1. The esophagus is a flap of tough tissue that prevents the food from entering the larynx and the trachea
2.**Peristalsis** is the wave of muscular contraction that helps move material through most of the digestive tract. It's an involuntary response to swallowing
3. By relaxing and contracting, the muscles can mix substances, in the lumen and control movement through the tract
- **Simple Diffusion**: Occurs when the concentration of a particular nutrient is higher in one area than an other. This form of transport does not require energy input. Many water-soluble vitamins, lipids, and some minerals are absorbed in the digestive tract by simple diffusion.
- **Facilitated Diffusion**: Enterocytes absorb some nutrients by facilitated diffusion, another process that does not require energy. Although the nutrient moves down its concentration gradient, it still needs to be carried by a special transport protein within the membrane of the enterocyte. Absorption of simple sugar fructose occurs by facilitated diffusion
- **Active Transport**: Some nutrients move from the lumen of the intestine and into an enterocyte against the concentration gradient; that is, from low to high concentration. Absorption of these nutrients requires both a unique transport protein and energy. Enterocytes rely on active transport to absorb glucose and amino acids.
- **Endocytosis**: In a few instances, a segment of a the cell membrane of an enterocyte surrounds and swallows relatively large substances, such as intact protein molecules. This process enables an infant's intestinal tract to absorb whole proteins in human milk that provide benefits to the immune system. However, endocytosis is not a common way for nutrients to enter enterocytes.
| Lumen | The open space inside of the digestive system |
| Chyme | A semiliquid mass that forms when food mixes with gastric juice. Occurs in the lower stomach |
| Motility | The ability of an organism to move independently |
| Gastrin | Secreted in response to food entering the stomach, it triggers parietal cells to release HCL and chief cells to release pepsinogen. *Stimulates stomach and small intestinal motility* |
| Secretin | Secreted from the small intestine in response to acidic chyme entering the duodenum and the first part of the jejunum, secretin *stimulates the release of a bicarbonate-rich solution from the liver and pancreas.* |
| Cholecystokinin | Secreted from the small intestine in response to fat and breakdown products of proteins (peptides) entering the small intestine, cholecystokinin *stimulates the release of bile from the gallbladder into the small intestine*. It also stimulates the release of pancreatic enzymes, decreases stomach secretions, and slows stomach motility. |
| Pancreas | An accessory organ of the GI tract that produces and secretes many of the enzymes that break down carbs, protein, and fat. It also secretes bicarbonate ions to neutralize the highly acidic chyme coming from the stomach. |
| Villi (singular villus) | Small finger like projections that line the inner surface of the small intestine. They help maximize the absorption of nutrients from food. They're covered in an outer layer of absorptive cells called enterocytes. |
