COMPOSITION OF SALIVA AND ITS MAJOR FUNCTIONS

Water content: Salivary fluid is predominantly water, constituting approximately 99% of its vol­ume. This aqueous base facilitates various physi­ological functions within the oral cavity.

Electrolytes: Saliva contains a variety of electrolytes essential for maintaining electrolyte balance and pH regulation. These include sodium, potassium, calcium, chloride, magnesium, bicarbonate, and phosphate ions.

Proteins: Saliva contains a diverse range of pro­teins, including enzymes, immunoglobulins (anti­bodies), and other antimicrobial factors. These proteins play crucial roles in digestion, immune defense, and maintaining oral health. Examples of enzymes present in saliva include salivary amy­lase for carbohydrate digestion and lysozyme for antibacterial action.

Mucosal Glycoproteins: Saliva also contains muco­sal glycoproteins, which contribute to the lubricat­ing and protective properties of saliva, helping to maintain oral mucosal integrity.

Traces of Albumin and polypeptides/oligopep- tides: Small amounts of albumin, along with various polypeptides and oligopeptides, are pres­ent in saliva. These substances may have roles in oral health and immune function.

Glucose and Nitrogenous products: Saliva may contain glucose, along with nitrogenous products such as urea and ammonia. These substances may originate from systemic circulation or local meta­bolic processes within the salivary glands.

The interactions among these components contribute to the multifunctional nature of saliva, which includes lubrication, digestion, antimicrobial defense, pH regulation, and main­tenance of oral health. Saliva’s diverse composition reflects its importance in oral physiology and overall well-being (Ekstrom et al., 2019).

The stomach is lined with mucus-secreting cells, but it also contains tubular gland types: the pyloric and oxyntic glands.

15.6.1 Pyloric Glands

These glands release gastrin and mucus. These cells secrete a mucus that facilitates the passage of digesta into the small intestine. Furthermore, it neutralizes the acid and shields the stomach wall from the hydrochloric acid’s corrosive effects. Mucus-secreting cells that release thick, viscous mucus that both protects the stomach wall and facilitates food movement line the whole luminal surface of the stom­ach. Because this mucus has an alkaline composition, it shields the stomach wall from the powerful hydrochloric acid.

15.6.2 Oxyntic Glands

The three cell types that make up this structure are the neck cells that secrete mucus, the peptic or chief cells that secrete pepsinogen, and the parietal cells that secrete hydrochlo­ric acid and intrinsic factor (Figure 15.1). The acid solu­tion secreted by activated parietal cells is isotonic with bodily fluids and has a pH of 0.8. When gastric acid and previously synthesized pepsin come into contact with the inactive form of pepsinogen, it gets activated. An active proteolytic enzyme with a pH range of 1.8 to 3.5 is called pepsin. Gastric lipase is a tributyrase; which breaks down butterfat but is not lipolytically active toward other types of fat. There is no statistical significance even with the tri­butyrase activity. While gelatinase aids in the liquefaction of some of the meat’s proteoglycans, gastric amylase has little effect on the digestion of carbohydrates. Along with hydrochloric acid, the parietal cells release the chemical called intrinsic factor, which is necessary for the absorption of vitamin B12. It is commonly recognized that the lack of intrinsic factor causes pernicious anemia because vitamin B12 is needed to stimulate erythropoiesis in the bone mar­row (Paxton, 2020).

15.6.3 Pancreatic Secretions

Chyme, a mixture of partially digested food, gastric, and salivary secretions, exits the stomach and is assisted in its further digestion in the small intestine by biliary and pan­creatic secretions.

As procarboxypeptidase, chymotrypsinogen, and tryp­sinogen, pancreatic proteolytic enzymes are secreted in an inactive state. They need to be secreted into the intes­tinal system before they become active. Trypsinogen is activated when the intestinal mucosa comes into contact with an enzyme known as enterokinase, which stimulates the chyme. It is also possible for trypsin to autocatalyti­cally activate trypsinogen. Trypsin stimulates the enzymes chymotrypsinogen and procarboxypeptidase. The primary enzyme in the pancreatic juice that breaks down carbohy­drates is pancreatic amylase. With the exception of cellu­lose, it hydrolyzes starches, glycogen, and most other carbs to produce disaccharides and a small number of trisaccha­ride (Agrawal & Aoun, 2014).

