Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin, M. R., & Sivoglazov, V. I. 2002

The Doctrine of Viscera (Splanchnology)
Digestive System
Digestion. Nutrients

For The Human Body to function properly, a regular intake of nutrients is essential—including Proteins, fats, CARBOHYDRATES, mineral salts, Vitamins, and Water. These nutrients serve as building blocks and an energy source necessary to replace dying Cells, support bodily growth, and sustain vital physiological Functions.

In its initial state, food cannot simply pass into the Blood AND Lymph to be utilized for various bodily functions. To be assimilated by the body, food undergoes mechanical and chemical Processing within the Digestive system. It is chewed and ground down, then mixed with digestive juices whose Enzymes break down nutrients into simpler elements that can be readily absorbed and utilized. Only water, minerals (salts), and vitamins are assimilated in their natural form.

The mechanical and chemical processing of food, along with its conversion into substances the body can assimilate, is known as Digestion.

All chemical compounds utilized by the body as building blocks and Energy Sources (proteins, carbohydrates, and fats) are referred to as nutrients.

Proteins consist of hydrogen, oxygen, carbon, nitrogen, sulfur, phosphorus, and other elements. In The Stomach and Small Intestine, dietary proteins are broken down into Amino Acids and their constituent parts, which are then absorbed and used to synthesize human-specific proteins. Of the 20 amino acids required by humans, nine are classified as essential because the human body cannot synthesize them. These are valine, Histidine, isoleucine, leucine, Lysine, Methionine, Threonine, Tryptophan, and phenylalanine.

These specific amino acids must be obtained through our diet.

Proteins containing a full Complement of amino acids, including all essential ones, are termed nutritionally complete proteins. The proteins found in milk, meat, fish, and eggs are among the most valuable. Conversely, proteins derived from corn, wheat, and barley are considered incomplete because they lack a full spectrum of Essential Amino Acids.

Carbohydrates, composed of hydrogen, oxygen, and carbon, enter the body primarily through vegetables, fruits, starches, and other plant-based foods. Complex carbohydrates are known as Polysaccharides. During digestion, polysaccharides are broken down into water-soluble Disaccharides and Monosaccharides. Monosaccharides (such as glucose and fructose) are absorbed into the bloodstream and utilized by the body as a source of energy and structural material.

Fats, consisting of carbon, oxygen, and hydrogen, possess a complex chemical Structure. Throughout digestion, fats are broken down into their fundamental components—glycerol and Fatty acids (such as oleic, palmitic, and stearic acids), which occur in fats in various combinations and proportions. The body can also synthesize fats from carbohydrates and protein breakdown products. Certain fatty acids cannot be produced internally (including oleic acid, as well as arachidonic, linoleic, and linolenic acids, which are found in vegetable oils). Fats are integral components of all cells, Tissues, and Organs, and also serve as rich energy reserves.

Dietary fiber, which consists of plant Cell wall material (Cellulose), is also present in food. Dietary fiber is not broken down by digestive enzymes, but it has a high water-holding capacity. This is crucial because, as swollen dietary fiber expands and stretches the walls of the Large Intestine, it stimulates peristalsis and the propulsion of intestinal contents toward the rectum.

Minerals likewise enter the body through food. These include salts containing calcium, phosphorus, potassium, sodium, sulfur, chlorine, iron, magnesium, and iodine. Many other elements are present in food in trace amounts and are therefore called Trace Elements.

A growing body requires a greater supply of mineral salts than an adult Organism, as these minerals participate in Bone tissue formation, organ growth, and the composition of blood Hemoglobin, gastric juice, Hormones, cell membranes, and nerve synapses.

Water, which accounts for up to 65% of total body mass in adults, forms a vital constituent of tissue fluid, blood, and the body's internal environment.

Vitamins are complex Organic compounds present in food in very small amounts. They are indispensable for metabolic processes, and their absence or deficiency leads to specific disorders known as avitaminoses.

Nutrition must fully support the body's plastic (anabolic) processes and meet its Energy Expenditure. The required quantity and nutritional Balance of Nutrients (proteins, fats, carbohydrates, minerals, and vitamins) depend on age, body weight, gender, and physical activity levels.

