Animal digestion

Monogastric animals

The digestive tract of an monogastric animal (animal with only one stomach) aims at breaking down the eaten feed, splitting the compounds into nutrients, transform them into usable form and excrete the non-digestable matter as feces or urine. This is achieved with digestive enzymes and/or microbes in the digestive tract.

Picture from lecture materials,
original source unknown

Digestion has roughly three phases: chewing or mincing, digestion and absorption. After absorption it is a matter of other tissues to use the nutrients for various chemical processes. The first phase, chewing, begins as the animal picks the feed to its mouth. Chewing adds the surface area of the feed, making it easier for enzymes to attach to it. Chewing also adds the excretion of saliva, which lubricates the feed and the esophagus. Saliva includes also growth factors, which support the renewal of the digestive tract epitelium. In some species, excluding ruminants, have amylase in their saliva, which breaks starch and glycogen into smaller polysaccharides. Mouth area has also several lymp nodules, which act as a first barrier against any bacteria in the feed.

Swallowing takes the feed in small chunks to the esophagus and down to the stomach. The stomach consists of four areas: cardia, fundus, corpus and pylorus. All areas have a different structure of mucous membrane: for example, cardia excretes mucus, but fundus excrete gastric juices. Pylorus has sensory nerves, which regulate digestion. Gastric juices include HCl, a strong acid, which lowers the pH of the stomach to 1-3 and this assists the denaturation of proteins and kills bacteria. Pepsinogen is secreted by the glands in the stomach, and transformed to pepsin by the HCl. Pepsin breaks proteins to small polypeptides. In addition to HCl and pepsinogen, the gastric juice includes water, inorganic salts and mucus.
In the stomach the HCl and pepsin are mixed with the feed as the stomach wall contracts, and the food mass becomes softer, warmer and more watery.

Pyloric sphincter acts as a gate between the stomach and the small intestine. The beginning of the small intestine is called duodenum. Pancreatic juices, intestinal gland liquids, bile and bile salts are all excreted to duodenum, where the feed mass is quickly neutralized (remember it's pH was about 1-3). Intestinal glands secrete a basic liquid which neutralizes the chyme (= food mass). Bile and bile salts from the liver mix water into fats, forming fat droplets. Lipase from the pancreas is actived by bile salts, and then it breaks the fat droplets into glycerol and fatty acids. Pancreas excrete also insulin and glucagon, which are used to transfer glucose from the blood into cells (insulin) or discharge stored glycogen into the blood as glucose (glucagon).

The next part of the small intestine is called jejunum. This is where most of the nutrients are absorbed, after they have been broken down to usable compounds in the duodenum. Pancreatic amylases break down polysaccharides into disaccharides, and disaccharidases into monosaccharides. Trypsin and chymotrypsin (from the pancreas) break polypeptides into smaller pieces, which are split into amino acids by peptidases:

Picture from lecture materials, original source unknown

From the small intestine the chyme moves to the large intestine, where microbes ferment it. Only water, electrolytes and volatile fatty acids (ruminants only) are absorbed from the large intestine. In addition, some B vitamins are formed in the large intestine.

Ruminants

The  saliva of ruminants doesn't have any amylase. Instead, it is rich in mucin, phosphates and carbonates, which balance the pH of the rumen. The esophagus differs from that of the monogastric animals in that it has two kinds of contractions: peristaltic, which move the feed pieces down to the rumen, and antiperistaltic, which bring boluses back to the mouth for the animal to ruminate. 

Ruminants have four stomachs, and only the last of them (the abomasum) functions like the stomach of monogastric animals. Ruminants have three fore-stomachs: rumen, reticulum and omasum. These are sites for microbial fermentation. Rumen is the first of the fore-stomachs, and in cows it can store up to 80 liters of feed. It hosts about 10¹¹-10¹² microbes / g of feed. The rumen contracts in three stages every 50-70 seconds. The contractions mix the feed (mixing the microbes into the contents of the rumen), releases methane as burps and brings boluses back to the mouth for the animal to ruminate. Rumen and reticulum are often considered as one, and called reticulorumen. The pH in the reticulorumen must stay between 5-8 for the microbes to work, and the gases must be let out as burps. Otherwise the rumen contractions are hindered and the animal may suffocate, as the methane fills the rumen and the lungs no longer have space to expand.

From the reticulorumen, water, Na, Cl, ammonium and water are absorbed. Omasum is the last of the fore-stomachs. It is filled with leafs, which sieve the feed, and prevent too large particles from reaching abomasum. From the omasum, water, minerals, ammonium and VFA are absorbed.

The final stomach for ruminants is the abomasum. It secretes pepsin and HCl, which lower its pH to 3-4. There the dead microbes from the rumen are digested enzymatically, and the proteins and fats in the microbes can be absorbed. Some of the proteins in the feed are not absorbed until in the abomasum. Abomasum also finalizes the digestion and absorption of other particles, which may not have been fermented in the fore-stomachs.