Lecture 26 - intro to GI

Digestive system

Purpose: Transfer nutrients, H2O, and electrolytes from the diet into the internal environment of the body (Also is another blood conditioner)

Salivary glands

Adds secretion to lubricate and begin digestion of oral contents

Exocrine pancreas

Adds digestive enzymes to breakdown nutrients in the small intestine, and adds bicarbonate to neutralize acid arriving from the stomach

Hepatic/Biliary system (liver, bile, and gallbladder)

• The liver makes bile; the gallbladder stores and concentrates it

• Bile enters the small intestine to help with fat digestion and absorption

• The liver also receives nutrient-rich blood from the gut for intial processing

Portal system (blood)

• Venous blood from the stomach and intestines travels through the livers before returning to the heart

• This gives the liver an early opportunity to process absorbed nutrients

• The liver also receives oxygenated blood thogh the hepatic artery

Layers of the gut: Serosa, Muscularis externa, submucosa, and mucosa

Serosa → Outer covering that reduces friction as digestive organs move

Muscularis externa → Circular and longitudinal muscle layers mix and proper content. The stomach has an additional oblique layer.

Submucosa → Supporting connective tissues containing blood vessels, lymphatics, and nerves

Mucosa → The lining facing lumen: Secretion, protection, digestion, and absorption

Major functions of the digestive system (Motility, secretion, digestion, and absorption)

Motility → Layers of muscle in gut have patterns to activate to retain, mix, and transport contents down the tract

Secretion → Regions of the tracts and accessory organs add secretion to ingested food

Digestion → Enzymes in secretion and at the muscle surface break large food molecules into smaller molecules

Absorption → Water, electrolytes, and digestion products cross the mucosa into the internal environments

4 main controls of the digestive system

Enteric (intrinsic) nervous system → Local circuits in the gut wall coordinate motility and secretion. Many local reflexes can operate without commands from the brain

Autonomic nervous system → Sympathetic and parasympathetic in and outs adjust gut activity and participate in longer reflex pathways

Sensory activity → The gut detects stretch, acidity, and nutrients in its contents

Hormonal control → Cells in the gut release hormones in response to those contents. The response helps match secretion and motility to the meal