The Tube
The digestive tract of dogs and cats is formed as a long tube containing lips, oral cavity, pharynx, oesophagus, stomach, small and large intestines, rectum, and anus. To perform the absorption of nutrients and fluids and the excretion of waste products it also includes the liver, gallbladder, and pancreas.
In the digestive phase food is taken into the mouth using lips, teeth, and tongue. The tongue and the buccinators, temporal and masseter muscles mix and move the food bolus. The rugae of the hard palate help directing the bolus towards the pharynx. Stimulation of the palate or touch receptors in the pharynx initiates the swallowing reflex. This reflex causes the epiglottis, the vocal cords, and the soft palate to close, the upper oesophageal sphincter to relax and oesophageal peristalsis then takes the bolus through the cardiac sphincter into the stomach. The oesophagus, similarly to the other portions of the gut tube, is composed of two layers of muscle tissue, an outer longitudinal and an inner circular one. The lumen of the tube is dilated by a contraction of the longitudinal muscle. This is followed by a contraction of the circular muscle. The effect is a moving ring of luminal constriction, called peristalsis. The stomach stores the chyme, and, in the more distal part, mixes the content and performs some mechanical digestion. Bit by bit the chyme leaves the stomach through the pyloric sphincter. The luminal face of the small intestines is covered with fingerlike structures called villi. This one cell layer of enterocytes, covered with microvilli, mucus, and glycocalyx, increases the surface area for absorption and secretion by tenfold or more. Segmentation, the non-propulsive pattern of intestinal motility, mixes the content by a periodic, repeating pattern of constrictions. By short waves of peristalsis the ingesta are moved aborally to be worked on again by segmentation. Finally, the ingesta enter the colon through the ileocecocolic valve/ileocecal sphincter. The cecum has almost no function in cats and dogs. Faeces in the colon is moved not only by segmentation and peristalsis but also by retropulsion/antiperistalsis initiated by pacemakers, and by mass action contraction. By this simultaneous contraction of smooth muscle over a large part of the colon faeces is moved through the colon and into the rectum. When the anal sphincter allows it, faeces leaves the body.
Regulation
In general, regulation of digestion is mediated by the autonomic nervous system. Stimulation of the parasympathetic part increases intestinal motility and tone, digestive secretion and relaxation of sphincters. Stimulation of the sympathetic part has the opposite effect. Also, there are two main ganglionated plexuses in the GI tract, the myenteric/Auerbach plexus and the submucosal/Messner plexus, that are considered to be a third portion of the autonomic nervous system called the enteric nervous system. The GI hormones gastrin, secretin, cholecystokinin, motilin, gastric inhibitory polypeptide/glucose-dependent insulinotropic peptide play a role in e.g., gastric secretion, emptying of the gallbladder, and gastric emptying. We should also note some negative feedback loops.
The Juices
Mastication, or chewing, would be difficult if the food bolus were dry. Therefore, salivary glands, such as the parotid, mandibular and lingual glands, deliver mucus. But enzymatic digestion also starts with the salivary glands. Feline saliva contains just a little amylase, canine saliva somewhat more. Amylase breaks down starch into simpler carbohydrates. Some saliva, gastric and pancreatic fluids start to get produced at the smell or vision of food, or any sign that is learned to be a precursor of a meal by classical conditioning. As soon as food is taken into the mouth saliva production increases. When the oral mucus membrane is triggered pancreatic fluid starts to flow.
Peristalsis in the oesophagus stimulates the release of fluid in the stomach as does the arrival of the bolus in the stomach. Dogs and cats have a “true” stomach: one with enzyme production. Chief cells, located at the bottom of the pits of the parietal and pyloric stomach mucosa, produce inactive pepsinogen, a zymogen. The acidic environment triggers the transformation into active pepsin which is important in protein digestion. Mucous cells all over the stomach, and especially in the cardiac mucosa, produce mucus to lubricate the chyme and to buffer the stomach tissue against the low pH. Parietal cells, halfway down the stomach pits of the parietal mucosa, secrete hydrochloric acid (HCl) after stimulation by histamine, gastrin, and acetylcholine. HCl helps to break down chyme and especially bacteria by virtue of its acidic nature. As a by-product of HCl production bicarbonate travels from the gastric cells into the bloodstream and raises blood pH. This is known as the alkaline tide. Gastrin is produced by G cells in the pyloric glands and the duodenum. Gastrin by itself and through production of histamine triggers parietal cells to increase HCl production. This works with a negative feedback loop after a certain point: the higher the blood pH and the lower the gastric pH, the lower the gastrin production.
Pancreatic cells are triggered by acetylcholine, cholecystokinin, and secretin to secrete more than 10 different enzymes and proenzymes to the duodenum, including amylase, trypsin, and lipase. In the luminal phase of digestion lipase breaks down fats, and trypsin divides proteins into amino acids. The hydrochloric acid from the stomach is being neutralized in the duodenum by sodium bicarbonate from the pancreas and in the bloodstream H+ ions from the pancreas bring balance to the alkaline tide that was created by gastric acid secretion.
Lipids are already warmed and broken into smaller droplets in the stomach. They are further emulsified by bile acids. These are produced in the liver and stored in the gallbladder. The arrival of ingesta in the duodenum triggers the release of bile. Only after emulsification lipase can work together with other pancreatic enzymes to break down fat in the jejunum. In the enterocytes chylomicrons are formed of the lipids. Because these are too big to pass into the capillaries they are transported through the abdominal lymph and thoracic duct to empty into the vena cava. Nearly all bile acids are reabsorbed in the ileum and circulate back to the liver to be reused and suppress bile production (negative feedback loop).
On the luminal side of the enterocytes anchored enzymes are completing hydrolysis in the membranous phase of digestion and the final products of carbohydrate and protein digestion are absorbed into the underlying epithelial cells. Some proteins still need to be broken down into amino acids inside the enterocytes before they can enter the bloodstream.
A part of absorption is passive osmotic movement, but we also see co transport and active transport, aided by a pumping function of the villi. On the lateral side of the enterocytes, through “tight” junctions, water and electrolytes pass relatively free between the intestinal lumen and the intestinal capillaries.
All the blood from the GI tract, except from the distal colon and the rectum, travels through the enterohepatic/portal circulatory system. The absorbed nutrients are processed in the liver and released to the general circulation or back to the GI tract together with waste products from the liver, as the liver filters toxins and breaks down medication. The enterohepatic circulation is easily impaired because it works with a small pressure gradient.
In the colon a good amount of fluid is resorbed and certain vitamins are synthesised. Bacteria and fungi help enzymes to process the ingesta.
The last fluid to be added is from the anal glands. This fluid actually has no function in digestion, but rather in communication.
References
1. Sturtz R, Asprea L. Anatomy and Physiology for Veterinary Technicians and Nurses. Ames, IA: Wiley-Blackwell; 2012.
2. Wanamaker BP, Lockett Massey K. Applied Pharmacology for Veterinary Technicians. St. Louis, MO: Elsevier Saunders; 2015.
3. Klein BG. Cunningham’s Textbook of Veterinary Physiology. St. Louis, MO: Elsevier Saunders; 2013.