The topic of this supporting document is PHYSIOLOGY FUNDAMENTALS and what we will be discussing is the path that nutrients take through the body, using the example of the nutrients from a chicken sandwich. This path involves four steps:
1) Digestion and Absorption
2) Transport through body
3) Function and metabolism in the cells and organs
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4) Elimination of metabolic wastes
Most of the material in this document is based on chapter 3 of the textbook.
Digestion and absorption refer to the processes of breaking food down into its component parts, specifically into small molecules that can be absorbed into the body. This process involves mechanical processes for breaking down food, into small particles, as well as the breakdown of chemical bonds in the molecules found in food. The body has a digestive system that is designed to optimize these processes.
The digestive system or gastrointestinal tract is essentially one very long tube starting at the mouth and ending at the anus – the many organs that make up this tube are marked with a star. In addition to this long tube the digestive system also includes accessory organs, marked with a blue arrow, that secrete important compounds that aid in the digestion of food.
The space inside the tube is called the lumen.
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It takes time for food to travel the whole length of this very long tube. This is called the transit time; food components that are digestible will travel from the mouth and be absorbed at some point along the length of the small intestine. Indigestible components, that cannot be absorbed, travel from the small intestine into the large and are eliminated in the feces.
One way of measuring transit time is to incorporate a non-absorbable dye into a meal and to measure how long it takes before that dye appears in the feces.
The nutrient that most influences transit time is dietary fibre.
Transit time
• time it takes food to
pass the length of the GI tract, typically 24 – 72 hours in a healthy adult.
Digestible food→small intestine →absorption
Indigestible food→full length of digestive tract→ elimination in feces.
Digestion of the chicken sandwich, begins in the mouth. Chewing results in the mechanical breakdown of the sandwich and the mixing of saliva and food, now referred to as a bolus ❶Saliva contains lysozyme, an enzyme that kills bacteria and salivary amylase which begins the chemical process of breaking down starch. The bolus, which no longer looks much like a chicken sandwich, passes into the pharynx ❷, forces the epiglottis to cover the opening to the lungs to prevent choking, and then enters the esophagus ❸.
Next the bolus travels down the esophagus, and is pushed down toward the stomach by a muscular action called persistalsis – consisting of alternating circular ❶ and longitudinal movement❷ of the smooth muscles in the esophagus.
Sphincter valves separate organs in the digestive tract, and help control the movement of digested food for one organ to the next. For example the gastroesophageal sphincter separates the stomach and the esophagus❸.
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The actions of the stomach are regulated by the brain; in fact it can be said that digestion begins in the brain because the sight and smell of food stimulates the secretion of gastric juices and increases the muscular activity (i.e. gastric motility) of the stomach❶. Stretching or distension of the stomach also send signals to the brain, that food is present in the stomach, and stimulates the secretion of a hormone called gastrin which also increases gastric juice secretion and motility. ❷
So a few minutes after you took your first bite out of the chicken sandwich, the bolus moves into the stomach which is ready to begin digestion. The stomach’s strong muscular structure shown here, allows for the mixing of the food, a mechanical process that facilitates the chemical reactions that hydrolyze food components. The stomach contains specialized cells that secrete hydrochloric acid (HCl) and enzymes which promotes hydrolysis of food components. The mixture of food and acidic gastric juices is called chyme.
The specialized cells that exist in the stomach are in structures called gastric pits❶; note that the word gastric describes anything related to the stomach. Parietal cells in the pits produce hydrochloric acid (HCl) and a protein called the intrinsic factor which is required for the absorption of vitamin B12 ❷. There are also chief cells ❸which produce an enzyme precursor, pepsinogen. The precursor is not able to catalyze reactions but when pepsinogen reacts with the HCl in the stomach, pepsin forms. This is an active enzyme, which begins the digestion of proteins to polypeptides. Pepsin begins to breakdown the proteins in the chicken sandwich, which is found in the chicken and bread. Salivary amylase no longer functions after exposure to the HCl in the stomach. Starch digestion resumes in the small intestine with enzymes secreted by the pancreas.
