Digestive Anatomy and Physiology of Domestic Animals

Carnivores

Tend to have rather simpler (shorter and less voluminous) digestive tracts.

Herbivores

generally have specialized regions of their gastrointestinal tracts (GIT) (e.g. the rumenoreticulum in ruminants, or the cecum and colon in horses), which provide a suitable environment ("fermentation vat") for specialized microorganisms that produce enzymes for digestion and utilization of more complex nutrients.

Less digestible feeds

are also fermented by the microflora producing fermentation end-products (organic acids such as VFA) which can be absorbed and utilized by the host animal as energy sources.

Glucose

Propionic acid is converted by the liver to __

Acetate and Butyrate

can be utilized for energy (via the citric acid cycle) or for lipogenesis (synthesis of long-chain fatty acids and their incorporation for storage in adipose tissue as triglycerides

Non-Ruminant and Ruminant

Classification of digestive system based on anatomy

Non-Ruminant

with simple stomach

Pigs

Non-ruminant omnivores, thus consume both plant and animal matter.

Poultry

Non-ruminants omnivores, and they have a complex foregut (three sections that replaces the normal stomach) and a relatively simple intestinal tract.

Dogs and cats

Non-ruminant carnivores.

Horses and rabbits

Non-ruminant herbivores and have a rather large and complex large intestine.

Ruminant

with complex stomach

• Capable to consume and digest plant materials and are herbivores

• Includes cattle, goats, sheep, buffaloes, camels

1. Pregastric/Foregut fermenters

2. Hindgut fermenters

3. Monogastric, non-functional cecum

Classification based on the site of microbial fermentation

Pregastric/foregut fermenters

• Ruminants

• Predominant site of microbial fermentation preceding the stomach (forestomach or gastric pouch) and small intestine

• Route of digesta: Mouth → esophagus reticulorumen abomasum (true stomach) → intestines → anus - omasum

• In young ruminants consuming predominantly milk, esophageal groove or reticular groove allows milk to pass directly from the esophagus through the omasum into the abomasum which permits efficient digestion and utilization of milk (in a manner similar to other young mammals without undergoing fermentation in the reticulorumen)

• Hindgut fermentation (in the cecum and colon) also occurs

Hindgut fermenters

• Non-ruminant herbivores

• Predominant site of microbial fermentation is the large intestine (cecum and colon)

• Horses, rabbits, and elephant

Monogastric, non-functional cecum

• (carnivores and non-ruminant omnivores)

• Do not rely upon plant tissues as a significant source of nutrients

• Have no major specialized site of microbial fermentation in their digestive tracts; some microbial fermentation occurs in colon (large intestine); carnivores (dog and cat) and non-ruminant omnivores (pig and poultry

Mouth, Pharynx, Esophagus, Stomach, Small Intestine, Large Intestine, Rectum, Anus

Monogastric digestive system

Mouth (oral cavity)

Proximal end of the digestive tract; prehension (bringing food into the mouth), mastication (chewing), partial digestion (thru the action of salivary amylase)

Contains the following structures: tongue, teeth and salivary gland (secretes saliva which functions to: moisten consumed feed, lubrication of feed to facilitate swallowing, serve as buffer, and initiate carbohydrate digestion)

In birds: beaks or bills replace the lips and teeth of mammals

Pharynx

musculomembranous organ which is a common passage for feed and air

Esophagus

• passage of ingested feed from mouth to stomach

• serves as temporary storage location for food and allows the food to be softened before it enters the stomach

Stomach

the highly muscular nature provides for significant mixing of ingested food, saliva, and gastric juices

Chief cells and Parietal cells (oxyntic cells)

Gastric mucosa contains

Chief cells

secrete proenzyme pepsinogen

Parietal cells (oxyntic cells)

secrete hydrochloric acid (HCI) that functions to denature proteins (to further promote enzymatic action on proteins), to kill most ingested microorganisms, and to activate the pepsinogen to pepsin (an endopeptidase)

chymosin (rennin)

