Digestive System

Three very different types of digestive systems will be studied in this unit: the monogastric (single stomach), the ruminant (multiple "stomachs") and the equine digestive system.
 Neonatal digestion will also be covered briefly. 
Monogastric digestion will be studied in depth since many of the structures and secretions are similar to the other species.

a) Give examples of monogastric and ruminant species

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The oral cavity

The mouth (oral cavity) is the beginning of the digestive system. Food is grasped and chewed by the teeth. It is broken into smaller more easily processed pieces. The salivary glands produce salvia which moistens and lubricates the food for easier swallowing. Some species' (hogs, humans and rats) salvia contains amylase, an enzyme which begins the chemical breakdown of carbohydrates. There is species variation in the number of salivary glands and the types of secretions from the glands. 
The functions of salivary glands include: 
moistening and lubrication of food 
secretion of small amounts of a bacterial static enzymes to help resist infections 
cooling the body through panting
protection of oral cavity from drying  
enzymatic digestion of carbohydrate by amylase (certain animals only).

b) Which species cools their body via the salivary glands?

c) Why do animals salivate before vomiting?

The tongue helps propel food into the esophagus and is used to "lap up" liquids in dogs and cats.

The esophagus

The esophagus is the muscular "food" tube which connects the oral cavity to the stomach. It has two muscular layers, one circular and the other longitudinal. Food is moved "along" in waves (longitudinal muscle layer) and "squeezed" or segmented by the circular muscle layer. There is both voluntary and smooth muscle in the esophagus, (the amount varies by species). The dog, cow and sheep have the most voluntary muscle.

Does the amount of striated muscle in the esophagus of cattle, sheep and dogs relate to any unique digestive actions of those species? 

The stomach

The stomach is a muscular and expandable pouch that regulates the movement of food into the intestines and begins the digestion of specific nutrients. The stomach contains two functional sphincters for regulation of food movement. One is at the entrance into the stomach (cardiac sphincter) and the other is at the exit (pyloric sphincter). These sphincters are composed of bands of smooth muscle which constrict to partially or fully close.

The stomach contains three muscular layers situated at right angles to each other. When these muscles contract the gastric contents are churned and mixed. The inner layer of the stomach is termed the mucosa. It is highly vascular and contains lots of glands that secrete digestive juices. The mucosa has deep folds called rugae which can stretch to increase the size of the stomach.

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Gastric digestion

There are several different gastric secretions from the various types of gastric glands located in different parts of the stomach. 
The secretions include: 
hydrochloric acid
pepsinogen
gastric lipase
mucus
gastrin (a hormone)

Hydrochloric acid combines with pepsinogen and is converted to pepsin, (the "active" chemical form) which begins the breakdown of protein. The enzyme gastric lipase begins the breakdown of lipids (dietary fats). The hormone gastrin acts as a messenger, signaling the small intestine to prepare for the arrival of food.  The small intestine does this by secreting appropriate enzymes. The pH of the stomach is low, about 2 (which is very acidic). The mucus which is secreted and coats the delicate lining of the stomach protects it from ulceration.

After gastric digestion is complete the pyloric sphincter opens allowing food into the intestines. The sphincter relaxes when the consistency of the food is "soupy" and the pH is very low.  Clear liquids are passed through the stomach in a short time, usually 30 minutes or less, while solid food may take hours before leaving the stomach. When food enters the stomach the gastric-colic "reflex" is stimulated, and the feces moves from the colon (the last part of the large intestine) to the rectum. This takes about 30 minutes (for evacuation of feces  from the body). This reflex action is caused by enervation between the stomach and colon. 

d) What does this mean in relation to house breaking puppies?

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a = esophagus, b =body of stomach, c = pancreas, d= transverse colon, e=jejunum, f=ileum , g=cecum, h =duodenum

The small intestine

The small intestine is a muscular tube composed of three parts: the duodenum, jejunum and ileum. There are two smooth muscle layers arranged in a similar manner to the esophagus, circular and longitudinal. When the longitudinal muscles contract a wave-like action termed peristalsis, food moves along the digestive tract. When the muscles that encircle the intestines contract, there is segmentation of the gut.

The duodenum receives acidic food from the stomach. It is then mixed with bile from the gall bladder and digestive enzymes from the pancreas (that enter the duodenum via ducts.) There are also enzymes and chemicals involved in digestion secreted by the duodenum. These substances act on specific types of food to digest it before absorption. Most of the chemical breakdown of food occurs in the duodenum, but there is very little absorption here.

