Nutrition, digestion, and absorption

Introduction

The gastrointestinal (GI) system supplies the fuel and the building blocks for the functioning of the body through the digestion and absorption of nutrients. Its machinery for doing so is the alimentary canal, also called the gastrointestinal tract, a highly specialized long tube that connects the mouth and the anus. While providing for the absorption of essential nutrients and elimination of waste products, the GI tract must also serve a critical barrier function in preventing the entry of toxic substances and organisms found in the environment (and therefore in the lumen of the digestive tract).

System structure

In general, the GI tract can be viewed as a hollow tube surrounded by a wall consisting of four layers starting from the inside ( Fig. 24.1 ):

The mucosa consists of an epithelial lining, a lamina propria (loose connective tissue, blood vessels, lymphatics), and a muscularis mucosae.
The submucosa is similar to the lamina propria but its connective tissue is denser and it contains nerves.
The muscularis propria contains two layers of smooth muscle.
- The inner layer of smooth muscle is circular (muscle fibers are oriented around the circumference of the gut).
- The outer layer of smooth muscle is arranged longitudinally (muscle fibers are oriented along the length of the gut).
- Interposed between these two layers is the myenteric nerve plexus. The neural innervation of the GI tract will be discussed in detail in Chapter 25 .
The serosa is a thin layer of connective tissue consisting of blood vessels, lymphatics, adipose tissue, and a simple squamous epithelium, sometimes called a mesothelium.

Fig. 24.1, Cutaway view of a segment of the gastrointestinal (GI) tract. The GI tract is essentially a hollow tube surrounded by a wall consisting of four main layers: from inside to out, the mucosa, submucosa, muscularis (composed of circular and longitudinal smooth muscle layers), and serosa.

In the intestine, the epithelial cells of the mucosa are called enterocytes. Enterocytes possess both a luminal (or apical) membrane, which faces the alimentary lumen, and a basolateral membrane, which faces the interstitial lamina propria and the blood supply.

Factors that aid in digestion are secreted across the luminal membrane, and nutrients are absorbed by facilitated diffusion and active transport across the luminal and then the basolateral membrane. Absorbed nutrients then enter the bloodstream and travel to the liver before they are distributed to the body tissues (see Fast Fact 24.1 ).

Fast Fact Box 24.1

This division of the intestinal epithelium into apical and basolateral membranes (with differing transporters on each side) is found in the epithelia of many organ systems. Other examples are the epithelium of the thyroid follicles and the epithelium of the renal tubules.

The abdominal portions of the digestive tract are supplied by the celiac and mesenteric arteries; venous blood collects into the portal vein, which then branches into a venous capillary bed within the liver. This is the portal circulation, which contain two capillary beds in sequence:

The capillaries supplying arterial blood to the digestive tract
The capillaries distributing the blood to the hepatic tissues for metabolism.

The oral cavity

Digestion begins in the oral cavity, which contains:

Tongue
Teeth
Salivary glands ( Fig. 24.2 ). Three paired salivary glands are associated with the oral cavity—the parotid, submandibular, and sublingual glands—and smaller salivary glands are scattered throughout the mouth.
- Within the glands are two types of secretory cells:
  - Serous (protein-secreting)
  - Mucous (mucus-secreting)

The stomach

The pharynx is a transitional zone conveying food from the oral cavity to the esophagus.

Food leaves the esophagus, passes through the lower esophageal sphincter, and enters the stomach.
The stomach consists of four main regions (in proximal to distal order) ( Fig. 24.3 ):
- Cardia
- Fundus
- Body
- Pylorus

On the macroscopic level, the stomach’s mucosa is bunched into ruggae (similar to the bladder), which increase the surface area of the stomach and allow for expansion and filling. On a microscopic level, the surface mucosa of the stomach invaginates to form gastric pits.

Emptying into the gastric pits are branched tubular gastric glands, whose function is different in each region of the stomach.
- The cardia, a narrow (<3 cm) band at the junction of the distal esophagus and stomach, contains glands that secrete mucus and lysozyme (which hydrolyzes bacterial cell walls).
- The glands of the fundus and body contain most of the stomach’s parietal cells, which populate the upper half of the gastric glands, and chief cells which populate the lower half.
  - Parietal cells which secrete hydrochloric acid (HCl).
  - Chief cells produce and secrete enzymes for the digestion of protein and fat (pepsinogen and lipase, respectively).

A small amount of absorption takes place in the stomach. However, most absorption of nutrients takes place after the partially digested food, or chyme, passes beyond the pyloric sphincter, which divides the stomach from the small intestine.

The small intestine

The small intestine is the last and most important site of food digestion and food absorption (but not the last site of water absorption). It consists of three segments (in proximal to distal order):

Duodenum
Jejunum
Ileum

These segments are coiled and tethered to the posterior wall of the abdominal cavity by the fatty mesentery, which contains the intestinal blood vessels.

The structure of the small intestine is well suited for digestion and absorption.

