Digestive Organs and Their Functions | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction

• The digestive system includes organs like the stomach and intestines.
• It aims to assimilate nutrients needed for life and discard harmful substances.

Secretions:

• Glands produce substances (saliva, gastric juices, etc.) aiding digestion.
• These maintain a balance in pH and provide mucus for protection.

Nutrients:

• Four essential nutrients: amino acids, fatty acids, vitamins, and minerals.
• Selection of food involves factors like taste, toxicity, and digestion efficiency.

Digestion of Carbohydrates, Fat, and Protein

Carbohydrates

• Amylases break down carbohydrates; pancreatic amylase is crucial.
• Disaccharides are broken down by specific enzymes at the intestinal mucosa.

Fat

• Bile salts emulsify fats for efficient digestion by lipases.
• Micelles formed include fatty acids, glycerides, and fat-soluble vitamins (A, D, E, K).
• Pancreatic lipase plays a vital role in fat digestion.

Protein

• Gastric enzymes (pepsinogens) work in a low pH environment.
• Enterokinase activates trypsinogen, leading to protein breakdown.
• End products (peptides, amino acids) are absorbed into the bloodstream.

Microorganisms in the Digestive Tract

Gut flora consists of microorganisms that aid digestion.

Categories:

• Autochthonous: Permanent residents beneficial under various conditions.
• Transient: Influenced by diet, antibiotics, and other factors.

Functions

• Microorganisms help in breaking down compounds, digesting enzymes, and producing acids.
• They play a role in reducing, dehydrating, and processing various substances.

Key Points for Layman

Digestive System Purpose:

The system helps the body absorb nutrients and eliminate harmful substances.

Digestion Steps:

• Chewing and stomach grinding break down food.
• Enzymes in the stomach and intestines further break down food.

Nutrients Breakdown:

• Carbs, fats, and proteins are broken into smaller units for absorption.
• This helps the body get energy and essential building blocks.

Importance of Microorganisms:

• Good bacteria in the gut aid digestion and keep the digestive system healthy.
• They also contribute to breaking down substances and producing useful compounds.

Digestion of carbohydrates, fats, and proteins involves a sequential enzymatic process under optimal conditions. In a simplified sequence, pepsin can generate small polypeptides, pancreatic endopeptidases are effective in digesting proteins, and carboxypeptidases can target larger peptide chains. Notably, the absence of any single enzyme in this process doesn't impede overall protein digestion. It's important to note that the diagram provided doesn't encompass all pancreatic enzymes involved in the breakdown of carbohydrates in the intestine.

Digestion and Microbes in Vertebrates

Microbial Aid in Digestion:

• Many animals rely on microbes in their stomach and intestines for digestion.
• This process helps break down both soluble and insoluble carbohydrates, like cellulose, into energy-absorbable organic acids.

Microbial Benefits:

• Microbes also produce B vitamins and essential nutrients from non-protein sources.
• They need anaerobic conditions, a neutral pH, and prolonged contact with food for optimal growth.

New Techniques for Understanding:

• Modern methods like fluorescent antibody procedures provide insights into the origin and activities of these microbes.

Peptides Affecting Digestive Processes

Secretin:

• Originally thought to trigger pancreatic juice, secretin is a 27-amino acid peptide.
• It shares similarities with glucagon and growth hormone, possibly influencing gastric secretion.

CCK-PZ (Cholecystokinin-Pancreozymin):

• A 33-amino acid peptide with a pentapeptide identical to gastrin.
• Stimulates gall bladder contraction, delays gastric emptying, and promotes pancreatic enzyme synthesis.

Gastrin:

• Known for stimulating stomach acid secretion, it's also secreted by the small intestine.
• Influences the mucosa of the stomach and intestine, cardiac sphincter tonus, and muscles in the large intestine.

Other Peptides:

• Enterogastrone, previously considered a gastric inhibitor, might be related to secretin and CCK-PZ.
• Villikinin aids intestinal villi movement, while enterokinin stimulates intestinal secretion.