Energy expenditure within the body is measured in calories (or Joules). One calorie is defined as The amount of energy required to raise the Temperature of water by 1°C (1 calorie = 4.2 J). During Biological Oxidation within the body, 1 g of protein yields 4.1 kcal (kilocalories), 1 g of carbohydrates yields 4.1 kcal, and 1 g of fats yields 9.3 kcal.

Data regarding the Energy Requirements of workers across various occupations are presented in Table 8.

Class="center">Table 8 Daily energy requirements for individuals in various occupational categories (in kcal)

To meet the body's daily physiological needs, a light-duty diet should provide at least 80–100 g of protein, whereas heavy physical exertion requires 120–160 g. For children, protein intake calculated per 1 kg of body weight must be higher than for adults, because synthetic and growth processes occur much more intensively in a growing child's body. The total daily fat intake should be at least 50 g, comprising both animal and plant fats. Daily Carbohydrate Requirements range from 400 to 500 g.

The breakdown (digestion) of proteins, fats, and carbohydrates is driven by digestive enzymes (contained in digestive juices)—secretory products of the Salivary Glands, stomach, small and large intestines, as well as The Liver and Pancreas. Over the course of a day, the digestive system receives approximately 1.5 L of saliva, 2.5 L of gastric juice, 2.5 L of intestinal juice, 1.2 L of Bile, and 1 L of pancreatic juice.

Enzymes are essential components of digestive secretions. Thanks to digestive enzymes, proteins are broken down into amino acids, fats into glycerol and fatty acids, and carbohydrates into monosaccharides. Digestive enzymes are complex organic molecules that readily react with food substrates. They also act as biological catalysts, accelerating The breakdown of nutrients. Enzymes are categorized based on their targets: proteases break down proteins, lipases break down fats, and amylases break down carbohydrates. For enzymatic reactions to proceed effectively, specific conditions are required, including optimal BODY TEMPERATURE AND a specific medium reaction (acidic or alkaline).

The Organs of the digestive system also perform a motor (motility) function. Within the digestive tract, food is mechanically broken down and thoroughly mixed with digestive juices, ensuring close contact between the food mass and enzymes. This mixing, combined with peristaltic propulsion, facilitates continuous and intimate contact with the absorptive surface of the intestine, ensuring more complete absorption of digested food components. The propulsion of digestive contents toward the rectum promotes The formation of fecal masses and ultimately results in their elimination from the body.

DIGESTION IN THE Oral Cavity

The mechanical and chemical processing of food begins in the oral cavity. Here, food is chewed and its taste qualities are analyzed. The food is moistened with saliva, initiating the Hydrolysis of polysaccharides and the formation of a bolus. The average duration of food retention in the oral cavity is 15–20 seconds.

In response to The stimulation of taste, tactile, and temperature receptors located in the mucous membrane of the Tongue and the walls of the oral cavity, both large and small glands secrete saliva.

Saliva is a slightly turbid fluid with a mildly alkaline reaction. It consists of 98.5–99.5% water and 1.5–0.5% dry matter. The main component of the dry matter is mucin, a type of mucus. The higher the mucin content, the more viscous and thick the saliva is. Mucin facilitates the formation and binding of the bolus, making it easier to swallow and pass from the oral cavity into the Pharynx. In addition to mucin, saliva contains enzymes such as amylase and maltase, as well as Na⁺, K⁺, Ca⁺, Cl^- ions, and others. Under the action of the enzyme amylase in an alkaline environment, the breakdown of carbohydrates into disaccharides (maltose) begins. Maltase then breaks down maltose into monosaccharides (glucose).

Different types of food trigger varying amounts and qualities of saliva secretion. Salivation occurs reflexively. When food acts upon mechanical, chemical, and temperature receptors located in the walls of the oral cavity, nerve impulses are transmitted from them to the salivatory centers in the Brain. From the brain, response signals are sent to the salivary glands via the sympathetic and parasympathetic fibers of the Autonomic Nervous system. The sympathetic effector nerve endings release noradrenaline, which stimulates the secretion of a small amount of thick saliva. The parasympathetic nerve endings release acetylcholine, prompting the salivary glands to secrete a large volume of watery saliva. Saliva is released not only upon the direct contact of food with the nerve endings of the oral mucosa (an unconditioned reflex), but also conditionally—in response to olfactory, visual, auditory, and other stimuli (such as the aroma or color of food, or even talking about eating).

