Digestion and Absorption Biology Notes - Alimentary Canal, Digestive Glands, Digestion, Absorption and Disorders

• Nutrition is the getting and utilization of energy-rich nutrients (food) by an organism.
• Food consists of carbohydrates, proteins, fats (lipids), vitamins, minerals, and water.
• Food provides energy for life activities, materials for growth, maintains body temperature, and repairs tissues.
• The water plays an important role in metabolic processes and prevents dehydration of the body.

I. Alimentary Canal (Gut)

It includes the following parts:

• Mouth: To receive the food (ingestion).
• Buccal Cavity (Oral or Mouth Cavity):
  • Consists of palate (roof), teeth, and muscular tongue.
  • Palate has anterior hard palate and posterior soft palate.
  • Tongue is a muscular organ attached to the floor of oral cavity by the frenulum. Tongue has small projections called papillae. Some papillae bear taste buds.
  • At the back, on either side of tongue, tonsils are present.
• Pharynx:
  • Common passage for digestive and respiratory systems.
  • When food materials pass through the pharynx, the cartilaginous epiglottis closes the glottis (opening of larynx) and prevents the entry of food into trachea.
• Oesophagus:
  • Muscular tube (30 cm) that conducts food into stomach.
  • Posterior part of the oesophagus has gastro-oesophageal sphincter (a circular muscle). It controls the opening of oesophagus into stomach.
• Stomach:
  • ‘J’ shaped structure for storage and digestion of food.
  • 4 parts: a cardiac portion into which the oesophagus opens, a fundic region, body (main central region), and a pyloric portion (antrum).
  • Pyloric stomach leads to small intestine by an opening called Pylorus, guarded by pyloric sphincter muscle.
  • Inner wall of stomach bears rugae (longitudinal folds).
• Small Intestine:
  • Longest part of gut (7 m long and 2.5 cm diameter).
  • Consists of 3 parts, namely duodenum (C-shaped first part), jejunum (middle part), and ileum (terminal part).
  • Finger-like villi are seen at the mucosa. Each villus has a brush-bordered columnar epithelial layer provided with microvilli. Villus consists of a capillary network and a small lymph vessel (lacteal).
• Large Intestine:
  • 1.5 m long. Consists of caecum, colon, and rectum.
  • Caecum is well-developed in herbivores but very small in man. Arising from the caecum is a finger-like vestigial organ, the vermiform appendix.
  • The colon consists of ascending colon, transverse colon, descending colon, and sigmoid colon.
  • Pelvic colon leads to rectum that opens out by anus. Anus is guarded by anal sphincter (circular voluntary muscles).
  • In some herbivores, the large intestine consists of several cellulose-digesting bacteria.

Teeth

Nature & mode of arrangement of teeth is called dentition. Human dentition is Thecodont, Heterodont & Diphyodont.

• Thecodont: It means teeth are placed in the jaw sockets.
• Heterodont: It means different kinds of teeth are present. They are incisors (I) for cutting, canines (C) for tearing, premolars (PM) & molars (M) for mastication. Premolars & molars are collectively called as cheek teeth which have cusps.
• Diphyodont: It means teeth appear twice in the lifetime. They are milk (deciduous) teeth and permanent teeth.

Milk teeth (20 in number) are erupted at 6-7 months of birth. They are replaced by permanent teeth (32 in number) at the age of 6-7. Last 4 molars (wisdom teeth) appear only at the age of 18. The hard chewing surface of teeth is made up of enamel.

Dental Formula: It explains the kinds and number of teeth.

Human dental formula (of permanent teeth):

I²/₂ C¹/₁ PM²/₂ M³/₃ × 2 = 32

Dental formula of milk dentition:

I²/₂ C¹/₁ PM⁰/₀ M²/₂ × 2 = 20

Histology of Human Gut (Transverse Section)

Human gut is formed of 4 layers:

1. Mucosa: Innermost, moist epithelial layer. Contains secretory and absorptive cells.
2. Submucosa: Soft connective tissue layer just outside the mucosa. Nerves and blood vessels are present.
3. Muscularis: Outer to submucosa. Smooth muscle layer (inner circular & outer longitudinal muscles).
4. Serosa: Outermost fibrous layer.

