HUMAN DIGESTIVE SYSTEM

Absorption of nutrients

• Absorption of end products of the digestion occurs mainly in the small intestine.
• By 3 main process

1) Diffusion

2) Facilitated diffusion – ATP is not required

3) Active transport – ATP is used

• Carbohydrate as disaccharide and protein as dipeptide and tripeptide are actively transported into the microvilli and the digestion is completed within the enterocytes.
• Carbohydrates as monosaccharides, protein as amino acid and fat as fatty acid and glycerol are absorbed into the enterocytes.

 

• Monosaccharide, amino acid, water soluble vitamins, minerals and water are absorbed into the blood capillaries in the villi.
• Some proteins such as antibodies in the mother’s milk are absorbed unchanged.
• Fatty acids and glycerol are absorbed and synthesized as triglycerides within the enterocytes.
• The triglycerides are absorbed into the lacteal. Fat soluble vitamins also absorbed along with the triglycerides.

Balance diet

• A diet containing essential nutrients, carbohydrates, proteins, lipids, dietary fibers, minerals, vitamins and water in the correct proportion is called as a balanced diet.
• A balanced diet is essential for health, if any nutrient is eaten in excess or is deficient, health may be adversely affected

Dietary fibers

• Fiber is not a nutrient as it is not digested or absorbed, but it has many beneficial effects on the digestive tracts.

1) Provides bulk to the diet and helps to satisfy the appetite

2) Stimulates peristalsis

3) Attracts water, increasing bulk and softness of faecus

4) Increases frequency of defecation, preventing constipation

 

• Prevents some gastrointestinal disorders

Example: – Colo-rectal cancer

 

Disorders in the alimentary canal

1. Gastritis

• When there is an excess of acid in the stomach causing damages to the mucosa.
• Due to the damages, blisters are formed.
• The causes of short-term gastritis are      1) Mental stress or tension

2) Prolonged starvation

3) Consumption of alcohol

4) Suffering from some diseases such as tuberculosis, syphilis etc.

5) Prolonged use of aspirin

• Chronic or long-term gastritis also associated with the bacteria called Helicobacter pylori

1. Constipation
   • Inhibition of the reflex action in defecation may lead to constipation.
   • Constipation causes pain of the anus and difficulties in defecating.

Control

• Behavioral adjustment should be developed to carry out defecation.
• Adequate fibers in the diet.

• the oral cavity  is opened to the outside and they relay by mouth which is bounded by the lips and is continuous with the pharynx posteriorly
• Superiorly it is bounded by palette
• Anterior part of the palate is Bony and is known as hard palate
• Posterior part is muscular and is called soft palate
• From the middle of the free posterior borders of the soft palate curved fold of muscle hangs down which is called ulva

• Tongue is attached to the bone by its base under the floor of the mouth by the frenulum made up of skeletal muscles and lined by non-keratinized stratified squamous epithelium
• Superior surface contains numerous papillae 
  • Vallate papillae
    • Largest one usually about 8 to 12
    • Arranged in inverted V shape towards the basal region of tongue
  • Fungiform papillae
    • Present mainly at the tip and edges
  • Filiform papillae
    • Smallest and most numerous occupying the anterior 2/3
  • Papillae contains taste buds which are situated in the epithelium of the upper surface of the tongue
  • Taste bud is a Goblet shaped organ made up of sensory cells and supporting cells
  • Microvilli of sensory cells are projected into taste pore
  • Four types of Taste buds are present to taste for basic tastes such as
    • sweet
    • Salt
    • Soar
    • Bitter
  • Taste buds are innervated by cranial nerves
  • Those sensitive to sweet and salt taste are mainly located at the terminal regions of the tongue
  • Those sensitive to soar taste are located on the side of tongue
  • Those sensitive to bitter taste are located at the basal region of the tank

 

Functions of the tongue

• Mixing of food in the process of mastication and forming bolus
• Swallowing
• Speech
• Sensing taste