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In the stomach mucus (also called the mucosa or the mucosal layer ❹- Fig 3.7 above) coats the inner surface of the stomach to protect it from the hydrochloric acid that is secreted. When this protective layer is broken down, the acid damages the cells of the stomach and peptic ulcers can result. One of the leading causes of ulcers is infection by acid-resistant bacteria called Heliobacter pylori (H. pylori). As a result, antibiotic therapy is now a common treatment for ulcers.
The next step in digestion is the movement of the chyme out of the stomach through the pyloric valve ❶, which separates the small intestine from the stomach. Distension of the small intestine slows the motility of the stomach so the chyme can enter the small intestine at a controlled rate❷. The chyme is released very slowly over 2 to 6 hours so that bicarbonate present in the small intestine can neutralize the acidic chyme and to ensure thorough digestion and absorption
Self-Assessment Questions:
1. What is the main function of lysozyme?
a) To kill bacteria that may have been ingested with food.
b) To begin the mechanical breakdown of food in the mouth. c) To begin the chemical process of starch digestion.
d) It is a precursor of an important enzyme.
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Before continuing to read try these self- assessment questions.
Answers in document: Physiology- Fundamental-Self-Assess-ANSWERS
2. Some types of dietary fibre increase transit time. What does this mean?
a) Diets with this type of fibre will shorten the amount of time food takes to travel through the digestive system.
b) This type of fibre increases gastric mobility.
c) Diets with this type of fibre will lengthen the amount of time food takes to travel through the gastrointestinal tract. d) This type of dietary fibre is indigestible.
3. Which hormone stimulates the secretion of digestive juices and increases the action of the muscles of the stomach? a) Chyme
b) Gastrin
c) Pepsinogen
d) Hydrochloric acid
4. Which compound is necessary to activate pepsinogen?
a) A compound secreted by the mucus.
b) A compound secreted by the parietal cells. c) An acid secreted by the chief cells
d) An enzyme found in the saliva.
We have followed our chicken sandwich from mouth to the small intestine. In the small intestine most of the digestion occurs and nutrients are absorbed.
The structure of the small intestine maximizes the absorption of nutrients by increasing the surface area of the intestinal wall.
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This is done by structures that have a series of folds. As you can see here the small intestine has large circular folds.
or brush border
These folds are covered by finger like projections called villi (singular, villus).
The villus contains a blood vessel and a lymph vessel ❶called a lacteal ❷ which is connected to the lymphatic system. The nutrients in the chicken sandwich pass from the lumen ❸through a layer of mucosal cells ❹ on the surface of the villi and from there they enter either the blood or the lymphatic system.
Many of these mucosal cells have a specialized surface called microvilli or brush border that increases the surface area for absorption. Mucosal cells with this specialized structure are called enterocytes. So there are several structures that increase the surface area for absorption in the small intestine: circular folds, villi and microvilli.
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Enterocyte
Enzymes secreted by the small intestine and by the accessory organs results in the chemical breakdown of the food in our sandwich. In the small intestine, two hormones, secretin ❶ and CCK ❷ (cholecystokinin) play an important role in controlling the secretion of these digestive enzymes. Secretin stimulates the secretion of bile from the liver to the gallbladder (1A) and the secretion of bicarbonate ions by the pancreas (1B). A second hormone cholecystokinin stimulates the release of bile from the gallbladder (2A) and digestive enzymes from the pancreas (2B).
Enzymes secreted by the small intestine and by the accessory organs which include the pancreas, the gallbladder, and the liver, results in the chemical breakdown of the nutrients in our sandwich. The pancreas secretes bicarbonate ions❶ which neutralize the stomach acid in the chyme. The pancreas also secrete enzymes which helped with the breakdown of the macronutrients in our chicken sandwich e.g. amylase ❷ which results in the digestion of starch which is found mainly in the bread in our sandwich. Protein digesting enzymes such as trypsin and chymotrypsin ❸ break down the protein in the sandwich in the chicken and bread into shorter polypeptides. Lipase❹ breaks down dietary fats specifically the triglycerides into fatty acids and monoglycerides, both of which can be absorbed.