Swallowing is a complex reflex act. Once chewed and moistened with saliva, food is formed in the oral cavity into a bolus, which is pushed toward the Base of the tongue by the Movements of the tongue, Lips, and Cheeks. The ROOT of the tongue and the soft palate contain numerous sensory nerve endings; when stimulated by food, these impulses are transmitted to the Medulla Oblongata to the Neurons of the swallowing center (The Nucleus ambiguus of the glossopharyngeal and vagus nerves). From there, motor (efferent) nerve impulses travel via these nerves to the pharyngeal Muscles, triggering the act of swallowing. At this moment, the entrance to the Nasal cavity is closed by the soft palate, the epiglottis seals off the entrance to the Larynx, and breathing is temporarily suspended. If a person talks while eating, the passage from the pharynx to the larynx does not close properly, and food may enter the laryngeal lumen and respiratory tract. This is why one should not talk while eating.

From the oral cavity, the food bolus enters the oral part of the pharynx. At this point, the longitudinal Muscles of the pharynx (stylopharyngeus and salpingopharyngeus) elevate the pharynx, effectively pulling it over the food bolus, while the circular muscles (pharyngeal constrictors) contract to propel the food from the pharynx into the Esophagus. Wave-like contractions of the esophageal muscles then push the food down into the stomach. The entire journey from the oral cavity to the stomach takes 6–8 seconds for solid food and 2–3 seconds for liquids.

Digestion in the Stomach

Food that passes from the esophagus into the stomach remains there for up to 4–6 hours. During this time, it is digested through the action of gastric juice.

Gastric juice, produced by the gastric glands, is a transparent, colorless liquid with an acidic reaction due to the presence of Hydrochloric acid (HCl) at concentrations up to 0.5% (pH 0.9–1.5). It contains digestive enzymes such as Pepsin, gastricsyn, and lipase, along with an Abundance of mucus (mucin). Thanks to hydrochloric acid, gastric juice possesses strong bactericidal properties. Given that the gastric glands secrete 1.5–2.5 liters of gastric juice per day, food in the stomach is transformed into a liquid chyme.

The enzymes pepsin and gastricsyn digest (break down) proteins into larger particles—Polypeptides—which are too large to be absorbed by the gastric capillaries. Pepsin also curdles milk casein, which then undergoes hydrolysis in the stomach. The emulsified fatty components of milk are broken down by lipase.

Mucus (mucin) plays a vital role in the stomach. It protects the gastric mucosa from self-digestion and contains the intrinsic factor of Castle, which is necessary for the absorption of vitamin B₁₂ and the Formation of the anti-anemic factor. When alcohol enters the stomach, the protective effect of the mucus is weakened, creating favorable conditions for the formation of mucosal ulcers and inflammatory conditions such as gastritis.

The secretion of gastric juice begins just 5–10 minutes after THE START OF a meal and continues for as long as food remains in the stomach. The COMPOSITION OF THE gastric juice and its secretion rate depend on the quantity and quality of the food. Fats, concentrated sugar solutions, and negative emotions (such as anger or sorrow) inhibit and slow down The production of gastric juice. Conversely, meat and vegetable extracts (such as meat and vegetable broths) dramatically accelerate the formation and secretion of gastric juice.

Stimulants of gastric juice secretion include hormones released by endocrinocytes located in the walls of the digestive system—particularly the stomach and duodenum—as well as digested food products absorbed into the bloodstream.

Gastric juice is secreted not only during eating, but also at the mere sight or smell of food, or even when discussing a meal. In these cases, gastric juice is produced As a result of the body's conditioned reflex activity.

Gastric motility plays a crucial role in food digestion. Contractions of the smooth muscles of the stomach occur when food is present, as well as in an "empty" stomach. Two Types of muscular contractions of the stomach walls are distinguished: peristole and peristalsis. When food enters the stomach, its musculature contracts tonically, and the stomach walls tightly embrace the food mass. This action is known as peristole. During peristole, the gastric mucosa comes into close contact with the food, allowing the secreted gastric juice to immediately moisten the food adjacent to the walls. Peristaltic contractions of the musculature originate in the cardiac region of the stomach and propagate as waves toward the pylorus. These peristaltic waves mix the food and propel it toward the exit of the stomach into the duodenum. This mixing facilitates better moistening of the food with gastric juice and enhances digestion.

