Digestive system of Animals

• The digestive system is a musculomembranous tube extending from the mouth to the anus
• Functions of the digestive system include:

  • Ingestion
  • Grinding
  • Digestion
  • Absorption of food
  • Elimination of solid waste
• The digestive system reduces food nutrients to compounds that can be absorbed and used for energy and building other compounds for metabolic use
• The wall of the digestive tract has four layers:

  • Mucosa
  • Submucosa
  • Muscularis
  • Visceral peritoneum (Serosa)
• Parts of the digestive system include:

  • Mouth
  • Pharynx
  • Esophagus
  • Glandular stomach
  • Small intestine
  • Large intestine
  • Accessory glands (salivary, liver, and pancreas)
• The mouth is the cranial opening of the digestive tract that contains teeth and tongue, surrounded by lips, cheeks, and muscles operating the jaws
• Functions of the mouth include:

  • Mastication
  • Insalivation
  • Bolus formation
  • Prehension in some animals
• The pharynx is a common passage for food and air, divided into nasal, oral, and laryngeal portions
• The esophagus is the direct continuation of the pharynx to the cardia of the stomach, with a muscular wall that changes from striated to smooth
• The non-ruminant stomach is subdivided into esophageal, cardiac gland, fundic gland, and pyloric gland regions
• Glands of the stomach include:

  • Cardiac glands
  • Fundic glands
  • Chief cells
  • Parietal cells
• The ruminant stomach is described as four compartmental:

  • Rumen
  • Reticulum
  • Omasum
  • Abomasum
• The reticulum is referred to as the honeycomb, located close to the heart, and can trap foreign materials
• The rumen, also called the "paunch," is a large muscular sac that almost fills the left side of the abdominal cavity
• The omasum, referred to as the "bookstomach," is a spherical organ filled with muscular laminae that help move food towards the abomasum
• The abomasum is the true stomach and glandular part of the ruminant stomach
• The small intestine is divided into three parts: duodenum, jejunum, and ileum
• The large intestine consists of the cecum, colon, and terminates as the rectum and anus
• Accessory digestive organs include the salivary glands, liver, and pancreas
• The pancreas is a compound tubulo-alveolar gland with both endocrine and exocrine portions
• The liver is a lobular organ involved in detoxification and bile production
• Absorption of food mainly occurs in the small intestine, with water absorption in the large intestine
• Water in the colon is absorbed osmotically, and mucus is secreted for lubrication and protection
• The fore stomach of ruminants absorbs various substances, including end products of digestion
• The small intestine of neonates can absorb macro-molecules, including intact protein molecules in colostrum
• In multicellular organisms, specialised exchange surfaces are required for efficient gas exchange of carbon dioxide and oxygen due to a higher surface area to volume ratio
• The mucosa of the adult intestine cannot absorb large molecules of carbohydrates, proteins, and fats to any extent
• End products of digestion like simple sugars, amino acids, fatty acids, and glycerol pass through the mucosa into the blood or lymph vessels
• Amino acids and simple sugars enter the portal vessels that are tributaries to the portal vein
• Portal vein carries substances to the liver sinusoids where they may be acted upon by liver epithelial cells
• Bile salts emulsify large fat droplets into smaller droplets, which are then degraded by pancreatic lipase
• Bile salts promote aggregation of free fatty acids, monoglycerides, and cholesterol into micelles for absorption
• Micelles, containing hundreds of molecules, are water-soluble and can enter the intestinal absorptive cells
• Triglycerides are re-synthesized inside the epithelial cells and released into the lymphatic lacteals by emiocytocis
• Triglycerides enter lacteals as chylomicrons, containing phospholipids, cholesterol, free fatty acids, and protein
• Small droplets of emulsified fat can be absorbed pinocytotically by the epithelial cells of the small intestine
• Contraction of the smooth muscle of the intestinal wall aids in digestion and absorption by churning the contents and pumping fluid from capillaries and lacteals
• Absorption involves physical and chemical forces like filtration, osmosis, diffusion, adsorption, and pinocytosis
• Much of absorption is an active process, indicated by the selective absorption of fructose, glucose, and galactose in different concentrations by epithelial cells
• Glucose is absorbed faster than galactose, and galactose is absorbed faster than fructose