Overview of the digestive system
What is the most metabolically active organ?
Liver
When nutrients are transported from intestinal epithelial cells to the vascular system, which organ is first to receive them?
Liver
Describe the process of blood moving towards the digestive system
1. Blood leaves the right side of the heart by way of the pulmonary artery 2. Blood loses carbon dioxide and picks up oxygen in the lungs and returns to the left side of the heart by the pulmonary vein 3. Blood leaves the left side of the heart by way of the aorta 4. Blood may leave the aorta to go to upper body and head or to go to the lower body 5. Blood may go to the digestive tract and then the liver 6. Blood returns to the right side of the heart 7. Lymph from most of the body's organs including the digestive system, enters the bloodstream near the heart
Describe how nutrients travel to the liver
1. Vessels gather up nutrients from the digestive tract 2. Vessels merge into the hepatic portal vein, which conducts all absorbed materials to the liver 3. Hepatic artery brings a supply of freshly oxygenated blood from the lungs to supply oxygen to the liver's own cells 4. A network of large capillaries branch all over the liver making nutrients and oxygen available to all its cells and giving the cells access to blood from the digestive system 5. Hepatic vein gathers up blood in the liver and returns it to the heart
What is the average meal digestion time?
3-4 hours
What is a bolus?
A portion of food swallowed at one time
What happens to nutrient molecules which are small enough to be absorbed?
Any nutrient molecule small enough to be absorbed will become trapped among the microvilli in crypts
How big is the small intestine?
Approximately 3-4 metres long with a diameter of 2-3 cm
Which substance controls the release of bile into the small intestine?
Cholecystokinin
Fat and protein take longer to digest than carbohydrate does. When fat or protein is present, intestinal motility slows to allow time for its digestion. How does the intestine know when to slow down?
Cholecystokinin is released in response to fat or protein in the small intestine. In addition to its role in fat emulsification and digestion, cholecystokinin slows GI tract motility. Slowing the digestive process helps to maintain a pace that allows all reactions to reach completion. Hormonal and nervous mechanisms like these account for much of the body's ability to adapt to changing conditions.
Pancreatic secretions contain a mixture of enzymes to digest carbohydrate, fat and protein. How does the pancreas know how much of each type of enzyme to provide?
Enzyme activity changes proportionately in response to the amounts of carbohydrate, fat and protein in the diet. If a person has been eating mostly carbohydrates, the pancreas makes and secretes mostly carbohydrases; if the person's diet has been high in fat, the pancreas produces more lipases; and so on. Presumably, hormones from the GI tract, secreted in response to meals, keep the pancreas informed about its digestive tasks. The day or two of lag between the time a person's diet changes dramatically and the time digestion of the new diet becomes efficient explains why dietary changes can 'upset digestion' and should be made gradually.
When fat is present in the intestine, the gall bladder contracts to squirt bile into the intestine to emulsify the fat. How does the gall bladder get the message that fat is present?
Fat in the intestine stimulates cells of the intestinal wall to release the hormone cholecystokinin (CCK). This hormone, travelling in the blood to the gall bladder, stimulates it to contract, releasing bile into the small intestine. Cholecystokinin also travels to the pancreas and stimulates it to secrete its juices, releasing bicarbonate and enzymes into the small intestine. Once the fat in the intestine is emulsified and enzymes have begun to work on it, the fat no longer provokes release of the hormone and the message to contract is cancelled. (Fat emulsification can continue even after a diseased gall bladder has been surgically removed because the liver can deliver bile directly to the small intestine.)
The stomach normally maintains a pH between 1.5 and 1.7. How does it stay that way?
Food entering the stomach stimulates cells in the stomach wall to release the hormone gastrin. Gastrin, in turn, stimulates the stomach glands to secrete the components of hydrochloric acid. When a pH of 1.5 is reached, the acid itself turns off the gastrin-producing cells. They stop releasing gastrin, and the glands stop producing hydrochloric acid
What are the GI hormones?
Gastrin, secretin, cholecystokinin
Which vessel carries blood from the liver to the heart?
Hepatic vein
What is the function of bile?
It emulsifies fats.
Which of the following is a feature of peristalsis?
It is caused by alternate tightening and relaxing of circular and longitudinal muscles.
The five different processes involved in muscular digestion
Mastication (chewing), peristalsis, stomach action, segmentation, sphincters
What is the primary site for absorption of nutrients?
Microvilli
What provides the large absorptive surface area in the small intestine?
Microvilli
What is the function of bicarbonate in the pancreatic juice?
Pancreatic juice contains sodium bicarbonate, which is alkaline, and this helps to neutralise the acidic chyme that enters the duodenum. From this point on, chyme remains at a neutral or slightly alkaline pH.
Where is the epiglottis located?
Pharynx
What are the accessory organs and their secretion?
Salivary glands - saliva, gastric glands - gastric juice, pancreas - pancreatic juice, liver - bile, gall bladder - bile, intestinal glands - intestinal juice
What is an important function of the intestinal villi crypts?
Secretion of juices into the small intestine
How do nutrients transverse the mucosal cell membrane?
Simple diffusion (water and small lipids), facilitated transport (water-soluble vitamins) require a specific carrier, active transport (glucose and amino acids) move against a concentration gradient which requires energy
Which action is associated with the presence of fat in the GI tract?
Slowing of the process of digestion and absorption
What does the large surface area of the small intestine facilitate?
The large surface area allows nutrients to enter mucosal cells and pass into the blood stream or lymphatic vessels (lacteals).
Why don't the digestive enzymes damage the pancreas?
The pancreas protects itself from harm by producing an inactive form of the enzymes. It releases these proteins into the small intestine where they are activated to become enzymes. In pancreatitis, the digestive enzymes become active within the infected pancreas, causing inflammation and damage to the delicate pancreatic tissues.
