Detailed Procedure of assimilation of food in body :MODERN VIEW

DETAILED PROCEDURE OF ASSIMILATION OF FOOD IN BODY : MODERN VIEW  AS ALL AS IN AYURVEDA 

Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. The human gastrointestinal tract is around 9 meters long and is divided into Upper and lower human gastrointestinal tract. Food digestion physiology varies between individuals and upon other factors such as the characteristics of the food and size of the meal. 
The process of digestion normally takes between 24 and 72 hours but it takes days for the nutrients to nourish body at cellular level.

There are three phases of digestions 

At CEPHALIC LEVEL, in upper GI TRACT and in lower GI TRACT

1.The first is the MECHANICAL DIGESTION of food with help of saliva ....

2. CHEMICAL DIGESTION or bio assimilation of food....

The first process consists of Physical breakdown of food or medicine that is oral mastication and stomach churning. In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication(chewing), a form of mechanical digestion with the wetting contact of saliva , Saliva is a liquid secreted by the salivary glands containing salivary amylase, an enzyme which starts the digestion of starch in the food. The saliva also contains mucus, which lubricates the food and hydrogen carbonate which provides the ideal conditions of ph(alkaline) for amylase to work. After undergoing mastication and starch digestion, the food will be in the form of a small, round slurry mass called a bolus.

It will then travel down the Oesophagus and into the stomach by the action of peristalsis.

Second phase consists of Chemical alteration of substances in the bloodstream by the liver or cellular secretions. Although a few compounds can be absorbed directly during digestion, but Bioavailability of many compounds is dictated by the second process since both the liver and cellular secretions are very specific in their metabolic action. This second process is where the absorbed food reaches the cells via the liver. Chemical breakdown is always done with enzymes and acids, which are secreted from stomach and liver. When we eat something and they get a trigger to produce them. The enzymes break the bigger substances of the food into smaller molecules so that they can go through the rest of the digestive tract easily.                                                                                                                                                                                          Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains Hydrochloric Acid and Pepsin. It also contains rennin in case of infants and toddlers. As the first two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the time protein digestion is occurring in stomach, mechanical mixing also occurs by peristalsis, which are waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.

After some time, typically 1–2 hours in humans the resulting thick liquid is formed which is called chyme. When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas and bile juice from the liver and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi and their epithelial cells is covered with numerous micro villi to improve the absorption of nutrients by increasing the surface area of the intestine.

In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place. Here water is absorbed and waste material stored as faeces to be removed by defecation via the anal canal and anus.

Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the Ph is slightly acidic about 5.6 to 6.9 . Some vitamins, such as biotin and vitamin K are produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation.

In the small intestine, the larger part of digestion takes place and this is helped by the secretions of bile, pancreatic juice and intestinal juice. The intestinal walls are lined with villi, and their epithelial cells  are covered with numerous micro villi to improve the absorption of nutrients by increasing the surface area of the intestine.

In the large intestine the passage of food is slower to enable fermentation by the gut flora to take place. Here water is absorbed and waste material stored as faeces to be removed by defecation via the anal canal and anus.

The  detail involvement of juices and enzymes and organs involved at every level are.....

 The cephalic phase occurs at the sight, thought and smell of food, which stimulate the cerebral cortex. Taste and smell stimuli are sent to the hypothalamus and medulla oblongata. After this it is routed through the vagus nerve and release of acetylcholine. Gastric secretion at this phase rises to 40% of maximum rate. Acidity in the stomach is not buffered by food at this point and thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.

The Second process The gastric phase takes 3 to 4 hours.... It is stimulated by distension of the stomach, presence of food in stomach and decrease in ph value. Distention activates long and myenteric reflexes. This activates the release of acetylcholine, which stimulates the release of more gastric juices. As protein enters the stomach, it binds to hydrogen ions, which raises the ph of the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid. Gastric acid is about 0.5% hydrochloric acid (HCL), which lowers the ph to the desired ph of 1–3. Acid release is also triggered by acetylcholine and histamine.

The intestinal phase has two parts ...the excitatory and the inhibitory. Partially digested food fills the duodenum. This triggers intestinal gastrin to be released. Enterogastric reflex inhibits vagal nuclei, activating sympathetic fibres causing the pyloric sphincter to tighten to prevent more food from entering  and inhibits local reflexes.

Breakdown into nutrients

Protein digestion :

Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas; break down food proteins into polypeptides that are then broken down by various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by pancreas in the form of trypsinogen, which is activated in the duodenum by enterokinase to form trypsin. Trypsin then cleaves proteins to smaller polypeptides.

Fat digestion

Fatty acid metabolism and Dietary sources of fatty acids, their digestion, absorption, transport in the blood and storage. Digestion of some fats can begin in the mouth where lingual lipase breaks down some short chain lipids into diglycerides. However fats are mainly digested in the small intestine.The presence of fat in the small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver which helps in the emulsification of fats for absorption of fatty acids.Complete digestion of one molecule of fat (a triglyceride) results a mixture of fatty acids, mono- and di-glycerides, as well as some undigested triglycerides, but no free glycerol molecules.

