Digestive System
Digestive System
Introduction
·
The digestive system is a complex group of
organs and glands that work together to break down food into simpler molecules,
enabling the body to absorb nutrients for energy, growth, repair, and
maintenance of vital functions.
·
It performs mechanical and chemical processes
to convert complex macromolecules into absorbable forms.
·
The process involves ingestion, propulsion,
digestion, absorption, and elimination.
·
It consists of the alimentary canal (GI
tract) and accessory digestive glands.
Main Functions
- Ingestion
– Intake of food and fluids.
- Digestion
– Mechanical (chewing, churning) and chemical (enzymatic, acid, bile)
breakdown of food.
- Absorption
– Transfer of digested nutrients into blood and lymph.
- Metabolism
– Nutrients used for energy, growth, and repair.
- Elimination
– Excretion of undigested residues as feces.
Anatomy of Digestive Organs
Alimentary Canal (Gastrointestinal Tract)
·
It is a continuous muscular tube about 8–9
meters long extending from mouth to anus.
(a) Mouth and Oral Cavity
- Structures:
lips, cheeks, palate, tongue, teeth, and salivary glands.
- Functions:
- Ingestion
of food.
- Mastication
(chewing) – mechanical breakdown.
- Mixing
food with saliva to form bolus.
- Tongue
helps in swallowing and taste sensation.
(b) Pharynx
- Common
pathway for food and air.
- Divided
into nasopharynx, oropharynx, and laryngopharynx.
- Function:
Swallowing (deglutition) – voluntary and involuntary phases.
(c) Esophagus
- A
25 cm long muscular tube.
- Lies
behind trachea, connects pharynx to stomach.
- Function:
Conduction of food bolus to stomach via peristalsis.
- Lower
esophageal sphincter prevents regurgitation of stomach contents.
(d) Stomach
- A
J-shaped organ located in left upper abdomen.
- Divisions:
Cardia, Fundus, Body, and Pylorus.
- Walls:
mucosa (gastric glands), submucosa, muscularis (three muscle layers),
serosa.
- Functions:
- Temporary
storage of food.
- Secretion
of gastric juice (HCl, pepsinogen, mucus, intrinsic factor).
- Conversion
of food into semi-liquid chyme.
- Initiation
of protein digestion.
(e) Small Intestine
- Longest
part (~6 meters). Divided into:
- Duodenum
(25 cm): receives bile and pancreatic juice.
- Jejunum
(~2.5 m): main site of absorption.
- Ileum
(~3.5 m): absorption of vitamin B12 and bile salts.
- Structure:
inner wall has plicae circulares, villi, and microvilli → increases
surface area.
- Functions:
digestion and absorption of nutrients.
(f) Large Intestine
- Length:
1.5 meters. Divisions: Cecum, Colon (ascending, transverse, descending,
sigmoid), Rectum, Anal canal.
- Functions:
- Absorption
of water and electrolytes.
- Formation,
storage, and expulsion of feces.
- Houses
gut microbiota which synthesize vitamins (K, B-complex).
(g) Anus
- Terminal
opening with internal (involuntary) and external (voluntary) sphincters.
- Function:
Defecation.
Accessory Digestive Glands
1. Salivary Glands
Three major paired glands:
- Parotid
gland → secretes watery, enzyme-rich saliva (contains
salivary amylase).
- Submandibular
gland → secretes mixed (serous + mucous) saliva.
- Sublingual
gland → secretes thick mucous saliva.
- Functions:
lubrication, initiation of starch digestion, antibacterial action
(lysozyme).
2. Liver
- Largest
gland (~1.5 kg). Located in right hypochondrium.
- Divided
into right and left lobes.
- Functions:
- Secretes
bile (bile salts, bile pigments, cholesterol).
- Carbohydrate,
fat, and protein metabolism.
- Detoxification
of drugs and toxins.
- Storage
of glycogen, vitamins, iron.
- Plasma
protein synthesis (albumin, clotting factors).
3. Gallbladder
- Pear-shaped
sac under liver.
- Stores
and concentrates bile, releases it into duodenum via bile duct.
4. Pancreas
- Mixed
gland (endocrine + exocrine).
- Exocrine
portion → secretes pancreatic juice (enzymes
+ bicarbonate).
- Endocrine
portion (Islets of Langerhans) → secretes insulin,
glucagon, somatostatin.
- Pancreatic
juice enzymes:
- Amylase
– starch digestion.
- Lipase
– fat digestion.
- Proteases
(trypsin, chymotrypsin, carboxypeptidase)
– protein digestion.
Mechanism of Digestion
The digestion process is both mechanical and chemical,
involving sequential steps:
1. Ingestion and Mastication (Mouth)
- Food
taken into mouth, broken into small particles by teeth.
- Saliva
moistens and lubricates food, salivary amylase begins breakdown of starch
→ maltose.
- Formation
of bolus.
2. Deglutition (Swallowing)
- Bolus
pushed into pharynx, then to esophagus.
- Peristalsis
propels bolus toward stomach.
3. Gastric Digestion (Stomach)
- Gastric
juice contains:
- HCl
→ provides acidic medium, kills microbes, converts pepsinogen to pepsin.
- Pepsin
→ protein → peptones and proteoses.
- Gastric
lipase → limited fat digestion.
- Intrinsic
factor → vitamin B12 absorption.
- Food
converted into chyme.
4. Intestinal Digestion (Small Intestine)
(a) Duodenum
– receives chyme, bile, and pancreatic juice.
- Bile
salts → emulsify fats into micelles.
- Pancreatic
enzymes:
- Amylase:
starch → maltose.
- Lipase:
fats → glycerol + fatty acids.
- Trypsin
& chymotrypsin: proteins → peptides.
- Carboxypeptidase:
peptides → amino acids.
(b) Jejunum & Ileum
– action of intestinal juice (succus entericus):
- Maltase,
sucrase, lactase → disaccharides → monosaccharides
(glucose, fructose, galactose).
- Peptidases
→ peptides → amino acids.
- Lipase
→ further fat digestion.
5. Absorption
- Small
intestine is primary site.
- Carbohydrates
→ monosaccharides absorbed into blood.
- Proteins
→ amino acids absorbed into blood.
- Fats
→ fatty acids + glycerol absorbed into lymph (lacteals) as chylomicrons.
- Large
intestine absorbs water, electrolytes,
vitamins.
6. Defecation
- Indigestible
substances, dead cells, bacteria, and waste products → form feces.
- Stored
in rectum, expelled via anus under reflex and voluntary control.
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