• The teeth   are embedded in the sockets of the jaws.
• This condition is known as thecodont
• Human teeth have 4 different types of teeth, which is called as heterodont
• the incisors and canine are the cutting teeth and are used for biting of pieces of food
• Premolars have two cups on the crown and molars usually have 4 cups with the broad surface. These are used for grinding of food
• Individual has two sets of teeth during the lifespan. This condition is known as diphyodont
• Temporary/deciduous/milk teeth and permanent teeth
• At birth the teeth of both conditions are present in immature form
• In the jaws the temporary teeth are 20 in number
• They begin to erupt when the child is 6 month old and should all be present after 24 months
• The permanent teeth consisting of 32 begins to erupt and replace the deciduous teeth when a child is above 6 years
• 4 molars which are known as wisdom teeth usually happy after the age of 17 and is usually complete by the age of 24
• Dental formula  of permanent tissue and deciduous tissue differs

 

 

DENTAL DISEASES

• There are two major common diseases in Sri Lanka
  • Periodontal disease
  • Dental carriers
• Periodontal disease
  • It is a disease of the gum caused by microorganisms that they normally present in the mouth.
  • At first very dental disease cause inflammation of the gum which is generally painless and allows to continue up to the root and destroys the fibres which anchor the tooth with the jaw
• Dental careers
  • The microorganisms convert Sugars in the mouth to acid
  • Initially the enamel is slowly dissolved by the acid. When this continues up to the pulp cavity it is accompanied by a severe pain or the tooth falling

 

 

Factors contributing to the dental disease

• Health problem fluoride level fruits tablets with fluoride brushing teeth which contain fluoride

• This is a narrow muscular tube  lined by stratified squamous epithelium
• In human it is about 25 cm long and conveys food and fluids by peristalsis
• Stomach
• The stomach in humans is roughly J shaped organ situated below the diaphragm and on the left side of the abdominal cavity
• It is a muscular bag with the foldings

• the stomach  is lined with secretory cells, Goblet cells, Parietal cells and chief cells
• Chief cells secrete the inactive enzyme pepsinogen and prorennin
• parietal cells secrete a dilute solution of HCl acid
• stomach has number of functions;
  • stores food temporary after meals
  • continues mechanical digestion
  • the acid in stomach kills many bacteria actors defence mechanism
  • It denatures many proteins
  • produces a thick muscle layer which stops the self-digestion of stomach
• Gastric juice it contains;
  • water
  • HCl
  • pepsinogen
  • mucus
  • intrinsic factor

• It is continuous with the stomach at the pyloric sphincter and leads in to the large intestine at the ileo-caecal valve. (ileum + caecum)
• It is situated in the middle and lower regions of the abdominal cavity, surrounded by the large intestine and below the stomach and liver.
• It is described in 3 parts.

1. Duodenum
2. Jejunum

• Ileum
• The inner surface of the small intestine is thrown into many circular faults. They are not smoothed out when the small intestine is distended.

Histological structure of the small intestine 

 

• The epithelium and the lamina propria projects into the lumen forming fingerlike macroscopic folds called villi.(1mm)
• The epithelium consists of simple columnar epithelial cells or Enterocytes With a number of tiny microvilli on their free surface. (Brush border)
• Goblet cells that secrete mucus are interspersed between the columnar shaped enterocytes.
• The intestinal glands or crypt of Liberkhun are situated below the surface between the villi.
• These glands secrete intestinal juice. These glands are in simple tubular shape.
• In the sub mucosa of duodenum are Bruner’s glands. They are compound tubular glands.
• They secrete an alkaline secretion with mucus. Their ducts open into the intestinal gland.
• Lamina propria of each villus contain a network of blood capillaries and the lymphatic vessels called Lacteal.
• Numerous lymph nodes are also present in lamina propria. Aggregated lymph nodes are also present in lamina propria. Aggregated lymph nodes or payer’s patches are Present in ileum.