The function of another accessory organ the gallbladder is to store bile (or bile salts) ❶ which is produced by the liver. Bile aids in the digestion of fats by emulsifying it, which means physically breaking it up into very small droplets called micelles ❷. This makes it easier for the lipase enzyme to act on the fat so it can be absorbed.
The small intestine secretes a watery and mucus-containing “juice” that help mix the food and digestive enzymes.
Finally there are enzymes on the brush border ❸ of the enterocytes that further to break down the nutrients in our chicken sandwich. Enzymes breakdown the polypeptides into amino acids and further break down carbohydrates into monosaccharides. As the enzymes works to break down the macronutrient components of the sandwich, vitamins and minerals are also being released from the food material and they are also absorbed in the small intestine.
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So we have tracked the nutrients from the mouth to the lumen of the small intestine. The carbohydrates and proteins are broken down to monosaccharides ❶ and amino acids ❷, respectively. These nutrients leave the lumen, and enter the enterocyte. They are now considered absorbed and from there go into the blood stream (❷ + ❹); this is possible because amino acids and monosaccharides are water-soluble. This figure also illustrates the path of lipids which includes an important processing step. After the fatty acids and monoglycerides enter the enterocyte they are resynthesized into triglycerides and are packaged into a particle called a chylomicron❺. The chylomicron then enters the lacteal ❻.
❶The large intestine is made up of the colon and the rectum. Food components that are not absorbed in the small intestine pass through the ileocecal valve to enter the large intestine. Most of the nutrients in the chicken sandwich have been absorbed. What remains of the sandwich are its indigestible components; the fibre found in the vegetable toppings and in whole grain bread. This, along with water, now enters the large intestine. One of the main functions of the large intestine is to reabsorb water. When the large intestine is not functioning properly, as with an infection, diarrhea results and significant amounts of water are lost from the body, causing dehydration.
❷The large intestine is also home to a great many bacteria, called the microbiome. These bacteria are believed to be important for human health and are the subject of a great deal of research. Some of these bacteria are able to break down or metabolize some of the fibre that was in our sandwich, a process that’s called fermentation. ❸The remaining fibre that cannot be broken down by bacteria along with dead intestinal cells, mucosa bacterial cells and small amounts of water are excreted in the feces.
❹Diets high in fiber when consumed in combination with adequate amounts of water helps to soften the feces and prevent constipation.
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In summary here are the paths that different nutrients in our original chicken sandwich took in its journey from the mouth through the digestive tract. ❶We can see carbohydrates such as starches are converted initially into smaller carbohydrate fragments and then ultimately by brush border enzymes into monosaccharides (shown here as simple sugars) which enter the bloodstream. ❷The proteins in the sandwich are broken down first into polypeptides and then into amino acids which also enter the bloodstream. Water-soluble vitamins and minerals will also enter the bloodstream. ❸Triglycerides are broken down into individual fatty acids and monoglycerides and then repackaged into chylomicrons which enter the lymphatic system. Fat-soluble vitamins are also incorporated into chylomicrons, but fatty acids with a very short carbon chains, which are water soluble, enter the blood stream. ❹Dietary fibre which is indigestible may be partially broken down by bacterial enzymes or eliminated in the feces.
The next step for the nutrients in our chicken sandwich is their transport throughout the body and into cells. But before continuing to read, try these self-assessment questions about the digestive system.
Answers in document: Physiology-Fundamental-Self-Assess-ANSWERS
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Self-Assessment Questions
5. Which structure of the gastrointestinal tract helps to maximize the absorption of nutrients by increasing the surface area of the intestinal wall?
a) Gastric pit
b) Parietal cell
c) Microvillus d) Lacteal
6. Which statement best describes the action of CCK?
a) The release of digestive enzymes from the pancreas and the secretion of bile acids from the liver to the gall bladder. b) The release of bicarbonate from the pancreas and the secretion of bile acids from the liver to the gall bladder.
c) The release of digestive enzymes from the gall bladder and the secretion of bile acids from the pancreas into the small intestine.
d) The release of digestive enzymes from the pancreas and the secretion of bile acids from the gall bladder into the small intestine.