As the chyme enters the intestine, the pancreas adds bicarbonate to it so that the intestinal contents always remain at a slightly alkaline pH. How does the pancreas know how much to add
The presence of chyme stimulates the cells of the duodenum wall to release the hormone secretin into the blood. When secretin reaches the pancreas, it stimulates the pancreas to release its bicarbonate-rich juices. Thus, whenever the duodenum signals that acidic chyme is present, the pancreas responds by sending bicarbonate to neutralise it. When the need has been met, the cells of the duodenum wall are no longer stimulated to release secretin, the hormone no longer flows through the blood, the pancreas no longer receives the message and it stops sending pancreatic juice. Nerves also regulate pancreatic secretions.
What is a function of the intestinal microvilli?
Transport of nutrient molecules
What is the name of the projections on the inner surface of the small intestine?
Villi
Which of the following results from reverse peristalsis?
Vomiting
The pyloric sphincter opens to let out a little chyme, then closes again. How does it know when to open and close?
When the pyloric sphincter relaxes, acidic chyme slips through. The cells of the pyloric muscle on the intestinal side sense the acid, causing the pyloric sphincter to close tightly. Only after the chyme has been neutralised by pancreatic bicarbonate and the juices surrounding the pyloric sphincter have become alkaline can the muscle relax again. This process ensures that the chyme will be released slowly enough to be neutralised as it flows through the small intestine. This is important because the small intestine has less mucus coating than the stomach and so is not as well protected from acid.
Chyme can best be described as:
a semi-liquid mass of partially digested food released by the stomach into the small intestine
Exchange of oxygen, nutrients and waste materials takes place across the walls of small vessels called:
capillaries
The presence of fat in the intestines stimulates cells of the intestinal wall to release:
cholecystokinin
What is the hepatic portal vein?
collects blood from the GI tract and delivers it to capillaries in the liver
What is segmentation?
inner circular muscles contract and relax in a way that churns the chyme and moves it in both directions (occurs in the small and large intestines and dual movement allows greater mixing with the secretions of the intestines)
What is peristalsis?
inner circular muscles contract in the oesophagus rhythmically and involves one-way motion - pushing chyme forward
The hepatic portal vein empties into the:
liver
What are the functions of the lymphatic system?
o Drains excess interstitial fluid from tissues o Transports chylomicrons from the GI tract to the bloodstream o Carries out specific immune responses (filters out waste products and foreign bodies, produces lymphocytes, and antibodies).
How does the gut relate to certain health conditions?
o Gut conditions (IBS, clostridium difficile) o Obesity o Metabolic disease o Markers of inflammation o Markers of CVD (eg. BP and cholesterol)
What are the components of the lymph system?
o Lymph o Lymphatic vessels o Lymphatic tissues and organs
The purpose of bicarbonate in the digestive process is to:
raise the pH of chyme
What is the function of the stomach?
· Adds juices to the bolus and grinds it up to chyme · Has the thickest walls and strongest muscles of all GI tract organs· Three sets of muscles (longitudinal, circular, diagonal) alternately contract and relax to force the chyme downwards · The stomach can stretch to hold more than 20 times its empty volume · Rugae (gastric folds) allows the stomach to stretch and accommodate a meal
Define digestion, absorption, ingestion and excretion
· Digestion = the mechanical and chemical breakdown of food into absorbable units · Absorption = uptake of nutrients by the cells of the small intestine for transport into either the blood or the lymph · Ingestion = the process of taking food, drink, or other substances into the body by swallowing · Excretion = removal of waste and excess water from the body
What are the intestinal cells that facilitate nutrient absorption?
· Mucosal cells at different sites of the small intestine are specialised to absorb certain nutrients · Those nutrients that are quickly digested are generally absorbed higher in the small intestine · Very little absorption occurs in the large intestine ("colon")
What is the function of the sphincters?
· Sphincters periodically open and close to allow the contents of the GI tract to move along at a controlled pace · Upper oesophageal sphincter opens in response to swallowing · Lower oesophageal sphincter (cardiac sphincter) prevents reflux of stomach contents · Pyloric sphincter at the bottom of the stomach stays closed most of the time o holding the chyme in the stomach long enough for it to be thoroughly mixed with gastric juice and liquified o Prevents the intestinal contents from backing up into the stomach · the ileocaecal valve allows the contents of the small intestine to empty into the large intestine · the tightness of the rectal muscle is a kind of safety device; together with the two sphincters of the anus, it prevents elimination until you choose to perform it voluntarily
How do nutrients enter the blood via the lymph?
• Large fatty acids and fat-soluble vitamins are insoluble in water and will not enter capillaries • Mucosal cells bundle these lipids into chylomicrons. • Chylomicrons are released into the lacteals within the villi. • Chylomicrons bypass the liver and enter the circulatory system in the right lymphatic duct and thoracic duct (near the heart). • The lymphatic system provides a one-way route for fluid from the tissue spaces to enter the blood. • Unlike the vascular system, the lymphatic system has no pump; instead, lymph circulates between the cells of the body and collects into tiny vessels. • The fluid moves from one portion of the body to another as muscles contract and create pressure here and there. • Ultimately, much of the lymph collects in the thoracic duct behind the heart. • The thoracic duct opens into the subclavian vein, where the lymph enters the bloodstream.
What is the function of the small intestine?
• The three segments of the small intestine are the duodenum, the jejunum and the ileum. • The small intestine receives bile and digestive juices from the gallbladder and the pancreas
What is the difference between absorption of water-soluble and fat-soluble nutrients in the small intestine?
• Water-soluble nutrients and small products of fat digestion are released to the bloodstream. • Fat-soluble vitamins and larger fats form chylomicrons and are released to the lymphatic system
