Physiology of digestive system is ultilized of food involves several processes , viz., Ingestion , digestion , absorption , assimilation , and egestion .
Ingestion means taking food into the oral cavity
through the mouth. Both solid and liquid material are taken by human.Hands are used for this purpose by man. Bite sized pieces are cut with into incisor teeth from larger food items. Thus, nutrition in humans is holozoic.
Role of Oral (Buccal) Cavity
In the oral (buccal) cavity, the food is
(i) tasted by the taste buds:
(ii) moistened with mucus and saliva to make chewing and swallowing easy:
(iii) masticated by the grinding teeth to smaller particles which get mixed with ptyalin and offer greater surface area for its action:
(iv) partially digested by salivary enzyme:
(v) made into a bolus or ball by working of the tongue against the palate; and
swallowed. Saliva also cleans the tongue. (vi)Mastication stimulates the salivary glands.
Digestion of food starts in the oral cavity in humans, continues in the stomach and is completed in the small intestine. The food undergoes no change in the pharynx and the oesophagus. These regions only convey the food from the oral cavity into the stomach by swallowing. “This motion can be seen in the bobbing of the ” Adam’s apple” during swallowing .
Meaning :- Swallowing also called regulation , involves the passage of food bolus from the oral cavity to the stomach.
Process :- Swallowing is started as a voluntary action by pressing the tongue against the hard palate .This pushes the ball into the pharynx. From here, swallowing continues by involuntary (reflex) action started by stimulation of sensory nerve endings in the pharynx. As the food reaches the pharynx, the soft palate and uvula rise to close the nasopharynx, and the larynx moves upward to meet the epiglottis for closing the glottis . This stops breathing for a moment and prevents the food from going the wrong way. Breathing during swallowing would choke the respiratory passage. Entry of a piece of food into the glottis may pose a threat to life. Fortunately, the coughing reflex quickly clears the air passage. This reflex is poor in an unconscious person. Hence, such a person should not be given liquids. Pharyngeal muscles contract, pushing the ball over the epiglottis into the oesophagus. The sphincter muscle at the beginning of the oesophagus relaxes for this purpose. Food undergoes no change in the oesophagus. The latter simply forces the food ball into the stomach by peristalsis that involves oral contraction of muscles just above the bolus and relaxation of muscles below it. The cardiac for sphincter opens as food ball approaches to let it slip with into the stomach. The sphincter then closes again to check food passing back into the oesophagus. Food does not merely slide down the oesophagus due to gravity, it is actively pushed along by peristalsis.
Meaning of Absorption
The digested food passes from the alimentary canal into the blood and absorption lymph through its mucous membrane. This is called absorption .
Site of absorption
Very little absorption occurs in the mouth and stomach. Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are absorbed into the blood capillaries lining them. Some water and salts, alcohol and a few drugs, such as aspirin, are, however, absorbed in the stomach. Small intestine is the principal organ for absorption of nutrients. After completion of digestion in small intestine, the products of digestion such as glucose , fructose , fatty acids, glycerol ,amino acid etc. are absorbed through the mucosa into the blood stream and lymph. Some absorption occurs in the large intestine. Water, sodium, small amounts of other ions and products of bacterial digestion (amino acid and vitamins B1 ,B2 and K ) are absorbed here.
Absorption mainly occurs in the small intestine . For this purpose , the internal surface area of the small intestine is greatly enlarged ( to about 300 m2) by four mechanisms: –
(i) Length -: Small intestine is about 6 m long and 3.5-4cm wide. It has enormous absorptive area.
(ii) Plicae Circulares -: These are circular folds that extend from the inner surface of the small intestine by 1/2 to 2/3 of the way into its lumen . The folds increase the absorptive surface .
(iii) Villi -: The villi are numerous small leaf , tongue or finger- shaped projections of mucosa , about 0.5 to 1 mm long. They make the absorptive surface still greater .
(iv) Microvilli -: Microvilli are countless minute, closely set projections from the free surface of most of the mucosal cells . They may be about 500 microvilli on each cell. The microvilli add to the absorptive surface.
Mechanism of absorption
Absorption occurs by two types of process :- passive and active absorption .
