Lecture 20: Digestion

What is the typical US diet like?

Energy-dense but nutrient-poor, high in refined grains, added sugars, and saturated fats.

High caloric intake + low nutrient density contribute to rising obesity, diabetes, and metabolic disorders.

What does the digestive system do?

Reduces macromolecules in food (proteins, fats, carbohydrates) down to their basic building blocks (amino acids, fatty acids, sugars).

What processes are involved in digestion?

Chemical: breaking chemical bonds with enzymes.

Mechanical: grinding food with teeth or muscular layers of the stomach.

What does nutrient absorption involve?

The movement of nutrients/chemicals into the body through intestinal epithelial cells (enterocytes).

This primarily occurs in the small intestine, but some absorption also occurs through the oral mucosa, stomach, and large intestine.

What is swallowing?

Swallowing is an involuntary reflex that begins when the tongue pushes a food bolus to the back of the mouth, triggering the pharynx to close off airways (epiglottis covers trachea; soft palate blocks nasal cavity).

Coordinated muscle contractions move food safely, with the upper esophageal sphincter relaxing to let the bolus enter the esophagus.

How does mechanical digestion work?

The stomach uses 3 layers of smooth muscle (longitudinal, circular, oblique) to powerfully churn and mix food, breaking it down into a semi-liquid mixture called chyme.

Coordinated contractions called peristaltic waves mechanically fragment food and enhance its mixing with gastric juices, increasing the efficiency of chemical digestion that follows.

How does chemical digestion work?

Parietal cells secrete hydrochloric acid (HCI), which creates an extremely acidic gastric environment (pH ~1-2) needed to denature proteins and kill most ingested microbes.

Chief cells release pepsinogen, an inactive precursor that is activated into pepsin by HCI. Pepsin then begins the enzymatic breakdown of proteins into smaller peptides.

Positive feedback: once formed, pepsin accelerates the conversion of more pepsinogen into pepsin, boosting protein-digesting efficiency.

Mucus-secreting cells protect the stomach lining by forming a barrier that prevents acid and pepsin from damaging the stomach wall.

What is the structure and function of the small intestine?

The microvilli expand the absorptive surface of the small intestine. Both capillary beds and lymphatic vessels are present in the villus.

The vast majority of nutrients in food are absorbed through the small intestine.

Tight junctions between epithelial cells regulate selective absorption, ensuring nutrients are transported into the bloodsteam while preventing pathogens and toxins from slipping between cells.

What is the structure and function of the large intestine?

The large intestine hosts dense populations of beneficial microbes that ferment undigested carbohydrates, producing short-chain fatty acids (SCFAs) which can be absorbed and used as energy sources.

The colon plays a role in electrolyte balance through the regulated absorption of Na+ and the secretion/absorption of K+, helping maintain overall fluid and electrolyte homeostasis.

Water absorption condenses and compacts the digestives into the feces.

What are carnivore digestive tracts like?

Carnivores like wolves, which feed primarily on highly digestible foods such as meats and fruits, have a short intestine (~6x body length) and small cecum.

What are herbivore digestive tracts like?

Herbivores like rabbits, which feed on difficult-to-digest plant tissues, have a relatively long intestinal tract (up to 10x body length). They also have specializations of the cecum or stomach that helps break down complex carbohydrates like cellulose.

Longer guts have more surface area and allow a longer retention time of the food. This increases the efficiency of nutrient digestion and absorption.