D.2 Digestion Flashcards
Diagram of digestive system
What kind of mechanisms control the secretion of digestive juices?
-Mechanical
-Nervous
-Hormonals
What actions can trigger the secretion of saliva and gastric juice?
-Not only will the presence or smell of food stimulate the digestive system, but also thinking about it will make your body secrete saliva and gastric juice.
-This would prepare the body for the future intake of food.
The nervous control is mainly provided by the ___
Autonomic nervous system.
What is the autonomic nervous system divided into?
The sympathetic and parasympathetic systems.
What does the sympathetic system control?
Processes involved in responses to danger
What does the parasympathetic system control?
Homeostasis and processes related to rest and digestion.
What are the functions of the parasympathetic and sympathetic systems in digestion?
The parasympathetic system speeds up digestion when food is ingested, while the sympathetic slows it down when there is no food available.
Diagram showing the control of the digestive processes
What does having food in your mouth and gut induce?
The presence of food in your moth will induce the secretion of saliva and the presence of food in the gut will induce the secretion of gastric juice (especially after eating proteins), intestinal juice, pancreatic juice and bile (especially if you have eaten fats).
What are the hormones involved in the chemical control of digestion?
-Gastrin
-Secretin
-Cholecystokinin (CCK)
How are the hormones involved in the chemical control of digestion secreted and how do they travel?
They are secreted in the digestive system and travel through the blood to the target organs.
How is gastrin produced?
It is produced by special cells (called G cells) in the stomach, duodenum, and pancreas, in response to physical stimulation due to the presence of food, as well as to chemical stimulation by protein
What is stimulated when gastrin is released?
When gastrin is released, it stimulates the production of gastric juice by the parietal cells in the gastric glands.
What is gastric juice made up of?
A mixture of water, hydrochloric acid, and other inorganic ions, enzymes (pepsin, rennin), mucus, various polypeptides, and intrinsic factor.
What is intrinsic factor necessary for?
For absorbing vitamin B12.
What is the effect of gastric juice on the food content?
It changes the pH of the food content from 6.7 to 2, providing acidic conditions that will enhance digestion.
What will happen when there is sufficient gastric juice present (around 1 to 1.5 liters)?
The production of gastrin will stop and therefore so will the secretion of gastric juice.
Hormonal control of digestion
- Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids
- If stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
- When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones:
- Secretin and cholecystokinin (CCK) stimulate the pancreas and liver to release digestive juices
- Pancreatic juices contain bicarbonate ions which neutralize stomach acids, while the liver produces bile to emulsify fats
Give an example of an inhibitory hormone
Somatostatin
How is acid secretion regulated?
-By the nervous system, which causes direct stimulation of the parietal cells to secrete acid, and indirect stimulation by decreasing somatostatin secretion.
-In addition, a regulatory feedback mechanism exists whereby the presence of acid in the lumen of the stomach stimulates somatostatin secretion, which in turn slows down acid secretion.
What are exocrine glands?
They are glands that have ducts that carry their secretory product to the surface of the body or the lumen of the gut.
Give examples of exocrine glands
The sweat, sebaceous, and mammary glands and the glands that secrete digestive enzymes.
Diagram showing the development of an exocrine gland
What is the function of gastrin?
Stimulates the production of gastric juice.
Gastrin stimulates the production of gastric juice by the gastric glands, providing acidic conditions that will enhance protein digestion.
What is a function of somatostatin?
Preventing the release of gastrin.
Somatostatin directly inhibits acid-producing cells. It also acts indirectly by preventing the release of gastrin, CCK and secretin, thus reducing the digestive process.
How were the first studies into digestion in the stomach performed?
-In 1822 Alexis St Martin, an 18-year-old Canadian, was accidentally wounded by the discharge of a musket.
-Although the wound was serious and left a hole in his body, he survived.
-His doctor, William Beaumont, made thorough observation of what occurred in the stomach and recorded his observations between the years 1825 and 1833.
-He was able to extract gastric juice by introducing an elastic tube into the stomach through the wound. He then observed which foods were digested and under which conditions.
How is acid in the stomach produced?
-By the combination of hydrogen ions (H+) and chloride ions (Cl-) produced by the epithelial cells (known as parietal cells) of the stomach lining.
-The release of hydrogen ions through protein carriers is coupled to the intake of potassium ions (K+) from the lumen of the stomach.
