Nutrition in man
Introduction:
We know that, there are many life processes like Nutrition, Respiration, Circulation, Excretion, Coordination, Reproduction etc. Nutrition and Respiration are the processes which provide us energy to perform different activities.
Nutrition is the sum total of all processes through which food is taken in, digested, absorbed, utilized and finally, the undigested matter is eliminated outside the body. Thus, nutrition includes the processes like ingestion, digestion, absorption assimilation and egestion (defecation). During ingestion, food is taken inside the body. This ingested food is then digested.
Digestion is defined as the process during which the complex, non-diffusible and non-absorbable food substances are converted into simple, diffusible and absorbable substances. During absorption, the simple substances get diffused into the blood. The absorbed food is then carried to each and every tissue cell of the body where it is assimilated.
Assimilation is defined as the process by which protoplasm is synthesized into each cell of the body by utilizing simple food substances. The undigested matter is released outside the body because if it remains in the body, it becomes toxic to the body.
Organs of Digestion:
Human digestive system consists of alimentary canal and associated digestive glands. Alimentary canal is a long tubular structure of varying diameter starting from mouth and ending with anus. The associated digestive glands include salivary glands, liver and pancreas.
Alimentary canal:
Alimentary canal is about 8-10 meters long and it consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, large intestine and anus.
1) Mouth:
This is the uppermost transverse slit-like opening of the alimentary canal. It is bounded by an upper lip and a lower lip. It leads to a large cavity called buccal cavity or oral cavity. Mouth is used to ingest the food.
2) Buccal cavity:
It is the space of which side walls are formed of cheeks, roof is formed of palate and floor by tongue. It is lined internally by a mucous membrane. Tongue, teeth and salivary glands are also present in the buccal cavity.
Tongue:
It is a muscular fleshy organ. It lies along the floor of the buccal cavity. It is roughly triangular in shape. The upper surface of the tongue bears numerous projections called papillae. These papillae contain sensory receptors called taste buds which are used to defect different tastes of the food. Besides, the tongue is used to mix saliva with the food. It helps in swallowing and in speech. It's secretions also keep the buccal cavity moist.
Teeth:
There are total 32 teeth in the bucal cavity of an adult human being. Each tooth is made up of two parts, the root embedded in the jaw bone and the crown as the visible portion. Crown has the hardest substances of the body covered by a white coating material called enamel. The body of the tooth is made up of dentine which is bone like substance. Beneath the dentine is the central cavity which contains blood vessels, nerves and pulp. The study of teeth with respect to their number, arrangement, development etc. is known as dentition.
In human beings, like in other nammals, dentition is of the heterodont type as there are four different types of teeth namely, lncisors(I), Cannines(C), pre molars (PM), and molars (M). The total number of teeth in the upper jaw is 16 and that in the lower jaw is also 16. Therefore the total number of teeth in the buccal cavity is 32. The teeth masticate the food into small particles and help to mix food with saliva.
3) Pharynx:
The buccal cavity leads to pharynx which is the meeting point of the digestive and respiratory systems. The opening of the respiratory system in pharynx is called glottis. It is guarded by a cartilaginous flap called epiglottis.
It prevents entry of food into the respiratory system. It closes during the swallowing action. Pharynx passes the food towards the oesophagus.
4) Oesophagus:
It is a long, thin and narrow tube connecting pharynx to the stomach. It is about 25 cms. long. It passes through the troracic cavity, pierces the diaphragm and leads to the stomach. The food moves through the oesophagus due to rhythmic contraction of longitudinal and circular muscles.
This wave of contraction and relaxation is called peristalsis. Mucuos secreted by the epithelial cells in the inner lining helps in the smooth passage of food.
5) Stomach:
It is a sac like, 'J' shaped, 25-30 cms. long organ. It can be divided into three parts namely:
a) Cardiac b) Fundus and c) Pyrolic stomach.
Cardiac stomach is the first part is called fundus and the remaining part is the pyloric stomach. The pyrolic stomach opens into the duodenus.
Sphincters are present at both the ends of stomach. The anterior sphincters present at the opening of the oesophagus into the stomach is called cardiac sphincter. The cardiac sphincter prevents the regurgitation of food i.e. the passage of food back into the buccal cavity.
The pyloric sphincter regulates the passage of food into the duodenus of intestine. The stomach stores the food and churns it. The churning breaks up the food into smaller pieces and also facilitates the mixing of the food with gastric juice secreted by the inner wall of the stomach.
Histology of the stomach:
Histologically the wall of stomach is made up of four distinct layers.
1) Serosa:
It forms the outer protective covering of the stomach. It has an outer layer of squamous epithelium and an inner layer of convective tissue.
2) Muscular layer :
It is a thick layer made up of three kinds of muscles: outer longitudinal muscles, middle circular muscles and inner oblique smooth muscles. This muscular layer is the thickest part responsible for churning movement in the stomach.
3) Submucosa:
It is a connective tissue layer containing blood vessels, lymph vessels and nerves. It supports mucosa.
4) Mucosa:
It is the innermost layer thrown into large folds called rugae. It is made up of three parts: Muscularis mucosa, lamina propria and epithelium. Its muscularis mucosa is prominent and formed of outer longitudinal and inner circular muscles.