 

Duodenum

• It occupies the mid region of the abdominal cavity. It lies posterior to transverse colon.
• It is about 25cm long,”C” shaped tube. Concave surface faces left, curving around the head of the pancreas.
• At the midpoint of the concave surface is an opening common to the pancreatic duct and the common bile duct. This opening is guarded by the “Sphincter of Oddi”.
• When acid chime passes into the duodenum, it is mixed with

1. Pancreatic juice
2. Bile

• Intestinal juice

1. Secretion from the Bruner’s gland
2. Mucus from goblet cells

• Pancreatic juice and bile enters the duodenum via relaxed sphincter of Oddi.

 

Intestinal juice

• About 1.5 l intestinal juice are secreted daily by the glands of small intestine.
• The pH is usually between 7.8 and 8.
• The intestinal juice consist of,

1. Water
2. Mineral salts

• Mucus

1. Enterokinase(an enzyme)
2. Nuceotidase
3. Sucrase

• Maltase Brush border enzymes
• Lactase

1. Peptidases(Amino peptidase and dipeptidase)

• Most of the digestive enzymes in the small intestine are contained in the enterocytes of the walls of the villi with their active sites exposed to the chyme. These are called brush border enzymes.
• The digestion is completed by direct contact between the substrate and the microvilli and within enterocytes.

 

• Jejunum and ileum are suspended from the posterior abdominal wall by the mesentery occupying the middle and lower areas of the abdominal cavity.

Adaptations of a small intestine for efficient digestion and absorption

• It is very long nearly 5 m
• Large number of villi
• Microvilli of enterocytes
• Circular folds
• The villi are richly supplied with blood capillaries and lacteals
• Large number of Mitochondria in the enterocytes
• The smooth muscles of muscularis mucosa able to constantly contract and relax. Thus bring the villi into close contact with the food.

Functions of small intestine

• Secretion of intestinal juice
• Completion of chemical digestion of Carbohydrates, proteins and fats in the enterocytes.
• Secretion of the hormones cholecystokinin-pancreozymin (cck-pz),secretin and enterogastrone.
• Absorption of nutrients
• Protection against infections by microbes by the aggregated lymph (payers patches) nodes
• Onwards movement of its contents.

• It extends from ileocecal Junction to the rectum 
• It is about 1.5 m long and forms and arch around the coil duct small intestine in the abdominal cavity
• For descriptive purposes it is divided into the following parts
• Caecum
• Ascending colon
• Transverse colon
• Depending colon
• Pelvic colon
• The caecum is the proximal part of the large intestine. It is situated in the right side of the pelvic cavity.
• Caecum is a dilated large blind sac. Ilium opens into the caecum via ileocecal opening which is guarded by the ileocecal valve.
• The vermiform appendix is a fine tube closed at one end which leave from the caecum.
• The ascending colon passes upwards from the caecum to the level of the liver where it curves acutely to the left to become the transverse colon.
• The transverse colon extends across the abdominal cavity (actually it is not transverse but hangs down as a Loop) in front of the duodenum and the stomach.
• At the area of the spleen, it curves acutely downwards to become the descending colon.
• The descending colon passes down the left side of the abdominal cavity then curves towards the midline.
• After it enters the true pelvis, it is known as the zygmoic colon or pelvic colon.

Histological structure of large intestine.

• Arrangement of the longitudinal muscle fibers is modified in the colon. They are collected into 3 bands called taenia coli, situated at regular intervals around the colon.
• As these bands of muscle tissue are slightly shorter than the total length of the colon. They give a sacculated appearance to the organ.
• In the epithelium, Goblet cells are interspersed between the columnar shaped cells.

Numerous lymph nodes are present in lamina propria and in submucosa.

• Some of the lymph nodes aggregated together which are known as payer’s patches.

Functions of large intestine

• Mineral salts, water, vitamins and some drugs are absorbed
• Semi solid consistency of faecus is formed.
• Certain amino acids, vitamin K, Folic acid and biotin are synthesized by certain types of bacteria which are heavily colonized in the large intestine. They are commonly known as coliform bacteria.

Eg:-        Escherichia coli

Enterobacter aerogenes.

• Protection against infection by microbes by the payer’s patches
• Mass moments (strong peristaltic movement exhibits in the transverse colon) force the contents into the rectum.

• It is a slightly dilated part which leaves from the pelvic colon.
• It temporarily stores faecus.