II. Digestive (Associated) Glands

• They secrete digestive juices.
• They include salivary glands, gastric glands, intestinal glands, pancreas, & liver.

a. Salivary Glands

• 3 pairs. They are:
  • Parotids (2): Largest salivary gland. Seen in cheeks.
  • Submaxillary/Submandibular (2): Seen in lower jaw.
  • Sublingual (2): Below the tongue.
• Salivary glands secrete saliva. It contains 99.5% water, mucin (mucus), enzymes like salivary amylase (Ptyalin) and Lysozyme, and electrolytes (Na⁺, K⁺, Cl^-, HCO₃^-, etc).

b. Gastric Glands

Tubular glands found on the inner wall. They consist of:

1. Mucus neck cells: Secrete mucus. Mucus and bicarbonates protect the stomach wall (mucosal epithelium) from HCl and prevent autodigestion.
2. Chief (zymogen or peptic) cells: Secrete proenzymes like pepsinogen & prorennin and some lipase.
3. Oxyntic (parietal) cells: Secrete HCl & Castle’s intrinsic factor (for absorption of vitamin B₁₂).

Gastric glands secrete acidic (pH 1.8-2.0) gastric juice.

c. Liver

• Largest gland (1.2 - 1.5 kg). Reddish brown.
• Situated in abdominal cavity, just below the diaphragm.
• Bilobed (large right lobe & small left lobe). Each lobe is formed of hepatic lobules (structural & functional units).
• A lobule has many hepatic cells arranged as cords around a central vein. They secrete alkaline bile juice.
• Liver lobule is covered by Glisson’s capsule.
• Bile is transported from liver to duodenum as follows:

  Bile → hepatic duct → gallbladder → cystic duct → common bile duct → common hepato-pancreatic duct → duodenum.

• Hepato-pancreatic duct is guarded by sphincter of Oddi.
• Bile has no enzymes but contains bile pigments (bilirubin & biliverdin), bile salts, cholesterol, and phospholipids.

d. Pancreas

• Second largest gland. Seen near duodenal loop.
• It is a cream-coloured heterocrine gland, i.e., it has both exocrine and endocrine parts.
• The exocrine part has a pancreatic duct that opens into duodenum along with bile duct (hepato-pancreatic duct).
• Exocrine part secretes alkaline pancreatic juice. It contains inactive protease enzymes (trypsinogen, chymotrypsinogen & procarboxypeptidases), amylases, lipases, & nucleases.

e. Intestinal Glands

• Simple tubular glands. 2 types:
  1. Crypts of Lieberkuhn: Consists of mucus-secreting Goblet cells and enzyme-secreting Paneth cells.
  2. Brunner (duodenal) glands: Confined to submucosa of duodenum. Secrete mucus only.
• Intestinal glands secrete alkaline intestinal juice (succus entericus). It contains enzymes (maltase, lactase, sucrase, dipeptidase, lipases, nucleotidases, nucleosidases, etc.).
• The bicarbonate and mucus provide alkaline medium and protect intestinal mucosa.

Digestion and Absorption of Food

Digestion is the conversion of complex insoluble food materials into simple and absorbable form. It includes mechanical processes such as mastication (chewing), deglutition (swallowing), & peristalsis (wave-like movement of food bolus through the gut by muscular contraction).

Digestion in buccal cavity

• Only starch digestion.
• Ptyalin converts starch (polysaccharide) into disaccharides. About 30% of starch is digested by ptyalin.

Digestion in Stomach

• Stomach stores food for 4-5 hrs. It is mixed with gastric juice by the churning movements and is converted into acidic pasty form (chyme).

• The gastric lipase hydrolyses a small amount of lipids.

Digestion in small intestine (in Duodenum)

• Chyme is mixed with succus entericus, pancreatic juice & bile juice.
• Action of bile: Bile helps in digestion by emulsification (conversion of fat into micelles or tiny droplets). It provides large surface area for the action of lipase on fat. Bile also activates lipase.
• Action of pancreatic juice: Amylopsin (Pancreatic amylase) hydrolyses remaining starch into disaccharides.

  

  Enterokinase (Enterokinin) secreted by intestinal mucosa activates trypsinogen to active trypsin. Trypsin activates chymotrypsinogen & procarboxypeptidase.