Role of Glucagon

Glucagon's Functions:

• Stimulates insulin secretion and glycogenolysis.
• Its discovery in the duodenum wall suggests a role in preparing the body for glucose absorption.

Development of Digestive Organs

Ontogenic Changes:

• Mammals initially receive a high-fat, low-carb diet from milk at birth.
• Digestive tract length and size increase during prenatal development, with muscle development in a specific sequence.

Weaning Transition:

• Changes occur during weaning, shifting to a diet low in fat and high in carbohydrates.
• The digestive system adapts both at birth and during weaning, with the digestive tract growing larger.

Special Cases

• Animals with complex stomachs, like ruminants, show compartmentalization early in gestation.
• Large intestine development tends to occur later in the developmental process.

Nervous control Parasympathetic

The cranial parasympathetic neurons innervate the esophagus, stomach, small intestine, and the initial part of the large intestine through the vagus nerve.

Neurohumoral Control of Digestive Organs - Sympathetic

• The distal part of the colon, rectum, and the sacrose segment of the anal sphincters receive innervation from the adrenergic sympathetic system, which releases norepinephrine.

Acetylcholine Stimulation

• Acetylcholine is generally known to stimulate motor and sensory activities in the digestive system.

Humoral Control

• Humoral control involves numerous hormones.
• Sympathetic preganglionic fibers originate in the thoracic and lumbar segments of the spinal cord, while post-ganglionic neurons originate in ganglia outside the viscera, impacting the entire digestive tract. The released norepinephrine inhibits secretion and motility.

Hormones Involved

• Gastrin - Released from the stomach.
• Secretin and 3. Cholecystokinin-Pancreozymin (CCK-PZ) - Produced by the intestinal mucosa.
• Glucagon - Originates from the alpha cells of the islets of Langerhans in the pancreas.
• Enterogastrone, 6. Enterokinin, and 7. Villikinin - Also play roles in humoral control.

Source of Hormones

• Gastrin originates in the stomach.
• Secretin and Cholecystokinin-Pancreozymin (CCK-PZ) are produced by the intestinal mucosa.
• Glucagon is released by the alpha cells of the pancreas' islets of Langerhans.

Changes in Gut Microflora After Weaning

Diet-Induced Changes:

• After weaning, animals experience significant, species-specific changes in gut microflora due to alterations in their diet and gastrointestinal content.

Microbial Digestion Focus

• Microbial digestion mainly occurs in the stomach and large intestine, as digesta moves slower through these sections.

Microbial Functions

• Microbes play a crucial role in converting poor-quality diets into more usable nutrients.
• In ruminants, microbes in the forestomach convert carbohydrates into organic acids like acetic, butyric, and propionic acids.

Humoral Control and Digestive Hormones

pH Regulation

• Saliva buffers maintain the pH in the forestomach despite the presence of volatile fatty acids (VFAs).
• Most VFAs produced are readily absorbed, contributing to the animal's energy needs.

Hormones in Digestion:

• Various hormones, including gastrin, secretin, cholecystokinin-pancreozymin (CCK-PZ), glucagon, enterogastrone, enterokinin, and villikinin, impact digestion.
• These hormones regulate peristaltic movements and sphincter contractions in the digestive tract.

Digestive Diseases and Diet-Related Issues

Disease Causes

• Changes in diet can lead to gastrointestinal diseases in domestic animals.
• Overfeeding or feeding high-concentrate diets may result in issues like tympany (bloat), inflammation, and ulceration in the ruminant forestomach.

Gastric Ulcers and Abomasal Displacement

• High-concentrate diets might lead to gastric ulcers in cattle and pigs.
• Abomasal displacement, a common issue in cattle, may result from high-concentrate diets affecting motility.

Importance of Microorganisms

Symbiotic Relationship

• Microorganisms in the digestive tract are crucial symbionts, protecting against pathogens, providing nutrition, and supporting various digestive functions.

Imbalance Consequences:

• Any disruption in this symbiotic relationship, caused by diseases or their treatment, can lead to serious malfunctions.