-This process requires energy in the form of ATP.
Diagram of the production of acid in the stomach
What does gastric juice contain?
Hydrochloric acid and pepsin
What is pepsin?
-A protease that hydrolyses proteins to peptides.
-Pepsin is found in the stomach in an inactive form as the pro-enzyme pepsinogen.
How is pepsinogen released?
It is released by the chief cells in the lining of the stomach and is activated into pepsin by the presence of acid.
What functions do the acidic conditions in the stomach serve?
- Assists in the digestion of food (by dissolving chemical bonds within food molecules)
- Activates stomach proteases (e.g. pepsin is activated when pepsinogen is proteolytically cleaved in acid conditions)
- Prevents pathogenic infection (stomach acids destroy microorganisms in ingested food)
Acid conditions in the stomach also favour the ___
Hydrolysis reactions by pepsin.
Diagram showing the effect of pH on pepsin activity
.
How do acidic conditions affect bacteria and other pathogens?
-Acidic conditions help to control pathogens in ingested food.
-The bacteria present in food cannot survive such acid conditions.
Explain how stomach ulcers form and how they can be treated
- Helicobacter pylori is a bacterium that can survive the acid conditions of the stomach by penetrating the mucus lining
- H. pylori anchors to the epithelial lining of the stomach, underneath the mucus lining
- An inflammatory immune response damages the epithelial cells of the stomach – including the mucus-secreting goblet cells
- This results in the degradation of the protective mucus lining, exposing the stomach wall to gastric acids and causing ulcers
- The prolonged presence of stomach ulcers may lead to the development of stomach cancer over many years (20 – 30 years)
- H. pylori infections can be treated by antibiotics (previously, stomach ulcers were considered stress related and not treatable)
What is a stomach ulcer?
This is when the lining of the stomach is disrupted.
What are the main symptoms of a stomach ulcer?
Stomach pain, heartburn, nausea and in some cases presence of blood in the stools.
What can cause a stomach ulcer?
-In some cases (either because the patient has eaten spicy foods or excess proteins, or is very stressed) the stomach will produce an excess of gastric acid.
-This can damage the mucus layer, producing gastric problems which might develop into a gastric ulcer.
-Another cause of gastric ulcers is the presence of the bacterium Helicobacter pylori in the stomach.
How can H. pylori cause a stomach ulcer?
-It produces toxins that cause continuous gastric inflammation.
-The inflammatory response by the immune system damages the stomach lining.
What medications are used to treat ulcers due to H. pylori
-Amoxicillin
-Clarithromycin
-Proton Pump Inhibitors (PPIs)
What is amoxicillin?
An antibiotic that directly inhibits the synthesis of bacterial cell walls.
What is clarithromycin?
An antibiotic that prevents bacteria from growing by inhibiting the translation of peptides in the ribosome, thus inhibiting their protein synthesis.
What are proton pump inhibitors (PPIs)?
These inhibit the acidification of the stomach.
Explain how PPIs work
- Proton pump inhibitors (PPIs) are drugs that irreversibly bind to the proton pumps and prevent H+ ion secretion
- This effectively raises the pH in the stomach to prevent gastric discomfort caused by high acidity (e.g. acid reflux)
- Individuals taking PPIs may have increased susceptibility to gastric infections due to the reduction of acid secretion
Diagram showing the effect of a proton pump inhibitor
What causes the stomach to be acidic?
Release of hydrogen ions.
What is the optimal pH for pepsin?
2
What happens once food products are digested?
They are absorbed by the digestive system into the blood or lymphatic system and carried to different organs, especially the liver, where they are metabolised into other molecules.
What are polysaccharides digested to?
Monosaccharides
What are monosaccharides digested by?
The small intestine
What is the destination of monosaccharides?
Villus through hepatic portal vein to liver.
What are proteins digested to?
Amino acids
What are amino acids absorbed by?
-Stomach
-Small intestine
What is the destination of the amino acids absorbed by the stomach?
Cells of stomach lining
What is the destination of the amino acids absorbed by the small intestine?
Villus through hepatic portal vein to liver.
What are lipids digested to?
-Short-chain fatty acids
-Long-chain fatty acids
What are short-chain fatty acids absorbed by?
Small intestine
What are long-chain fatty acids absorbed by?
Small intestine
What is the destination of short-chain fatty acids?