Water and bicarbonate ions are secreted into the pancre­atic juice by the epithelial cells of the ductules and ducts that lead from the acini. The concentration of bicarbonate ions in pancreatic juice can rise five times higher than that in blood plasma when the pancreas is stimulated to release large amounts of its juice.

15.6.4 Bile

Bile is an alkaline secretion that has a greenish or greenish- yellow color and contains bile salts, bilirubin, cholesterol, lecithin, and the regular electrolytes found in plasma.

FIGURE 15.1 Schematic diagram of the stomach showing different layers and various cell types in stomach.

bile acids are lithocholic, chenodeoxycholic, deoxycholic, and cholic. They are combined to create taurocholic and gly­cocholic acids by conjugating either with taurine or glycine. Bile contains the salts of these acids, primarily the sodium salts. Within the gastrointestinal tract, the bile salts serve two crucial purposes: (i) Bile salts’ emulsifying or detergent action: This lowers fat’s surface tension and enables intes­tinal movement to break fat globules into tiny pieces. (ii) They facilitate the intestinal tract’s absorption of fatty acids, monoglycerides, cholesterol, and other lipids. They accom­plish this by joining the lipids to form tiny complexes. The soluble compounds are known as micelles. (Rao, 1998).

15.6.5 Regulation of Bile Secretion

It is regulated by chemicals, hormones, and the nervous sys­tem. Although total hepatic denervation does not completely stop bile production, vagal stimulation does. Pancreatic and bile secretions run parallel to one another because secre­tin and chemicals that stimulate secretin also stimulate bile production. The second part of the bile, which is high in water and electrolytes, is secreted when secretin stimulates the duct cells.

The sphincter of Oddi relaxes, and the gall bladder contracts to release the stored bile, which is caused by cholecystokinin, vagal fibers, and ENS fibers. The most significant choleretics (that induce bile secretion) are bile salts.

15.6.6 Secretions of the Small Intestine

The first few centimeters of the duodenum include a large number of compound mucus glands known as Brunner’s glands. In response to vagal activation, secretin, and tac­tile or irritating stimuli of the mucosa above, these glands release copious volumes of alkaline mucus. The duodenum wall is shielded from the extremely acidic stomach juice by this mucus. Furthermore, the extra bicarbonate ions in this mucus complement those released in pancreatic juice and bile and balance the hydrochloric acid that enters the duo­denum from the stomach. The Brunner’s glands are inhib­ited by sympathetic activation (Carr & Toner, 1984).

Small pits known as Lieberkuhn crypts are found throughout the small intestine. The small intestine’s inter­stitial spaces are home to these crypts. In the crypts, a large number of enterocytes secrete large amounts of water and electrolytes (especially bicarbonate and chloride, which are actively secreted followed by sodium and water passively), while the villi’s intestinal surface is covered by a moderate number of goblet cells that secrete mucus, which lubricates and protects the small intestinal surface. As the chyme comes into contact with the villi, the fluid that circulates from crypts to villi provides a wet medium for the contents to be absorbed. Enzymes are absent from pure small intes­tine secretions that do not contain cellular detritus. On the other hand, the enterocytes on the villi do contain enzymes that break down nutrients as they are absorbed. These enterocytes comprise the following enzymes: (i) modest levels of lipases, which divide neutral fat into glycerol and fatty acids; (ii) sucrase, maltase, isomaltase, and lactase, which split disaccharides into monosaccharides; and (iii) peptidases, which split short peptides into amino acids.

15.6.7 Secretions in the Large Intestine

Despite the large intestine having several Lieberkuhn crypts as well, the epithelial cell lacks villi and enzymes. Mucus-secreting cells are the only type of cells found in the large intestine crypts. Bicarbonate released by cells other than those that secrete mucus is present in this mucus in rather moderate levels. Mucus secretion is regulated by tactile stimulations and local nerve responses. When the spinal cord stimulates the pelvic nerves, parasympathetic innervation reaches the crypts, resulting in abundant mucus secretion.