Variations in Digestive Tract

Diet Influence

• The structure and function of an animal's digestive tract are often influenced by its diet and feeding habits.

Prehension Mechanisms

• Animals use prehensile organs like forelimbs, trunks, tongues, or lips for food intake, aiding in careful food selection.

Dental Adaptations

• Dental arrangements, such as large incisors and canines in carnivores or specialized molars in herbivores, are linked to their respective diets.

Eating and Digestive Processes

Food Mastication

• Carnivores swallow large prey pieces, ruminants poorly masticate fibrous material initially, while omnivores and herbivores carefully chew their food before swallowing.

Deglutition Process

• The act of swallowing initiates peristalsis, influenced by stimuli from the esophagus.
• Caudal esophageal sphincter relaxation is maintained until after the last swallow.

Vomiting Mechanisms

• Carnivores and omnivores easily vomit, while ruminants don't vomit but eject abomasal contents into the forestomach.
• Horses rarely vomit, and retching involves inspiratory muscle contraction and abdominal muscle contraction.

Expulsion Process

• Vomiting is brought about by strong rectus abdominis muscle contraction, raising intra-thoracic pressure against a closed glottis.
• This allows rapid expulsion of vomitus through the mouth when the soft palate reflexively elevates, closing the posterior nares.

Eructation or Belching

• In ruminants, eructation is a well-developed reflex occurring approximately once or twice a minute, closely linked with forestomach contractions.
• Volume receptors in the rumen, stimulated during contractions, trigger a reflex relaxation of the cardia.
• Gas enters the esophagus due to rumen contraction and abdominal pressure, then expelled through antiperistalsis.

Regurgitation

• Unlike emesis, regurgitation is a normal part of a ruminant's digestive process and is the initial stage of rumination.
• It involves inspiration against a closed glottis simultaneous with the submergence of the cardia.
• As ingested material is aspirated into the lower esophagus, it is propelled to the mouth through antiperistalsis.

Esophagus Variations

• The structure of the esophagus varies among species.
• In certain birds like doves and pigeons, it produces a nutrient fluid known as "crop milk" under prolactin control.
• In some birds, it serves as a storage receptacle, particularly in those with crops, and its emptying is reflexively regulated based on tract distension.

Esophagus in Ruminants and Horses

• In ruminants, the esophagus is effectively continuous with the reticular groove.
• Suckling in young ruminants reflexively closes the groove, directing swallowed milk directly to the abomasum.
• Horses have a long, thick, smooth-muscled esophagus with a distinct cardiac sphincter.

Swallowing Mechanism

• Afferents from the palate, pharynx, and epiglottis connect tactile receptors to a swallowing center in the medulla.
• Bolus movement into the upper esophagus is driven by compressive forces exerted by hyoid, lingual, and pharyngeal musculature.
• Motor neurons innervate striated muscle in overlapping sequences, causing a peristaltic wave upon bolus entry.
• The cardia relaxes during the first swallow, remaining open until the oesophageal peristaltic wave reaches it after the last swallow.

Stomach Function and Reflexes

• Upon sensing, smelling, or ingesting food, reflex vagal tone to the stomach increases, leading to increased blood flow, motor activity, and acid and pepsin secretion.
• Vagal stimulation induces the release of gastrin from the pyloric mucosa, synergizing with acetylcholine to enhance gastric mucosal blood flow and HCl and pepsinogen secretion.
• Gastrin has a tropic effect on the mucosa of the stomach and intestine, reducing gastric motility when digesta enters the duodenum.
• The stomach acts as a reservoir and the primary site for protein and fat digestion, with variations in structure and composition among species.

The cow's stomach is notably enlarged and compartmentalized. In the bovine stomach, the increased surface area is predominantly covered with a layer of stratified squamous epithelium. Within the gastric mucosa, there are compound tubular glands responsible for secreting hydrochloric acid (by parietal or oxyntic cells) and pepsinogen (by neck chief cells). In the pyloric region, these glands release mucus and a certain amount of pepsinogen. Additionally, they produce a serous fluid with a low bicarbonate content, contributing to a partial neutralization of acidity.