Villus through hepatic portal vein to liver.
What is the destination of long-chain fatty acids?
Lacteal to lymphatic system.
Where does most absorption occur?
In the small intestine
What is the wall of the small intestine lined with?
Villi
What is the overall benefit of villi?
They increase the surface area for the absorption of substances
Each villus has a central lacteal whose function is mainly ___
The absorption of fats
What is the function of the capillaries that villi have?
They carry the rest of the absorbed molecules to other organs.
What do most capillaries in the villus join to form?
The hepatic portal vein (that carries blood to the liver)
The structure o the cells in the ___ is adapted to the absorption of food
Epithelium of the villus
What are the main adaptations of the cells in the epithelial cells of the villus?
-Increased surface area due to the presence of villi and microvilli.
-Many mitochondria to provide energy for active transport.
-Presence of capillaries in villi for absorption of digested foods.
-Presence of lacteal for absorption of absorbed lipids.
-Presence of pinocytotic vesicles aid the uptake of fluids.
-Presence of tight junctions to ensure molecules do not escape through the membrane, and to maintain a concentration gradient.
-Enzymes (peptidases and disaccharidases) bound to epithelial membranes to complete hydrolysis/digestion.
Diagram showing the structure of the villus
Electron micrograph of the epithelium of a villus showing epithelial cells with many microvilli (arrows) and goblet cells (asterisk)
How many layers does the wall of the small intestine have?
5
What are the five layers of the cross-section of the wall of the small intestine?
-Mucosa
-Submucosa
-Circular muscle layer
-Longitudinal muscle layer
-Serosa
Describe the mucosa
-Contains the epithelium formed by enterocytes, goblet cells, and endocrine cells.
-Enterocytes are cells that have microvilli; they digest and absorb substances.
-Goblet cells produce mucus and endocrine cells secrete hormones.
Describe the submucosa
Contains blood vessels and connective tissue.
Describe the circular muscle layer
Smooth muscle arranged in a circular manner
Describe the longitudinal muscle layer
Smooth muscle arranged in a longitudinal manner
Describe the serosa
Single layer of epithelial cells with connective tissue
Diagram of a cross-section of the small intestine
How are polysaccharides digested?
They are mainly hydrolysed to glucose and absorbed by epithelial cells of the small intestine.
How is glucose absorbed?
-It is co-transported with sodium (Na+) by facilitated diffusion.
-For this to happen, Na+ must first be transported out of the cell by active transport (along the basal membrane).
-This powers the co-transport of glucose and Na+ into the epithelial cells via the apical membrane.
-The glucose is then carried along the epithelial cell to a sugar transporter protein that allows its transport into the capillaries, which will carry it to the hepatic portal vein.
How are amino acids absorbed?
-Proteins are digested by proteases into amino acids, which are transported by co-transport with Na+ into the epithelial cells.
-Amino acid carrier proteins then carry them by facilitated diffusion into the capillaries.
Digestion of fats (lipids)
-Fats (or lipids) are emulsified in the intestines by bile.
-This transforms them into smaller droplets, allowing lipases to digest them into fatty acids and glycerol.
How is glycerol absorbed?
The glycerol will be absorbed and used in carbohydrate metabolism.
How are fatty acids absorbed?
-The fatty acids will diffuse into the small intestine epithelial cells.
-Long-chain fatty acids will combine with proteins to form chylomicrons.
-The chylomicrons travel to the border of the cell in the smooth endoplasmic reticulum and are expelled by exocytosis to later enter the lacteal, thus entering the lymphatic system.
Diagram summarizing the absorption of foods in the epithelial cells of the villus
Exocytosis in absorption
-Exocytosis is a form of active transport.
-Active transport requires energy; therefore villi epithelial cells are rich in mitochondria, as a lot of ATP is required.
What is a chylomicron?
A lipoprotein particle used to transport fats.
What is a function of enterocytes?
Absorption of glucose.
Glucose is co-transported with Na+ by facilitated diffusion into enterocytes.
By the time food reaches the large intestine, most of it ___
Has already been digested and absorbed.
What is the function of the large intestine?
The absorption of water, minerals, and vitamin K.
Insoluble fiber
-Insoluble fiber (e.g. wheat bran, fruit skin) is not digested so it has no nutritional value.
-Fiber is important because it increases fecal mass, thus stimulating peristaltic movements.
