Human Physiology - Digestion, Absorption, and Egestion

Digestion is the process of breaking down large, insoluble food molecules into small, soluble molecules that can be absorbed into the bloodstream. It occurs through mechanical means (chewing, churning) and chemical means (hydrolysis catalyzed by enzymes).

Chemical digestion involves enzymes, which are biological catalysts. The general reaction for hydrolysis of a macronutrient is: $Substrate + H_{2}O \xrightarrow{Enzyme} Products$.

Carbohydrate digestion: Starch is broken down by Amylase into Maltose, and then by Maltase into Glucose. The simple sugar Glucose has the formula $C_{6}H_{12}O_{6}$.

Protein digestion: Proteins are broken down by Proteases (like Pepsin in the stomach and Trypsin in the small intestine) into Polypeptides and eventually into Amino Acids.

Lipid digestion: Fats are emulsified by bile and then broken down by Lipase into Glycerol and Fatty Acids. The reaction consumes water: $Triglyceride + 3H_{2}O \xrightarrow{Lipase} Glycerol + 3(Fatty Acids)$.

Absorption occurs primarily in the small intestine. The inner surface is covered in Villi and Microvilli to maximize the surface area-to-volume ratio ($SA:V$), facilitating faster diffusion and active transport.

Bile is produced in the liver, stored in the gallbladder, and acts in the duodenum. It performs two roles: Emulsification (increasing lipid surface area) and neutralization of stomach $HCl$ to create an alkaline environment ($pH \approx 8.0$).

Egestion is the discharge of undigested and unabsorbed food as feces through the anus. This is distinct from excretion, which is the removal of metabolic waste (like $CO_{2}$ or urea).