What are the sites of nutrient digestion and absorption in the human GI tract?

What are the sites of nutrient digestion and absorption in the human GI tract?

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Where does nutrient digestion and then absorption take place for different nutrients along the alimentary tract?

The saliva from your mouth will slowly start to digest starches.

Then the stomach specializes in digesting protein with enzymes and its highly acidic content.

The small intestine will digest proteins, starchs and carbs (with the help of the enzymes traveling from the stomach).

The liver and pancreas secretions will join in the small intestine will help break down fats and complete digestion.

The small intestin is responsible of absorbing the vast majority of digested nutrients and the water is then mostly absorbed by the colon.

Source: Your Digestive System and How It Works

Figure 15.5.1 shows some of the cells of what has been called “the last human organ to be discovered.” This “organ” weighs about 200 grams (about 7 oz) and consists of a hundred trillion cells, yet scientists are only now beginning to learn everything it does and how it varies among individuals. What is it? It’s the mass of bacteria that live in our lower gastrointestinal tract.

Most of the bacteria that normally live in the lower gastrointestinal (GI) tract live in the large intestine. They have important and mutually beneficial relationships with the human organism. We provide them with a great place to live, and they provide us with many benefits, some of which you can read about below. Besides the large intestine and its complement of helpful bacteria, the lower GI tract also includes the small intestine . The latter is arguably the most important organ of the digestive system. It is where most chemical digestion and virtually all absorption of nutrients take place.


This chapter emphasizes the physiology of intestinal absorption. The transport protein responsible for absorptive function of the gastrointestinal tract resides in the apical side of the villous structure, which is involved in facilitating the transport of nutrients across the length of the small intestine. Recent advances in molecular cloning have led to further understanding of these transport proteins and how they are altered in the settings of health and disease states. The chapter will also discuss the intestinal transport of ions, short chain fatty acids, sulfate, oxalate as well as major nutrients such as carbohydrates, proteins, and fats. The chapter concludes with a discussion of the absorption of water-soluble vitamins, fat-soluble vitamins, minerals, and micro-nutrients.

3.2 Mouth to the Stomach

Digestion begins in the mouth, both mechanically and chemically. Mechanical digestion is called mastication, or the chewing and grinding of food into smaller pieces. The salivary glands release saliva, mucus, and the enzymes, salivary amylase and lysozyme.

Figure 3.21 The mouth1

Salivary amylase cleaves the alpha 1-4 glycosidic bonds in the starch molecules, amylose and amylopectin. However, salivary amylase cannot cleave the branch points in amylopectin where there are alpha 1-6 glycosidic bonds, as shown in the figure below. Overall this enzyme accounts for a minor amount of carbohydrate digestion.

Figure 3.22 Enzymatic action of salivary amylase. Purple arrows point to alpha 1-4 glycosidic bonds that can be cleaved. The yellow arrows point to the alpha 1-6 glycosidic bonds that cannot be cleaved

Lysozyme helps break down bacteria cell walls to prevent a possible infection. Another enzyme, lingual lipase, is also released in the mouth. Although it is released in the mouth, it is most active in the stomach where it preferentially cleaves short-chain fatty acids in the sn-3 position. Lingual lipase has a small role in digestion in adults, but may be important for infants to help break down triglycerides in breast milk2.

Now that the food has been thoroughly chewed and formed into a bolus, it can proceed down the throat to the next stop in digestion. It will move down the pharynx where it reaches a “fork in the road”, with the larynx as one road and the esophagus as the other. The esophagus road leads to the stomach this is the direction that food should go. The other road, through the larynx, leads to the trachea and ultimately the lungs. This is definitely not where you want your food or drink going, as this is the pathway for the air you breathe.

Figure 3.23 Cross section of face. The epiglottis covers larynx to prevent food and drink from entering the lungs3

Fortunately, our body was designed in such a way that a small tissue, called the epiglottis, covers the opening to the trachea. It directs the food down the correct road as shown below.

Figure 3.24 Epiglottis is like a traffic cop guiding food down the correct digestion road.