What is dietary fiber?
The non-digestible carbohydrates, especially cellulose, and lignin that are intrinsic and intact in plants.
Table showing the adequate fiber intake (AI) for children
What happens to materials that are not absorbed?
They are egested as feces.
What materials are not absorbed?
Cellulose and lignin from plant matter, bile pigments, the remains of intestinal epithelial cells and bacteria.
Peristalsis in the large intestine
Peristalsis of the walls of the large intestine will push the fecal matter towards the rectum and eventually eliminate it through the anus.
Relationship between the rate of transit of materials and their fiber content
The rate of transit of materials through the large intestine is positively correlated with their fibre content.
Graph showing the relationship between dietary fiber intake on the prevalence of constipation
Define constipation
Unsatisfactory defecation, characterised by infrequent bowel movement, difficult stool passage, or both.
What can infection of the intestines, due to a virus, bacterium, or parasite, cause?
-Inflammation of the epithelial cells lining the intestine.
-This will reduce the absorption of water, causing the feces to be loose or liquid (diarrhea).
-If the infection persists for several days it can cause dehydration.
What is cholera?
A disease caused by the ingestion of food or water contaminated with the bacterium Vibrio cholerae.
Why does cholera cause watery diarrhea?
-When cholera toxin is released from the bacteria in the infected intestine, it binds to the intestinal epithelial cells (enterocytes), triggering endocytosis of the toxin.
-The toxin then becomes an active enzyme that activates ions and water to leave the infected enterocytes, leading to watery diarrhea.
3D illustration of microscopic cholera bacteria infection
How many people are there in the world every year with cholera?
Approximately 4 million
Symptoms of cholera
Vomiting, muscle cramps, and profuse diarrhea.
What can cholera eventually lead to?
Given that the volume of water lost by the body is very large, this causes severe dehydration that can lead to death (approximately 100,000 per year worldwide).
Signs and symptoms of dehydration
-Increased thirst
-Dry mouth
-Swelling of brain
-Weakness
-Dizziness
-Palpitations
-Seizures
-Drop in blood pressure
-Fainting
-Decreased urine output
-Kidney failure
-Coma
-Death
What is the treatment for cholera?
The use of antibiotics to kill the bacteria and oral or intravenous rehydration.
What is mainly absorbed in the large intestine?
Water
What causes cholera infection?
A bacterium (Vibrio cholerae)
Outline how infection by Vibrio cholerae can lead to dehydration
-V. cholerae produces toxin
-«toxin» causes ions to be pumped into «small» intestine
-Drawing water into the intestine through osmosis
-Leading to water loss through diarrhea/vomiting
-Leading to dehydration
What happens if the stomach pH becomes too acidic?
Gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
Outline the role of nerves in the secretion of gastric juices
-Sight/smell of food stimulates brain
-Food entering stomach stimulates chemoreceptors/stretch receptors to send impulses/signals to brain
-Impulse/signal from brain causes cells in stomach lining/parietal cells to secrete
acid/HCl/gastric juice
-Brain sends impulses/signals «via vagus nerve» to endocrine cells in wall of stomach to release gastrin
-Gastrin stimulates «more» production of acid/HCl/gastric juice
Hormonal control of digestion
-Gastrin is secreted into the bloodstream from the gastric pits of the stomach and stimulates the release of stomach acids
-If stomach pH drops too low (becomes too acidic), gastrin secretion is inhibited by gut hormones (secretin and somatostatin)
-When digested food (chyme) passes into the small intestine, the duodenum also releases digestive hormones:
-Secretin and cholecystokinin (CCK) stimulate the pancreas and liver to release digestive juices
-Pancreatic juices contain bicarbonate ions that neutralize stomach acids, while the liver produces bile to emulsify fats
What is the difference between exocrine and endocrine glands?
Exocrine glands secrete their substances through ducts onto your body’s surface, while endocrine glands secrete their substances directly into your bloodstream.
The thought/sight of food acts on the ___ in the brain, sending an impulse to the stomach along the ___
Medulla oblongata
Vagus nerve
When is hormonal control of digestion activated?
Only when food is present in the stomach
Explain how stretch receptors work
-When food enters the stomach it causes distension, which is detected by stretch receptors in the stomach lining
-This acts on the medulla oblongata and it sends an impulse to the stomach via the vagus nerve to stimulate the gastric glands