In addition to protecting the mucosa, the mucus released in the large intestine serves as an adhesive medium, keep­ing the feces together. It acts as a barrier to keep the acids produced by bacterial activity from damaging the intestinal wall and shields the wall from the significant amount of bacterial activity that occurs inside the feces. In addition to mucus, there is a strong production of water and elec­trolytes if there is severe irritation of the large intestine, as occurs in cases of bacterial infections, food poisoning, etc. Diarrhea is caused by the large intestine’s excessive motil­ity, which is brought on by the extra water and electrolytes. This has a preventive effect since it removes the irritating substance (Shashikanth et al., 2017).

15.6.8 Digestion

The majority of mammals feed primarily on carbohydrates, proteins, and lipids. They must be broken down into absorb­able form because they cannot be absorbed in their original state. The process of moving digested food from the GIT lumen to the circulation (blood/lymph) is called absorption. The absorbed materials are transported by circulation to the tissues for processing, storing, or decomposition.

Limited absorption occurs in mouth and stomach because the digestion is far from complete in these organs. Small intestine is the chief site of absorption both in car­nivores and omnivores. In non-ruminant herbivores also small intestinal absorption is of considerable importance. Except for colon, which absorbs water and some electro­lytes, large intestines undertake negligible absorption. However, in non-ruminant herbivores like the horse and the pig, caecal microbial fermentation gives rise to large quan­tities of volatile fatty acids which are absorbed from the large intestine in these animals. The absorptive surfaces of small intestine show many folds called “folds of Kerckring” or “valvulae conniventes” which increase the surface area by three times. Positioned all over these folds starting from duodenum to illeocecal junction are millions of specified structures known as villi. The villi project into the lumen of the small intestine up to 1-8mm. They further increase the surface area of the small intestinal mucosa by 10 times. On the luminal side of each villus, there are about 1000 micro­villi protruding into the intestinal lumen, which increase the surface area, another 20-fold. Thus, a combination of folds of Kerckring, villi and microvilli together increase the surface a 1000-fold. In an adult man the absorptive surface of the small intestine is approx. 250 sq. meters or slightly more than the surface area of a tennis court (Petras, 2013).

The villi (villus) are the most important structural ele­ments involved in absorption in the small intestine. At the core of each villus is a large lymph capillary known as the central lacteal. Each villus has several small arteries, which enter the base and form dense capillary network immediately under its epithelium. Prominent smooth mus­cle strands are also found in the core of each villus. Villi undergo rhythmic (pumping) contractions including pen­dular movements and tonic contractions all of which help absorption and transfer of absorbed materials. Contractions of villi are brought about by Meissner’s plexus, sympathetic nervous system and a hormone called villi kinin produced by the duodenal mucosa in response to presence of acid chyme.

15.6.9 Mechanisms of Absorption

Absorption through the gastrointestinal mucosa occurs by active transport, diffusion and solvent drag. Transport by solvent drag means that any time a solvent is absorbed because of physical absorptive forces, the flow of solvent shall drag dissolved substances along with the solvent.

15.6.10 Absorption in the Small Intestine

Diffusion is the only mechanism that moves water over the gut barrier. The principles of osmosis apply to this dis­persion. Water so absorbs from chyme when it is diluted, but flows into chyme from plasma when it is concentrated. Water is therefore absorbed by isoosmotic absorption through the intestinal mucosa.

Each day, the intestines can absorb up to one-fifth of the body’s entire sodium content. Feces lose only 0.5% of the salt that is lost in the gastrointestinal secretions. However, the body can quickly become fatally low in salt when sig­nificant amounts of digestive secretions are lost in feces, as occurs in diarrhea.

The renal tubules’ method of sodium absorption is similar to that of the intestinal epithelium. As a result, a concentration gradient is formed between the interior of the epithelial cells and the intestinal lumen as sodium is actively transferred from within the cells via the lateral and basal borders. As a result, sodium diffuses more easily over the brush border and into the epithelial cells as it moves down the concentration gradient. In return for potassium and hydrogen ions, some sodium is also absorbed. When released, adrenaline improves the body’s ability to absorb salt from the intestine, particularly the colon.