Before being correctly guided into the esophagus, the bolus of food will travel through the upper esophageal sphincter. Sphincters are circular muscles that are found throughout the gastrointestinal tract that essentially serve as gates between the different sections. Once in the esophagus, wavelike muscular movements, known as peristalsis, occur, as shown in the animation and video in the links below.

At the end of the esophagus the bolus will encounter the lower esophageal sphincter. This sphincter keeps the harmful acids of the stomach out of the esophagus. However, in many people this sphincter is leaky, which allows stomach acid to reflux, or creep up, the esophagus. Stomach acid is very acidic (has a low pH). The ruler below will give you an idea of just how acidic the stomach is. Notice that the pH of gastric (term used to describe the stomach) fluid is lower (more acidic) than any of the listed items besides battery acid.

Figure 3.26 pH of some common items4

The leaking of the very acidic gastric contents results in a burning sensation, commonly referred to as “heartburn.” If this occurs more than twice per week and is severe, the person may have gastroesophageal reflux disease (GERD). The following videos explain more about these conditions.

Table 3.21 Review of Chemical Digestion in the Mouth

Macronutrient Action
Carbohydrates Salivary amylase cleaves 1,4-glycosidic bonds
Lipids Release of lingual lipase
Protein None

References & Links

  1. Alan Hoofring,
  2. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.

Peristalsis Animation –

Acid Reflux –
GERD 101 –

From the Small Intestine to the Large Intestine

The process of digestion is fairly efficient. Any food that is still incompletely broken down (usually less than ten percent of food consumed) and the food’s indigestible fiber content move from the small intestine to the large intestine (colon) through a connecting valve. A main task of the large intestine is to absorb much of the remaining water. Remember, water is present not only in solid foods and beverages, but also the stomach releases a few hundred milliliters of gastric juice, and the pancreas adds approximately 500 milliliters during the digestion of the meal. For the body to conserve water, it is important that excessive water is not lost in fecal matter. In the large intestine, no further chemical or mechanical breakdown of food takes place unless it is accomplished by the bacteria that inhabit this portion of the intestinal tract. The number of bacteria residing in the large intestine is estimated to be greater than 1014, which is more than the total number of cells in the human body (1013). This may seem rather unpleasant, but the great majority of bacteria in the large intestine are harmless and many are even beneficial.

Digestion and Absorption Assignment Help

A common study topic for biology students, the concept of digestion can be defined as the process in which food consumed by human body reached the abdomen, excreting all the important nutrients and getting the same absorbed whereas the unused stuff gets excreted out. Digestive system is the entire subject that teaches the students about the breaking of food, its dissolution in water that further helps in generating energy for the human body. Although it might sound simple but understanding this process from scratch and writing the same in theory is not everyone’s cup of tea. Understanding the digestive system and its related functioning needs basic familiarity with morphology and anatomy of human body, along with understanding the function of organs which are associated with GI tract. BookMyEssay’s qualified assignment help tutors are capable and efficient enough to handle complicated matters like Digestion and Absorption assignment help so as to enable the student glide smoothly through his or her academic curriculum.

How Does Digestion Take Place?

As a common conception, humans assume that digestion of food happens in the stomach itself which is not entirely true. The process initiates from mouth and is taken forward to the esophagus finally making its way to the stomach. The entire process is accomplished when unnecessary materials get passed via anus travelling through intestines. The process of digestion is further classified into the following major steps which include:

  • Ingestion – This involves taking the food through the mouth.
  • Secretion – Generation of different fluids that simplifies the process of food breakup.
  • Mixing ad movement – This stage involves travel of food from mouth to stomach where it is mixed and moved into further smaller particles.
  • Digestion – This phase involves the process wherein important food components gets absorbed in the stomach.
  • Absorption – Post the process of digestion, absorption happens that starts from stomach wherein water and alcohol get absorbed pushing the rest of the process to be done in small intestine. Large intestine absorbs water and Vitamin B & K.
  • Excretion – The entire process ends when remaining stuff gets excreted from the body.

