Digestive System Anatomy

THE DIGESTIVE SYSTEM

PROCESSES OF THE BODY RELATED TO THE DIGESTIVE SYSTEM

  • Digestion = Breakdown of ingested food

*Absorption of nutrients into the blood

*Assimilation of nutrients into cells

*Cellular Respiration : A.Liberation of energy (ATP) from nutrients

CELLULAR ACTIVITIES
  • Constructive (Anabolic) and (Catabolic) degradative cellular activities

  • Defaecation

FUNCTIONS OF THE DIGESTIVE SYSTEM
Ingestion—placing food into the mouth
Propulsion—moving foods from one region of the digestive system to another

  • Peristalsis—alternating waves of contraction and relaxation that squeezes food along the GI tract

  • Segmentation—moving materials back and forth to aid with mixing in the small intestine

Food breakdown as mechanical digestion

Examples:

  • 1.Mixing food in the mouth by the tongue

  • 2.Churning food in the stomach

  • 3.Segmentation in the small intestine

Mechanical digestion prepares food for further degradation by enzymes

Food breakdown as chemical digestion

Enzymes break down food molecules into their building blocks

Each major food group uses different enzymes

  • 1.Carbohydrates are broken to simple sugars

  • 2.Proteins are broken to amino acids

  • 3.Fats are broken to fatty acids and alcohols


Absorption

A.End products of digestion are absorbed in the blood or lymph

B.Food must enter mucosal cells and then into blood or lymph capillaries

Defecation

Elimination of indigestible substances from the GI tract in the form of feces







ORGANS OF THE DIGESTIVE SYSTEMIN PROGRESSSS
Two main groups

  • A.Alimentary canal ̵ continuous coiled hollow tube
  • B.Accessory digestive organs

DigestiveSystFull.jpg

Digestive Tract/Alimentary Canal
  • A.Mouth
  • B.Pharynx
  • C.Esophagus
  • D.Stomach
  • E.Small intestine
  • F.Large intestine
  • G.Anus

THE ORAL CAVITY/BUCCAL CAVITY
  • Lips ̵ protect the anterior opening
    • tonsils.jpg


  • Cheeks ̵ form the lateral walls


  • Hard palate ̵ forms the anterior roof

  • Soft palate ̵ forms the posterior roof

  • Uvula ̵ fleshy projection of the soft palate

  • Vestibule ̵ space between lips and teeth and gums internally

  • Oral cavity ̵ area contained by the teeth
  • Tongue ̵ attached at hyoid and styloid processes of the skull, and by the lingual frenulum


Processes of the Mouth

Teeth cut food - bite size to fit in mouth

Mastication (chewing) of food

Mixing masticated food with saliva

Allowing for the sense of taste

Initiation of swallowing by the tongue

Formation of bolus


THE TEETH

MouthFulabeld.jpg




Pharynx Anatomy and Functions

Nasopharynx ̵ not part of the digestive system

Oropharynx ̵ posterior to oral cavity

Laryngopharynx ̵ below the oropharynx and connected to the esophagus

Functions:
Serves as a passageway for air and food

Food is propelled to the esophagus by two muscle layers
  • A.Longitudinal inner layer
  • B.Circular outer layer

Food movement is by alternating contractions of the muscle layers (peristalsis)

OESOPHAGUS
Food Runs from pharynx to stomach in the esophagus throught diaphragmatic hiatus

Esophagus Conducts food by peristalsis (slow rhythmic squeezing)

Esophagus is the Passageway for food only (respiratory system branches off after the pharynx)

Layers of Walls of AC Organs
Mucosa

Innermost layer

Moist membrane
  • 1.Surface epithelium

  • 2.Small amount of connective tissue (lamina propria)

  • 3.Small smooth muscle layer

Submucosa
  • 1.Just beneath the mucosa

  • 2.Soft connective tissue with blood vessels, nerve endings, and lymphatic

Muscularis externa ̵ smooth muscle

  • Inner circular layer
  • Outer longitudinal layer

Serosa
  • Outermost layer ̵ visceral peritoneum
  • Layer of serous fluid-producing cells


