Digestive system

DIGESTIVE SYSTEM
Objectives:
Identify the organs of the digestive system and list their major functions. (stomach, small intestine, large intestine and salivary glands, pancreas, liver, gall bladder)
Explain the processes by which materials move through the digestive tract.
Outline the mechanisms that regulate digestion.
The digestive system consists of a muscular tube, the digestive tract, also called the gastrointestinal tract (GI) or alimentary canal, and various accessory organs.
The nutrition for the cells of the body must be in a simple form: amino acids, simple sugars, and fatty acids.

The digestion process: The digestive system takes the complex organic molecules of the foods we ingest —proteins, carbohydrates, and fats — and break them down into their simple form. The simple molecules (nutrients) are absorbed from the digestive system by the cardiovascular and lymphatic systems and transported to cells throughout the body.

The digestive system may divide into two parts:

- A muscular tube, known as the alimentary canal or digestive tract: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
- The accessory digestive organs and glands that help in the digestive process include the tongue, teeth, salivary glands, pancreas, liver, and gall bladder.
Functions of the Digestive System :
- Ingestion occurs when materials enter the digestive tract via the mouth. Ingestion is an active process involving conscious choice and decision making.
- Mechanical processing is crushing and shearing that makes materials easier to propel along the digestive tract. It also increases their surface area, making them more susceptible to enzymatic attack.
- Digestion refers to the chemical breakdown of food into small organic fragments suitable for absorption by the digestive epithelium. Simple molecules in food, such as glucose, can be absorbed intact, but epithelial cells cannot to absorb molecules the size and complexity of proteins, polysaccharides, or triglycerides. These molecules must be splitted by digestive enzymes prior to absorption.
- Secretion is the release of water, acids, enzymes, buffers, and salts by the epithelium of the digestive tract and by glandular organs.
- Absorption is the movement of organic substrates, electrolytes (inorganic ions), vitamins, and water across the digestive epithelium and into the interstitial fluid of the digestive tract.
- Excretion is the removal of waste products from body fluids.
Peristalsis :
- The muscularis externa propels materials from one portion of the digestive tract to another by contractions knows as peristalsis.
- Peristalsis consists of waves of muscular contractions that move a bolus, or small oval mass of digestive contents, along the length of the digestive tract.
- During a peristaltic movement, the circular muscles contract behind the bolus while circular muscles ahead of the bolus relax. A wave of contraction in the circular muscles then forces the bolus forward.

Segmentation :
- Most areas of the small intestine and some portions of the large intestine undergo cycles of contraction that churn and fragment the bolus, mixing the contents with intestinal secretions.
- This activity, called segmentation, does not follow a set pattern, and thus does not push materials along the tract in any one direction.
The mouth
Food enters the body through the mouth, or oral cavity. The mouth performs the first step of digestion that is mastication (chewing).
Three pair of salivary glands open at various points into the mouth, about 1 to 2 liters of saliva are secreted each day.
Sympathetic nerves stimulate saliva production.
Saliva has many functions :
- It contains amylase which breaks down polysaccharides (a type of carbohydrate)
- It contains mucus which moistens and lubricates the food which helps in swallowing.
- It contains lysozyme which is an anti-bacterial enzyme.
- It helps in the stimulation of the taste buds.
- It aids speech by moistening the lips and tongue.
- It helps keep mouth and teeth clean.
- It buffers acidic foods.
The pharynx:
A common passage way for solid food, liquids, and air.
Food normally passed through the oropharynx and laryngopharynx on its way to the esophagus.
The epithelial lining and regions of the pharynx – the nasopharynx, the oropharynx, and laryngopharynx.
The esophagus:
- The esophagus is a hollow muscular tube with a length of approximately 25 cm (10 in.) and a diameter of about 2 cm (0.80 in.) at its widest point.
- The primary function of the esophagus is to convey solid food and liquids to the stomach.
The stomach

The stomach performs 4 major functions :
1) storage of ingested food
2) mechanical breakdown of ingested food
3) disruption of chemical bonds in food material through the action of acids and enzymes
4) production of intrinsic factor, a glycoprotein whose presence in the digestive tract is required for the absorption of vitamin B12 in the small intestine.


+ In an “average” stomach, the lesser curvature has a length of approximately 10 cm, and the greater curvature measures about 40 cm.

+ In the fundus and body of the stomach, each gastric pit communicates with several gastric glands. Gastric glands are dominated by 2 types of secretory cells :
+ parietal cells: secrete hydrochloric acid (HCl).
+ chief cells: are more abundant near the base of a gastric gland. These cells secrete pepsinogen, an inactive proenzyme. Pepsinogen is converted by the acid in the gastric lumen to pepsin, an active proteolytic enzyme.
Gastric juice contains hydrochloric acid and pepsin.

- Pepsin is an enzyme that breaks down proteins.
- Hydrochloric acid kills microorganisms and breaks down cell walls and connective tissue in food.