Much of the absorption of food takes place in the jejunum. The surface area of the inside of the duodenum and jejunum is increased by innumerable tiny projections, the villi. The villi are covered with even smaller microscopic projections on their surface termed microvilli (brush boarder).  
Within the microvilli are extensive capillary networks and lymphatic vessels. Nutrients are absorbed from the lumen of the intestine into intestinal cells and then into the capillaries or lymphatic vessels inside the microvilli and villi. 
By this time the large chunks of food that were ingested have become tiny molecules and can move across cell membranes. Lipids are absorbed into the lymphatic vessels.   
The last portion of the small intestine is the ileum. The ileum is the major area for absorption of water and water soluble vitamins from the small intestine. It joins with the large intestine, a small "pouch" also diverges off the intestine here termed the cecum.

e) What is the cecum called in humans?

The length of the small intestine varies among species, herbivores have the longest (relative to their size). A cow can have as much as 50 yards of small intestines, the average sized dog has about 4 1/2 yards of small intestine. But the dog has such an extensive network of villi and microvilli that the actual surface area for absorption is estimated at 100 square meters!

The large intestine

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Descending colon cross section

The colon is divided into three parts based location in the abdomen. The first part, the ascending colon, is directed cranially. The transverse colon crosses from the right to the left side of the abdomen and the last part the descending colon is directed caudally.

The rectum is the last portion of the large intestines and the storage area for feces. The exit of the digestive system to the environment is the anus.

The large intestine has no villi, but in horses and hogs contains outpouchings or saccules. There is bacteria in the large intestines of all species, which ferment and assist in the breakdown of plant fiber. There are also mucus secreting glands which lubricate the feces.
The absorption here is similar to that of the ileum, primary water, electrolytes and vitamins (but NOT the fat soluble A, D, E & K) are absorbed.

Movement of food through the intestinal tract.

Food is propelled through the digestive tract by the contraction of smooth muscles. The longitudinal muscles of the intestines cause peristalsis, the slow, wave like rhythmic contractions that moves food through the system. 
The contraction of the muscles encircling the intestines causes segmentation and the mixing of food with digestive juices. 
These contractions are influenced by three factors: 
1) the autonomic nervous system (primarily the vagus nerve of the parasympathetic portion)
2) hormones released from one part of the digestive system acting on another portion
3) the intrinsic system of the intestinal tract

The intrinsic system is a specialized nervous system located within the digestive tract which acts somewhat similar to, but separate from, the regular nervous system. Throughout the digestive system pacemaker tissue (similar to the pacemaker tissue in the heart) is located. This tissue triggers activity in the smooth muscles which causes regular peristalsis and segmentation.

The last part of the digestive process is evacuation of the rectum (defecation). It involves the defecation reflex. When the rectum is distended with food it causes contraction of smooth muscle and the relaxation of the anal sphincter muscles, which are voluntary striated muscles. In very young animals the sphincter muscle is not well developed and between the gastric-colic reflex and the defecation reflex there is little control over bowel movements. There is considerable species variation in numbers of bowel movements per day, cows may defecate 20 times daily while carnivores generally defecate 2-3 times daily.

Accessory Organs of digestion

The Pancreas    
The pancreas is a long, thin delicate organ that sits behind the stomach and next to duodenum. It is a pinkish gray color and appears glandular. The pancreas produces the following substances which are essential in the digestive process: 
1) amylase, an enzyme, involved in carbohydrate digestion
2) lipase, an enzyme, involved in lipid digestion
3) trypsinogen which is activated to trypsin, also an enzyme, and is involved in protein digestion 
4) sodium bicarbonate which acts to neutralize acid is also secreted. 
A duct leaves the pancreas carrying these substances and empties into the duodenum. In some species the pancreatic duct joins with the bile duct (forming the common bile duct) and other species have a secondary pancreatic duct.

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The Liver  
The liver is the largest organ in the body (discounting the skin) and it is very complex. It performs many life sustaining functions. It is located behind the diaphragm and is divided into lobes which are visible with the naked eye. Lobes are composed of microscopic lobules.

