Unit 5 Human Physiology

Introduction:

Systems biology is the computational and mathematical modelling of complex biological systems. According to Systems biology, all living phenomena are developing several 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 or developing properties. In the chapters under this unit, major human physiological processes like digestion, exchange of gases, blood circulation, locomotion and movement are described in cellular and molecular terms.  

Story box: Alfonso Corti, Italian anatomist began his scientific career studying the cardiovascular systems of reptiles. Later, he turned his attention to the mammalian auditory system. He published a paper describing a structure located on the basilar membrane of the cochlea containing hair cells that convert sound vibrations into nerve impulses, the organ of Corti.

Preview:

• As the name suggests, the chapter mainly revolves around digestion.
• Digestion is the process by which biomacromolecules in the food have to be broken down and converted into simple substances in for absorption.
• We shall then describe the digestive system, which is otherwise called the alimentary canal in detail. It starts from the anterior mouth trough which food is consumed and ends with the posterior anus through which food is egested.
• Then we shall discuss comprehensively, the steps involved in digestion. Digestion is followed by absorption which involves entry of the end products of digestion through the intestinal mucosa into the blood or lymph. These absorbed products are then conveyed to tissues where they can be utilized for specific functions by another process called, assimilation. Finally, the undigested food wastes are egested out as faeces to the outside through the anus and the process is termed, defaecation.
• We come to the end of the chapter with the discussion of symptoms of few digestive disorders that occur in humans; such as: Jaundice, Vomiting, Diarrhoea, Constipation and Indigestion.

16.0 Introduction:

Food is one of the basic requirements of all living organisms.

Components of food: The major components of our food are carbohydrates, proteins and fats. Vitamins and minerals are required in small quantities.

Importance of food and water: 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.

Digestion: 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.

Digestion is carried out by our digestive system by mechanical and biochemical methods. The organisation of human digestive system is shown in Figure 16.1.

16.1 Digestive System

The human digestive system consists of the alimentary canal and the associated glands.

16.1.1 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 of the teeth is embedded in a socket of jaw bone (Figure16.2). This type of attachment is called thecodont. Formation of teeth in majority of mammals including human beings occurs twice 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 is in the order I, C, PM, M and is represented by a dental formula: 2123/ 2123 (In humans). The hard chewing surface of the teeth, made up of enamel, helps in mastication the process of crushing and grinding food by teeth) 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 pharynx opens into the oesophagus and the trachea (wind pipe). A cartilaginous flap called, epiglottis prevents the entry of food into the glottis by shutting the entry of the windpipe while 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 called, gastro-oesophageal sphincter 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 (where the oesophagus opens), a fundic region (uppermost region) and a pyloric portion (which opens into the first part of small intestine) (Figure 16.3).

• Small intestine is distinguishable into three regions a C 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. The large intestine 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 part, a transverse part and the descending part.
• The descending part of the colon opens into the rectum which opens out through the anus.

Glands and layers:

• The wall of alimentary canal from oesophagus to rectum possesses four layers (refer Figure 16.4 below) 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. The muscle layer is sometimes placed obliquely in some regions.
• The submucosal layer is formed of loose connective tissues containing nerves, blood and lymph vessels. The innermost layer lining the lumen of the alimentary canal is the mucosa.

Specialization of the layers in different regions of the alimentary canal: In duodenum, glands are also housed in the sub-mucosal layer.

Mucosa forms irregular folds called, rugae in the stomach and small finger-like foldings called villi , in the small intestine (refer Figure 16.5 below).

• The cells lining the villi produce numerous microscopic projections called microvilli which give a brush border appearance. These modifications in the form of folds and projections 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 help in lubrication. The mucosa also forms crypts or chambers in between the bases of villi in the intestine called, crypts of Lieberkuhn.
• Mucosa also forms glands in the stomach (gastric glands).
• Thus, the four layers show different modifications in different parts of the alimentary canal.

16.1.2 Digestive Glands

The digestive glands associated with the alimentary canal include: the salivary glands, the liver and the pancreas.

Saliva:

• Saliva is mainly produced by three pairs of salivary glands the parotids (cheek), the sub-maxillary or sub-mandibular (lower jaw) and the sublinguals (below the tongue).
• These glands are situated just outside the buccal cavity. They secrete salivary juice into the buccal cavity.

Liver:

• 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 that secretes bile. Each of the hepatic lobules 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 called, cystic duct along with the hepatic duct from the liver forms the common bile duct (refer Figure 16.6 below).

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.

Pancreas:

• The pancreas is a compound (made of both, an exocrinic and an endocrinic gland) elongated organ situated between the two 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.

16.2 Digestion of Food

The process of digestion is accomplished by mechanical and chemical processes as described:

• In the buccal cavity, the teeth and the tongue along with saliva help masticate and mix up the food thoroughly.
• Mucus in saliva helps in lubricating and sticking the masticated food particles together to form a solid mass, called bolus.
• The chemical process of digestion is initiated by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase present in saliva. About 30 per cent of starch is hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide maltose. Lysozyme which is also present in saliva acts as an antibacterial agent that prevents infections.
  $$ \ce { Starch ->[{Salivary Amylase}][{pH 6.8}]Maltose}$$
• The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition.
• The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.

The gastro-oesophageal sphincter controls the passage of food into the stomach. Furthermore, 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 $B_{12}$).