Its great length (approximately 5 m) permits prolonged contact between food and enzymes and between digested foodstuffs and the absorptive lining.
The small intestine also has a large surface area for digestion and absorption, achieved through numerous folds in the mucosa ( Fig. 24.4 ).
- The folds of the mucosa and submucosa are visible to the naked eye and are called plicae circulares.
  - The plicae circulares are in turn ruffled into finger-shaped outgrowths of mucosa called villi, which are around 1 mm long. The plicae circulares are most prominent in the jejunum, and their presence triples the surface area of the intestine.
  - Finally, microvilli are located on the luminal (apical) surface of absorptive cells in the small intestine. Each absorptive cell bears about 3000 microvilli (see Fast Fact Box 24.2 ).

Fast Fact Box 24.2

Each microvillus (1 µm tall by 0.1 µm in diameter) is a cylindrical protrusion of apical cytoplasm and cell membrane-enclosing actin filaments. The villi increase the surface area by approximately 10-fold and the microvilli by 20-fold.

Microvilli bear enzymes bound to their membranes. These enzymes hydrolyze complex carbohydrates and peptides into simple sugars and amino acids.

In between the villi, tubular glands (also called the crypts of Lieberkuhn) open onto the intestinal lumen.
The glands consist primarily of stem cells that divide and replace old epithelial cells and the other intestinal cells found on the villi:
- Mucus-producing cells (goblet cells)
- Lysozyme-producing cells (Paneth cells)
- Enteroendocrine cells.

Carrier molecules are present in the apical membrane of intestinal enterocytes, as are Na ⁺ ,K ⁺ -adenosine triphosphatase (ATPase) molecules. These proteins enable specific substances in the lumen to be absorbed into the enterocyte. In addition to absorption through the various carrier mechanisms, some nutrients or ions can be absorbed through the tight junctions between epithelial cells.

The blood vessels that remove the products of digestion from the small intestine penetrate the muscularis to form a submucosal capillary network. From there, branches of the plexus penetrate the muscularis mucosae and lamina propria to supply the villi.

At the tip of each villus is a capillary network that drains into veins of the submucosal plexus ( Fig. 24.5 ). (These veins drain into the mesenteric veins, which in turn empty into the portal vein that carries blood to the liver.)
Lymphatics (lymph vessels), which are important for the absorption of lipids, are also located in each villus. Lymphatics begin as blind-ended vessels within the villus. These vessels join and are known as lacteals.
- Lacteals anastomose to form the lymphatic drainage of the intestine, which eventually drains into the thoracic duct. The thoracic duct empties into the systemic venous circulation at the left subclavian and brachiocephalic veins.

The pancreas, liver, and gallbladder

The pancreas and liver are important secretory organs that empty their products into the duodenum.

The pancreas is both an exocrine organ, secreting substances out of the body and into the GI lumen, and an endocrine organ-secreting hormones into the body via the bloodstream. The exocrine pancreas is the portion that participates in digestion.

Like the parotid gland, the exocrine pancreas is organized into acini, which are small groups of serous (protein-secreting) cells clustered around a secretory duct.
Pancreatic acinar cells contain granules loaded with inactive digestive enzyme precursors called zymogens.
When stimulated, the acini dump their zymogens into the pancreatic duct, which empties into the ampulla of Vater.
The ampulla in turn leads directly to the duodenum.

The liver also plays a key role in digestion, mainly through its production of bile acids , which are essential for the digestion of lipids.

Once bile is produced, the gallbladder, a hollow pear-shaped organ on the undersurface of the liver, concentrates and stores it.
Bile is collected from the liver into the common hepatic duct, which communicates with the gallbladder via the cystic duct. At the point where the cystic and hepatic ducts join, they are called the common bile duct.
The common bile duct joins the pancreatic duct at the ampulla of Vater, which conducts bile and pancreatic secretions into the duodenum.
Some of the other critical roles of the liver will be discussed in Chapter 26 .

The large intestine

When all the foodstuffs that the body requires have been digested and absorbed, the remaining water, salt, and solids pass through the ileocecal valve into the large intestine ( Fig. 24.6 ).

The main function of the large intestine, also called the colon, is to absorb water and ions (Na ⁺ , Cl ⁻ ) that escape absorption in the small intestine.
Once the process of absorption of salt and water is complete, feces are formed from dietary residua.

Fig. 24.6, Anatomy of the large intestine (colon).

System function

A typical diet consists of carbohydrates, fats, and proteins, which are used to meet the maintenance, growth, and energy needs of the body. However, before the body is able to absorb and use what is ingested, these nutrients must first be broken down from macromolecules into their component building blocks. The process of breaking down ingested nutrients is called digestion.

Polysaccharides and disaccharides are processed into monosaccharides.
Triglycerides are digested into glycerol and fatty acids.
Protein is digested into its component amino acids.
Nucleic acids (deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]) are degraded into their constituent bases, which in some cases are further modified.

Although different enzymes and different intestinal locations are involved in the digestion of the various classes of nutrients, one basic chemical process is used in the digestion of all the major types of food: hydrolysis.

The general formula for hydrolysis is as follows, where R represents an undefined organic group:

R′′ − R′ + H ₂ O → R′′OH + R′H

During this process, substrate-specific enzymes catalyze the addition of water to a macromolecule, leading to its breakdown. The chemical structure of macromolecules and the process of carbohydrate, protein, and lipid hydrolysis is shown in Fig. 24.7 .