Muscular contractions also occur in an empty stomach. These "hunger contractions" appear every 60–80 minutes and are believed to be triggered by the sensation of hunger.

When spoiled food or strongly irritating substances enter the stomach, reverse peristalsis (antiperistalsis) occurs, resulting in vomiting, which serves as a protective reflex mechanism of the body.

The propulsion of food from the stomach into the small intestine occurs via peristaltic waves that drive the chyme toward the pylorus. If the pyloric sphincter is relaxed and the opening to the duodenum is open, the food passes into the duodenum. If the opening is closed, the chyme is thrust back from the pylorus deeper into the stomach to continue digestion. Once a portion of food enters the duodenum, its mucosa is stimulated by the acidic contents and the mechanical action of the food. In response, the pyloric sphincter reflexively closes the opening leading from the stomach into the intestine. After an alkaline reaction is established in the duodenum due to the release of BILE AND PANCREATIC juice, a new portion of acidic contents from the stomach is admitted. Thus, chyme is expelled from the stomach into the duodenum in discrete portions.

Digestion in the Small Intestine

The duodenum plays a unique role in digestion. Secretions not only from its own glands, but also bile and pancreatic juice, are poured into this initial section of the small intestine. The enzymes produced by the duodenal glands play an active role in digestion. Their secretion contains mucin, which protects the mucous membrane, as well as protein-digesting enzymes and enterokinase, which converts the inactive pancreatic enzyme trypsinogen into active Trypsin.

Pancreatic juice (the secretion of the pancreas) is colorless and has an alkaline reaction (pH 7.3–8.7). It contains various digestive enzymes that break down proteins, fats, and carbohydrates. Under the action of trypsin and Chymotrypsin, proteins are digested down to amino acids. Lipase breaks down fats into glycerol and fatty acids, while amylase and maltase digest carbohydrates into monosaccharides.

The secretion of pancreatic juice occurs reflexively in response to signals originating from receptors in the oral mucosa, beginning 2–3 minutes after the start of a meal. Subsequently, pancreatic juice is secreted in response to the irritation of the duodenal mucosa by the acidic chyme arriving from the stomach. The secretion of pancreatic digestive enzymes is also stimulated by the hormones secretin and pancreozymin, which are released by duodenal endocrinocytes in response to chemical and mechanical stimuli from food. These hormones reach the pancreas via the bloodstream from the Blood Vessels of the duodenum.

Bile, produced by the liver between meals, flows into the Gallbladder in a liquid state, where it is concentrated 7- to 8-fold through the absorption of water. During digestion, when food enters the duodenum, bile is released into it from the gallbladder. Golden-yellow in color, bile contains bile acids, bile pigments, Cholesterol, and other substances. Between 0.5 and 1.2 liters of bile are produced daily. Bile emulsifies fats into microscopic droplets to facilitate their absorption, activates digestive enzymes, slows down putrefactive processes, and enhances peristalsis in the small intestine.

Bile formation and its release into the duodenum are stimulated by the presence of food in the stomach and duodenum, as well as by the sight and smell of food, and are regulated via nervous and humoral pathways. Driven by peristalsis, the chyme moves from the duodenum into the jejunum and subsequently into the ileum. Intestinal juice, secreted by intestinal glands in response to mechanical and chemical stimuli (up to 2.5 liters per day), breaks down Peptides into amino acids and sugars into glucose and fructose. Intestinal juice contains 22 digestive enzymes, including enterokinase (the activator of pancreatic trypsinogen), peptidase, lipase, amylase, phosphatase, and sucrase. Digestion takes place both within the lumen of the small intestine (cavitary digestion) and On the surface of the microvilli of the brush border of the intestinal epithelium (parietal, or membrane, digestion) (Fig. 48). Parietal digestion represents The final stage of food breakdown, after which absorption begins.

The final digestion of food and the absorption of its breakdown products occur as the food mass progresses from the duodenum into the ileum and further toward the cecum. As a result of the contraction of the circular and longitudinal Muscle layers of the small intestinal walls, two types of movement take place: peristaltic and pendular movements. Peristaltic movements of the small intestine manifest as contraction waves that originate in its upper segments and travel all the way to the cecum. This process mixes the food mass with intestinal juice, accelerating both digestion and its propulsion toward the large intestine. During pendular movements, the muscle layers of a short segment of the small intestine alternately contract and relax, causing the food mass to shift back and forth within the lumen, which results in intensive mixing of the food.