Important Glands and Their Participation in the Process of Digestion

The process of digestion also involves functioning of different types of salivary glands that majorly include buccal, labial, and parietal gland that are located in cheeks, lips, and palate respectively teamed up with lingual gland that is found in the tongue. Together all these gland work towards forming the saliva in a small amount that consists of dissolved gases, water, solutes, and organic substances namely lysozymes, uric acid, urea, mucous and immunoglobulin A, and salivary amylase. The water in saliva dissolves food, breaking down starch, whereas mucous works on lubricating the food, lysozymes kills bacteria and immunoglobulin A safeguards against attachment of microbes. Some of the important components related to digestion are described below:

  • Pharynx: A funnel-shaped tube, it has three parts namely laryngopharynx, nasopharynx, and oropharynx. It also features epiglottis that prevents food from reaching windpipe and whereas rest pushes the food from pharynx to esophagus.
  • Esophagus: It is located subsequent to trachea, and secrets mucous that transports food to the stomach.
  • Stomach: Located at the end of esophagus and left to the liver, it is also the largest portion in a human body. Different sphincters present in the stomach helps in controlling the course of food within as well as outside of the stomach.
  • Liver and Gallbladder: Also known as the heaviest gland is the recognised as a pear-shaped organ that has left lobes and right lobes connecting to the gallbladder which has 3 parts namely fundus, body and neck.
  • Small intestine: It is a long tube which assists the body in absorption of digested food. Absorption that is left undone in earlier compartment gets successfully completed in the small intestine.
  • Large intestine: lastly, as the food reaches to this last part, it gets final absorption, resulting in the production of vitamins, followed by excretion

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Milk digestion & absorption

Cows’ milk contains protein (whey and casein), fat (saturated, monounsaturated and polyunsaturated), carbohydrate (namely the disaccharide lactose, which is the only milk sugar present in natural milk), vitamins (thiamin, riboflavin and vitamin B12) and minerals (calcium, magnesium, phosphorus, potassium, selenium and zinc).

Milk composition can vary considerably between different cow breeds but also between cows of the same breed and herd.

Digestion releases the nutrients from cows’ milk in a usable form 1,2

The gastrointestinal (GI) system processes ingested milk into its molecular forms (digestion), ready for absorption and distribution by the circulatory system, as the larger macromolecules contained in milk are generally unable to cross the intestinal epithelium. This is achieved by the action of hydrochloric acid in the stomach, bile from the liver and a variety of digestive enzymes secreted by the exocrine glands into the lumen of the GI tract.

The mammalian GI tract is also inhabited by a large and complex community of commensal bacteria (the microbiota), which has co-evolved with its host and provides digestive benefits, protection against pathogens and regulation of the immune system, among various other functions.

How The Digestion And Absorption Of Vitamins Occurs?

Have you ever thought about importance of vitamins? How they work to keep you healthy and what happens if they don’t?

From a medical point of view neither vitamins are food nor medications but the most important thing we need to learn is that our life would not be possible without them. While fungi, bacteria and plants produce vitamins for themselves we are not able to, so we must provide another way of intake. These ways include varied diet. Both animal and plant sources are essential to ensure sufficient amounts of all kind of vitamins. Although, chemists have found a way to synthesize the same or similar compounds that compensate food intake through supplements. The deficiency leads to disease called hypovitaminosis (lack of vitamins) or avitaminosis (absence of vitamins).

Vitamins come in two types: fat-soluble (lipid-solubile) and water-soluble. The main differences between them are: different ways of transport, storage in body and elimination of the excess. While minerals are stored in your body only few of vitamins also stay as our guests in liver ( fat-soluble are stored in fatty tissues also) . These are fat-soluble vitamins and vitamin B12, as an exception.

