food stimulate parasympathetic nervous system reflexes
• This increases secretion of gastric juice

Gastric Juice

• Secretion regulated by neural and hormonal factors

Gastrin

• Presence of food and a rising pH in the stomach stimulates the stomach cells to release gastrin
• Gastrin prods stomach glands to produce protein-digesting enzymes, mucus, and hydrochloric acid

Pepsinogen

• The hydrochloric acid provides an acidic environment to activate pepsinogen to pepsin
• Pepsin is the active protein-digesting enzyme

Rennin

• The 2nd protein-digesting enzyme produced by the stomach
• Works mostly on milk protein, converting it to a sour milk substance

Food Entry

• As food enters the stomach, its wall begins to stretch

Stomach Wall Activation

• Then the three muscle layers of the stomach wall compress and pummel the food
• This breaks it apart, while mixing it with enzyme-containing gastric juice, semifluid chyme is formed

Food Propulsion

• Peristalsis moves food towards the digestive site until the intestines

Pyloric Passage

• The pylorus of the stomach holds 30 ml of chyme
• Acts like a meter that only allows liquids and small particles to pass through
• Each stomach muscle contraction squirts 3 ml or less of chyme into the small intestine

Enterogastric Reflex

• It helps the gastric activity, putting the brakes on" to slow the emptying activity on the stomach

Small Intestine (Alimentary Canal)

• The small intestine is the body's major digestive organ

Location

• A muscular tube extending from the pyloric sphincter to the large intestine

Size

• The longest section of the alimentary tube, with an average length of 2.5 to 7 m (8 to 20 feet) in a living person

Subdivisions

• The small intestine has three subdivisions: the duodenum, the jejunum, and the ileum

Ileocecal Valve

• The ileum meets the large intestine at the ileocecal valve

Hepatopancreatic Ampulla

• The main pancreatic and bile ducts form this at the duodenum

Duodenal Papilla

• From there, the bile and pancreatic juice travel to the duodenal papilla and enter the duodenum together

Microvilli

• These are projections of the plasma membrane of the mucosa cells
• This gives the cell surface a fuzzy appearance that gives the mucosa

Villi

• Finger-like projections that gives it a velvety appearance and fee, much like the soft nap of a towel.

Lacteal

• Within each villus is a rich capillary bed and a modified lymphatic capillary called a lacteal.

Circular folds

• Also called plicae circulares, and are deep folds of both mucosa and submucosa layers
• They do not disappear when food fills the small intestine

Peyer's Patches

• Collections of lymphatic tissue in the submucosa increase toward the end of the small intestine

Small Intestine Actions

• Food Breakdown and Absorption and food propulsion

Food Breakdown and Absorption

Explore the digestive system's primary function, components, and processes like peristalsis. Learn about the roles of the alimentary canal, oral cavity, and related structures in breaking down food and nutrient distribution. Test your knowledge of digestive anatomy and physiology.