Chapter 25: The Digestive System

Digestive System

• The digestive system processes food, extracts nutrients from it, and eliminates the waste. It does this in five stages:

1. Ingestion

2. Digestion – The chemical and mechanical breakdown of food.

3. Absorption – The uptake of nutrients into the epithelial cells of the digestive tract and then into the blood or lymph.

4. Compaction – Absorbing water and consolidating the waste.

5. Defecation

Digestive System Anatomy

• Consists of the digestive tract (mouth, pharynx, esophagus, stomach, small intestine, and large intestine) and the accessory organs (salivary glands, liver, gallbladder, and pancreas).

Mouth

• The teeth and tongue are used to mechanically break up food into smaller pieces.

• Enzymes (salivary amylase, salivary lipase) in the saliva initiate chemical digestion.

Stomach

• The stomach functions as a food storage organ as well as a site for further digestion.

• The process of chemical digestion, which was initiated in the mouth, continues in the stomach.

• It also functions to mechanically break up food.

• This mechanical and chemical digestion produces a watery mixture of semi-digested food that is called chyme.

Gastric Epithelium

• The gastric epithelium is the inner lining of the stomach.

• The epithelium has many depressions called gastric pits. These pits lead down into gastric glands.

• These gastric pits and gastric glands contain a variety of glandular cell types, each of which produces different substances.

Cells of the gastric glands

1. Mucous Cells – Produce mucus

2. Parietal Cells – Secrete hydrochloric acid and, when the stomach is empty, an appetite- stimulating hormone called ghrelin. HCl breaks down food and helps to destroy ingested pathogens. Also produce intrinsic factor, which binds to vitamin B12 and allows for its absorption in the small intestine. Vitamin B12 is needed for normal RBC production. Without intrinsic factor, develops due to the inability to synthesize hemoglobin.

3. Chief Cells – Secrete gastric lipase and pepsinogen (pepsinogen gets converted to pepsin by HCl, which then digests proteins).

4. Enteroendocrine Cells - Secrete hormones and paracrine messengers that regulate digestion.
   There are at least eight kinds of enteroendocrine cells in the stomach, and there are probably about 20 different chemical messengers produced. One of these, G cells, produce gastrin, a hormone that stimulates HCl release and increases gastric motility.

Protection of the stomach

• The environment inside the stomach is harsh and caustic. How is the lining of the stomach protected?

  1. Mucous Coat

  2. Tight Junctions

  3. Rapid Cell Replacement – The epithelial cells of the mucosa layer only last for 3 to 6 days.

Gall Bladder

• The gall bladder is a sac that stores bile until it is released into the small intestine.

• Bile is initially secreted from the liver.

• Bile aids in the digestion of fats. In essence, the contents of bile act to separate fat molecules form one another so they do not form clumps. This allows pancreatic lipase to digest the fat more efficiently.

The Pancreas

• The pancreas is both an endocrine and an exocrine organ,

• The endocrine part, the pancreatic islets (a.k.a Islets of Langerhans), secrete insulin and glucagon.

• About 99% of the pancreas is exocrine tissue that produces pancreatic juice. It consists of acini which open into ducts that eventually converge into the main pancreatic duct, which carries materials to the small intestine.

• Pancreatic juice is an alkaline mixture that contains water, electrolytes sodium bicarbonate, and digestive enzymes.

• The enzymes or enzyme precursors that
  are found in pancreatic juice include:

  1. Trypsinogen – Converted to trypsin in the SI; digests proteins.

  2. Chymotrypsinogen – Converted to chymotrypsin in the SI; digests proteins.

  3. Procarboxypeptidase – Converted to carboxypeptidase; digests proteins.

  4. Pancreatic Amylase – Digests starch

  5. Pancreatic Lipase – Digests fats

• The release of pancreatic juice (and bile) are under the regulation of:

  A. Acetylcholine – From the vagus nerve. Causes secretion of pancreatic enzymes, even in anticipation of eating.

  B. Cholecystokinin (CCK) – Secreted by the SI in response to fats in the SI. Strong effect on the release of bile; also causes secretion of pancreatic enzymes. Inhibits gastric secretions and motility.

  C. Secretin – Secreted by the SI in response to acidity in the SI. Causes an increase in the release of sodium bicarbonate from the pancreas to neutralize the acid. inhibits gastric secretions and mobility.

The small intestine:

• Divided into the duodenum, jejunum, and ileum.

• The duodenum is the area where secretions from the gall bladder and pancrease are added the chyme.

• Most digestion and nutrient absorption occur in the jejunum.

• The surface area of the inner lining is greatly increased by folds, villi, and microvilli.

• Villi are covered with absorptive cells and goblet cells, which produce mucus. The wall of the SI produces its own alkaline secretion, which is called intestinal juice.

• Each villus will contain an arteriole, capillaries, a venule, and a lacteal (which is absorbs lipids).

Intestinal Motility

• Contractions of the small intestine serve to a) mix chyme with intestinal juice, pancreatic juice, and bile, b) churn the chyme to break it up mechanically and promote absorption, and c) move materials toward the large intestine.

• Segmentation

• When absorption of nutrients is mostly complete, the SI releases the hormone motilin, which causes waves of peristalsis called the migrating motor complex.

Carbohydrate Digestion / Absorption

• Starch is digested into glucose, which is what gets absorbed by the SI.

• About 50% of starch is digested by the actions of salivary amylase and HCl from the stomach before it reaches the SI.

• In the SI, pancreatic amylase breaks down the rest of the starch into glucose.

• Sucrose and lactose are broken down by sucrase and lactase (both embedded in the wall of the SI) and the resulting monosaccharides (glucose and fructose glucose and galactose) are absorbed.

• The effectiveness of lactase diminishes with age in most individuals

• All of the monosaccharides are absorbed by blood capillaries of the villi, and the hepatic portal vein transports them to the liver.

Protein Digestion / Absorption

• Enzymes that digest proteins are called proteases. these include pepsin in the stomach and trypsin, chymotrypsin, carboxypeptidase, amino-peptidase, and dipeptidase in the SI. (the last two are embedded in the wall of the SI)

• Amino acids are absorbed by blood capillaries of the villi, and the hepatic portal vein transports them to the liver.

Lipid Digestion / Absorption

• Fats are digested by lipases. these include salivary lipase, gastric lipase, and pancreatic lipase. Only about 10% of lipids are digested by the chyme reaches the SI.

• The churning of the stomach emulsifies the fats (breaks up into smaller droplets).

• In the SI, these droplets are broken up into smaller pieces by the bile, and pancreatic lipase digests the lipids in the droplets.

• The products of lipid digestion (fatty acids) are absorbed and taken up by the lacteals.

• From there, the lipids travel through the lymph system to be deposited in the circulatory system at the subclavian veins. eventually they move to adipocytes for storage or to other body cells for use in production of ATP.

The Large Intestine

• The main function of the LI is reabsorption of water and electrolytes.

• It also compacts of the waste into feces.