Digestion and absorption of vitamins

Our body is clever, it acts like a machine that crushes food, processes, uses and stores vitamins and minerals. This process starts in mouth by chewing food. Saliva is responsible for chemical breakdown and allows next step – swallowing. Once you swallow it travels to your stomach where hydrochloric acid and other digestive enzymes are dealing with solid parts, shredding them and sends to small intestines where further digestion and nutrient absorption occurs. In the upper regions of small intestine begins the final part of digestion. Bile and pancreas enzymes give us fine particles able to pass between intestinal cells wall. This is called absorption and now through capillaries vitamins go to bloodstream. Journey continues by sending nutrients in liver to be used immediately or stored or sent to kidneys for elimination through urine.

Fat-soluble vitamins

There is a common belief that fats are unhealthy. If you are convinced in it too I must break it to you. It´s opposite, fats are necessary for usage of vitamin A, D, E and K. The whole process wouldn´t have been possible if fats are missing and the body would just eliminate this group of vitamins. Each type is responsible for different functions in organism.

  1. A – supports good vision and immune system functioning. We can find it in fish liver oil, butter, liver of animals, carrots, spinach. Deficiency can cause hair loss, dry eyes, blindness, reduce immune function.
  2. D – aids in bone health and development and supports immune system. Dietary sources : fish oil, fatty fish, egg yolks, mushrooms, fortified foods. Lack of vitamin D leads to increased bone fractures, weakened immune system and muscles, hair loss, more prone to infections, tiredness.
  3. E – as antioxidant protects body from free radicals that can cause cancer. It is found in sunflower seed and oil, almonds, hazelnuts… Deficiency leads to muscle weakness, vision problems, anemia, heart disease.
  4. K – main role is in blood clotting. Also, reduce risk of heart disease, high concentration of calcium in the blood… Dietary sources : kale, liver, butter, spinach, eggs. If this vitamin is missing it can cause bleeding and reduced bone density that can lead to fractures.

If you are thinking now how essential they are for your overall health and you maybe did not take this seriously you are right! But on the other hand, I would like to turn your attention to the fact that they are stored in our body so overdose can cause toxic conditions. It is important to follow recommended intake.

Water-soluble vitamins

  1. Vitamin B1 ( thiamine)
  2. Vitamin B2 (riboflavin)
  3. Vitamin B3 (niacin)
  4. Vitamin B5 (pantothenic acid)
  5. Vitamin B6
  6. Vitamin B7 (biotin)
  7. Vitamin B9
  8. Vitamin B12 (cobalamin)
  9. Vitamin C

You should try to provide them from your diet or supplements because they are generally not stored in the body. But also you shouldn´t worry about overdose because there is small probability for toxic effects because excretion is fast.

Functions, diet sources and deficiency

Thiamine is important for many essentials chemical reactions just like riboflavin and niacin. Pantothenic acid is the key for various metabolic functions. Vitamin B6 helps red blood cell formation, energy metabolism, synthesis of several neurotransmitters. Biotin as well as the others uses chemical reactions for fatty acid synthesis, glucose formation and amino acid metabolism. Vitamin B9 is necessary for the cell growth, DNA formation and amino acid metabolism. Cobalamin is involved in brain function maintenance, DNA synthesis… Vitamin C has a role in neurological functions, formation of red blood cells, organism defence. Dietary sources include various fruits, vegetables and meat. Deficiency cause severe problems, opposite of their function.

Be wise without help of years. Think now and take care about your health, because what more important than that?

The Digestive System Process

The digestive system consists of the gastrointestinal (GI) tract, the liver, the pancreas and the gallbladder. Food enters the mouth and passes through the body to the anus via the hollow organs of the GI tract, which consists of the mouth, esophagus, stomach, small intestine and large intestine. The organs work with a combination of nerves, hormones, bacteria and blood to complete the complex process of digestion.