Nervous Networks in A.C.
All are part of the autonomic nervous system

Three separate networks of nerve fibers
  • A.Submucosal nerve plexus
  • B.Myenteric nerve plexus
  • C.Subserous plexus

THE STOMACH

StomachWall.jpg
Regions of the stomach

  • A.Cardiac region ̵ near the heart
  • B.Fundus
  • C.Body
  • D.Pylorus ̵ funnel-shaped terminal end

Food empties into the small intestine from the pyloric sphincter


Layers of peritoneum attached to the stomach

  • A.Lesser omentum ̵ attaches the liver to the lesser curvature

  • B.Greater omentum ̵ attaches the greater curvature to the posterior body wall

  • C.Contains fat to insulate, cushion, and protect abdominal organs


FUNCTIONS OF THE STOMACH
Site of food breakdown
Chemical breakdown of protein begins
Delivers chyme (processed food) to the small intestine
The body’s major digestive organ
Site of nutrient absorption into the blood


Specialized mucosa of the stomach
Simple columnar epithelium

  • A.Mucous neck cells ̵ produce a sticky alkaline mucus
  • B.Gastric glands ̵ secrete gastric juice
  • C.Chief cells ̵ produce protein-digesting enzymes (pepsinogens)
  • D.Parietal cells ̵ produce hydrochloric acid
  • E.Endocrine cells ̵ produce gastrin
Gastric pits formed by folded mucosa
Glands and specialized cells are in the gastric gland region

THE SMALL INTESTINE
Muscular tube extending form the pyloric sphincter to the ileocecal valve

Suspended from the posterior abdominal wall by the mesenteryLiverDuoPancreas.jpg

Regions of the small intestine
Duodenum

  • Attached to the stomach
  • Curves around the head of the pancreas
Jejunum : Attaches anteriorly to the duodenum

Ileum : Extends from jejunum to large intestine

Properties of the small intestine
Source of enzymes that are mixed with chyme: A.Intestinal cells B.Pancreas

Bile enters from the gall bladder - does not contain any enzymes

Fingerlike structures formed by the mucosa give the small intestine more surface area

IntestinalMucosa.jpg
Contains Absorptive cells .Blood capillaries Lacteals (specialized lymphatic capillaries)

Folds of the Small Intestine
Called circular folds or plicae circulares

Deep folds of the mucosa and submucosa

Do not disappear when filled with food

The submucosa has Peyer’s patches (collections of lymphatic tissue)



smalIntcolon.jpg



THE LARGE INTESTINE [COLON]

Larger in diameter, but shorter than the small intestine
Frames the internal abdomen
FUNCTIONS OF LARGE INTESTINE
Absorption of water Eliminates indigestible food from the body as feces
ileocaecaljunction.jpg Does not participate in digestion of food Goblet cells produce mucus to act as a lubricant Bacteria produce vitamins

STRUCTURES OF THE LARGE INTESTINE
Caecum ̵ saclike first part of the large intestine
  • Appendix Hangs from the caecum .
  • Contains an accumulation of lymphatic tissue that sometimes becomes inflamed (appendicitis)


Colon : Ascending , Transverse , Descending , S-shaped sigmoidal
Rectum
Anus ̵ external body opening
Smooth muscle is reduced to three bands (teniae coli) V.Muscle bands have some degree of tone
Walls are formed into pocketlike sacs called haustra




ACCESSORY GLANDS OF THE DIGESTIVE SYSTEM



Salivary glands , Teeth , Pancreas , Liver , Gall bladder

THE SALIVARY GLANDS



SalivaryGlands (2).jpg



Three pairs of salivary glands empty secretions into the mouth

A.Parotid glands .Found anterior to the ears

B.Submandibular glands

C.Sublingual glands

Both submandibular and sublingual glands empty saliva into the floor of the mouth through small ducts

