Digestion in intestine. Absorbtion in alimentary tract.

After corresponding processing in stomach food passes in small intestine, its first part - duodenum. Food undergoes to action of 3 alimentary juices in it:

· pancreatic;

· bile;

· proper intestinal.

In small intestine, mainly, nutrients decomposition to ending products (monomeres) occurs. Digestion in this part is origined from cavity (cavital), then it continues near intestine wall (parietal).

Pancreas secretory function is linked with its exocrine function – pancreatic juice releasing. It is colourless transparent liquid, containing up to 98,7 per cent of water. Its pH is from 7,5 to 8,5, amount – 1,5-2,5 l per 24 hours. Pancreatic juice contains proteolytic enzymes:

1) endopeptidases – they decompose internal peptide links of proteins forming peptides and aminoacids:

· trypsine;

· chemotrypsine;

· elastase;

2) exopeptidases - they decompose ending links in proteins and peptides releasing aminoacids:

· carboxipeptidase “A” and “B”;

· aminopeptidase;

3) enterokinase – it is secreted in duodenum mucosa and transforms non-active proteases in their active state; enterokinase activates trypsinogene activation into trypsine which, in turn, causes all rest enzymes activation.

Lipolytic enzymes are also released in their unactive state as prophospholipase “A” (it decomposes phospholipids) and active as lipase (lecitinase) (it causes neutral fats hydrolysis to fat acids and monoglycerides). Both lipases are activated under bile (biliary acids) and calcium ions presence.

Amylolytic enzymes:

· alpha-amylase (decomposes starch and glycogen to mono- and disacharides);

· maltase – maltose decomposition to glucose;

· lactase – lactose decomposition to glucose.

Nucleolytic enzymes - nucleases:

· ribonuclease;

· desoxyribonuclease.

Kallikrein - is activated by trypsine.

Pancreatic juice different amount is secreted on every food type. For example, on carbohydrate and protein food (bread, meat) secretion is increased during first 2 hours from the digestion beginning and lasts 4-5 hours (on meat) and 9-10 hours (on bread). On milk secretion maximum is on the 3-rd hour and lasts to 5 hours. Fat food enforces pancreatic juice secretion.

Pancreatic juice releasing regulation has complicated character and consists of 3 phases: conditioned-reflectory, gastric and intestinal.

1) Conditioned-reflectory phase – complex of conditioned (occuring on food appearence, smell, situation connected with eating) and unconditioned (originating from oral cavity, stomach and duodenum receptors) reflexes. All of them are ended by one and the same efferent link – vagus secretory fibres, carrying the information to pancreas secretory cells.

2) Gastric phase – is delt with stomach receptors mechanical, chemical irritations and hormones releasing from it (for instance, gastrines). Main chemical irritators encouraging pancreatic juice releasing are the following: hydrochloric acid, vegetable juices and fats.

3) Intestinal phase- is linked with chimus passage in duodenum; it is developed both under nervous impulses from intestinal mechanoreceptors and intestinal hormones. Secretine and cholecystokinine belong to such hormones. Secretine is formed from prosecretine under hydrochloric acid action and stimulates pancreatic juice releasing in big amount but is is pour in enzymes. Cholecystokinine causes juice secretion rich in enzymes. It is released under fat and protein metabolism products influence.

Pancreatic secretion is inhibited by autonomic nervous system sympathetic part excitement, noradrenaline, adrenaline and their analogues. Such reactions as sleep, tensed physical and mental activity, pain – decrease pancreatic secretion.

Liver role in digestion is in following: it possesses cholepoietic (bile formation) and choleretic (bile secretion) function. Bile formation occurs constantly by the way of several substances (water, glucose, electrolites) filtration from blood in biliary capillaries as well as by means of hepatocytic biliary acids salts and sodium ions secretion. Bile final formation occurs as a result of water and mineral salts reabsorbtion in biliary capillaries, ducts and biliary vesicle.

Bile contains products not only of secretory but also excretory liver activity, directed on excretion some substances (urea, ureic acid and others) out off organism. Twenty-four-houred bile secretion in human being is 0,5-1,5 liters. Main bile components are the following:

· biliary acids;

· biliary pigments;

· cholesterol.

Biliary acids - specific metabolism products in liver. In hepatocytes primary biliary acids (cholic and deoxycholic) are formed from cholesterol. Connected in liver with aminoacids glycine and taurine they are released as glycocholic acid sodium salt and taurocholic acid potassium salt. They are transformed in intestine under bacterial flora action into secondary biliary acids – deoxycholic and lithocholic. Up to 90 per cent of biliary acids are actively reabsorbed from intenstine to blood. They return to liver through portal vessels.

Biliary pigments - bilirubin, biliverdin are haemoglobine metabolism products and they give its colour to bile. Bilirubin predominates in human being determining bile gold-yellow colouring. Urinary and faecal pigments - urobilin, urochrome and stercobilin – are formed from biliary pigments.

Bile can pass into common biliary duct and biliary vesicle. Vesicular bile differs by its dark colour, more significant special weight, decreased active reaction (pH) - 6,0-7,0. Hepatic bile has pH - 7,8-8,6. Bile is concentrated in gall bladder in 7-10 times.