Fig. 24.7, Hydrolysis of macronutrients. Hydrolysis degrades the macronutrients into their building blocks.

Digestion is followed by absorption, wherein nutrients are transferred from the intestinal lumen into the bloodstream for delivery to the periphery. There are several recurring absorptive mechanisms, including:

Active transport: An energy-requiring process in which a substance is transported against its concentration gradient.
Diffusion: A substance is transported along its concentration gradient, thus requiring no energy expenditure.
Solvent drag: Water (the primary physiologic solvent) is absorbed in bulk quantities, “dragging” with it the solutes in the water.

We will consider, in turn, the mechanisms of digestion and absorption for each class of nutrients, along with the importance of nutrients to physiologic functioning and their sources in the diet.

Metabolism: The reason we eat

Metabolism encompasses the sum of all the energy transactions by which living tissues are produced and maintained. Metabolic processes are divided into two main categories:

Anabolic reactions: The synthesis of macromolecules from building blocks, a process requiring energy
Catabolic reactions: The breakdown of macromolecules into simpler substances, thereby releasing chemical energy which in turn fuels mechanical, heat, and electrical energy (see Fast Fact Box 24.3 ).

Fast Fact Box 24.3

Because the forms of energy are interchangeable, dietary energy is expressed in terms of heat—calories or kilocalories—even though most of that dietary chemical energy will be put to other uses. A calorie is defined as the amount of heat required to raise 1 g of water 1 °C in temperature. A kilocalorie (kcal) is defined as the amount of heat needed to raise 1 kg of water 1 °C in temperature.

All organisms store captured energy in various forms, such as starch (in plants) or glycogen and fat (in animals). If the diet does not provide enough calories to fuel metabolism, the body will catabolize these internal energy stores. When the internal energy stores are exhausted, metabolism will require catabolic breakdown of tissues that are not meant to store energy but nevertheless contain energy, such as the protein in muscle. Organic molecules yield energy in the following amounts:

1 g of protein yields 4 kcal
1 g of carbohydrate yields 4 kcal
1 g of fat yields 9 kcal
1 g of alcohol yields 7 kcal

Carbohydrates

Carbohydrates are a class of organic compounds made of carbon, hydrogen, and oxygen with hydrogen and oxygen present in the ratio of water (two atoms of hydrogen for each atom of oxygen).

Carbohydrates are made from various species of monosaccharides, which often have six carbons and form a ring.
Chains of monosaccharides are called polysaccharides.

Carbohydrates in the diet

Carbohydrates are the major source of calories in a typical diet. Although oxidation of carbohydrates is the preferred mode of energy extraction, carbohydrates are not essential in the diet because they can be manufactured by the body.

Food often contains both digestible and nondigestible carbohydrates.

The digestible dietary carbohydrates are starches, monosaccharides, and disaccharides (two monosaccharides linked together).
- Starches are complex polysaccharides composed of long chains of glucose molecules linked by α-1,4 glycosidic bonds.
  - The most abundant starch is amylopectin, a plant starch.
  - Another dietary starch is glycogen, an animal starch.
- The main monosaccharides in the diet are glucose and fructose, found in fruit, soft drinks, and processed foods.
- Disaccharides include lactose (found in dairy products), maltose (found in beer), and sucrose (table sugar).
Nondigestible carbohydrates, referred to collectively as dietary fiber, include cellulose, a β-1,4-linked glucose polymer.
Cellulose is a major source of dietary fiber found in grasses and leaves.
It is not digestible because the enzyme capable of hydrolyzing β-1,4 glucose linkages is not present in the human intestine.

Carbohydrate digestion

The digestion of carbohydrates is critical because the intestines can absorb only monosaccharides, such as glucose, galactose, and fructose.

Carbohydrate digestion begins in the mouth, where food is ground into smaller pieces by the action of the teeth and tongue.

Salivary secretions contain salivary α-amylase, an enzyme that hydrolyzes α-1,4 glycosidic bonds (but not α-1,6 branchpoint linkages).
Fig. 24.8 summarizes the digestion of carbohydrates by α-amylase. Before swallowing occurs, approximately 5% of dietary starch is digested.

Carbohydrate digestion continues in the stomach until salivary α-amylase is inactivated by the stomach’s acidic environment.

It is estimated that 30% to 40% of dietary starch is digested before food leaves the stomach.
After leaving the stomach, the acidic and partially digested food and enzyme mixture called chyme enters the small intestine and mixes with pancreatic secretions containing pancreatic α-amylase, which is much more powerful than the salivary form.

Digestion to monosaccharides occurs in the duodenum and jejunum, which contain intestinal disaccharidases in the brush border of the epithelial membrane.

The major brush border enzymes are α-dextrinase, maltase, sucrase, and lactase, which hydrolyze disaccharides to glucose, fructose, and galactose ( Fig. 24.9 ).
About 80% of ingested carbohydrates are digested to and absorbed as glucose.
Most carbohydrate absorption occurs in the upper small intestine (duodenum and upper jejunum).

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