Digestion and Absorption

Food is one of the basic requirements of all living organisms. The major components of our food are carbohydrates, proteins, and fats. Vitamins and minerals are also required in small quantities. Food provides energy and organic materials for growth and repair of tissues. The water we take in plays an important role in metabolic processes and also prevents dehydration of the body. Biomacromolecules in food cannot be utilized by our body in their original form. They have to be broken down and converted into simple substances in the digestive system. This process of conversion of complex food substances to simple absorbable forms is called digestion and is carried out by our digestive system by mechanical and biochemical methods.

• Overview: The alimentary canal is a continuous muscular tube running from the mouth to the anus. It begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.
• Buccal Cavity (Oral Cavity): The mouth leads to the buccal cavity or oral cavity. The oral cavity has a number of teeth and a muscular tongue. Each tooth is embedded in a socket of jaw bone. This type of attachment is called thecodont.
• Dentition:
  - Diphyodont: Majority of mammals including human beings form two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth.
  - Heterodont: An adult human has 32 permanent teeth which are of four different types: Incisors (I), Canines (C), Premolars (PM), and Molars (M).
  - Dental Formula: Arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula which in human is 2123/2123.
  - Structure: The hard chewing surface of the teeth, made up of enamel (the hardest substance in the body), helps in the mastication of food. Enamel is secreted by Ameloblasts (ectodermal).
• Tongue:
  - The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum.
  - The upper surface of the tongue has small projections called papillae, some of which bear taste buds.
  - Papillae Types:
    • Circumvallate: Largest, form a V-shape at the back.
    • Fungiform: Mushroom-shaped, red dots, contain taste buds.
    • Filiform: Smallest, most numerous, NO taste buds, provide tactile sensation.
    • Foliate: Vestigial in humans.

• Pharynx: The oral cavity leads into a short pharynx which serves as a common passage for food and air. The oesophagus and the trachea (wind pipe) open into the pharynx. A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the wind pipe – during swallowing (deglutition).
• Oesophagus:
  - The oesophagus is a thin, long tube (approx. 25cm) which extends posteriorly passing through the neck, thorax, and diaphragm and leads to a ‘J’ shaped bag-like structure called the stomach.
  - Sphincter: A muscular sphincter (gastro-oesophageal) regulates the opening of the oesophagus into the stomach.
  - Histology Note: The upper 1/3rd is skeletal muscle (voluntary), middle 1/3rd is mixed, and lower 1/3rd is smooth muscle (involuntary).
  - No digestion occurs here; only peristalsis transfers food (bolus) to the stomach.

• Location & Structure: The stomach, located in the upper left portion of the abdominal cavity, has four major parts:
  1. Cardiac: Portion into which the oesophagus opens.
  2. Fundic: Dome-shaped region, usually filled with gas/air.
  3. Body: The main central region where most digestion occurs.
  4. Pyloric: Opens into the first part of the small intestine.
• Sphincters:
  - Gastro-oesophageal (Cardiac) Sphincter: At the junction of oesophagus and stomach.
  - Pyloric Sphincter: Guards the opening of the stomach into the duodenum.
• Storage: The stomach stores food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.

• Small Intestine: Distinguishable into three regions:
  - Duodenum: 'C' shaped (or U-shaped as per older texts). Shortest and widest part. Receives the hepatopancreatic duct.
  - Jejunum: A long coiled middle portion. Redder and more vascular.
  - Ileum: A highly coiled region. Opens into the large intestine. Contains Peyer's Patches (lymphoid tissue).
• Large Intestine: Consists of caecum, colon, and rectum.
  - Caecum: A small blind sac which hosts some symbiotic micro-organisms. A narrow finger-like tubular projection, the vermiform appendix, which is a vestigial organ, arises from the caecum.
  - Colon: Divided into four parts – an ascending, a transverse, descending part and a sigmoid colon.
  - Rectum: The descending part opens into the rectum which opens out through the anus.