According to the National Institute of Diabetes and Digestive and Kidney Diseases, digestion is important for breaking down food into nutrients, which your body uses for energy, growth and cell repair. In order for your blood to absorb vitamins and minerals and carry them to cells throughout your body, food and drink must be converted into smaller molecules. The nutrients include:

  • Carbohydrates: This macronutrient is grouped into two categories: simple carbohydrates, which include sugars found in foods such as fruit, vegetables and milk, and complex carbohydrates, which consists of starches and fiber found in whole grain breads and cereals, starchy vegetables and legumes.
  • Protein: A macronutrient found in foods such as meat, eggs and beans, which your body digests into smaller molecules called amino acids.
  • Fats: A macronutrient that's categorized into healthy fats, which consist of oils such as corn, canola, olive, safflower, soybean and sunflower, and less-healthy fats that are found in butter shortening and snack foods.
  • Vitamins: Micronutrients that are classified by the fluid in which they dissolve water-soluble vitamins include all B vitamins and vitamin C, while fat-soluble vitamins include vitamins A, D, E and K.
  • Minerals: Micronutrients that are elements in food that your body needs to function normally. Some minerals that are essential for health include calcium, potassium, magnesium and iron.

Biology and Chemistry in the Human Digestive System

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body.

The digestive system is constructed to perform its specialized function of turning food into energy needed to survive and packaging the residue for waste disposal. There are a variety of organs that work together, and here is an overview of the structure and function of this system.

The mouth is the beginning of the digestive tract, where digestion starts right after taking the first bite of food. Chewing breaks the food into pieces that are more easily digested, while saliva mixes with food to begin the process of breaking it down into a form your body can absorb and use.

Located in your throat (near the trachea (windpipe)), the esophagus receives food from your mouth when you swallow. The esophagus delivers food to your stomach through peristalsis, a series of muscular contractions.

The stomach is a hollow organ, allowing it to hold food while it is in the process of mixing with enzymes that continue the process of breaking down food into a usable form. Cells that line the stomach secrete a strong acid and powerful enzymes, which are responsible for the breakdown process. After the contents have been sufficiently processed, they are released into the small intestine.

The small intestine is made up of three segments: the duodenum, jejunum, and ileum. It is a 22-foot long muscular tube whose role is to break down food using enzymes released by the pancreas and bile from the liver. Peristalsis also plays a role, by moving food through and mixing with digestive secretions from the pancreas ad liver. The duodenum is responsible for the continuous breaking-down process, while the main role of the jejunum and ileum is the absorption of nutrients into the bloodstream.

Contents of the small intestine start out semi-solid, and end in a liquid form after passing through the organ. Water, bile, enzymes, and mucous contribute to the change in consistency. Once the nutrients have been absorbed and the leftover-food residue liquid has passed through the small intestine, it then moves on to the large intestine, or colon.

The pancreas secretes digestive enzymes into the duodenum, the first segment of the small intestine. These enzymes break down protein, fats, and carbohydrates. The pancreas also makes insulin, secreting it directly into the bloodstream. Insulin is the chief hormone for metabolizing sugar.

The liver has multiple functions, however its main function within the digestive system is to process the nutrients absorbed from the small intestine. Bile from the liver secreted into the small intestine plays an important role in the digestion of fat. In addition, the liver is the body&rsquos chemical &ldquofactory.&rdquo It takes the raw materials absorbed by the intestine and makes all the various chemicals the body needs to function. The liver also detoxifies potentially harmful chemicals. It breaks down and secretes many drugs.

The gallbladder stores and concentrates bile, and then releases it into the duodenum to help absorb and digest fats.

The rectum is a chamber that connects the colon to the anus. It is the rectum&rsquos job to receive stool from the colon, to let you know there is stool to be evacuated, and to hold the stool until evacuation happens. When anything comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not.

The anus is the last part of the digestive tract. The anus is the last part of the digestive tract. It is a 2-inch long canal consisting of the pelvic floor muscles and the two anal sphincters (internal and external). The lining of the upper anus is specialized to detect rectal contents. It lets you know whether the contents are liquid, gas, or solid. The anus is surrounded by sphincter muscles that are important in allowing control of stool. It consists of the pelvic floor muscles and the two anal sphincters. The anus is what ultimately excretes our feces.

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body.

Watch the video: Το πεπτικό σύστημα του ανθρώπου- Η πορεία της τροφής (July 2022).


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