The acid is strong enough to burn a hole in carpet, yet the mucus produced by the mucous membrane protects the lining of the stomach. Even so, the cells of the mucous membrane wear out quickly: the entire stomach lining is replaced every three days. Mucus also aids in digestion by keeping food moist.
Regulation of Gastric Activity :
The production of acid and enzymes by the gastric mucosa can be :
1) controlled by the CNS
2) regulated by short reflexes of the enteric nervous system (in the wall of the stomach)
3) regulated by hormones of the digestive tract.

- The Cephalic Phase:
The cephalic phase of gastric secretion begins when you see, smell, taste, or think of food. This stage, which is directed by the CNS, prepares the stomach to receive food.
- The Gastric Phase:
The gastric phase begins with the arrival of food in the stomach.
- The Intestinal Phase:
The intestinal phase of gastric secretion begins when chyme first enters the small intestine. The intestinal phase generally starts after several hours of mixing contractions of the stomach.
Digestion and Absorption in the stomach:
+ The stomach performs preliminary digestion of proteins by pepsin and, permits the digestion of carbohydrates and lipids by salivary amylase and lingual lipase.
+ As the stomach contents become more fluid and the pH approaches 2.0, pepsin activity increases in the stomach.

Key :
+ The stomach is a storage site that provides time for the physical breakdown of food that must precede chemical digestion.
+ Protein digestion begins in the acid environment of the stomach through the action of pepsin.
+ Carbohydrate digestion, which began with the release of salivary amylase by the salivary glands before swallowing.
The pancreas:
+ The pancreas is primarily an exocrine organ, producing digestive enzymes and buffers. The large pancreatic duct (duct of Wirsung) delivers these secretions to the deodenum.
+ The pancreas has 2 distinct functions, one endocrine and the other exocrine.
- The endocrine cells of the pancreatic islets secrete insulin and glucagon into the bloodstream.
- The exocrine cells include an acinar cells and the epithelial cells that line the duct system.
The exocrine pancreas produces a mixture of buffers and enzymes essential for normal digestion.
- Pancreatic secretion occurs in response to the release of regulatory hormones (CCK and secretin) by the duodenum.
Physiology of the Pancreas : (exocrine)
+ Each day, the pancreas secretes about 1000 ml of pancreatic juice. The secretory activities are controlled primarily by hormones from the deodenum.
+ The specific pancreatic enzymes involved include the following :
- Pancreatic alpha-amylase, a carbohydrase - an enzyme that breaks down certain starches, pancreatic alpha-amylase is almost identical to salivary amylase.
- Pancreatic lipase, which breaks down certain complex lipids, releasing products (such as fatty acids) that can be easily absorbed.
- Nucleases, which break down nucleic acids.
- Proteolytic enzymes, which break certain proteins apart. The proteolytic enzymes of the pancreas include proteases, which break apart large protein complexes, and peptidases, which break small peptide chains into amino acids.
Proteolytic enzymes account for about 70% of total pancreatic enzyme production. The enzymes are secreted as inactive proenzymes and are activated only after they reach the small intestine.
The liver :
The liver, the largest visceral organ, is one of the most versatile organs in the body. Most of its mass lies in the right hypo-diaphragm and epigastric regions.
The liver weighs about 1.2 kg. This large, firm, reddish-brown organ performs essential metabolic and synthetic functions.
The Physiology of the liver :
The liver is responsible for 3 general categories of functions :
1) Metabolic regulation
2) Hematological regulation
3) Bile production.
Metabolic Regulation : The liver is a primary organ involved in regulating the composition of circulating blood.
+ Liver cells extract nutrients or toxins from the blood before it reaches the systemic circulation through the hepatic veins.
+ The liver removes and stores excess nutrients, and it corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities. The liver’s regulatory activities affect the following :
- Carbohydrate metabolism: The liver stabilized blood glucose levels at about 90-110 mg/dl. If blood glucose levels drop, hepatocytes break down glycogen reserves and release glucose into the bloodstream.
- Lipid metabolism: the liver regulated circulating levels of triglycerides, fatty acids, and cholesterol. When those levels decline, the liver breaks down its lipid reserves and releases the breakdown products into the bloodstream.
- Amino acid metabolism: The liver removes excess amino acids from the bloodstream. These amino acids can be used to synthesize proteins or can be converted to lipids or glucose for storage.
- Waste product removal: When converting amino acids to lipids or carbohydrates, or when breaking down amino acids to get energy, the liver strips off the amino groups, a process called deamination.
- Vitamin storage: Fat-soluble vitamins (A, D, E and K) and vitamin are absorbed from the blood and stored in the liver. These reserves are called on when your diet contains inadequate amounts of those vitamins.
- Mineral storage: The liver converts iron reserves to ferritin and stores this protein-iron complex.
- Drug inactivation: The liver removes and breaks down circulating drugs, thereby limiting the duration of their effects.
Hematological regulation: The liver, the largest blood reservoir in your body, receives about 25% of cardiac output. As blood passes through it, the liver performs the following functions:
- Phagocytosis and antigen presentation: Kupffer cells in the liver sinusoids engulf old or damaged red blood cells, cellular debris, and pathogens, removing them from the bloodstream. Kupffer cells are antigen-presenting cells that can stimulate an immune response.
- Synthesis of plasma proteins: Hepatocytes synthesize and release most of the plasma proteins, including the albumins (which contribute to the osmotic concentration of the blood), the various types of transport proteins, clotting proteins, and complement proteins.
- Removal or circulating hormones: The liver is the primary site for the absorption and recycling of epinephrine, norepinephrine, insulin, thyroid hormones, and steroid hormones, such as the sex hormones (estrogens and androgens) and corticosteroids.
- Removal of antibodies: The liver absorbs and breaks down antibodies, releasing amino acids for recycling.
- Removal or storage of toxins: Lipid-soluble toxins in the diet, such as the insecticide DDT, are absorbed by the liver and stored in lipid deposits, where they do not disrupt cellular functions. Other toxins are removed from the bloodstream and are either broken down or excreted in the bile.