The wall of the alimentary canal from oesophagus to rectum possesses four layers:

• 1. Serosa: Outermost layer made up of thin mesothelium (epithelium of visceral organs) with some connective tissue. Absent in the oesophagus (replaced by tunica adventitia).
• 2. Muscularis: Formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer. An oblique muscle layer may be present in the stomach (inner to circular).
  - Auerbach’s Plexus (Myenteric): Controls peristalsis.
• 3. Sub-mucosa: Formed of loose connective tissues containing nerves, blood, and lymph vessels. In the duodenum, glands are also present in sub-mucosa (Brunner’s Glands).
  - Meissner’s Plexus (Submucosal): Controls secretion.
• 4. Mucosa: Innermost layer lining the lumen.
  - Stomach: Forms irregular folds (rugae/gastric folds).
  - Small Intestine: Forms finger-like foldings called villi. The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance. These modifications increase the surface area enormously.
  - Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.
  - Mucosal epithelium has goblet cells which secrete mucus that helps in lubrication.
  - Forms glands in the stomach (gastric glands) and crypts in between the bases of villi intestine (Crypts of Lieberkuhn).

• Salivary Glands: Saliva is mainly produced by three pairs of salivary glands.
  1. Parotids: (Cheek) Largest, duct is Stensen's duct. Viral infection causes Mumps.
  2. Sub-maxillaries/Sub-mandibular: (Lower jaw) Duct is Wharton's duct. Produces max saliva (70%).
  3. Sub-linguals: (Below the tongue) Smallest, duct is Duct of Rivinus.
  - Saliva composition: Water (99.5%), electrolytes (Na+, K+, Cl-, HCO3-), salivary amylase (ptyalin), and lysozyme. pH 6.8.
• Liver:
  - Largest gland of the body weighing about 1.2 to 1.5 kg in an adult human.
  - Situated in the abdominal cavity, just below the diaphragm and has two lobes.
  - Hepatic Lobules: Structural and functional units containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule (characteristic of mammalian liver).
  - Bile: Secreted by hepatic cells, passes through hepatic ducts and is stored and concentrated in the gall bladder.
  - Duct System: Cystic duct (gall bladder) + Hepatic duct (liver) = Common Bile Duct. Common Bile Duct + Pancreatic Duct = Hepato-pancreatic duct (opens into duodenum), guarded by Sphincter of Oddi.
• Pancreas:
  - Compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘C’ shaped duodenum.
  - Exocrine part: Secretes an alkaline pancreatic juice containing enzymes.
  - Endocrine part: Secretes hormones, insulin and glucagon. Islets of Langerhans.

• Digestion in Mouth:
  - Mastication of food and facilitation of swallowing. Mucus helps in lubricating and adhering the masticated food particles into a bolus.
  - Chemical Process: About 30% of starch is hydrolysed here by salivary amylase (optimum pH 6.8) into a disaccharide – maltose.
  - Starch + Salivary Amylase (pH 6.8) → Maltose
  - Lysozyme acts as an antibacterial agent that prevents infections.
• Digestion in Stomach:
  - The mucosa of the stomach has gastric glands with three major types of cells:
    1. Mucus neck cells: Secrete mucus.
    2. Peptic or Chief cells: Secrete the proenzyme pepsinogen.
    3. Parietal or Oxyntic cells: Secrete HCl and intrinsic factor (essential for absorption of vitamin B12).
  - Action: HCl provides the acidic pH (pH 1.8) optimal for pepsin. Pepsinogen (inactive) + HCl → Pepsin (active).
  - Pepsin converts proteins into proteoses and peptones (peptides).
  - Rennin: Protelytic enzyme found in gastric juice of infants for digestion of milk proteins (Casein → Calcium Paracaseinate).
  - Small amounts of lipases are also secreted by gastric glands.
• Digestion in Small Intestine:
  - The bile, pancreatic juice, and intestinal juice are the secretions released into the small intestine.
  - Pancreatic Juice Action: Contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, and nucleases.
    • Trypsinogen + Enterokinase (from intestinal mucosa) → Trypsin.
    • Trypsin activates other pancreatic enzymes.
    • Proteins/Peptones + Trypsin/Chymotrypsin/Carboxypeptidase → Dipeptides.
    • Polysaccharides (Starch) + Amylase → Disaccharides.
    • Fats + Lipases → Diglycerides → Monoglycerides.
    • Nucleic acids + Nucleases → Nucleotides → Nucleosides.
  - Bile Action: Bile helps in emulsification of fats, i.e., breaking down of the fats into very small micelles. Bile also activates lipases. Bile contains bile pigments (bilirubin and biliverdin), bile salts (sodium glycocholate/taurocholate), cholesterol and phospholipids but no enzymes.
  - Succus Entericus Action: Secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice.
    • Dipeptides + Dipeptidases → Amino acids.
    • Maltose + Maltase → Glucose + Glucose.
    • Lactose + Lactase → Glucose + Galactose.
    • Sucrose + Sucrase → Glucose + Fructose.
    • Nucleotides + Nucleotidases → Nucleosides + Nucleosidases → Sugars + Bases.
    • Di/Monoglycerides + Lipases → Fatty acids + Glycerol.
  - The breakdown of biomacromolecules occurs in the duodenum. The simple substances formed are absorbed in the jejunum and ileum.