 

• This is a short passage, it leaves from the rectum to the exterior.
• There are two sphincter muscles which controls the anus. The internal sphincter consisting of smooth muscle fibers is under control of the autonomic Nervous system and the external sphincter formed by striated muscle (skeletal muscle) is under voluntary nerve control.

 

• It is situated in the umbilical epigastric left hypochondriac region of the abdominal cavity.
• It consists of a broad head, a body and a narrow tail.
• The head lies in the curve of the duodenum, the body behind the stomach and the tail lies in front of the kidney.
• This is a compound alveolar gland, it acts as an exocrine and endocrine gland.
• The exocrine pancreas consists of a large number of lobules made up of small alveoli, the walls of it consists of secretary cells.
• Their function is to provide pancreatic juice containing enzymes.
• Each lobule is drained by a tiny ductule and these unite eventually to form the pancreatic duct.
• The pancreatic duct extend the whole length of the gland and opens into the duodenum. Just before entering the duodenum, this joints a common bile duct to form the hepato-pancreatic ampulla.
• The endocrine pancreas distributed throughout the gland are group of specialized cells called the islets of Langerhans.
• They have no ducts, they are well supplied with blood capillaries. So, the hormones diffuse directly into the blood.
• There are two types of cells in the islets of Langerhans.
• α cells -secrete the hormone glucagon
• β cells -secrete the hormone insulin.

Pancreatic juice

1. Water
2. Mineral salts (mainly bicarbonates)
3. Trypsinogen
4. Chymotrypsinogen Proteolytic enzyme precursers(inactive form of enzyme)
5. Procarboxy peptidase
6. Pancreatic amylase
7. Pancreatic lipase
8. Nuclease

Bicarbonates

• pH is raised by bicarbonates and ions by this effect,

1. Pancreatic and intestinal enzymes act most effectively
2. Stops the action of pepsin
3. Neutralize the acidity of chime

functions of enzymes in the pancreatic juice includes 

 

Location

• It is situated in the upper part of the abdominal cavity and just below the diaphragm occupying the greater part of the right hypochondriac region, part of the epigastric region and extending into the left hypochondriac region

Or

• Right upper quarter and upper middle of the abdominal cavity.

Structure

• The liver is the largest gland in the body weighing 1-2 kg.
• It is second organ in size (first one is skin)
• It is wedge shaped and reddish brown in colour.
• It’s upper and anterior surfaces are smooth and curved to fit the undersurface of the diaphragm.
• Its posterior surface is irregular in outline.
• The liver has 4 lobes
• Right lobe
• Left lobe
• Quadrate love
• Caudate lobe
• Right lobe is the largest lobe. It is separated from the smaller left lobe by falciform ligament.
• This ligament attaches the liver to the inferior surface of the diaphragm.
• Caudate lobe is positioned inferior posteriorly and quadrate lobe inferior anteriorly.
• Gall bladder lies on the inferior surface right to the quadrate lobe.
• The portal fissure is the region on the posterior surface where various structures enter and leave the gland.
• Portal vein
• Hepatic artery Enter the gland
• Nerve fibers
• Right and left hepatic duct
• Lymphatic vessel Leave the gland
• Hepatic vein.

Histological structure of liver

• The lobes of liver are made up of tiny lobules. These are the structural and functional unit of liver.
• In the center of each lobule is a central vein which is a branch of hepatic vein.
• These lobules are hexagon in outline and are formed by cuboidal shaped cells, hepatocytes arranged in pairs of columns radiating from the central vein.
• Between two pairs of columns of cells, there are sinusoids containing a mixture of blood from the tiny branches of the portal vein and hepatic artery.
• Sinusoids are large blood capillaries with incomplete walls of simple squamous epithelium.
• The hepatic macrophages (kupffer cells) are you found scattered in the lining of sinusoids.
• Their function is to ingest and destroy any foreign particles and mature RBCs.
• Blood drains from the sinusoids into Central vein, then these are join with each other and becomes as the hepatic vein.
• The bile canaliculi join up to form bile ductules until eventually they form right and left hepatic ducts which drain by from the liver.
• These ducts joins to form the common hepatic duct. It is joined with the cystic duct from the gall bladder to form the common bile duct.
• The hepatocytes synthesis the bile.
• The bile canaliculi runs between the columns of liver cells of pair. This means that each column of hepatocyte has blood sinusoids on one side and a bile canaliculus on the outer side.
• Bile is stored in the gall bladder.

