INTRODUCTION :
HUMAN PHYSIOLOGY :
The reductionist approach to study of life forms resulted in increasing use of physico-chemical concepts and techniques. Majority of these
studies employed either surviving tissue model or straightaway cell- free systems.
Anexplosion of knowledge resulted in molecular biology.
Molecular physiology became almost synonymous with biochemistry
and biophysics. However, it is now being increasingly realised that
neither a purely organismic approach nor a purely reductionistic
molecular approach would reveal the truth about biological processes
or living phenomena. Systems biology makes us believe that all living
phenomena are emergent properties due to interaction among
components of the system under study. Regulatory network of molecules,
supra molecular assemblies, cells, tissues, organisms and indeed,
populations and communities, each create emergent properties. In the
chapters under this unit, major human physiological processes like
digestion, exchage of gases, blood circulation, locomotion and movement are described in cellular and molecular terms. The last two chapters point to the coordination and regulation of body events at the organismic level.
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 utilised 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.
DIGESTIVE SYSTEM :
The human digestive system consists of the alimentary canal and the associated glands
ALIMENTARY CANAL :
The alimentary canal begins with an anterior opening – the mouth, and 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 (I), 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 is
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.
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. The oesophagus is a
thin, long tube which extends posteriorly
passing through the neck, thorax and
diaphragm and leads to a ‘J’ shaped bag like
structure called stomach. A muscular
sphincter (gastro-oesophageal) regulates the
opening of oesophagus into the stomach.
The stomach, located in the upper left
portion 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
part 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. 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
opens into the rectum which opens out
through the anus.
The wall of alimentary canal from
oesophagus to rectum possesses four layers
namely serosa, muscularis,
sub-mucosa and mucosa. Serosa is the
outermost layer and is made up of a thin
mesothelium (epithelium of visceral organs)
with some connective tissues. Muscularis is
formed by smooth muscles usually
arranged into an inner circular and an outer
longitudinal layer. An oblique muscle layer
may be present in some regions. The sub-
mucosal layer is formed of loose connective tissues containing nerves, blood and lymph
vessels. In duodenum, glands are also present in
sub-mucosa. The innermost layer lining the
lumen of the alimentary canal is the mucosa. This
layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine 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 gobletcells which secrete mucus that help in lubrication.
Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn). All the four layers show modifications in different parts of the
alimentary canal.
DIGESTIVE GLANDS :
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 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
and the endocrine portion secretes hormones, insulin and glucagon.
DIGESTION OF FOOD:
- The process of digestion is accomplished by mechanical and chemical processes.
The buccal cavity performs two major functions, mastication of food
and facilitation of swallowing. The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly. Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition. the oesophagus by successive waves of muscular contractions called peristalsis. The gastro-oesophageal sphincter controls the passage of food into the stomach. The saliva secreted into the oral cavity contains electrolytes (Na+, K+, Cl–, HCO–) and enzymes, salivary amylase andlysozyme. The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase. About 30 per cent of starch is hydrolysed here by this enzyme(optimum pH 6.8) into a disaccharide – maltose. Lysozyme present in
saliva acts as an antibacterial agent that prevents infections.The mucosa of stomach has gastric glands. Gastric glands have three
major types of cells namely –
(i) mucus neck cells which secrete mucus;
(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and
(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor
(factor essential for absorption of vitamin B12).
The stomach stores the 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. The proenzyme pepsinogen,
on exposure to hydrochloric acid gets converted into the active enzyme
pepsin, the proteolytic enzyme of the stomach. Pepsin converts proteins
into proteoses and peptones (peptides). The mucus and bicarbonates
present in the gastric juice play an important role in lubrication and
protection of the mucosal epithelium from excoriation by the highly
concentrated hydrochloric acid. HCl provides the acidic pH (pH 1.8)
optimal for pepsins. Rennin is a proteolytic enzyme found in gastric juice
of infants which helps in the digestion of milk proteins. Small amounts of
lipases are also secreted by gastric glands.
Various types of movements are generated by the muscularis layer of
the small intestine. These movements help in a thorough mixing up of
the food with various secretions in the intestine and thereby facilitate
digestion. The bile, pancreatic juice and the intestinal juice are the
secretions released into the small intestine. Pancreatic juice and bile are
released through the hepato-pancreatic duct. The pancreatic juice
contains inactive enzymes – trypsinogen, chymotrypsinogen,
procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is
activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the
pancreatic juice. The bile released into the duodenum contains bile
pigments (bilirubin and bili-verdin), bile salts, cholesterol and
phospholipids but no enzymes. Bile helps in emulsification of fats, i.e.,
breaking down of the fats into very small micelles. Bile also activates lipases.
The intestinal mucosal epithelium has goblet cells which secrete mucus.
The secretions of the brush border cells of the mucosa alongwith the
secretions of the goblet cells constitute the intestinal juice or
succus entericus. This juice contains a variety of enzymes like
disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.
The mucus alongwith the bicarbonates from the pancreas protects the
intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner’s glands) also help in this.
Proteins, proteoses and peptones (partially hydrolysed proteins) in
the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice - Carbohydrates in the chyme are hydrolysed by pancreatic amylase
into disaccharides. - Fats are broken down by lipases with the help of bile into di-and monoglycerides.
- Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides.
- The enzymes in the succus entericus act on the end products the respective simple absorbable forms. These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.
- The breakdown of biomacromolecules mentioned above occurs in the
duodenum region of the small intestine. The simple substances thus
formed are absorbed in the jejunum and ileum regions of the small
intestine. The undigested and unabsorbed substances are passed on to
the large intestine.
No significant digestive activity occurs in the large intestine. The
functions of large intestine are:
(i) absorption of some water, minerals and certain drugs;
(ii) secretion of mucus which helps in adhering the waste (undigested)
particles together and lubricating it for an easy passage.
The undigested, unabsorbed substances called faeces enters into the
caecum of the large intestine through ileo-caecal valve, which prevents
the back flow of the faecal matter. It is temporarily stored in the rectum
till defaecation.
The activities of the gastro-intestinal tract are under neural and
hormonal control for proper coordination of different parts. The sight,
smell and/or the presence of food in the oral cavity can stimulate the
secretion of saliva. Gastric and intestinal secretions are also, similarly,
stimulated by neural signals. The muscular activities of different parts of
the alimentary canal can also be moderated by neural mechanisms, both
local and through CNS. Hormonal control of the secretion of digestive
juices is carried out by the local hormones produced by the gastric and
intestinal mucosa. - 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. However, 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, monosacharides like glucose,
electrolytes like Na+ are absorbed into the blood by this mechanism.
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.
Absorption of substances takes place in different parts of the alimentary
canal, like mouth, stomach, small intestine and large intestine. However,
maximum absorption occurs in the small intestine. - DISORDERS OF DIGESTIVE SYSTEM :
- 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 tape worm, round worm, thread worm, hook
worm, pin worm, 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 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.