• Definition: Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.
• Mechanisms:
  - Passive Transport (Simple Diffusion): Small amounts of monosaccharides like glucose, amino acids, and some electrolytes like chloride ions depend on concentration gradients.
  - Facilitated Transport: Substances like fructose and some amino acids are absorbed with the help of carrier ions like Na+.
  - Active Transport: Occurs against the concentration gradient and requires energy. Nutrients like amino acids, monosaccharides like glucose, electrolytes like Na+ are absorbed into the blood by this mechanism.
• Absorption of Fats: Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.
  - They are first incorporated into small droplets called micelles which move into the intestinal mucosa.
  - They are re-formed into very small protein-coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi.
  - These lymph vessels ultimately release the absorbed substances into the blood stream.
• Summary of Absorption Sites:
  - Mouth: Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are absorbed into the blood capillaries lining them.
  - Stomach: Absorption of water, simple sugars, and alcohol takes place. Note: Aspirin is absorbed here.
  - Small Intestine: Principal organ for absorption of nutrients. The digestion is completed here and the final products of digestion such as glucose, fructose, fatty acids, glycerol, and amino acids are absorbed through the mucosa into the blood stream and lymph.
  - Large Intestine: Absorption of water, some minerals, and drugs takes place.
• Assimilation & Egestion:
  - The absorbed substances finally reach the tissues which utilise them for their activities. This process is called assimilation.
  - The digested wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal. The egestion of faeces to the outside through the anal opening (defaecation) is a voluntary process and is carried out by a mass peristaltic movement.

• The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections. The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pinworm, etc.
• Jaundice: The liver is affected, skin and eyes turn yellow due to the deposit of bile pigments.
• Vomiting: It is the ejection of stomach contents through the mouth. This reflex action is controlled by the vomit centre in the medulla. A feeling of nausea precedes vomiting.
• Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.
• Constipation: In constipation, the faeces are retained within the colon as the bowel movements occur irregularly.
• Indigestion: In this condition, the food is not properly digested leading to a feeling of fullness. The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.
• PEM (Protein Energy Malnutrition):
  - Kwashiorkor: Protein deficiency alone. Occurs in children > 1 year. Edema (water retention), pot belly, wasted muscles, thin limbs. Psychomotor development retarded.
  - Marasmus: Deficiency of both Protein and Calories. Occurs in infants < 1 year. Extreme emaciation, no edema, dry wrinkled skin.

• Gastrin: Secreted by G-cells (Pyloric stomach). Stimulates gastric glands to secrete HCl and Pepsinogen.
• Secretin: First hormone discovered. Secreted by duodenal mucosa (S-cells). Acts on exocrine pancreas to release water and bicarbonate ions (neutralizes acid).
• CCK-PZ (Cholecystokinin-Pancreozymin): Secreted by duodenum. Stimulates Gall bladder contraction (bile release) and Pancreas (enzyme release).
• GIP (Gastric Inhibitory Peptide) / Enterogastrone: Inhibits gastric secretion and motility.
• Duocrinin: Stimulates Brunner's glands for mucus secretion.
• Villikinin: Stimulates movement of villi for faster absorption.

Thecodont is a type of dentition found in mammals, including humans, and crocodiles. In this arrangement, each tooth is firmly embedded in a deep socket of the jaw bone (alveolus). This provides strong anchorage for the teeth, enabling them to withstand the high mechanical stresses generated during the mastication and crushing of food particles.