Functions of liver

1) Synthesis or production

1.  Plasma proteins

Eg :- albumin, globulin, fibrinogen

1.  Prothrombin and some blood clotting factors
2.  Vitamin A – it can be synthesized from carotene which is found mostly in green vegetables
3.  Red blood cells in fetus
4.  Cholesterol
5.  Bile
6.  Nonessential amino acids

 

2) Storage Fat soluble vitamins (A, D, E, K)

1.  Some water soluble vitamins (vitamin B₁₂ )
2.  Minerals (Fe, Zn, Cu, Mo, Co)
3.  Glycogen
4. Blood (around 300 ml – 1500ml blood is stored in the blood vessels of liver. This is not a static If necessary these vessels can constrict and make more blood available to the general circulation)

 

3) Protection

1. Defends against microbes is carried out by phagocytic hepatic macrophages (kupffer cells) in the sinusoids.
2. detoxification of drugs and noxious substances.

Example :- alcohol, Nicotin, microbial toxins.

4) Formation of excretory substances.

1. Ammonia is produced by the deamination and is converted into urea by a cyclic reaction known as ornithine cycle.
2. Nucleic acid (nucleoproteins) are broken down into uric acid.

1.  Hemoglobin is broken down and forms the bile pigment bilirubin as a waste product.

5) Production of heat.

• Liver is the main heat producing organ of the body.
• It has a high metabolic rate and produces nearly 80 of the heat in the body.

6) Elimination or destroying

1. a) Steroid hormones are converted into inactive and more soluble forms by the liver.
2. b) The old erythrocytes are engulfed by the activity of kupffer cells.

7) Regulation

1. a) Carbohydrate metabolism

• The liver has an important role in maintaining blood plasma Glucose level.
• When level rise, glucose is converted to glycogen for storage under the influence of the hormone insulin. When Glucose level falls, the hormone glucagon stimulates conversion of glycogen into glucose again, keeping level within the normal range.

1. b) Fat metabolism

• Stored fat can be converted to a form in which it can be used by the tissues to provide energy

 

1. c) Protein metabolism

1) The body is unable to store absorbed amino acids and those not required for protein synthesis are deaminated in the liver. The keto acids can be used for respiration.

2) Transamination – Removes the Amine groups of amino acids and attaches to form the new non-essential amino acids.

 

Bile

• Bile is secreted by the liver which is stored in the gall bladder. It is a greenish yellow alkaline fluid. It has a pH of 8.
• The main constituents of bile are,

1. Water
2. Mineral salts(HCO₃^–)

• Mucus

1. Bile salts – Sodium taurocholate

Sodium glycoholate

1. Bile pigments(mainly bilirubin)
2. Cholesterol

• The bile pigment Bilirubin is a waste product of the breakdown of erythrocytes and is excreted in bile rather than in the urine because of its low solubility in water.

Functions of the bile

• To aid the action of lipase, bile salts emulsify fats. That is reduce the size of the globules and increasing the surface area.
• The pH is raised by bicarbonate ions (HCO₃^–) to bring the duodenal contents to a suitable level for digestion.
• The control of bile and pancreatic secretion
• Secretin, cholecystokinin/Pancreazymin (CCK-PZ) are the hormones produced by endocrine cells in the wall of the duodenum.
• Secretin stimulates the production of bile by liver and also stimulates secretion of pancreatic juice rich in bicarbonates.
• CCK-PZ stimulates the contraction of gall bladder to release the bile and also increases the secretion of pancreatic juice rich in enzyme.
• Para sympathetic stimulation increases the secretion of pancreatic juice and bile. But sympathetic stimulation depresses the both secretions.