Human Physiology - Digestion and Absorption

Human dentition is described as Thecodont (teeth embedded in jaw sockets), Diphyodont (two sets of teeth: deciduous and permanent), and Heterodont (different types of teeth: Incisors, Canines, Premolars, and Molars).

The wall of the alimentary canal consists of four layers from outer to inner: Serosa, Muscularis (inner circular, outer longitudinal), Sub-mucosa (contains nerves and vessels), and Mucosa (forms rugae in stomach and villi in small intestine).

Salivary amylase (Ptyalin) initiates carbohydrate digestion in the mouth, converting approximately $30\%$ of starch into the disaccharide maltose at an optimum $pH$ of $6.8$.

In the stomach, $HCl$ provides an acidic $pH$ of $1.8$, which is optimal for the conversion of inactive Pepsinogen into active $Pepsin$ for protein digestion.

Bile, secreted by the liver, contains bile salts (sodium glycocholate and sodium taurocholate) but no enzymes. It is essential for the emulsification of fats into small micelles.

Pancreatic juice contains inactive enzymes: Trypsinogen, Chymotrypsinogen, Procarboxypeptidases, Amylases, Lipases, and Nucleases. Trypsinogen is activated by Enterokinase (secreted by intestinal mucosa) into $Trypsin$.

End products of digestion (glucose, amino acids) are absorbed by active transport or facilitated diffusion, while fatty acids and glycerol are first converted into $Chylomicrons$ (protein-coated fat globules) before entering the lacteals.

The Gross Calorific Value of carbohydrates, proteins, and fats are $4.1 \text{ kcal/g}$, $5.65 \text{ kcal/g}$, and $9.45 \text{ kcal/g}$ respectively.