Majority of mammals including human beings form two sets of teeth during their life. The first set is the temporary milk or deciduous teeth (20 in number) which appears during childhood. These are later replaced by a set of permanent or adult teeth (32 in number). This condition of having two successive sets of teeth is called diphyodont.

An adult human has 32 permanent teeth which are of four different types structurally and functionally. This condition is known as Heterodont dentition. The types are Incisors (for cutting), Canines (for tearing), Premolars (for crushing), and Molars (for grinding). This ensures efficient mechanical breakdown of various food types.

The arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, M is represented by a dental formula. For an adult human, it is 2123 / 2123. This means in each quadrant there are 2 Incisors, 1 Canine, 2 Premolars, and 3 Molars. The total number of teeth is calculated as (2+1+2+3) x 4 = 32.

Enamel is the hard, visible outer surface of the tooth and is chemically the hardest substance in the human body. It is highly mineralized (mostly Hydroxyapatite crystals) and helps in the mastication of food. Enamel is secreted by special cells called Ameloblasts which are ectodermal in origin. Once formed, enamel cannot regenerate.

The tongue is a freely movable muscular organ attached to the floor of the oral cavity by a fold of tissue called the frenulum. The upper surface of the tongue has small projections called papillae, some of which bear taste buds. It aids in mixing saliva with food, swallowing, and articulation of speech.

The pharynx is a common passage for food and air. During swallowing (deglutition), a cartilaginous flap called the epiglottis covers the glottis (the opening of the windpipe or trachea). This reflex action prevents the entry of food or liquid into the respiratory tract, thereby preventing choking.

The oesophagus is a thin, long, muscular tube (approx. 25 cm in length) which extends posteriorly passing through the neck, thorax, and diaphragm. It conveys the food bolus from the pharynx to the stomach using wave-like muscular contractions called peristalsis. It does not play any role in the chemical digestion of food.

A muscular ring called the gastro-oesophageal sphincter (or cardiac sphincter) regulates the opening of the oesophagus into the stomach. It relaxes to allow the passage of bolus into the stomach and contracts to prevent the acidic reflux (regurgitation) of stomach contents back into the oesophagus, which can cause heartburn.

The pyloric sphincter is a band of smooth muscle at the junction between the pylorus of the stomach and the duodenum of the small intestine. It acts as a valve to control the flow of partially digested food (chyme) from the stomach to the small intestine, releasing it in small amounts for efficient digestion.

The stomach is J-shaped and has four parts. The Fundic region (fundus) is the upper dome-shaped part located superior to the opening of the oesophagus. It typically contains swallowed air or gas and can accommodate food when the stomach is full. It is one of the distinct anatomical regions alongside the cardiac, body, and pyloric parts.

The duodenum is the first and shortest segment of the small intestine, shaped like a 'C' (or 'U' in older texts). It receives the chyme from the stomach and digestive secretions from the pancreas (enzymes) and liver (bile) via the hepatopancreatic duct. It is the primary site for the mixing of chyme with digestive enzymes.

The jejunum is the long, coiled middle portion of the small intestine connecting the duodenum to the ileum. It has a thick intestinal wall, is redder, and more vascular than the ileum. It is the principal site of absorption for sugars, amino acids, and fatty acids that were digested in the duodenum.

The ileum is the final and longest segment of the small intestine, which is highly coiled. It opens into the large intestine (caecum) via the ileo-caecal valve. It contains Peyer's patches (lymphoid tissue) which play an important role in immunity. It absorbs Vitamin B12 and bile salts.

The caecum is a small blind sac situated at the junction of the small and large intestines. It hosts some symbiotic micro-organisms (gut flora) that aid in the synthesis of certain vitamins (like Vitamin K and B12) and the fermentation of undigested carbohydrates. It is more developed in herbivores (rabbit) for cellulose digestion.

A narrow finger-like tubular projection called the vermiform appendix arises from the caecum. It is considered a vestigial organ in humans, meaning it has lost its original function (cellulose digestion) during evolution. However, in modern biology, it is believed to act as a safe house for good bacteria.

The colon is divided into four main parts: an ascending colon (moves up right side), a transverse colon (crosses abdomen), a descending colon (moves down left side), and a sigmoid colon (S-shaped). The main function of the colon is to absorb water, electrolytes, and form solid faeces from indigestible residues.