The roles of various gastric juices have been discussed below:

a) HCl: The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice (containing HCl) of the stomach by the churning movements of its muscular-wall. The food will now be called, the chyme. HCl also provides the acidic pH (pH 1.8) optimal for pepsins to act.

b) Pepsinogen: The proenzyme, pepsinogen on exposure to hydrochloric acid gets converted into the active enzyme: pepsin, the proteolytic enzyme (the enzyme that breaks protein chains) of the stomach. Pepsin converts proteins into proteoses and peptones (peptides).

c) Mucus: 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.  

d) Rennin: 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.

The simplified forms of food then enter the small intestine. Various types of movements are generated by the muscularis layer of the small intestine, that help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitating digestion. The secretions that are released into the small intestine include: the bile, pancreatic juice and the intestinal juice. The pancreatic juice and bile are released through the hepato-pancreatic duct. 

Intestinal juices and their roles:

a) Pancreatic juice: 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. Trypsin activates the other enzymes in the pancreatic juice.

b) Bile: 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 by activating lipases. Lipases break down the fats into very small micelles of di-and monoglycerides. The reaction is depicted below:
$$ \ce { Fats ->[{Lipases}] Diglycerides->Monoglycerides}$$

c) Mucus: The intestinal mucosal epithelium has goblet cells which secrete mucus.

d) Intestinal juice or Succus entericus: 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. This juice contains a variety of enzymes like disaccharidases (for example, maltase), dipeptidases, lipases and nucleosidases. The mucus, along with 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 of the intestine called, Brunner s glands also help in protecting the duodenal layer while it also provides an alkaline medium for enzyme action.

e) Proteolytic enzymes: Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme on reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice to give dipeptides as given below:

f) Pancreatic amylase: Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides as depicted by the reaction:

$$ \ce {Polysaccharides (starch)->[{Amylase}]Disaccharides}$$

g) Nucleases: Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides.

$$ \ce { Nucleic acids ->[{Nucleases}]Nucleotides->Nucleosides}$$

e)The enzymes in the succus entericus again come into play. They act on the end products of the above reactions to form the respective simple absorbable forms. These steps in digestion occur very close to the mucosal epithelial cells of the duodenum region of the small intestine..

$$ \ce {Dipeptides ->[{Dipeptidases}] Amino acids}$$

$$ \ce { Maltose->[{Maltase}] Glucose + Glucose}$$

$$ \ce { Lactose ->[{Lactase}] Glucose + Glucose}$$

$$ \ce { Sucrose ->[{Sucrase}] Glucose + Fructose}$$

$$ \ce {Nucleotides->[{Nucleotidases}]Nucleosides->[{Nucleosidases}] Sugars + Bases}$$

$$ \ce {DiandMonoglycerides ->[{Lipases}] Fatty acids + Glycerol}$$

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 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.

Roles of the Nervous system in digestion:

• The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
• The sight, smell 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 are also moderated by neural mechanisms, both local and through CNS. Hormonal control of the secretion of digestive juices however, is carried out by local hormones produced by the gastric and intestinal mucosa.

Questions from sections 16.1 and 16.2:

1. What is digestion?

2. Describe the alimentary canal in humans with a labelled diagram.

3. Which are the layers present in the alimentary canal? Discuss their modifications and functions in different parts of the alimentary canal briefly.

4. Write a note on the following digestive glands:

a) Saliva

b) Liver

c) Pancreas.

5. Explain in detail, the steps in digestion of food (with the action of enzymes) starting from buccal cavity until it reaches the large intestine.

6. Explain the roles of the nervous system in digestion.

16.3 Absorption of Digested Products

Definition box: 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 and facilitated transport mechanisms.

How do different substances get absorbed?

• Small amounts of monosaccharides like glucose, amino acids and electrolytes like chloride ions are generally absorbed by simple diffusion. The passage of these substances into the blood is governed by the concentration gradient.
• Some substances like glucose and amino acids are absorbed with the help of carrier proteins. Since diffusion here is facilitated by the presence of carrier proteins, apart from a concentration gradient, this mechanism is called the facilitated transport or facilitated diffusion.
• Similarly, transport of water depends upon the osmotic gradient.
• Active transport occurs against the concentration gradient and hence requires energy while passive occurs along the concentration gradient (from higher concentration to lower concentration). Various nutrients like amino acids, monosaccharides like glucose and electrolytes like, Na⁺ are absorbed into the blood by active transport.
• Fatty acids and glycerol being insoluble cannot be absorbed into the blood. Hence, they are first incorporated into small droplets called micelles which move into the intestinal mucosa where 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.

Note box: 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. A summary of absorption (sites of absorption and substances absorbed) is given in Table 16.1.

• Absorption is followed by assimilation and egestion.
• Assimilation is the process by which the absorbed substances finally reach the tissues which utilise them for their activities.
• The digestive wastes, solidified into coherent faeces in the rectum initiates a neural reflex causing an urge or desire for its removal.
• Egestion or Defaecation is the process of elimination of faeces to the outside through the anal opening. It is a voluntary process and is carried out by a mass peristaltic movement.

16.4 Disorders of Digestive System

a) 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 and pin worm.

b) Jaundice: During jaundice, the patient s liver is affected; skin and eyes turn yellow due to the deposit of bile pigments.

c) 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.

d) Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhoea. It reduces the absorption of food.

e) Constipation: In constipation, bowel movements occur irregularly as the faeces are retained within the rectum.

f) 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.

Questions from sections 6.3 and 6.4:

1. Define absorption. Write a note on how different substances get absorbed.

2. Define:

a) Assimilation

b) Egestion

3. List and explain the different digestive disorders that occur in humans.