Secretion of enzyme __ in nursing mammals which causes the coagulation of casein (milk protein) to prevent rapid passage of milk out of the stomach

Horses

have a stomach that is small relative to its body size and is less muscular hence no extensive muscular movement to cause mechanical breakdown of ingested feed

1. Proventriculus

2. Ventriculus (gizzard)

In birds, stomach has two parts:

Proventriculus

glandular portion; secretes HCl, mucus and pepsin

Ventriculus (gizzard)

muscular portion; performs the same function of the mammalian teeth (grinding of feed); contains grits (sand grains or small rocks) in most birds to aid the grinding process

Small Intestine

• Final site of digestion

• Receives pancreatic enzymes and bile

Villi

Small intestine walls has fingerlike projections called _ that greatly increase its surface area for absorption of nutrients

Duodenum, Jejunum, and, Ileum

three parts of small intestine

Duodenum

site of digestion; secretion of enterokinase (enteropeptidase) that activates trypsinogen (a zymogen from pancreas) to trypsin

Jejunum

major site of nutrient absorption

Ileum

site of nutrient absorption

Brunner's and Lieberkuhn's glands

• Secretions of intestinal juice by the __; enzymatic components include aminopeptidase, dipeptidases, disaccharidases (maltase, sucrose, lactase)

• Trypsin

• Chymotrypsin

• Elastase

• Carboxypeptidase

• a-Amylase

• Pancreatic lipases

• Phospholipase A2 (lecithinase A)

• Cholesteryl ester hydrolase

• Deoxyribonuclease (DNase)

Pancreatic enzymes

Trypsin

an endopeptidase; secreted as trypsinogen and activated by enterokinase (from duodenum).

Chymotrypsin

an endopeptidase; secreted as chymotrypsinogen and activated by trypsin.

Elastase

an endopeptidase; secreted as proelastase and activated by trypsin.

Carboxypeptidase

an exopeptidase; secreted as procarboxypeptidase and activated by trypsin.

a-Amylase

a carbohydrase which primarily cleaves alpha - 1 - 4 linked glucose residues of starch (from consumed plants) and glycogen (from consumed meat and liver).

Pancreatic lipases

cleaves the primary ester linkages in triglycerides

Phospholipase A2 (lecithinase A)

acts on lecithins and cephalins

Cholesteryl ester hydrolase

an esterase acting on cholesterol esters

Deoxyribonuclease (DNase) and ribonuclease (RNase)

nucleases acting on DNA and RNA, respectively

Bile

• produced by the liver, stored in the gall bladder and secreted into the duodenum

• serves a similar neutralizing role as that of the pancreatic secretions

• serves as an excretory route for bile acids, cholesterol, bile pigments, drugs/toxins, and several minerals (e.g. copper, zinc and mercury).

bile salts

bile contains __, which reduce surface tension and aid the emulsification of fats to aid/promote the efficient digestion and absorption of fat and the fat-soluble vitamins).

Horse

__ have no gall bladder hence bile is continuously secreted from the liver to small intestine

Peristalsis and Segmentation

• Intestinal movements:

Peristalsis

movement that tends to propel the ingesta along the intestine in a direction towards the anus

Segmentation

movement that does not move the contents along the intestine, but merely mixes it; facilitates absorption by bringing the ingesta into contact with the villi and by stimulating the flow of blood and lymph in the intestinal wall

Large intestine

• Includes the cecum and colon

• the undigested food residues and endogenous secretions pass from the ileum to the colon.

• Site of significant absorption of water

Microbial fermentation

__ of undigested and unabsorbed carbohydrates from the small intestine results in the production of VFA, CO₂ and CH4.

Microbial degradation

__ of proteins and amino acids results in the release of NH3 that is absorbed via the portal blood and converted to urea in the liver.

Tryptophan

Certain amino acids such as __ can be metabolized by the microflora to indole and 3-methylindole (skatole) responsible for a portion of the characteristic odor of feces.

