Anatomy and Function of the GIT

Study Notes

The gastrointestinal tract (GIT) is composed of Upper and Lower GIT organs, and accessory organs.
Upper GIT organs include the esophagus, stomach and part of the duodenum.
Lower GIT organs include the small intestine (duodenum, jejunum, ileum), large intestine (cecum, colon, rectum), and anus.
Accessory organs include salivary glands, gallbladder, pancreas, and spleen.

Saliva is an isotonic fluid secreted by parotid, submandibular, and sublingual salivary glands.
Daily saliva volume is approximately 1.5 liters.
Saliva's pH is neutral to slightly alkaline (due to bicarbonate).
Saliva's composition includes water, electrolytes, amylase, mucus, and antibodies.
Saliva initiates carbohydrate digestion (by amylase).
Saliva aids in swallowing and speech.
Saliva provides defense against microbes.
Saliva neutralizes acids produced by bacteria.

The stomach stores food for up to 4 hours and controls its release into the duodenum.
It digests proteins and lipids using pepsin and gastric lipase enzymes respectively.
It protects against bacteria by producing hydrochloric acid (HCl).
It produces the intrinsic factor (IF) for vitamin B12 absorption.
It plays a role in iron absorption by maintaining iron in the ferrous state.
It absorbs small amounts of water, ions, alcohol, and certain drugs.
It produces hormones such as gastrin, glucagon, and somatostatin.

The small intestine is responsible for digestion and absorption of nutrients from food.
It absorbs about 80% of water, vitamins, minerals, carbohydrates, proteins, and lipids.
It secretes digestive enzymes.
Nutrients are absorbed into the bloodstream.

The respiratory system participates in acid-base balance within seconds.
It responds to changes in blood pH through chemoreceptors.
Peripheral chemoreceptors are stimulated by low oxygen (Po2), high carbon dioxide (Pco2), and low pH.
Central chemoreceptors are stimulated by high Pco2 and low pH.
Stimulation of chemoreceptors leads to hyperventilation, removing excessive CO2, thus correcting low pH.
Conversely, high pH decreases stimulation, leading to hypoventilation, raising CO2, and correcting high pH.

Ingestion of food and water
Digestion of food to absorbable nutrients
Absorption of nutrients, vitamins, minerals, water and electrolytes into the blood to supply the tissues
Elimination of undigested materials in waste products.

Approximately 9 liters of fluid enter the small intestine per day.
This fluid comes from saliva (1.5L), stomach (2.5L), pancreas (1.5L), bile (0.5L), and intestinal secretions (1L).
Almost all (99%) of this fluid is reabsorbed.

Digestion breaks down complex proteins, fats, and carbohydrates.
Absorption is the process whereby digested products, vitamins, minerals, and water cross the mucosa to enter the lymph or blood.

The liver and biliary system play a vital role in secretion, metabolism, and detoxification processes.

Bile is alkaline and approximately 1.5 liters are produced per day
Bile color arises from bile pigments from heme catabolism
Bile components include water, electrolytes, bile salts, cholesterol, phospholipids, and phosphatase enzymes.

Pancreatic enzymes include amylase for carbohydrate digestion, lipase and co-lipase for fat digestion, and proteolytic enzymes for protein digestion.

The colon is responsible for water absorption and feces formation.
Defecation is the elimination of bowel contents as feces.
Defecation involves a reflex involving stretching receptors in the rectum triggering the defecation reflex.
Voluntary control is also possible.