The serosa is the outermost layer of the alimentary canal wall. It is made up of a thin mesothelium (epithelium of visceral organs) with some underlying connective tissue. It provides protection and reduces friction with other organs. Note that serosa is absent in the oesophagus, where it is replaced by tunica adventitia.

The muscularis layer is formed by smooth muscles. These are usually arranged into an inner circular and an outer longitudinal layer. The coordinated contractions of these layers produce peristalsis, which propels food through the gut, and mixing movements that blend food with digestive juices.

In the stomach, there is an additional layer of muscle fibers called the oblique muscle layer, situated inner to the circular muscle layer. This extra layer allows the stomach to perform vigorous churning movements to mix food thoroughly with gastric juice, turning it into a semi-fluid mass called chyme.

The sub-mucosa layer is formed of loose connective tissue containing nerves, blood, and lymph vessels. This layer supports the mucosa and joins it to the muscularis. The nerve network here (Meissner’s plexus) controls mucosal secretions. In the duodenum, special glands (Brunner’s glands) are also present in this layer.

Brunner’s glands (or duodenal glands) are compound tubular submucosal glands found in that portion of the duodenum which is above the hepatopancreatic sphincter (Sphincter of Oddi). They secrete an alkaline mucus which protects the duodenal wall from the acidic chyme and raises the pH to allow enzyme action.

The mucosa of the stomach forms irregular folds (rugae/gastric folds) when the stomach is empty. These rugae allow the stomach to stretch and expand significantly as it fills with food. They also increase the surface area for the secretion of gastric juices, but they flatten out when the stomach is full.

In the small intestine, the mucosa forms small finger-like foldings called villi. These villi greatly increase the surface area available for absorption. The cells lining the villi produce numerous microscopic projections called microvilli, giving a brush border appearance, which further amplifies the absorptive surface area.

Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal. While sugars and amino acids enter the blood capillaries, fatty acids and glycerol are absorbed into the lacteals. This lymph fluid (chyle) eventually drains into the bloodstream, bypassing the liver initially.

Mucosal epithelium contains specialized goblet cells which secrete mucus. This mucus plays a crucial role in lubrication, allowing the easy passage of food through the alimentary canal. It also forms a protective layer on the mucosal lining, preventing excoriation by harsh chemical secretions like HCl.

The mucosa in the intestine forms crypts in between the bases of the villi called Crypts of Lieberkuhn. These crypts contain stem cells that renew the epithelial lining, and Paneth cells (in some species) that secrete antimicrobial peptides. The secretion of the crypts contributes to the Succus Entericus (intestinal juice).

Parotids are the largest of the three pairs of salivary glands, situated in the cheek area (near the ears). They secrete saliva entering the oral cavity via Stensen’s duct. Viral infection of the parotid glands causes a painful swelling known as Mumps. They produce a serous type of saliva containing Amylase.

The liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human. It is situated in the abdominal cavity, just below the diaphragm and has two lobes (left and right). It performs vital functions including detoxification, protein synthesis, and production of bile.

Hepatic lobules are the structural and functional units of the liver containing hepatic cells arranged in the form of cords. Each lobule is covered by a thin connective tissue sheath called Glisson’s capsule. This capsule is a characteristic feature of the mammalian liver, providing structural integrity to the lobules.

The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder. The gall bladder absorbs water from the bile, making it more potent for digesting fats. Upon stimulation (by CCK), it contracts to release bile into the cystic duct.

The duct of the gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct. This duct and the pancreatic duct open into the duodenum as the common hepato-pancreatic duct, which is guarded by a sphincter called the Sphincter of Oddi. It regulates juice flow.

About 30% of starch is hydrolysed in the oral cavity by the enzyme salivary amylase (ptyalin). It works at an optimum pH of 6.8 (slightly acidic). It breaks down starch (a polysaccharide) into Maltose (a disaccharide). The action stops in the stomach due to the high acidity of gastric juice.

Saliva contains an enzyme called Lysozyme. It acts as an antibacterial agent that prevents infections. It works by breaking down the cell walls of bacteria (specifically the peptidoglycan layer), thereby protecting the oral cavity and the upper digestive tract from pathogenic bacteria ingested with food.

