Small Intestine Absorption and Membrane Phase

Epithelial cells contain enzymes that break down carbohydrates into monosaccharides.
Enzymes can change with age: lactase is less active in adults.
Water is absorbed in the small intestine and follows an osmotic gradient.
The small intestine absorbs a majority of ingested water.
Water follows the absorption of sodium, glucose, amino acids, and volatile fatty acids.

Glucose and galactose absorption is linked to Na+ absorption, which occurs against the concentration gradient (active process).
Absorbed glucose is processed by the liver and stored as glycogen.
Fats are absorbed in the small intestine.
Bile salts from the liver emulsify fat, increasing surface area for lipase action.
Micelles are formed from degraded monoglycerides and free fatty acids, which can enter the spaces between microvilli.
Fatty acids and monoglycerides are fat-soluble and can diffuse out of micelles and across the epithelial membrane.
Inside the epithelial cell, fatty acids and monoglycerides recombine to form triglycerides.
Triglycerides form chylomicrons, which are lipoproteins containing cholesterol and a protein coat.
Chylomicrons are released into interstitial fluid by exocytosis and are too large to enter capillaries.
Chylomicrons are transported via lymph to the central circulation.

Hepatocytes process amino acids, monosaccharides, and triglycerides from the digestive tract.
The liver adjusts the concentration of incoming nutrients and distributes them to the tissues.
Excess nutrients are stored as glycogen and fat.
Glucose is used as energy in hepatocytes and stored as hepatic glycogen.
Triglycerides are synthesized in hepatocytes and adipocytes.
Amino acids are processed by the liver, synthesized into non-essential amino acids, and used for gluconeogenesis.
Ammonia (NH4) is broken down into urea and excreted via the kidneys.

Nutrients are no longer absorbed from the intestines.
Energy stores are mobilized: glycogen is broken down (glycogenolysis) and new glucose is synthesized from amino acids (gluconeogenesis).
Fatty acids are mobilized by the degradation of stored fats.
Fatty acids are oxidized to water and carbon dioxide, releasing energy.
Ketones are synthesized in the liver.
The body utilizes lipids as the main energy source.

Insulin and glucagon from the pancreas regulate metabolism.
Hormones from the gastrointestinal tract (GIT) also play a role in regulating metabolism.
The autonomic nervous system influences regulation of metabolism.
Sympathetic nerves stimulate glycogenolysis but not gluconeogenesis.
Glucocorticoids (e.g. cortisol) from the adrenal gland are important during stress.
Cortisol stimulates gluconeogenesis and lipid mobilization from adipose tissue.

The muscular wall of the small intestine is responsible for mixing food and ensuring contact with the lumen.
Stomach filling induces intestinal motility.
Crypt cells at the base of the villi in the small intestine secrete intestinal juice (water, bicarbonate, and mucus) and enzymes that remain bound to the epithelial cells.
Small intestine has a large reserve capacity, meaning most nutrients are absorbed long before the end of the small intestine.

The immune system protects the body from disease-causing invaders, removes injured cells, and attacks cancer cells.
Immune cells are present in all tissues and the immune response is quick.
Pathogenic microbes have strategies to establish themselves in the body.
Commensal microbes live in symbiosis with the body.
Allergies result from an inappropriate or over-reaction of the immune system.

Pathogens can invade the body through various routes: skin, mucous membranes, respiratory tract.
Some pathogens require physical damage to the skin for entry.
Others rely on biting insects (vectors) for transmission, like tick bite fever or fleas carrying worm eggs.

The immune system has two branches: the innate and adaptive immune systems, working synergistically.
The innate immune system is non-specific, inborn, and provides a rapid response.
The innate system lacks specificity, meaning it acts against any pathogen.
The innate system has no memory.
The adaptive immune system is specific, develops after birth, and has a slower response to the first infection.
The adaptive immune system has a high degree of specificity and develops memory cells.
Memory cells allow for a rapid response to a recognized pathogen upon subsequent infection.

Bacteria are single-celled organisms.
Bacteria are able to reproduce on their own, using enzymes for metabolism, growth, and reproduction.
White blood cells (WBCs) called phagocytes engulf and destroy bacteria.

Viruses are not cells, consisting of a protein coat and nucleic acid.
They lack cell membranes and cytosol and cannot replicate on their own.
They require host cells to replicate.
They insert their genetic material into the host cell's DNA, disrupting the host cell's normal function.