Absorption in the Small Intestine
The absorption of nutrients occurs partially by diffusion through the wall of the small intestine.
Describe the role played by the small intestine in the absorption of nutrients
- Digested food is able to pass into the blood vessels in the wall of the small intestine through the process of diffusion.
- The inner wall, or mucosa, of the small intestine is covered in wrinkles or folds called plicae circulares that project microscopic finger-like pieces of tissue called villi, which in turn have finger-like projections known as microvilli.
- The function of the plicae circulares, the villi, and the microvilli is to increase the amount of surface area available for the absorption of nutrients.
- Each villus transports nutrients to a network of capillaries and fine lymphatic vessels called lacteals close to its surface.
- villi: Tiny, finger-like projections that protrude from the epithelial lining of the intestinal wall.
- plicae circulares: These circular folds (known as the valves of Kerckring or the valvulae conniventes) are large, valvular flaps that project into the lumen of the bowel.
- diffusion: The act of diffusing or dispersing something, or the property of being diffused or dispersed; dispersion.
Examples of nutrients absorbed by the small intestine include carbohydrates, lipids, proteins, iron, vitamins, and water.
The Small Intestine
The small intestine is the part of the gastrointestinal tract between the stomach and the large intestine where much of the digestion of food takes place. The primary function of the small intestine is the absorption of nutrients and minerals found in food.
Intestinal villus: An image of a simplified structure of the villus. The thin surface layer appear above the capillaries that are connected to a blood vessel. The lacteal is surrounded by the capillaries.
Digested nutrients pass into the blood vessels in the wall of the intestine through a process of diffusion. The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue.
Structurally, the mucosa is covered in wrinkles or folds called plicae circulares—these are permanent features in the wall of the organ. They are distinct from the rugae, which are non-permanent features that allow for distention and contraction.
From the plicae circulares project microscopic finger-like pieces of tissue called villi (Latin for shaggy hair). The individual epithelial cells also have finger-like projections known as microvilli. The function of the plicae circulares, the villi, and the microvilli is to increase the amount of surface area available for the absorption of nutrients.
Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries ( amino acids and carbohydrates) and lacteals (lipids).
The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances, such as the proteins required by our body. The food that remains undigested and unabsorbed passes into the large intestine.
Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:
- Iron is absorbed in the duodenum.
- Vitamin B12 and bile salts are absorbed in the terminal ileum.
- Water and lipids are absorbed by passive diffusion throughout the small intestine.
- Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport.
- Fructose is absorbed by facilitated diffusion.
Section of duodenum: Section of duodenum with villi at the top layer.
Absorption of Monosaccharides, Amino Acids, Dipeptides, Tripeptides, Lipids, Electrolytes, Vitamins, and Water
Glucose, amino acids, fats, and vitamins are absorbed in the small intestine via the action of hormones and electrolytes.
Describe the process of absorption of nutrients in the small intestine
- Proteins are degraded into small peptides and amino acids (di- and tripeptides) before their absorption by proteolytic and digestive enzymes such as trypsin.
- Lipids (fats) are degraded into fatty acids and glycerol by pancreatic lipase.
- Carbohydrates are degraded into monosaccharide or oligosaccharide sugars by the action of amylase. Carbohydrates, such as cellulose, pass through the human intestinal tract undigested.
- Water and some water-soluble vitamins are absorbed by diffusion. Some electrolytes and water non-soluble vitamins require an active uptake mechanism.
- trypsin: A digestive enzyme that cleaves peptide bonds (a serine protease).
- lipase: Any of a group of enzymes that catalyses the hydrolysis of lipids.
- amylase: Any of a class of digestive enzymes that are present in saliva and that break down complex carbohydrates, such as starch, into simpler sugars, such as glucose.
During breastfeeding, the lactase enzyme breaks down lactose (milk sugar). However, lactase production ceases after weaning in most populations, so adults in those populations experience gastric discomfort or distress when eating dairy products.
Digestive Enzymes and the Small Intestine
The small intestine is where most chemical digestion occurs. Most of the digestive enzymes that act in the small intestine are secreted by the pancreas and enter the small intestine via the pancreatic duct.
The enzymes enter the small intestine in response to the hormone cholecystokinin, which is produced in the small intestine in response to the presence of nutrients. The hormone secretin also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid that comes from the stomach.
Small intestine: This image shows the position of the small intestine in the gastrointestinal tract.
The three major classes of nutrients that undergo digestion are:
- Proteins. These are degraded into small peptides and amino acids before absorption. Their chemical breakdown begins in the stomach and continues in the large intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time. Aminopeptidase and dipeptidase free the final amino acid products.
- Lipids (fats). These are degraded into fatty acids and glycerol. Pancreatic lipase breaks down the triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and the gall bladder. Bile salts attach to triglycerides to help emulsify them and aid access by pancreatic lipase. This occurs because the lipase is water soluble, but the fatty triglycerides are hydrophobic and tend to orient towards each other and away from the watery intestinal surroundings. The bile salts are the main thing that holds the triglycerides in their watery surroundings until the lipase can break them into the smaller components that can enter the villi for absorption.
- Carbohydrates. Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose ). Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Other carbohydrates pass undigested into the large intestine for further handling by intestinal bacteria.
Brush Border Enzymes
Brush border enzymes take over from there. The most important brush border enzymes are dextrinase and glucoamylase that further break down oligosaccharides. Other brush border enzymes are maltase, sucrase, and lactase.
Lactase is absent in most adult humans and so lactose, like most polysaccharides, is not digested in their small intestine. Some carbohydrates, such as cellulose, are not digested at all despite being made of multiple glucose units. This is because the cellulose is made out of beta-glucose that makes the inter-monosaccharidal bindings different from the ones present in starch, which consists of alpha-glucose. Humans lack the enzyme for splitting the beta-glucose-bonds, something reserved for herbivores and the bacteria from the large intestine.
The fat-soluble vitamins A, D, and E are absorbed in the upper small intestine. The factors that cause the malabsorption of fat can also affect the absorption of these vitamins. Vitamin B12 is absorbed in the ilium and must be bound to intrinsic factor, a protein secreted in the stomach, in order to be absorbed. If intrinsic factor is missing, then Vitamin B12 is not absorbed and pernicious anemia results.
Of the water-soluble vitamins, the transport of folate and B12 across the apical membrane are independent from sodium (Na+), but the other water-soluble vitamins are absorbed by Na+ co-transporters. In physiology, the primary ions of electrolytes are sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl−), hydrogen phosphate (HPO42−), and hydrogen carbonate (HCO3−).
Sodium is the main electrolyte found in extracellular fluid and is involved in fluid balance and blood pressure control. Electrolyte balance is regulated by hormones, generally with the kidneys flushing out excess levels. In humans, electrolyte homeostasis is regulated by hormones such as the antidiuretic, aldosterone, and parathyroid hormones.
Serious electrolyte disturbances, such as dehydration and over-hydration, may lead to cardiac and neurological complications that, unless they are rapidly resolved, will result in a medical emergency. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic—about 5.6 to 6.9.
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