Microscopic anatomy: 
To understand the functioning of the liver, it is important to appreciate the complicated microscopic anatomy of the liver, remember - form follows function. Lobules are microscopic six sided hexagon like structures, formed by tall columns (or slabs) of cells radiating outward from a central vein. These columns are arranged somewhat like the spokes of a wheel, with a central vein in the center. There is a small space between the columns of cells, the sinusoids. The sinusoids are where the liver cells, hepatocytes, are exposed to blood.

Liver lobule

a=central vein, b=hepatocytes (liver cells in "slabs"), c= sinusoid, d=arteriole(from hepatic artery), e=lymphatic vessel, f= venule from portal vein

Hepatic blood supply:
The blood supply to the liver is unique. Three main vessels run to and from the liver.
1) The hepatic artery, which supplies oxygen and nutrients for liver metabolism.
2) The hepatic vein, which drains unoxygenated blood from the liver to the vena cava.
3) The portal vein, part of the portal hepatic system. A portal system connects a capillary bed to another capillary bed.  The portal vein connects capillary beds between the gastrointestinal system and the liver. 

Although the portal vein brings blood to the liver, this blood has very little oxygen so the liver has another source of oxygenated blood, the hepatic artery. The hepatic artery branches off the aorta, about 30% of the total cardiac output flows to the liver. The hepatic vein leaves the liver carrying nutrients for the body (made by the liver) and waste products. It empties into the vena cava.

The large prominent portal vein brings blood to the liver, remember arteries usually bring blood "to" organs. The portal vein receives blood from the gastrointestinal tract capillaries (stomach, pancreas, spleen and intestines). The intestinal capillary bed attaches to another capillary bed, inside the liver. The blood coming from the intestinal capillaries contains nutrients (molecules) from digested and absorbed food. These nutrients are further processed in the liver as they circulate through the sinusoids. The nutrient molecules (proteins, carbohydrates, fat) undergo further enzymatic digestion termed, intermediate metabolism, by enzymes secreted from liver hepatocytes. The fully metabolized nutrients become amino acids (units of protein), simple sugars and fatty acids that are utilized by the body for energy and building blocks. 

A=aorta, B=hepatic artery, C=portal vein, D=liver, E=Vena cava

In addition to intermediate metabolism the liver also produces bile and stores it in the gall bladder. The liver also manufactures and stores glycogen, which is the only storage form of sugar in the body.

Main functions of the liver:
The liver has dozens of important functions in the body. Many of its functions are related to the numerous liver enzymes. Some enzymes "break down" molecules while others work to "combine" molecules and form new essential substances for the body.
Below are some of the non digestive functions of the liver:  
1) detoxification of most medicines, alcohol and harmful substances from the body 
2) production of blood clotting factors
3) storage of glycogen (the only storage form of sugar in the body)
4) destruction of old non functional red blood cells
5) storage of vitamins and minerals
6) removal of hormones from there body
7) formation of urea (thus removal of ammonia from the body) 
8) formation of plasma proteins globulin and albumin (used for the building blocks of the body and the immune system.)

Don't forget about intermediate metabolism!

Ruminate digestion

a=small portion of rumen, b=rectum, c=spiral colon

Ruminates provide milk, meat and wool for much of the world. They do this by converting food that is useless for carnivores into energy.

Ruminants have a unique digestive system that allows for the digestion of hay, grass, corn cobs, and even a small amount of urea (i.e. chicken droppings!). They convert  these substances to proteins, vitamins, fatty acids, methane gas and energy.

Ruminates have three extra "stomachs" where millions of bacteria and protozoa (single celled organism) live. These microorganisms produce special enzymes that breakdown cellulose (plant fiber).

The rumen stomachs and cross sections of stomachs

A=interior of rumen, B=interior of reticulum, C=interior of omasum, D=abomasum, E=caudal diverticulum of rumen, F=reticulum

Only ruminant animals have thes microorganisms to digest cellulose living in the three extra digestive "vats". 
The three specialized stomachs of ruminants are termed the forestomachs (i.e. located before the true stomach, not 4 stomachs)
1) reticulum
2) omasum
3) rumen  

The most cranial stomach is the reticulum, it's lining looks like a honeycomb. 
The largest compartment, the rumen occupies an area from the diaphragm to the pelvis, primarily on the left side of the abdomen. In a large cow the rumen can hold up to 150 liters It is partially divided into two chambers by muscular pillars and within each chamber is a diverticulum. These separations distribute the weight more evenly, think how hard it would be for a wild ruminant to run and jump if there was a hundred pound ball food churning in the rumen! Most of the fermentation (microbial digestion) takes place in the rumen. 
The omasum is a rounded compartment, filled with muscular laminae which descend from the top downward. The lamina have been compared to the pages of a book in appearance. The lamina are covered with papilla and function to grind food.  