Bile formation is changed at alimentary tract interoreceptors excitement. These receptors irritators are the following: bile, secretine, glucagon, gastrine, cholecystokinine. Bile formation is activated at hyperparasympathicotony.

Bile secretion – is a periodic process occuring as a result of non-coordinated gall-bladder wall muscular activity, Oddi sphincter’s and sphincter of vesicular and common biliary duct (choledoch) fusion place. Common bile duct sphincter is closed out off digestion process and bile comes into gall bladder. In course of gall bladder contraction choledoch sphincter is relaxed and bile passes to duodenum. This reflectory mechanism is triggered by oral cavity, stomach and duodenum receptors irritation with food. Excitement sygnals through medulla oblongata and vagus fibres cause gall bladder muscular contraction and Oddi sphincter relaxation as a result of which bile comes into duodenum. Cholecystokinine is gall bladder contractile activity main stimulator. Bile secretion strong stimulators are yolks, milk, meat, fats.

Bile functions:

· fats emulgation;

· fats transformation in soluble form in water environment;

· encouraging fats digestion and absorbtion;

· pancreatic lipases activation;

· connection with pepsine (it prevents trypsine destruction with pepsine);

· pancreatic proteases action enforcement;

· pancreatic and intestinal juice secretion activation;

· iron and copper absorbtion providing;

· motor activity regulation;

· bacteriostatic action;

· it enforces formation of itself.

Liver main functions:

1. Metabolic role (participation in all substances exchange):

a) protein exchange:

· albumine synthesis;

· globuline synthesis;

· fibrinogen synthesis;

· vit K-dependent coagulation factors synthesis;

· urea formation and some others;

b) carbohydrate exchange:

· glycogenogenesis;

· glyconeogenesis et al.;

c) pigment exchange – due to biliary pigments;

d) hormonal exchange;

e) vitamines exchange.

2. Desintoxicational function:

a) biotransformation:

· conjugation to glucuronic acid;

· conjugation to sulfuric acid;

b) biologically-active substances inactivation:

· adrenaline;

· noradrenaline;

· dophamine;

· serotonine;

· estrogens;

· androgens.

3. Alimentary - due to bile formation and bile secretion.

Small intestine glands secretion. Intestinal juice is Lieberkuhn’s crypts and Brunner’s glands activity product as well as all small intestine mucosa. Twenty-four-houred stomach juice amount is up to 2,5 l. Intestinal juice enzymes releasing is delt with glandulocytes death. Intestinal mucosa dead cells form intestinal juice solid part. Intestinal epitheliocytes are regenerated for 24-36 hours, secret pH is about 7,2-7,5 and at intensive secretion it reaches 8,6. There are about 20 enzymes of all types in juice. Their releasing is activated mainly by humoral-chemical way:

I. Activators: 1. Hormones:

· secretine;

· vasoactive-intestinal peptide (VIP).

2. Mediators (acethylcholine).

3. Chemical substances:

· hydrochloric acid;

· proteins and fats digestions products.

II. Inhibitors: 1. Hormones:

· somatostatine;

· adrenaline.

2. Mediators (noradrenaline).

3. Autonomic nervous system sympathetic part excitement.

Special place in this part of alimentary tract occupies membrane (parietal) digestion, realized in strigillate margin zone formed by microvillies. Pancreatic and intestinal juice enzymes are adsorbed on microvillies glycocalix. These enzymes perform mainly intermediate stages of all nutrients hydrolysis. Proper enterocytes membrane intestinal enzymes do in biggest extent proteins, fats and carbohydrates decomposition final stages. These enzymes active centers are oriented by definite way as for small intestine membrane and cavity. Enzymes catalytic centers free orientation as for hydrolyzed objects is impossible due to it that is membrane digestion distinguishing feature. Digestion initial stages are realized extremely in intestinal cavity and mainly monomeres are formed as a result of membrane hydrolysis which are transported in circulation. It is the very beginning of adsorbtion.

Small intestine motor function is digestion essential stage. One can differentiate 2 movements types in small intestine: peristaltic and pendulum-like. Pendulum-like movements are expressed in following: gut on its short locus is shortened and then is prolonged again. Thus, content is passed to one and then to other direction. Food is mixed due to this. Peristaltic movements: above the food piece isthmus (girdle) is formed, below – gut cavity dilation. These movements encourage food passage through intestine. Sometimes these movements are performed with big velocity and such movements are called peristaltic pushings (grumbling in one’s abdomen).

Intestinal motor activity is regulated by neuro-reflectory and humoral way. Emotions of wrath, fear, pain usually lead to intestinal contraction inhibiting (autonomic nervous system sympathetic part excitement). But at several strong emotions, for example, of fear sometimes wild intestinal peristalsis is observed (nervous diarrhoea). Autonomic nervous system parasympathetic part enforces intestine peristalsis.

Humoral factors influencing on peristalsis:

1. Mediators:

· acetylcholine – increases intestinal secretion;

· noradrenaline – inhibits intestinal secretion.

2. Hormones:

· adrenaline - inhibits intestinal secretion;

· secretine - increases intestinal secretion.

In course of first year of life parietal digestion in children predominates comparatively to cavital one compensating its low development. Babies feeding up increases small intestine devlopment.