1- Passive Absorption
It occurs down the concentration gradient. It takes place by physical processes such as simple diffusion, osmosis and facilitated diffusion.
a- Simple Diffusion – Small nutrient molecules dissolved in water diffuse into the blood across the mucous membrane till their concentration is higher in the intestinal contents than in the blood. When the concentration difference disappears, diffusion stops. Thus, no nutrient can be fully absorbed by diffusion. Moreover, diffusion is a slow process.
b- Osmosis – Water is absorbed by osmosis. This process occurs so long as the solute concentration and the consequent osmotic pressure) is higher in the blood than in the intestinal contents. Absorption of any solute raises the osmotic pressure of the blood, and brings about absorption of an equivalent amount of water. Water is absorbed partly in the small intestine and mostly in the large intestine.
c- Facilitated Diffusion – Fructose is absorbed by this process. As it occurs only down the concentration gradient, it cannot absorb the entire amount of the nutrient in the intestinal contents. It is, however, more rapid than simple
This process occurs by biological processes called active transport and endocytosis.
a- Active Transport – This process is independent of concentration gradient. It can move materials not only down the concentration but also against the concentration gradient. It is brought about by the carrier protein molecules of cell membrane. lt utilizes energy derived from hydrolysis of ATP. Active transport is far more rapid than diffusion. Nutrients can be absorbed completely from the intestinal contents by active transport. Substances of high nutritional importance, such as glucose, galactose and amino acids, are absorbed by this process.
b- Endocytosis – Cells lining the small intestine take up small portions of chuke by endocytosis. How important this process is in the absorption of food is not clear .
Meaning of Assimilation
Assimilation means the incorporation of the absorbed food materials into the tissue cells as their integral and homogeneous component.
Mechanism of Assimilation
The various food itself under: molecules taken up by the cells are transformed as under :
i. Amino acids
Amino acids reach the liver by way of hepatic portal vein. The surplus amino acids get deaminated in the liver by separation of amino group (NH2). The amino group combines with carbon dioxide to form urea, CO (NH2), which is excreted in the form of urine. The rest of the amino acid molecules after deamination are converted into sugar or fat for use as source of energy. Other amino acids are released by the liver to the elimination cells of the body. Here, they are transformed into the characteristic proteins of human body by intracellular enzymes. Some of these proteins finally compounds change into living material, the protoplasm, for repair and growth, and some act as enzymes to catalyse chemical reactions. Proteins are not stored in the body .
Glucose, fructose and galactose also reach the liver by way of hepatic portal vein . Here , fructose anda galactose are converted into glucose . Excess of glucose is converted into glycogen in the liver and stored for supply at the time of need ; and the rest released to the body cells . Some glucose is also stored in the muscles as glycogen and other glucose is changed into fats that are stored in the liver and muscles.
Fasting – Glycogens stored in the liver and muscles would supply energy needs of the body for only about 12 hours if they were used alone. How- ever, a man of normal weight can usually survive without food for about six weeks, using fat reserves for energy. However, thirst can lead to death within a few days.
The absorbed fats change into characteristic fats of human body by rearrangement of fatty acids. Some fats are stored in the liver, bones, survive muscles, mesenteries, and adipose tissue, whereas some enter into the composition of cell organelles ,especially cell and nuclear membranes. Fats are used as fuel also. For this, they are split into fatty acids and glycerol . Fats are also changed into glucogen in the liver cells .
Meaning of Egestion
The elimination of the indigestible remains of the food from the alimentary canal is called egestion, or defecation or evacuation. The waste matter discharged from the alimentary canal is called faeces, or stool.
Mechanism of Egestion
Egestion is carried out by the large intestine. In the colon, water is absorbed into the blood, thus concentrating chyle into faeces. Faeces can remain in the colon for about 36 hours, then moving into rectum. This movement is spontaneous and involuntary. It typically occurs after a meal when food from the small intestine passes into large intestine. Faeces remains in the rectum briefly before discharge via anus. This activates involves coordinated action of muscles of large intestine, abdominal muscles, diaphragm and anal sphincter. The act is partly voluntary and partly involuntary. The urge to pass out faeces is produced by widening of rectum on the entry of faeces and the resulting stimulation of nerves in its wall. The urge may occur one or more times each day.
Delayed defaecation hardens the faeces by further absorption of water from them. This results in constipation.
Composition of Faeces
The faeces consist of about three – fourth water and one – fourth solid matter. Of the solid matter, about 3% is bacteria, about 30% is undigested roughage, about 20% is fat, about a 15% is inorganic matter , and about 3% is protein. Dead mucosal cells , mucus and cholesterol also occur in the faeces. Its color is due to brown pigments , stercobilinogen ,and stercobilin , formed by modification of bile pigments by colon bacteria. Faeces are formed even when no food is taken during fasting.
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