In the upper part of intestine chloride is rapidly absorbed by diffusion due to the electrochemical gradient created between the chyme and the inside of epithelial cells by the absorption of sodium. Thus absorption of chloride follows absorption of sodium.

Pancreatic and bile secretions release large amounts of bicarbonate ions into the intestine. The upper section of the small intestine is where they must be absorbed. Carbon dioxide is absorbed in a way akin to that of the renal tubules when bicarbonate ions are absorbed. As previously stated, in exchange for one hydrogen ion, a certain amount of sodium is absorbed. As a result, the bicarbonate and hydro­gen ion released into the chyme mix to generate carbonic acid. After then, the carbonic acid split into carbon dioxide and water. Carbon dioxide readily enters the blood and is exhaled through the lungs, but water stays in chyme.

The duodenum in particular actively absorbs calcium ions from the small intestine. The body precisely meets its daily calcium demands through regulation of calcium absorption. Vitamin D and parathyroid hormone improve the intestines’ ability to absorb calcium. From the duo­denum, iron ions are actively absorbed. The ferrous state absorbs it more easily than the ferric state. Iron joins forces with transferrin upon intestinal absorption to facilitate cir­culation. Ninety percent of the iron in the blood is depos­ited in the liver, where it joins forces with apoferritin to produce ferritin. Because apoferritin and iron are loosely combined, iron can be easily released into the system when needed. Iron absorption from the duodenum is significantly decreased when the whole apoferritin in the liver is satu­rated with iron.

The intestinal mucosa is also capable of actively trans­porting a number of other ions, including phosphate, potassium, and magnesium. Monovalent ions are gener­ally readily absorbed in large amounts. Bivalent ions, how­ever, are only slightly absorbed. For instance, calcium ion absorption is only 1∕50th of sodium ion absorption.

Monosaccharides are primarily absorbed as carbohy­drates. Disaccharides are also absorbed in very minute amounts, but not as bigger polymers. Approximately 80% of the monosaccharides that are absorbed from the intestine are glucose. Galactose and fructose make up the remaining 20% of absorbed carbohydrates. Active transport facilitates the absorption of nearly all monosaccharides. Sodium and glucose travel through the brush border together. Galactose is conveyed in a comparable manner. Nevertheless, fructose is absorbed by enhanced diffusion rather than the sodium co-transport route. Fructose absorbs at half the rate of glu­cose or galactose because it is not co-transported with salt.

The majority of proteins are absorbed as tri- and di-pep­tides, as well as some free amino acids. Similar to glucose, these compounds are absorbed through the sodium co­transport process. Some free amino acids are also absorbed through the method of assisted diffusion, similar to fruc­tose. Because different amino acids and peptides have varied binding properties, the intestine has at least five dif­ferent types of amino acid and peptide transport proteins.

Glycerol, free fatty acids, and monoglycerides are the byproducts of fat digestion. All of these byproducts of fat digestion disintegrate in the bile micelles’ inner lipid layer. The micelles’ charged exterior and tiny diameter make them soluble in chyme. These micelles travel to the brush border where they get trapped in the nooks and crannies created by the stirring, moving microvilli. Due to their solubility in the lipid membrane of the cells, fatty acids and monoglycerides here both permeate into the epithelial cells right away.

This results in the micelles remaining in the chyme, where they continue to transport progressively more fatty acids and monoglycerides. The fatty acids in the epithe­lial cells are converted into new triglycerides, which are then carried into the bloodstream by lymph chylomicrons. Because they dissolve in water, small amounts of short and medium chain fatty acids are also absorbed straight into the portal circulation.

15.6.11 Absorption in the Large Intestine

The colon absorbs the majority of the electrolytes and water in chyme. The proximal half of the colon, also known as the absorbent colon, is where the majority of this absorption takes place. The term “storage colon” refers to the func­tion of the distal part of the colon, which stores the undi­gested food. The large intestine mucosa is highly capable of actively absorbing salt and the resulting chloride. This is especially true in cases where elevated levels of aldosterone are released. Water follows the osmotic gradient that is cre­ated throughout the large intestine mucosa by the absorp­tion of sodium and water (Oake et al., 2023).