Carbohydrate digestion

Carbohydrate metabolism and Carbohydrate catabolism .In humans, dietary starches are composed of glucose units arranged in long chains called amylose, a polysaccharide. During digestion, bonds between glucose molecules are broken by salivary and pancreatic amylase, resulting in progressively smaller chains of glucose. This results in simple sugars glucose and maltose (2 glucose molecules) that can be absorbed by the small intestine.

Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose and galactose. Glucose and galactose can be absorbed by the small intestine. Approximately 65 percent of the adult population produce only small amounts of lactase and are unable to eat unfermented milk-based foods. This is commonly known as lactose intolerance. Lactose intolerance varies widely by genetic heritage; more than 90 percent of peoples of east Asian descent are lactose intolerant, in contrast to about 5 percent of people of northern European descent.

Sucrase is an enzyme that breaks down the disaccharide sucrose, commonly known as table sugar, cane sugar, or beet sugar. Sucrose digestion yields the sugars fructose and glucose which are readily absorbed by the small intestine.

DNA and RNA digestion

DNA and RNA are broken down into mononucleotides by the nucleases deoxyribonuclease and ribonuclease (dnase and rnase) from the pancreas.

Non-destructive digestion

Some nutrients are complex molecules (for example vitamin B12) which would be destroyed if they were broken down into their functional groups. To digest vitamin B12 non-destructively, haptocorrin in saliva strongly binds and protects the B12 molecules from stomach acid as they enter the stomach and are cleaved from their protein complexes.

After the B12-haptocorrin complexes pass from the stomach via the pylorus to the duodenum, pancreatic proteases cleave haptocorrin from the B12 molecules which rebind to intrinsic factor (IF). These B12-IF complexes travel to the ileum portion of the small intestine where cubilin receptors enable assimilation and circulation of B12-IF complexes in the blood.

Digestive hormones

Action of the major digestive hormones

There are at least five hormones that aid and regulate the digestive system in mammals. There are variations across the vertebrates, as for instance in birds. Arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.

Gastrin – is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low ph.

Secretin – is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver. This hormone responds to the acidity of the chyme.

Cholecystokinin (CCK) – is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder. This hormone is secreted in response to fat in chyme.

Gastric inhibitory peptide (GIP) – is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach. Another function is to induce insulin secretion.

Motilin – is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.

Significance of ph

Digestion is a complex process controlled by several factors. Ph plays a crucial role in a normally functioning digestive tract. In the mouth, pharynx and esophagus, ph is typically about 6.8, very weakly acidic. Saliva controls ph in this region of the digestive tract. Salivary amylase is contained in saliva and starts the breakdown of carbohydrates into monosaccharides. Most digestive enzymes are sensitive to ph and will denature in a high or low ph environment.

The stomach's high acidity inhibits the breakdown of carbohydrates within it. This acidity confers two benefits: it denatures proteins for further digestion in the small intestines, and provides non-specific immunity, damaging or eliminating various pathogens.

In the small intestines, the duodenum provides critical ph balancing to activate digestive enzymes. The liver secretes bile into the duodenum to neutralize the acidic conditions from the stomach, and the pancreatic duct empties into the duodenum, adding bicarbonate to neutralize the acidic chyme, thus creating a neutral environment. The mucosal tissue of the small intestines is alkaline with a ph of about 8.5

Detailed Physiology of the gastrointestinal system

Gastro intestinal  tract

Upper   tract  

Chief cells           Pepsinogen

Parietal cells     Gastric acid

Intrinsic factor

 aloveolar cells ,HCO3 , Mucus ,Goblet cells ,Mucus 

UPPER GI TRACT RESPONSIBLE FOR  TWO PROCESSES :

Swallowing / Vomiting

Fluids   added for chewing :

Saliva

Gastric acid

Gastric acid secretions :

 Gastrin

G cells

Histamine

ECL cells

Somatostatin

D cells

Food shifts to intestines :

Lower  gastro intestinal tract

Endocrine/paracrine

Bile and pancreatic secretion:

Enterogastrone

Cholecystokinin

I cells

Secretin

S cells

Glucose homeostasis (incretins):

GIP

K cells

GLP-1

L cells

Endocrine cell types

Enteroendocrine cells

Enterochromaffin cell

APUD cell

Fluids 

Intestinal juice

Processes          

Segmentation contractions

Migrating motor complex

Borborygmus

Defecation

Enteric nervous system

Submucous plexus

Myenteric plexus

Processes                          

Peristalsis

(Interstitial cell of Cajal

Basal electrical rhythm)

Gastrocolic reflex

Digestion

Enterocyte

Accessory Fluids            

Bile

Pancreatic juice

Processes

Enterohepatic circulation

Abdominopelvic

Peritoneal fluid