Honors Anatomy and Physiology Unit 13 Review: Digestive System

Digestion uses two kinds of breakdown. Mechanical digestion includes chewing, stomach churning, and segmentation in the small intestine. It makes pieces smaller and raises surface area, but it breaks no chemical bonds. Chemical digestion uses enzymes to hydrolyze bonds, which splits polymers into monomers small enough to absorb.

Track each macronutrient in order. Starch starts with salivary amylase in the mouth and continues with pancreatic amylase in the duodenum. Brush border enzymes (maltase, sucrase, lactase) finish the job, releasing glucose, fructose, and galactose.

Protein digestion starts in the stomach. There, HCl denatures proteins and converts inactive pepsinogen into pepsin. Trypsin, chymotrypsin, and carboxypeptidase from the pancreas cut protein chains into smaller peptides and amino acids. Brush border peptidases break peptides down further. Intestinal cells absorb amino acids and some dipeptides and tripeptides, then break the absorbed peptides into amino acids.

Fat changes very little until the duodenum. There, bile emulsifies it and pancreatic lipase splits triglycerides into fatty acids and monoglycerides.

Each enzyme has an optimal pH. Pepsin works near pH 2, while pancreatic enzymes need about pH 7 to 8. That's why the pancreas releases bicarbonate.

Nerves and hormones control the process. Parasympathetic signals through the vagus nerve speed digestion, and sympathetic signals slow it. Gastrin from the stomach increases acid secretion. Acidic chyme entering the duodenum triggers secretin release. Secretin signals the pancreas to release bicarbonate-rich fluid. Fats and proteins trigger CCK release, which stimulates pancreatic enzyme secretion and gallbladder contraction.

Nearly all nutrient absorption happens in the small intestine. Circular folds and villi increase its surface area. Microvilli on absorptive epithelial cells form the brush border and increase surface area further.

The route depends on the molecule. Glucose and galactose enter cells along with sodium through SGLT1, which is secondary active transport. Fructose enters by facilitated diffusion through GLUT5. Amino acids also enter with sodium. Sugars and amino acids then move into capillaries and travel through the hepatic portal vein to the liver.

Most dietary fats take a different path. Bile salts help carry fatty acids and monoglycerides to the cell surface in micelles. Inside the cell, these products are rebuilt into triglycerides and packed into chylomicrons. Chylomicrons enter lacteals and reach the blood later through the thoracic duct. Vitamins A, D, E, and K follow this route. Short-chain and medium-chain fatty acids can enter portal blood directly. Vitamin B12 needs intrinsic factor from the stomach and is absorbed in the ileum.

Worked example. About 9.0 L of fluid enters the gut daily, roughly 2 L from food and drink and 7 L from secretions. The small intestine absorbs about 7.5 L, so $9.0 - 7.5 = 1.5$ L reaches the colon. The colon absorbs about 1.4 L, which leaves $1.5 - 1.4 = 0.1$ L in feces. So the small intestine absorbs $7.5 / 9.0 \approx 83\%$ of the water. The colon absorbs $1.4 / 1.5 \approx 93\%$ of what it receives.

When the rectum stretches, a reflex relaxes the involuntary internal anal sphincter. The external anal sphincter is skeletal muscle, so you control when it relaxes.

Metabolism is the sum of every chemical reaction in your body. Catabolism breaks molecules down and releases energy, as when glycolysis splits glucose. Anabolism builds molecules and uses energy, as when cells link glucose into glycogen or amino acids into proteins. ATP, made mostly by cellular respiration, carries energy between the two.

After a meal, insulin rises and cells take in glucose. The liver and muscles store extra glucose as glycogen, and adipose tissue stores extra fat as triglycerides. Between meals, glucagon signals the liver to break down glycogen and make new glucose (gluconeogenesis) so your brain keeps a steady supply.

Basal metabolic rate (BMR) is the energy you use at rest just to stay alive. Total daily energy expenditure adds physical activity and the energy spent digesting food. When intake regularly exceeds expenditure, the extra gets stored, mostly as fat.

Macronutrients (carbohydrates, proteins, fats) supply energy and building blocks. Micronutrients (vitamins, minerals) are needed in small amounts. Fat-soluble vitamins (A, D, E, K) can build up in body fat. Water-soluble vitamins (B vitamins, C) mostly leave in urine.

Worked example. A granola bar has 30 g carbohydrate, 5 g protein, and 8 g fat. Carbohydrate and protein give 4 kcal/g, and fat gives 9 kcal/g. So carbohydrate gives $30 \times 4 = 120$ kcal, protein gives $5 \times 4 = 20$ kcal, and fat gives $8 \times 9 = 72$ kcal, for a total of $212$ kcal. Fat supplies $72 / 212 \approx 34\%$ of the calories, even though it's only 8 of the 43 grams.

The gastrointestinal (GI) tract, or alimentary canal, is one continuous tube. It runs from the mouth through the pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), and large intestine (cecum, colon, rectum) to the anus. Accessory organs (salivary glands, liver, gallbladder, pancreas) send secretions into the tube through ducts, but food never passes through them.

From the esophagus onward, the wall has four layers. The mucosa lines the lumen and handles secretion and absorption. The submucosa holds blood vessels, lymph vessels, and the submucosal nerve plexus. The muscularis externa usually has inner circular and outer longitudinal muscle layers, with the myenteric plexus between them. These layers produce peristalsis. The muscle is smooth in most of the tract, but the upper esophagus contains skeletal muscle. Serosa, or visceral peritoneum, covers the outside of some sections. Other sections, including most of the esophagus, have fibrous adventitia instead.

Each organ adjusts this plan. The stomach adds an oblique muscle layer for churning and has rugae that flatten as it fills. The small intestine adds circular folds and villi. The large intestine has no villi, and three bands of longitudinal muscle (teniae coli) bunch it into pouches called haustra.

Sphincters (lower esophageal, pyloric, ileocecal, anal) control flow between sections. Bile leaves through the common bile duct, which joins the pancreatic duct to empty into the duodenum. The pancreas, most of the duodenum, and the ascending and descending colon are retroperitoneal, meaning they sit behind the peritoneum.