Peptic or Chief cells are one of the types of cells found in the gastric glands of the stomach mucosa. They secrete the proenzyme Pepsinogen. Pepsinogen is an inactive precursor which is converted into the active proteolytic enzyme Pepsin only upon exposure to the acidic environment created by HCl.

Parietal or Oxyntic cells are located in the gastric glands. They secrete Hydrochloric acid (HCl) and Intrinsic Factor. HCl provides the acidic pH (1.8) necessary for pepsin activity and kills bacteria. Intrinsic Factor is essential for the absorption of Vitamin B12 in the small intestine; its absence causes pernicious anaemia.

Pepsin is the principal proteolytic enzyme of the stomach. It is secreted as pepsinogen and activated by HCl. Its optimum pH is 1.8. Pepsin converts proteins into proteoses and peptones (peptides). It is an endopeptidase, meaning it cleaves internal peptide bonds within protein chains.

Rennin (with double 'n') is a proteolytic enzyme found in the gastric juice of infants. It helps in the digestion of milk proteins (casein) by curdling milk (converting casein to calcium paracaseinate), which slows its passage through the stomach allowing better digestion by pepsin. It is absent in adults.

Enterokinase is an enzyme secreted by the intestinal mucosa. Its sole key function is to convert inactive Trypsinogen (from pancreatic juice) into active Trypsin. Once Trypsin is formed, it autocatalytically activates more trypsinogen and also activates other enzymes like chymotrypsinogen and procarboxypeptidases.

Bile contains bile salts (Sodium glycocholate and Sodium taurocholate). These salts help in the emulsification of fats, i.e., breaking down large fat globules into very small micelles. This increases the surface area for the action of lipases. Bile salts also activate lipases but bile itself contains no enzymes.

The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus. It contains a variety of enzymes like disaccharidases (maltase), dipeptidases, lipases, nucleosidases, etc., for the final digestion of food substances.

Fatty acids and glycerol are re-esterified in the epithelial cells to form triglycerides. These are then coated with protein to form small fat globules called chylomicrons. Chylomicrons are too large to enter blood capillaries, so they are transported into lymph vessels (lacteals) in the villi and eventually reach the blood.

Fatty acids and glycerol being insoluble, cannot be absorbed into the blood directly. They are first incorporated into small, water-soluble droplets called micelles with the help of bile salts. These micelles move into the intestinal mucosa where the fats are reformed and packaged into chylomicrons.

Jaundice represents a liver dysfunction where the liver fails to process bile pigments (bilirubin) efficiently, or the bile duct is blocked. The excess bile pigments accumulate in the blood and get deposited in the skin and eyes, causing them to turn yellow. It is often a symptom of hepatitis or liver damage.

Vomiting is the forceful expulsion of stomach contents through the mouth. This is a complex reflex action controlled by the vomit centre located in the medulla oblongata of the brain. A feeling of nausea usually precedes vomiting. It can be caused by motion sickness, irritation of the stomach, or certain drugs.

Kwashiorkor is produced by protein deficiency unaccompanied by calorie deficiency. It typically results from the replacement of mother’s milk by a high-calorie, low-protein diet in a child more than one year in age. Symptoms include extensive oedema (swelling due to water retention) and swelling of body parts, along with wasted muscles.

Marasmus is produced by a simultaneous deficiency of proteins and calories. It is found in infants less than a year in age. Symptoms include extreme emaciation of the body, thinning of limbs, and dry, thin, and wrinkled skin. Unlike Kwashiorkor, edema and ascites are absent. Growth rate declines considerably.

The gross calorific value is the amount of heat liberated from complete combustion of 1g of food in a bomb calorimeter (with oxygen). Values are: Carbohydrates: 4.1 kcal/g, Proteins: 5.65 kcal/g, and Fats: 9.45 kcal/g. These values are higher than what the body actually derives (Physiological value).

The physiological value is the actual amount of energy liberated from consumption of 1g of food in the human body. Values are: Carbohydrates: 4.0 kcal/g, Proteins: 4.0 kcal/g, and Fats: 9.0 kcal/g. Fats are the most energy-dense nutrient, providing more than double the energy of carbs or proteins.

Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas. Its release is stimulated by peptides in the lumen of the stomach.