Lamina propria contains gastric pits and gastric glands. Epithelium of mucosa is columnar and folded to form gastric glands.
5) Gastric glands:
These are simple, tubular, branched or unbranched glands, embedded in the lamina propria. Each is made up of the following types of cells:
a) Chief cells or Peptic cells-
They are pyramid shaped and present deep in the base of the glands. They secrete the digestive enzyme pepsinogen.
b) Parietal cells or Oxyntic cells-
They are large oval cells bulging out of the mucous membrane. They secrete dilute HCl and an intrinsic factor.
c) Mucous cells:
They are present in the neck region and secrete mucous.
6) Small Intestine:
It is about 6 meters long and 2.5 cms. broad tube compactly coiled within abdominal cavity. The coils are held together by connective tissue membrane called mesenteries supporting the blood vessels, lymph vessels and nerves. It is divided into three parts:
a) Duodenum:
It is 'I'll shaped and is about 25cms long. The bile duct and the pancreatic duct open into the duodenum.
b) Jejunum:
It is about 2.5 meters long and narrower than the duodenum.
c) Ileum:
It is about 3.5 meters long. It is a little bit broader than the jejunum.
Histologically the mucosa, the innermost layer of the small intestine is highly folded into numerous ginger like projections called villi. Each villus is lined by columnar epithelium cells.
These cells have microvilli to increase the area of absorption. The core of each villus has loose connective tissue with a central lymph vessel called lacteral. It absorbs fats. There are a large number of capillaries in the core for absorption.
7) Large Intestine:
It is broader than the small intestine. It is about 1.5 meters long. It can be divided into two parts: a) colon and b) rectum.
a) Colon:
It is the anterior region and is differentiated into three parts: the ascending colon, the transverse colon and the descending colon. The colon is lined internally by mucosal cells which secrete mucous makes the passage of undigested material easy.
b) Rectum:
It is the posterior region of large intestine. Undigested material called faecal matter is stored in the rectum temporarily before it is expelled through the anus.
At the junction of the ileum and colon there is a blind pouch called caecum. An elongated worm like vermiform appendix arises from the caecum. It is a vestigial organ in human beings. It is functional in herbivorous animals for the digestion of cellulose.
8) Anus:
The rectum opens to the outside by the opening called Anus. Anus is guarded by a sphincter. It removes undigested matter to the outside by the process known as defecation egestion.
Digestive glands:
They are the salivary glands, liver and pancreas.
1) Salivary glands:
These are multicellular glands which open into the buccal cavity. There are three pairs of salivary glands:
a) Sub-lingual glands below tongue.
b) Sub-mandibular glands below lower jaw.
c) Parotid glands in front of the ears.
The salivary glands have two types of secretory cells. Serous cells and mucous cells. The serous cells produce a watery fluid which contains the digestive enzyme salivary amylase or ptyalin. The mucous cells produce mucous. It binds the food and makes it slippery so that it can be easily swallowed. The secretion of the salivary glands is called saliva.
2) Liver:
It is the largest gland of the body. It is dark reddish brown in colour. It weighs about 1.5 kg. in an adult. It is located on the right side, just below the diaphragm. It is made up of two lobes. Cells of the liver produce the bile juice which is stored in a small bag like structure called gall bladder.
Histology of liver:
Liver is covered by a thin membrane of connective tissue called Glissons's capsule. But it is not very distinct in man. Internally the liver is made up of hexagonal units called hepatic lobules. Between these lobules is connective tissue, containing blood vessels and bile ducts. Each hepatic lobule has a central vein around which a number of hepatic cells or hepatocytes are arranged in single rows forming hepatic cords. Hepatic cells are large, polygonal cell with distinct nucleus and granular cytoplasm.
The cytoplasm stores glycogen and fat droplets. Hepatic cells are responsible for secretion of bile. This bile is collected by bile capillaries which carries it to the bile duct. The bile duct opens into the hepatic duct which deposits it into the gall bladder. Blood sinusoids are large irregular dilated blood vessels lying between the hepatic cords.
Functions of the liver:
1) Liver secretes bile which emulsifies fats and makes the food alkaline.
2) Liver stores excess of glucose in the form of glycogen which is used during starvation.
3) Excess amino acids are converted into ammonia by the process called determination.
4) It synthesizes vitamin A, D, K and B12.
5) Blood proteins like prothrombin and fibrinogen are formed in the liver.
6) It produces red blood cells during early development thus working as a haemopoietic organ.
7) Kupffer cells digest old R.B.Cs.
3) pancreas:
It is a leaf shaped gland lying in the gap between the duodenum and stomach. It is a mixed gland because some part of it is exocrine in function i.e. its secretion is carried by ducts while the other part is endocrine i.e. its secretion is poured directly into the blood stream. The exocrine part is made up of pancreatic lobules called acini. They are flask shaped and formed of a single layer of large glandular pyramid shaped cells. These cells secrete the pancreatic juice.
The endocrine part is made up of groups of cells present in the connective tissue between the acini. These are called islets of Langerhans. They are made up of 'A' or alpha cells that secrete glycogon and 'B' or beta cells that secrete insulin. Both these hormones together control the blood sugar level. In addition the delta cells secrete somatostatin hormone, which decreases glycogon and insulin secretion.
