Gastrointestinal Tract: Function and Structure

• The gastrointestinal (GI) tract is also known as the digestive tract
• The structural and functional adaptations of this system is the function
• The aim is to outline function and structure of the digestive tract and it's regional variations
• It will describe fluid balance, hormonal properties and enteric nervous system

Function of the GIT system

• Digestion converts food into a sterile, neutral, isotonic structure containing small sugars, amino acids, small peptides, lipids, and other small molecules
• Absorption and excretion: This process is facilitated by hydrochloric acid (HCl), bile, and various enzymes secreted by exocrine glands including secretion
• Motility: Contractions of smooth muscles in the tract walls mix and move food from mouth to anus.
• Absorption: Digestion products are absorbed into the blood

Initial Steps

• Initial physical disruption through ingestion to storage
• Initial chemical disruption and creation of suspension (chyme)
• Disinfection, Controlled release of chyme, Dilution, and neutralization
• Completion of chemical breakdown, Absorption of nutrients and electrolytes, Final absorption of water
• Produces faeces for controlled excretion.

Digestion Phases

• Digestion is divided into non-specific and specific phases
• The non-specific phase physically reduces the size of the food particles
• The specific phase chemically breaks down food molecules via enzymes into small molecules
• This process creates chyme which is a semiliquid form of ingested food and is kept sterile due to the stomach acid.

Absorption Details

• Absorption is the specific active or passive uptake of nutrient molecules, water, and electrolytes
• Fluid moves slowly through the small intestine which has a high surface area due to villi and microvilli, increasing absorption
• Remaining water and electrolytes get absorbed in the large intestine
• Slow transit through the large intestine reabsorbs water, leaving dry faeces to accumulate in the sigmoid colon and anal canal
• The small intestine absorbs approximately 12.5 L of fluid
• The large intestine absorbs approximately 1.5 L of fluid

Excretion Process

• Faeces are periodically propelled into the rectum, triggering the urge to defecate
• Relaxation of the sphincters around the anus results in the expulsion of faeces
• Diarrhoea and vomiting cause dehydration, as water cannot be reabsorbed
• GI disturbances, such as constipation may also occur

Mouth and Oesophagus

• Mastication
• Saliva contains protective elements plus helps lubricate the mouth,
• Wet substances that are bacteriostatic, alkaline, and high in Ca2+ protect the mouth
• Wet mucus lubricates food for mastication
• Saliva begins the digestion of sugars
• Swallowing forms a bolus, which allows for rapid oesophageal transport

Stomach Function

• Storage accommodates food
• Initial disruption involves contractions to mix and disrupt food
• Secretes acid and proteolytic enzymes to break down tissues and disinfect as chyme
• Delivers chyme slowly into the duodenum

Duodenum

• Dilution and neutralisation of chyme
• Achieved by water drawn in from the extracellular fluid (ECF), since the duodenum is permeable while the stomach is impermeable
• Bile alkali from the liver and pancreas helps.
• Enzymes added from the pancreas also help

Gut Fluid Balance

• Eating 1 kg of food per day results in 1 L of water in the GI tract lumen
• Adding 1.5 L of saliva increases it to 2.5 L
• Adding 2.5 L of gastric secretion increases it to 5 L
• The addition of 9 L of fluid for isotonic and neutral conditions results in 14 L
• The small intestine absorbs 12.5 L of water, leaving 1.5 L
• The Large intestine absorbs 1.35 L, leaving 0.15 L
• Remaining faeces weigh 0.15 kg

Small Intestine Role

• Absorption of nutrients and electrolytes
• Fluid passes very slowly due to large surface area
• Epithelial cells absorb molecules actively or passively, where the absorption is often coupled with Na+
• This passes through the hepatic portal circulation as a first pass
• It absorbs most of the water (1.5L vs 0.15L in the large intestine)

Large Intestine Action

• Final absorption of water (0.15 L)
• Very slow transit
• Faeces form and accumulate in the descending and sigmoid colon

Rectum Action

• Faeces are propelled periodically into the rectum, causing an urge to defecate
• This results in Controlled relaxation of sphincters, expelling the faeces

Enteric nervous system

• The enteric nervous system is a subdivision of the autonomic nervous system for direct control of GI motility
• It comprises two nerve plexuses in the gut wall, acting independently of the CNS
• Parasympathetic control coordinates secretion and motility using neurotransmitters

Hormonal properties

• A dozen or more peptide hormones are released by endocrine cells in the walls of the gut

There are two classes

• Paracrine action hormones
• True endocrine action hormones
• Hormones comprise two structurally related "groups": gastrin and secretin groups. These hormones are released from one part of the gut to affect secretions or motility in other parts.