Cecum

Many herbivorous and omnivorous mammals have an intestinal sac/pouch, called the __ (located near the junction of the ileum and colon; in the human, the appendix is the vestigial remnant of the cecum; in many birds, a pair of ceca exist

Cecum and Large colon

In horse, microbial fermentation much like in ruminants occurs in the __ (hindgut fermentation) where production of absorption of VFA occurs; however, fiber digestion is less efficient than ruminants due to faster passage rate

Small intestine and Cloaca

In birds large intestine is reduced to a short connection between the __

Rectum

Terminal segment of the digestive system in which feces accumulate just prior to discharge

Cloaca

Birds have __ (bulbous final holding area for the waste products of digestion, as well product of the reproductive system in hen (egg) and waste product of the urinary system, until they are voided through the vent).

Anus

terminal opening of the digestive tract through which fecal material is excreted.

also called vent which serves a common opening for excretion waste products of digestive and urinary systems as well as of expulsion of egg.

Dental pad

Ruminants have no incisor teeth on the upper jaw, they have a — instead.

Incisors and Molars

there is a large gap between the — a configuration which allows harvesting and chewing a large amount of fibrous feed.

Esophagus

functions bi-directionally allowing them to regurgitate their cud further chewing

1. Reticulum (honeycomb)

2. Rumen (Paunch)

3. Omasum (Manyplies)

4. Abomasum

Four compartments of stomachs of ruminants digestive system

Reticulum

• most cranial compartment and not completely separated from rumen

• involved with rumination

• acts as a trap for foreign objects ingested

• Its location behind the diaphragm places it almost in apposition to the heart so any swallowed foreign objects (e.g. wire or nails) lodged in the reticulum are in good position to penetrate into the heart ("hardware disease")

Rumination

process of bringing food material back from the ruminant stomach to the mouth for further mastication

1. Regurgitation

2. Remastication

3. Reinsalivation

4. Reswallowing

rumination is a cycle of activity composed of four phases:

Rumen

• largest compartment

• With projections called papillae

• No enzyme secretion

• contains billions of bacteria, protozoa, molds, and yeasts which live in a symbiotic manner with the animal, and they are the reason ruminant can eat and digest large amounts of roughage.

60-80%

The microorganisms digest the plant fiber and produce VFA which are absorbed directly through the rumen wall and supply __ of the energy needed

6 and 7

Rumen fluid normally has pH between __.

10 to 50

Each milliliter of rumen content contains roughly: __ billion bacteria, 1 million protozoa, and variable numbers of yeasts and fungi.

B and K

The microbes also synthesize vitamins

Omasum

• Has laminae

• it functions as the gateway to the abomasum, filtering large particles back to the reticulorumen and allowing fine particles and fluid to be passed to the abomasum.

• It does aid in water resorption and recycling of buffers for the saliva, and may also absorb some volatile fatty acids

Eructation (belching)

• Process of elimination/releasing of gaseous products of fermentation process in the rumen (CO₂, CH, and H₂)

• Facilitated by the contractions of the upper sacs of the rumen to force the gasses to escape

Ruminal tympani

Failure to do eructation will lead to excessive gas accumulation in the rumen leading to __ (bloat) and eventual death of the animal due to asphyxiation as expanding rumen interferes with breathing

Abomasum

is also known as the "true stomach" It functions much like the non-ruminant stomach producing acid and some enzymes to start protein digestion.

Microbial cells as well as nutrients which did not undergo rumen digestion (by-pass nutrients) are prepared for further digestion in the small intestine

Small intestine

Digestion and absorption of by-pass nutrients and microbial protein

Enzymes to digest proteins, sugars, and starch flow into the small intestine from the pancreas, while the gall bladder releases stored bile to help digest fats.

Produces some enzymes to aid in digestion, but its major function is absorption of digested nutrients including amino acids from microbial protein

Except for the VFA, most of the nutrients are absorbed in the small intestine similar to non-ruminant mammals.

Large intestine

Absorption of water and a second phase of microbial digestion

The volatile fatty acids produced in the large intestine are absorbed, but microbial cells are excreted together with undigested food components in the feces.