In Chapter One, we saw that the digestive tract is one long continuous tube running through the body from mouth to anus, and that the inside of that tube is outside of the body, with the gut wall separating the two. We followed food through the stomach and are set to enter the small intestine. Here again is our map:
A reminder that even when not specifically mentioned, we should imagine water as part of the discussion, as the major component of blood and of the interstitial fluid and other body fluids, the fluid outside the blood vessels.
The small intestine is the major digestion and absorption site. As a tubular organ, the inner space of the small intestine is called the lumen. Lumen is not the term for the gut wall but for the space inside the gut.
The small intestine is long, about two and a half times the length of the cat on average. Not every cat is average and the individual length may vary. This length is cleverly arranged to fit in a smaller space. The 'map' above shows the small intestine as a folded affair in order to indicate relative length in relationship to the cat. Other illustrations may show the small intestine looking more like a string of sausage, as in the first illustration below.
In fact the small intestine is a creatively ruffled affair, as we see more clearly in the second image, with the arrangement of the rufflings and riffles of the small intestine varying from individual to individual.
In the third, a photo of cat small intestine with the small intestine partially unruffled, we now see evidence of the mesentery, a membrane that anchors the small intestine to the back wall of the abdominal wall cavity.
Below are two illustrations of the mesentery which show the mesentery's physical connection to the small intestine along with the blood vessels which pick up and transport nutrients throughout the body after digestion in the gut. This mesenteric arrangement is anchored to the back wall of the abdominal cavity. The small intestine is secured yet able to give and stretch as chyme flows through its lumen.
It is enough to know that the small intestine is not flopping around loose inside one's cat.
Immediately after the chyme enters the duodenum, the first part of the small intestine, the acid from the stomach is neutralized by a buffer, a bicarbonate, delivered from the pancreas via the pancreatic duct. Only the stomach is designed to handle its strong gastric acid, not the esophagus nor the small intestine.
Enzymes from the pancreas are delivered to the duodenum to digest carbohydrates, fats, and to complete the digestion of protein. Different enzymes are required to break down different substances, they are specialists. Cells in the villi of the gut wall also synthesize enzymes which assist with breakdown and absorption of various nutrients.
Bile also has neutralizing effects on the stomach acid. Bile, synthesized by the liver and stored in the gall bladder, enters into the duodenum via the bile duct to emulsify edible fats present in the chyme, resulting in a milky mix called chyle.
The neutralized chyme flows through the small intestine like a slow swirling river, propelled along by both peristalsis and segmentation. These are different but complementary actions which move chyme along (peristalsis) while simultaneously mixing it (segmentation).
As the chyme swirls and flows through the jejunum, molecules of digested nutrients contact the villi and are absorbed via the mesentery into the blood system for distribution to all parts of the body where they are put to use.
In the last section of the small intestine, the ileum, Vitamin B12 from meat sources is absorbed.
A common theme develops. Digestion breaks bigger pieces into smaller and smaller pieces. Mechanical digestion uses physical means to break down big pieces, for greater surface area and better enzymatic access. Chemical digestion employs enzymes to snip into smaller molecules by breaking chemical bonds that hold compounds together. Food needs to be broken down before it can cross the gut wall barrier into the blood stream.
The inner surface of the intestines is tightly packed with millions of little fingerlike projections called villi. Villi increase surface area manyfold. If a human small intestine were unfolded to flatten all the villi and microvilli, it would cover more area than a football field!
The villi are in turn covered with microvilli. This greatly increased surface area and the richness of the blood supply allow more nutrients to be absorbed without the need to make the small intestine even longer.
Not only do the villi and their microvilli expand the inner surface area of the small intestine, they also serve as loading docks and gateways.
Here is a greatly magnified view of villi which gives a hint of the microvilli covering them.
The villi also synthesize enzymes which further break down nutrients until they are small enough to be absorbed by the absorptive cells of the villi. Absorptive cells, as their name implies, absorb or take in nutrients from the nutrient-rich chyme. Nutrients are absorbed from the fluid environment of the lumen into the internal fluid environment of the villi where they are picked up by the villi's blood or lymph capillaries and thus into their respective circulatory systems.
Here is an illustration to bring the various concepts together. There is the mesentery, the fold of the peritoneum which supports and anchors the small intestine. We see the enteric nervous system and the various layers of the structure, and we see the villi carpeting the interior. There is the lumen, the space inside the gut.
The health of the cat depends on the health of the villi who themselves depend on good nutrition and nonexposure to toxic substances. Villi also require the physical passage of food in a use-it-or-start-to-lose-it response. In a cat (or a human or any species with small intestinal villi) who is not eating, the villi shrink. They can grow again, they can recover, but in the meantime health is at risk because both the gut barrier function and the ability to absorb nutrients are compromised.
The villi are also subject to damage by toxic substances, intestinal parasites, food allergies, food poisoning and anything that limits or cut off the blood flow or oxygenation to an area. Fortunately the fast cellular turnover in the villi means the gut wall has great healing capability when negatives are eliminated and a healthful diet is served and eaten regularly. Regularly for a cat means small frequent meals.
One of the important aspects about the small intestine is that it evolved to transport liquidy contents, not solid objects. The small intestine is flexible and expandable and it works hard at its job but still, it was designed for chyme, not hefty solid objects such as formed hairballs.
A linear foreign object, such as a string, is particularly risky because as it moves along, its length may occupy more than one turn of the small intestine and could cinch several sections together in a strangle-hold, cutting off the blood supply to the area.
Bit by bit, as chyme reaches the end of the small intestine, it is released into the large intestine. By then, most everything which counts as a nutrient for the cat has been absorbed upstream and what’s left, besides water, are the parts of the diet which can't be digested. These remainders are called dietary fiber and play a feature role in the large intestine.
We leave digestion and absorption at this point since our focus is in the gut, not inside the body. Readers interested in internal metabolism can find a remarkable amount of information on the web, in books stores and in the public library.
Like the small intestine, the large intestine is anchored by its own mesentery which is called the mesocolon.
The first part of the large intestine is called the cecum. In cats, this section is very small in comparison to most other species and its function in cats is presently unknown.
The large intestine or colon wraps around the coiled-up small intestine as these graphics illustrate.
When the chyme reaches the large intestine or colon, electrolytes and excess water begin to be drawn off from the soupy chyme and resorbed into the body. The large bowel makes poop out of soup! If water and electrolytes weren't reclaimed, we'd all be dehydrated by constant diarrhea.
The large intestine or bowel is also where any fermentable dietary fiber in the chyme is fermented by the trillions of gut bacteria who live in the bowel. These and other processes, discussed in the next chapters, result in what we call feces or stool or poop.
Formed poop is stored until time to move the bowels. Pressure from the poop, moving into the rectum and pressing from the inside out on the nervous system of the bowel, the enteric nervous system, signals the bowel when it is time to move.
Please do not skip over the next sections. It is important to understand the function of water and fiber in the whole process, and what poop is and is not, so that constipation can be treated and prevented as effectively as possible. It is especially important to read about the gut bacteria and their role in bowel health. The best treatment for a constipated cat is a caring human who has a general understanding of gut workings.
“Cats require purity and simplicity.” – SEM