The Synthesis and Secretion of Bile:
Bile is synthesized in the liver and excreted into the lumen of the duodenum. Bile consists mostly of water, with minor amounts of irons, bilirubin (a pigment derived from hemoglobin), cholesterol, and lipids collectively known as a bile salts.

Bile is essential for the digestion of lipids; it breaks down large lipid droplets so that individual lipid molecules can be attacked by digestive enzymes.
The Gallbladder:
Physiology of the Gallbladder:
- A major function of the gallbladder is bile storage. Bile is secreted continuously – about 1 liter is produced each day – but it is released into the deodenum only under the stimulation of the intestinal hormone CCK.
- The gallbladder also functions in bile modification. When full, the gallbladder contains 40-70 ml of bile. The composition of bile gradually changes as it remains in the gallbladder. Much of the water is absorbed, and the bile salts and other components of bile become increasingly concentrated.

The small intestine
+ The small intestine plays the key role in the digestion and absorption of nutrients. 90% of nutrient absorption occurs in the small intestine; most of the rest occurs in the large intestine.
- The duodenum, 25 cm in length, is the segment closest to the stomach. This portion of the small intestine is a “mixing bowl” that receives chyme from the stomach and digestive secretions from the pancreas and liver.
- The jejunum is about 2.5 meters long. The bulk of chemical digestion and nutrient absorption occurs in the jejunum.
- The ileum, the final segment of the small intestine, is also the longest, averaging 3.5 meters in length.
+ The small intestine receives and raises the pH of materials from the stomach.
+ It then absorbs water, ions, vitamins, and the chemical products released by the action of digestive enzymes produced by intestinal glands and the exocrine glands of the pancreas.
The small intestine:
The intestinal tract secretes a variety of peptide hormones with similar chemical structures. Many of these hormones have multiple effects in several regions of the digestive tract, and in the accessory glandular organs as well.
Duodenal enteroendocrine cells produce the following hormones known to coordinate digestive functions:
- Secretin in released when chyme arrives in the duodenum. Secretin’s primary effect is an increase in the secretion of bile and buffers by the liver and pancreas.
- Cholecystokinin (CCK) is secreted when chyme arrives in the duodenum, especially when the chyme contains lipids and partially digested proteins, has a very potent effect in increasing contractility of the gallbladder, thus expelling bile into the small intestine.

- Gastric inhibitory peptide (GIP) is secreted when fats and carbohydrates – especially glucose – enter the small intestine, has a mild effect in decreasing motor activity of the stomach, therefore slowing the gastric contents into the duodenum when it is oversupplied with food products.
- Vasoactive intestinal peptide (VIP) stimulates the secretion of intestinal glands, dilates regional capillaries, and inhibits acid production in the stomach.
- Gastrin in secreted by G cells in the duodenum when they are exposed to large quantities of incompletely digested proteins. The functions of gastrin include promoting increased stomach motility and stimulating the production of acids and enzymes.
- Enterocrinin, a hormone released when chyme enters the small intestine, stimulates mucin production by the submucosal glands of the duodenum.
Physiology of the Large Intestine:
- Less than 10% of the nutrient absorption in the digestive tract occurs in the large intestine.
- The large intestine also prepares fecal material for ejection from the body.
- The reabsorption of water is an important function of the large intestine. Although about 1500 ml of material enters the colon each day, only about 200 ml of feces is ejected.
- In addition to reabsorbing water, the large intestine absorbs a number of other substances that remain in the feces or were secreted into the digestion tract along its length.
- Most of the bile salts entering the large intestine are promptly reabsorbed in the cecum and transported in blood to the liver for secretion into bile.
- Vitamins or organic molecules that are important as cofactors or coenzymes in many metabolic pathways. The normal bacterial residents of the colon generate 3 vitamins that supplement out dietary supply:
+ Vitamin K: a fat-soluble vitamin the liver requires for synthesizing 4 clotting factors, including prothrombin. Intestinal bacteria produce about half of your daily vitamin K requirements.
+ Biotin: a water-soluble vitamin important in various reactions, notably those of glucose metabolism.
+ Pantothenic acid: a water-soluble vitamin required in the manufacture of steroid hormones and some neurotransmitters.
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