Study Notes on Digestion and Absorption
Study Notes on Digestion and Absorption
- The human digestive system consists of the alimentary canal and the associated glands.
- The alimentary canal begins with an anterior opening – the mouth.
- It opens out posteriorly through the anus. 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. Majority of mammals including human being forms two sets of teeth during their life.
- A set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth.
- This type of dentition is called diphyodont. An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (). canine (C). premolars (PM) and molars (M).
- 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 2123/2123.
- The hard chewing surface of the teeth, made up of enamel. helps in the mastication of food.
- 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.
- A cartilaginous flap the entry of food pen into pharynx epiglottis prevents into the glottis during then passing through the neck, thorax and structure called stomach.
- A muscularning of then wind pipe -catled glottis swallowing.
- The oesophagus is a long tube extends posteriorly and leads to bag like structure to regulates the Function of oesophagus into the stomach.
- The stomach, located in the upper left notion of the abdominal cavity, has three major parts – a cardiac portion into which
- the oesophagus opens, a fundic region and a pyloric portion which opens into the first Dart of small intestine Small intestine is distinguishable into three
- regions, a ‘U’ shaped duodenum, a long coiled middle portion jejunum and a highly coiled ileum.
- The opening of the stomach into the duodenum is guarded by the pyloric sphincter.
- Ileum opens into the large intestine. It consists of caecum, colon and rectum.
- The Caecum is 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.
- The caecum opens into the colon. The colon is divided Into three parts – an ascending, a transverse and a descending part.
- The descending part pens into the rectum which opens Out through the anus.
- The digestive glands associated with the alimentary canal include the salivary glands, the liver and the pancreas.
- Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek)the sub-maxillary/sub-mandibular (lower jaw) and the sub-linguals (below the tongue).
- These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.
- 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.
- The hepatic lobules are the structural and functional units of liver containing hepatic cells arranged in the form of cords.
- Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule.
- 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 duct of gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct The duct systems of liver, gall bladder.
- The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.
- The pancreas is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘U’ shaped duodenum.
- The exocrine portion secretes an alkaline pancreatic Juice containing enzymes.
- the endocrine portion secretes hormones, insulin and glucagon
DIGESTION OF FOOD
ABSORPTION OF DIGESTED PRODUCTS
- Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.
- It is carried out by passive, active or facilitated transport mechanisms.
- Small amounts of monosacharides like glucose, amino acids and some of electrolytes like chloride ions are generally absorbed by simple diffusion.
- The passage of these substances into the blood depends upon the concentration gradients.
- The some of the substances like fructose and some amino acids are absorbed with the help of the carrier ions like Na’.
- This mechanism is called the facilitated transport.
- Transport of water depends upon the osmotic gradient.
- Active transport occurs against the concentration gradient and hence requires energy.
- Various nutrients like amino acids, monosaccharides like glucose. electrolytes like Na* are absorbed into the blood by this mechanism.
- Patty 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 vill.
- These lymph vessels ultimately release the absorbed substances into the blood stream.
- The Absorption of substances takes place in different parts of the alimentary canal, like mouth, stomach, small intestine and large intestine.
- The maximum absorption occurs in the small intestine. A summary of absorption.
DISORDERS OF DIGESTIVE SYSTEM
- 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 rectum 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, over eating, and spicy food.