The last stomach, the abomasum is the true stomach and is similar in function to the monogastric stomach.

For the successful digestion of roughage, like hay, the food needs to be exposed to the microbes for long periods. To achieve this ruminants regurgitate their food and chew it again, this is rumination. They have to eructate or belch to release the huge quantities of methane gas produced by microbial digestion.

g)  What would happen if a cow did not eructate?

Ruminates also secrete copious amounts of saliva, which is needed for the rumination process. A 190 liters of salvia was measured  in a day, via cannulation of salivary ducts in dairy cow.

It is sometimes possible to feel/palpate the ruminations of cattle by placing your hand in their paralumbar fossa (flank region) and waiting to feel the contractions (lifting) of abdominal muscles which signal the rumination. Normally cattle ruminate about 3 times a minute after eating. Ruminant animals need to spend about 8 hours a day ruminating to digest and utilize their food. When cattle are stressed or sick they stop ruminating. Often the first question the bovine practitioner asks cattleman/dairyman when called out to treat a sick cow is "is she still ruminating?"

Equine digestion

Members of the equine family are monogastric herbivores. They have a specialized fermentation vat, the cecum, for the microbial digestion of cellulose. The cecum in the horse is very different than in carnivores. All food traveling through the intestines must enter the cecum, via the ileocecal valve. This allows roughage to be digested by the enzymes of the bacteria and protozoans living in the cecum. The cecum extends from the flank region to the diaphragm and is coma shaped, it holds about 15 quarts in the average horse.

Horses have a complex large intestine which allows the absorption of the feed processed by the cecum.

Other unique equine digestive feature include: 
1) low levels of amylase in saliva
2) absence of a  gall bladder (there is a constant release of bile into the intestine) 
3) a relatively small stomach with approximately a 3 to 4 quart capacity

Horses are "designed" to be grazers, to ingest small volumes of feed frequently. If wild equine species are compared with today’s horses (that often spend many hours daily in stalls and are fed large "meals" of carbohydrate rich food) it is easy to see why many domestic horses have digestive problems.

h)  What is the name of the most serious equine disease?

Developmental digestion

New born animals (neonates) of different species have very similar digestive tracts, in fact, a foal, puppy and calf are more like one another than adults of their own species (relative to the digestive system)! 
If not for several adaptations it would be difficult for neonates to ingest enough nutrients for growth. 
They have relatively longer and more distendable intestines than adult animals, this allows for a large capacity and more chance for absorption of nutrients. 
They also have special enzymes to digest milk sugars (lactase and maltose). 
For the first day or so after birth they can absorb maternal antibodies (large protein molecules in the "first milk" of the mother) which fights off infection. 
They also lack proteolytic enzymes, so these antibodies are not destroyed.

i)  What is the first milk called?

Neonates have a "suckling reflex" which stimulates peristalsis and digestive secretions when suckling begins. The liver does not become fully functional in neonates until several weeks after birth.
Although they have not ingested anything a newly born animal often has bowel movements immediately after birth; this fetal fecal matter is called meconium.

A unique feature of young ruminates is the esophageal groove. It is a groove like structure formed by contraction of smooth muscle extending from the esophagus to the omasum. This temporary tube allows milk to bypass the rumen and reticulum (which are useless for milk digestion). The groove is formed when the calf suckles. The rumen doesn't become functional until a few months of age. The young ruminant ingests the needed bacteria and protozoa by licking and chewing objects, by grazing and by eating the mother's feces.

Answers to the italicized questions:

a) Give examples of monogastrics  Dogs, cats, humans, horses; ruminants: cows, sheep, goats.
b) Which species cool their body through help from the salivary glands? canines - some rodents also wet their bodies by licking to cool themselves via evaporation
c) Why do animals salivate before vomiting?   To protect esophagus from acidic vomit
d) What does this mean in relation to house breaking puppies? Puppies have to go within 1/2 hour of eating and they can't hold it!
e) What is the cecum called in humans?  appendix
f) Where are they absorbed?  Via the lymphatics
g)  What would happen if a cow did not eructate?  Bloat
h)  What is the name of the most serious equine disease?  Colic
i)  What is the first milk called?  Colostrum 

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