  

• Action of intestinal juice: At duodenum region, the intestinal enzymes act on the products of above reactions.

  

• In large intestine, there is no significant digestive activity.
• The functions of large intestine are:
• Absorption of water, minerals, and certain drugs.
• Secretes mucus for adhering waste (undigested) particles together and lubricating it for an easy passage.

• Fully digested semi-fluid and alkaline food formed in small intestine is called chyle.
• The digestive activities like gastric & intestinal secretions are controlled by neural and hormonal mechanisms. The sight, smell, or presence of food in buccal cavity stimulate salivary secretions. Gastric & intestinal mucosa secretes digestive hormones. They control secretion of digestive juices.

Absorption of Digested Products

Absorption is the transfer of end products of digestion through the intestinal mucosa into blood & lymph.

It is 2 types - passive and active.

• Passive absorption (Passive transport):

  Absorption of nutrients from higher concentrated region to lower concentrated region without the expenditure of energy.

  It includes osmosis (absorption of water) and diffusion (absorption of solute molecules).

  Diffusion is 2 types:

  • Simple diffusion: Molecules alone can be diffused. E.g., small amounts of monosaccharides like glucose, amino acids, vitamins, electrolytes like Cl^-, etc.
  • Facilitated diffusion: Diffusion with the help of carrier proteins. E.g., glucose, amino acids, etc.

• Active absorption (Active transport):

  Absorption of nutrients from lower concentrated region to higher concentrated region (i.e., against concentration gradient). It needs energy. E.g., absorption of amino acids, monosaccharides like glucose, electrolytes like Na⁺, etc.

Absorption of Lipids

• Monoglycerides, diglycerides, and fatty acids cannot be absorbed directly as they are insoluble in water.
• Bile salts and phospholipids convert them into small spherical water-soluble droplets called micelles.
• They are reformed into small protein-coated fat globules (chylomicrons). They are transported into lacteals in the villi. From the lymph, the chylomicrons enter the blood.

Absorption in Different Parts of Alimentary Canal

• Mouth: Certain drugs.
• Stomach: Water, simple sugars, some drugs & alcohol.
• Small intestine (mainly Jejunum & Ileum): All nutrients including minerals & vitamins. It is the chief area of absorption due to the presence of villi, its great length, and coiled nature.
• Large intestine: Water, some minerals & drugs.

The absorbed materials are incorporated into tissues for their activities. It is called assimilation.

The undigested substances like plant fibres, dead bacteria, etc., form faeces. It enters caecum through the ileo-caecal valve, which prevents back flow of faeces.

Faeces are temporarily stored in rectum and are eliminated through anus. It is called egestion (defaecation).

Disorders of Digestive System

• Jaundice:

  The skin and eyes turn yellow due to the deposition of bile pigments. It indicates liver damage.

• Vomiting:

  Ejection of stomach content through the mouth. It is controlled by the medulla oblongata.

• Diarrhoea:

  Frequent elimination of watery faeces. It reduces the absorption of food.

• Constipation:

  Infrequent elimination of dry stool. It is due to decreased peristalsis in the colon.

• Indigestion:

  Condition leading to a feeling of fullness due to improper digestion. It is due to anxiety, inadequate enzyme secretion, food poisoning, spicy food, etc.

• Protein-Energy Malnutrition (PEM):

  Dietary deficiencies of proteins & food calories.

  PEM causes Marasmus & Kwashiorkor in children.

  • Marasmus: Due to deficiency of both proteins and calories. Found in infants less than a year in age.

    Reason: Replacement of mother’s milk by foods with poor proteins and caloric value. This often happens if the mother has a second pregnancy or childbirth when the older infant is still too young.

    Symptoms: Impaired growth and replacement of tissue proteins; extreme emaciation of the body, thin limbs, dry, thin, and wrinkled skin, declined growth rate and body weight, impaired growth and development of brain and mental faculties.

  • Kwashiorkor: Due to protein deficiency only.

    Reason: Replacement of mother’s milk by a high-calorie, low-protein diet in a child more than one year in age.

    Symptoms: Like Marasmus, it shows wasting of muscles, thinning of limbs, failure of growth & brain development. Unlike Marasmus, some fat is still under the skin; extensive oedema and swelling of body parts are seen.