COMPOSITION OF SALIVA

Approx 98% water

Contains solutes: Na+ , K + , Cl - , PO4=- ,HCO3-

Has pH of 6.7 - 7.00

Contains lysozyme, IgA, defensin, cyanidic compound

Some metabolic waste

Mixture of mucus and serous fluids

FUNCTIONS OF SALIVA

Helps to form a food bolus

Contains salivary amylase to begin starch digestion

Dissolves chemicals so they can be tasted

Intrinsic salivary glands - continuous secretions - keep mouth moist Small amounts from intrinsic glands

REGULATION OF SALIVATION

Stimulation by food - chemoreceptors in mouth send signal to Salivatory nuclei in Pons and MO

Parasympathetic stimulation via Facial & Glossopharyngeal to salivary glands

Extrinsic glands secrete large qty of saliva I.Approx 1 - 1,5 litre saliva per day

Smell or sight of food enough to set off salivatory nuclei

Sympathetic stimulation - thick saliva - dry mouth syndrome




THE TEETH

Moutheeth.jpg
THE DENTAL FORMULA
Primary Dentition / Milk Teeth
2 I 1 C 2 M (5 teeth left / also 5 right)

2 I 1 C 2 M “ ” “ ”

Thus a total of 20 deciduous teeth

Permanent Dentition
2 I 1 C 2 PM 3 M

2 I 1 C 2 PM 3 M

A total of 32 permanent teeth


STRUCTURE OF A TYPICAL TOOTH

Crown—exposed part

  • A.Enamel—hardest substance in the body

  • B.Dentin (Dentyne) —found deep to the enamel and forms the bulk of the tooth

  • C.Pulp cavity—contains connective tissue, blood vessels, and nerve fibers

  • D.Root canal—where the pulp cavity extends into the root

Neck

  • A.Region in contact with the gum

  • B.Connects crown to root

Root

Cementum—covers outer surface and attaches the tooth to the periodontal membrane



THE LIVERLiverDuoPancreas.jpg
Largest gland in the body

Located on the right side of the body under the diaphragm

Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament

Connected to the gallbladder via the common hepatic duct


Mass about 1,2 - 1,5 kg

Functional units = lobules = hexagonal
Hepatocytes radiate outward from central vein
Each corner of hexagonal lobule has triad: = branch of :hepatic artery, hepatic portal vein and bile duct

Between hepatocytes = large sinusoidal capillaries--- form liver sinusoids
Sinusoids contain blood from HA and HPV
Blood collects in central vein
Contains bile canaliculi through which bile flows to larger ducts

The liver has several roles in digestion
  • Manufactures bile

  • Detoxifies drugs and alcohol

  • Degrades hormones

  • Produces cholesterol, blood proteins (albumin and clotting proteins)

  • Contains star-shaped macrophages (Kupffer cells) that remove debris -bacteria , worn out rbc.

  • Can regenerate if part of it is damaged or removed

THE GALL BLADDER

Sac found in hollow fossa of liver

When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder

When digestion of fatty food is occurring, bile is introduced into the duodenum from the gallbladder

Gallstones are crystallized cholesterol which can cause blockages

BILE
Produced by cells in the liver

Bile leaves the liver through the common hepatic duct

Composition is:
  • Bile salts

  • Bile pigments (mostly bilirubin from the breakdown of hemoglobin)

  • Cholesterol , Phospholipids , Electrolytes

Function—emulsify fats by physically breaking large fat globules into smaller ones


THE PANCREAS

Large part located posterior to the parietal peritoneum

Its location is partially retroperitoneal

Extends across the abdomen from spleen to duodenum

“Behind” greater curvature of the stomach

STRUCTURE OF THE PANCREAS

Pancreatic acini = clusters of secretory cells

Extensive RER - Zymogen granules with digestive enzymes

Pancreatic Islets (Langerhans) - Alpha cells,,, Beta cells

FUNCTIONS

Produces a wide spectrum of digestive enzymes that break down all categories of food

Enzymes are secreted into the duodenum

Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from stomach

Hormones produced by the pancreas : Insulin , Glucagon

Approx 1,2- 1,5 litre produced daily

Clear watery fluid

Has pH of about 8

Contains enzymes and electrolytes