Digestive Systems
• Functions of digestive system:
– Accessory organs
• Pancreas
– Exocrine gland between stomach and small intestine – Produces several digestive enzymes: » trypsin: digests proteins » pancreatic amylase: digests starches » lipase: digests fats – Also acts as endocrine gland » produces hormones to regulate glucose levels in blood (insulin and glucagon)
Digestive Systems
• Functions of digestive system:
– Accessory organs
• Liver
– Large exocrine gland lying above stomach – Produces bile that empties into small intestine : » Bile pigments of old red blood cells eliminated in feces » Bile salts break fats into small droplets (=emulsifies) for lipase
Digestive Systems
• Functions of digestive system:
– Accessory organs
• Liver
– Liver also removes toxins, poisons and alcohol from blood » Converts toxic ammonia produced by bacteria in large intestine into urea eliminated in urine » Also regulates levels of many substances produced in body (e.g. steroids hormones)
Digestive Systems
• Functions of digestive system:
– Accessory organs
• Gall bladder
– Exocrine gland that stores and concentrates bile » Empties directly into small intestine
Digestive Systems
• Regulation of digestion:
– Digestion regulated by neural and hormonal signals
• Nervous system stimulates salivary stomach secretions • When food reaches stomach, hormone gastrin released, which in turn causes release of pepsinogen and HCl
Digestive Systems
• Regulation of digestion:
– negative feedback systems
• Protein present in stomach, gastrin released • Gastrin causes release of pepsinogen and HCl • HCl lowers stomach pH and inhibits release of gastrin
Digestive Systems
• Regulation of digestion:
– Small intestine releases two hormones to regulate digestion
• Cholecystokinin (CCK) released if fat in chyme, triggers gall balder to release bile • Secretin released in response to low pH of chyme, stimulate pancreas to release bicarbonate
Digestive Systems
• Vertebrate digestive systems vary:
– Protein diets are easily digested; carnivores have short digestive tracts – Herbivores require long digestive tracts with specialized organs to digest cellulose in plants
• 4-chambered stomach (ruminants) • Large cecum
Digestive Systems
• Herbivore digestive systems:
– 4-chambered ruminant stomach
• Plant enters 2 chambers were plants partially digested
– Rumen acts as fermentation vat where bacteria and protists break down cellulose – Similar symbiotic relationships occur in termites
• ‘cud’ regurgitated and rechewed • ‘cud’ enters final two chambers into which gastric juices released
Digestive Systems
• Herbivore digestive systems:
– Non-ruminant herbivores
• Cecum enlarged and serves as fermentation vat
– Does not allow for reguritation and rechewing – Improve digestion of plants accomplished by eating feces (= coprophagy)