Stomach Function

• The stomach has special glands that release bicarbonate and absorb chloride ions.
• Studies on certain animals show that stratified squamous tissue in the stomach helps absorb important acids.

Ulcer Formation:

• Gastric ulcers often occur in the stratified squamous region of the stomach.
• In cattle, ulcers are found in the oxyhtic and pyloric mucosal regions.

Mid Gut Process:

• The pylorus in the stomach controls the passage of particles into the intestine.
• The speed at which food leaves the stomach is controlled by nerves and hormones.

Small Intestine:

• Small intestine receives bile, pancreatic juice, and its own secretions from the stomach.
• Dog's duodenum is more resistant to acid than the jejunum.
• Intestinal movements help in nutrient absorption, hormone release, and maintaining a low microbe concentration.

Pancreatic Exocrine Secretion:

• Pancreas releases a fluid with bicarbonate, enzymes, and hormones when stimulated.
• Some bicarbonate is reabsorbed in exchange for chloride as the fluid moves through the pancreatic ducts.

Liver and Bile Formation

• Liver does not filter like kidneys but actively forms bile using chemical energy.
• Active secretion of bile salts increases bile volume and concentration.

Stomach Development in Ruminants

• In embryo development, the ruminant stomach starts as a spindle-shaped enlargement of the foregut.
• The stomach later divides into four parts: rumen, reticulum, omasum, and abomasum.

Structure of Rumen

• Rumen lining in the first three parts is layered and has papillae.
• Rumen has an S-shaped structure, with pillars and flaps dividing it into upper (dorsal) and lower (ventral) sacs.
• Rumen and reticulum aid in microbial digestion of plant material.

Function of Each Stomach Chamber

• Rumen and reticulum: Specialized for breaking down plant material using microbes.
• Omasum: Specialized for absorbing water from saliva used in digestion.
• Abomasum: Functions as the true stomach, killing bacteria with acids and preparing them for further breakdown.

Oesophageal Groove and Reticulo-Ruminal Fold

• Oesophageal groove directs food from the oesophagus to the omasum.
• Reticulo-ruminal fold is an invagination aiding in the movement of food.

Omasum Structure

• Omasum has numerous laminae (folded structures) inside.
• Canal connects reticulo-omasal orifice with omaso-abomasal orifice.

Abomasum

• Has two folds on either side of the orifice, possibly acting as "trap" valves.
• Fundic and pyloric parts: Different parts with varying gland structures.

Summary for Layman:

• Ruminant stomach evolves from a simple shape to four parts during development.
• Each part has a specific role: breaking down plants, absorbing water, and acting as a true stomach.
• Specialized structures like grooves and folds help in directing and processing food.
• Abomasum has distinct regions with specific gland functions.

Stomach Development in Young Animals

• By six months old, the stomach compartments in animals are similar to adults.
• The speed of development depends on the diet, with a faster development on solid food compared to a milk diet.

Oesophageal Grooves

• In young animals, a groove directs milk straight to the true stomach (abomasum) while nursing.
• The groove closes during nursing, forming a tube. It can be stimulated to close by substances like sodium chloride and bicarbonate.

Salivation in Ruminants

• Saliva is produced by various glands in the mouth.
• Glands can be categorized into serous (e.g., parotid), mucous (e.g., buccal), and mixed (e.g., submaxillary).
• Parotid glands secrete continuously, with the rate influenced by feeding and rumination.

Saliva Production Rates

• In cattle, a single parotid gland can produce 30-50 ml per minute during feeding.
• Total saliva production in cattle ranges from 90 to 190 liters per day.
• Diet affects saliva production, with the slowest rate observed after 24 hours of not eating.

In summary, as young animals grow, their stomach compartments develop, influenced by their diet. Grooves in the esophagus help direct milk to the stomach, and saliva production is influenced by different glands, with rates varying based on diet and feeding habits.