Fig. 48. Relationship between luminal and membrane digestion (diagram) (Kositsky G.I., 1985):

a — intestinal lumen without food content, b — intestinal lumen with food content; 1 — enzymes in the intestinal lumen, 2 — microvilli, 3 — enzymes on the microvilli surface, 4 — pores of the brush border, 5 — microbe, 6,7 — food masses at various stages of hydrolysis

Digestion in the Large Intestine

From the small intestine, food residues that have not been absorbed into its Blood and Lymphatic capillaries pass through the ileocecal valve into the large intestine.

The large intestine absorbs water and digested food residues, and forms feces, which are subsequently eliminated from the body. The Glands of the large intestine also secrete digestive juices containing low amounts of enzymes and a large quantity of mucus, which is essential for the formation and excretion of fecal masses. Additionally, the large intestine harbors Bacteria whose enzymes break down and digest fiber (cellulose). At the same time, colonic bacteria synthesize vitamin K and B-complex vitamins.

Due to the peristaltic and antiperistaltic movements of its musculature, food masses are retained in the large intestine for up to two days. This promotes a more complete absorption of water and nutrients.

Up to 10% of ingested food (on a mixed diet) is not assimilated by the body. Food residues are bound together by mucus in the large intestine and compacted. The distension of the rectal walls by feces triggers the defecation reflex. The center for defecation is located in the sacral segment of the Spinal Cord.

Absorption

Digestion products—nutrients dissolved in water, salts, and vitamins—are absorbed into the blood and lymphatic capillaries of the small intestinal mucosa. The numerous villi of the mucous membrane and the microvilli of the small intestinal enterocytes create a massive absorptive surface area (approximately 200 m²). Thanks to their contracting and relaxing smooth muscle cells, the villi function like miniature suction pumps. Therefore, absorption is an active process requiring metabolic energy expended by enterocytes. Enterocytes transport amino acids and glucose from the intestinal lumen into the bloodstream while blocking unabsorbed proteins; fats, which are digested (broken down) in the intestine into glycerol and fatty acids, are absorbed into lymphatic capillaries. On their way to the lymphatic capillaries, glycerol and fatty acids within the epithelial cells form microscopic, water-soluble fat droplets. Fatty acids are rendered soluble with the help of bile acids, whereas glycerol is directly soluble in water. In the absence of bile acids in the intestine—for instance, due to biliary obstruction or liver disease—fat absorption fails, and fats are excreted in the feces.

The large intestine absorbs water and salts. Certain medications, such as Glauber's salt (sodium sulfate) and other sulfas, are poorly absorbed through the intestinal mucosa. When such drugs are ingested, the osmotic pressure in the intestine rises sharply, causing water to move from the blood into the bowel, where it distends the organ, enhances peristalsis, and exerts a laxative effect.

The stomach absorbs alcohol and certain medicinal substances, such as hypnotics (barbiturates) and aspirin (acetylsalicylic acid). Nutrients are not absorbed in the stomach because they have not yet undergone sufficient digestion. Minor absorption already takes place in the oral cavity. Among medicinal substances, nitroglycerin is absorbed through the oral mucosa.

REVIEW AND SELF-Control Questions:

1. What physiological processes are referred to as digestion? Describe these processes.

2. Which substances are classified as nutrients? What is their significance for the human body?

3. What do you know about the body's energy expenditure during various types of activity? Give Examples if you remember any.

4. What are digestive enzymes, and what role do they play in digestion?

5. Characterize the digestive processes taking place in the oral cavity, stomach, small intestine, and large intestine.

6. Explain what you know about the composition of saliva, gastric juice, bile, intestinal juice, and pancreatic juice.

7. Into what constituent compounds are proteins, fats, and carbohydrates broken down in the body? Why is the breakdown (digestion) of these substances necessary?

8. What is luminal digestion and membrane (contact) digestion? Describe these processes.

9. What functions do the muscle layers of the digestive organs perform during the digestion process? What are peristalsis, antiperistalsis, and pendular movements of the intestine?

10. Describe the absorption of digested food into the blood and lymphatic capillaries.