Human digestive Physiology

Human digestive physiology.

Development embryologic of the digestive tract

Anatomy of the digestive system

The digestive tract

Mouth
Pharynx
Esophagus
Small intestine Stomach
Large intestine

(see the articles detailed on each body)

Associated bodies

Tooth S, Language and Lip S
salivary Glands
Pancreas
Liver and Gall bladder

(see the articles detailed on each body)

Digestive processes

Usually, digestion implies mechanical and chemical actions. At the majority of the Vertebrate S, it is held in several phases in the digestive system after the Ingestion (generally of other organizations). After being absorbed, the nutrients are transported in the circulatory Système (Sang and Lymphe) to meet the requirements in Glucide S, Lipide S, Protéine S, Vitamine S, Rock salt and Eau of the cells of the organization. The not-assimilable parts as the Fiber S are rejected with deposit.

The majority of the chemical reactions carried out during the process of digestion are catalyzed by Protéine S, the Enzyme S.

Mechanical, secretary aspect and absorbent

Mechanical aspect

Digestive tract of 5 meters with Sphincter S at each end.

opening coordinated of the sphincters
progression of the bolus by Peristalsis
regulating system according to the food
impregnation of the bolus by digestive secretions
put in contact of the nutrients with absorbing surfaces.

Secretary aspect

It is the action of the enzymes, water, and different the pH implemented.

Absorbing aspect

The absorption of the nutrients is done all along the digestive tract and the small intestine.

Various phases of digestion of food

Oral cavity

Role of the chewing

Destruction of the solid structures (walls of the vegetable cells), reduction of the size (the passage of the bolus facilitates in the pharynx) and increase in the surface of contact with the enzymes

Role of saliva

hydration,
beginning of sterilization,
action of alpha-amylase or Ptyaline, which acts only on the Amidon, by cleaving it in Maltose and isomaltose. It also includes/understands the lingual lipase which ensures digestion at the nourisson lipids of the mother's milk. It has a modest roleplus in the adult.

Stomach

role of mixing thanks to the contractions of the muscles of the wall of the stomach
role of storage

Gastric secretions

They are orde of 1,5 liters per day.

Hormones

Enzymes

hydrolysis of proteins thanks to Endoprotéase: the Pepsin

Pepsin

it cleaves proteins in Peptide S,
its pH optimum of action ranges between 1,8 and 4,4; it is thus inactivated by alkaline bicarbonates of the pancreatic Suc. A low temperature delays and even suspends its action. The alcohol precipitates this enzyme.

Others

the Hydrochloric acid acidifies with pH 2 in order to allow the enzymes degrading sugars and proteins to function to their optimum pH,
the intrinsic Facteur of Castle.

The bolus becomes the chyme in the stomach.

Duodenum

discharge of secretions of the Pancreas by the pancreatic channel
discharge of secretions of the liver by the cholédoque channel

In addition villosities and microvilosities will increase the heat-transferring surface.

The duodenum is the place where flow:

food chyme,
the bile,
pancreatic juice,
intestinal juice.

The latter transform the food chyme into nutrients.

The nutrients are:

water,
rock salt,
vitamins,
glucose,
amino-acids,
fatty-acids,
glycerol.

Liver

It secretes the Bile, made up for a quarter of proteins and biliary salt three-quarters. It makes it possible to emulsify the fat contents in microgouttelettes and thus to facilitate their absorbtion and their digestion by the enzymes. The bile is then stored in the gall bladder, according to the animal species (for example, the Rat does not have a gall bladder).

Small intestine

It is here that the processes of absorption take place. The structure of the small intestine is made crypts and villosities, allowing the increase in the heat-transferring surface between the bolus and fabric. The cells specialized in the absorption of the nutrients are the entérocytes, which present to their surface Microvillosité S which, like villosities of fabric, have as a role to increase the heat-transferring surface. Thus, it is considered that the human intestine has a surface of contact with the bolus equivalent to the surface of two tennis courts (400m2).

Colon (large intestine)

Storage
Absorption of water
Total colony count:

The colon has a microbial flora commensale abundant, this is why an antibiotic treatment often induced a acute Diarrhée, consequence of the destruction of part of the total colony count of the colon. This flora protects the host from the aggression of pathogenic micro-organisms, and plays a major physiological part by degrading the food for which the man does not have the enzymatic equipment necessary. Thus, the food fibers (cellulose of the plants for example) are degraded by the bacteria, and not by our own digestive enzymes. The relation flora-colon rests on a symbiotic balance

Adaptation of secretions

Several factors can activate the secretion of the various elements implied in digestion, like saliva or the gastric juice and pancretic.

One can distinguish several phases:

Cephalic phase

The odor, sight, or the simple imagination of a meal or a food can suffir with the activation of secretions (in particular salivary).

Oral phase

The food catch and swallowing involve a certain number of reflexes of secretion.

The body has the capacity to adapt its secretions to various food. We will see the limits further from this capacity. The observations of Pavlov on the reactions of the gastric pockets of the dogs to the meat, the bread and milk show obviously that the mechanism of gastric secretion can adapt to food. What makes possible this adaptation, in fact gastric secretions emanate from approximately five million microscopic glands dissimulated in the internal walls of the stomach. Several of these glands secrete various parts of the gastric juice. The quantities and variable proportions of the various elements which use the composition of this juice give a fluid to the multiple properties and which contributes to the digestion of the various kinds of food. Thus, according to the needs, the reaction of juice can be practically neutral, slightly or strongly acid, and contain pepsin more or less. The factor time plays too. At one time of digestion, the character of juice can be very different from what it is at another time, always according to what requires food to digest.

Saliva adapts, it also, with various food and digestive needs. Thus, the weak acids cause a copious flood of saliva, whereas the alkaline weak ones do not cause any of it. Even unpleasant and harmful substances cause a salivary secretion, but then is to facilitate their rejection. The physiologists point out that the action of at least two types different of oral glands can bring a considerable range of corresponding changes to the character of the composed secretion, which is finally released.

The dog provides us an excellent example of this capacity which has the body to modify and to adapt its secretions according to the character of various food. Nourish the meat dog, and its submaxillary especially secrete a thick and viscous saliva. Nourish it of powder of meat and of its parotid an abundant aqueous secretion will run. Thick secretion lubricates the bolus and thus facilitates swallowing, while the other, as fluid as water, involves the powder dries far from the mouth. Thus, the service to be returned determines the character of juice.

The ptyaline does not act on the Sucre. When sugar is eaten, saliva abounds, but it is deprived of ptyaline. The Amidon S soaked do not receive saliva. No the ptyaline either on the meat or grease. They are only some of the easily verifiable adaptations there among those which one could mention and one believes that gastric secretion produced a range of adaptations even wider than that of salivary secretion.

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