7. Which statement best describes the action of pancreatic lipase? a) Breaks down triglycerides into fatty acids and glycerol.
b) Breaks down a protein into peptides.
c) Breaks down triglycerides into fatty acids and monoglycerides. d) Breaks down starches into fatty acids and glucose.
8. What is the chylomicron?
a) It is a structure that contains bile acids and it aids the absorption of dietary fat.
b) It is a transporter protein that aids in the absorption of monosaccharides.
c) It is a structure that forms in the enterocyte and contains triglycerides and fat-soluble vitamins.
d) It is a brush border enzyme that catalyzes the formation of triglycerides from fatty acids and monoglycerides.
9.What does research indicate about the microbiota?
a) They probably have no impact on human health.
b) They can ferment dietary fibre.
c) They include the microorganisms that grows in the digestive tract. d) Both b) and
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Water-soluble and fat-soluble nutrient follow different paths after being absorbed. Amino acids, glucose, water- soluble vitamins and other water-soluble food components enter the blood stream and follow the hepatic portal vein❶ to the liver before being distributed to other tissues and organs.
Fat soluble components are packaged into chylomicron particles, enter the lacteal, the lymphatic system❷, then enter the blood stream, tissues and organs, before reaching the liver.
Here we see the how the capillaries from the digestive tract merge into the hepatic portal vein❶. Nutrients go to the liver – where some are stored, some metabolized, and others distributed to the rest of the body. For this reason the liver is often called a “gatekeeper” ❷.
For example, after a meal, the levels of absorbed glucose in the hepatic portal vein are very high, as expected. This glucose goes first to the liver, where In the case of glucose, after a meal, it some of it is converted to a polysaccharide called glycogen and stored in the liver. The remaining glucose is then released into the bloodstream to be used as a source of energy by the cells. Because some of the glucose was stored in the liver, the amount of glucose entering the blood from the liver is at a healthy level.
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The lymphatic system mainly functions as part of the immune system as lymphocytes or white blood cells circulation through its vessels. But it also functions in the absorption of fat. Absorbed dietary fat are incorporated into particles called chylomicrons in the enterocyte; these chylomicrons are too large to enter the capillaries in the villus, so they enter the lacteal, shown in green ❶. The chylomicrons travel in the lymphatic vessel until it can enter the blood stream at the left shoulder ❷. Chylomicrons continue in circulation and fat is removed from the particle and enters cells. What remains of the chylomicron after the fat has been removed is called a chylomicron remnant❸ and is processed in the liver.
Chylomicron remnants
Once in the cell, the nutrients participate in metabolism. In order to understand the key points of metabolism there are five components of the cell that will come up in lectures throughout the course:❶ the cell membrane, which is selectively permeable allowing some compounds to enter the cell easily while controlling or restricting the entry of others,❷ the cell nucleus, which contains DNA,❸ the cytosol, which contains various organelles and such as ❹ribosomes, which function in protein synthesis and ❺the mitochondria which generates energy for the cell, in the form of the compound, ATP.
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Metabolic Pathway
Nutrients are involved in metabolic pathways. A metabolic pathway, like the one shown opposite, is a series of reactions that convert compounds from one form into another and nutrients have important roles in these many pathways. Some nutrients take the role of compound 1 ❶, others support the conversion of compound 1 to product X, by being part of the enzymes that catalyze the pathways ❷.
There are two major types of metabolic pathways:
❶Catabolic pathways that breakdown macronutrients such as the glucose, amino acids and fatty acids into smaller molecules while creating ATP❷, adenosine triphosphate, an energy-containing compound. The ATP, g
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