Physiology of Gastrointestinal Tract

Gastrointestinal Tract (GIT) Physiology

• Functions:
  • Digestion of food
  • Absorption of nutrients, electrolytes, vitamins, and water

Mouth Physiology

• Mechanical and chemical digestion of food
• Source of unconditioned reflexes
• Control of physical and chemical properties of food
• Mechanical activity:
  • Mastication (chewing)
  • Anterior teeth: cutting action
  • Posterior teeth: grinding action
  • Maximal closing force: 15 kg (incissors), 50 kg (molars)
  • Innervations: 5th, 8th, 12th cranial nerves
  • Centers: near brain stem and cerebral cortex centers for taste
• Act of mastication:
  • Movement of the lower jaw down: contraction of muscles
  • Movement up: stretching of the muscles
  • Rebound of antagonists: inhibition of the jaw drops
  • Compression of the bolus of food against the linings of the mouth
• Functions of mastication:
  • Grinding the food
  • Mixing with saliva
  • Preventing excoriation of the GIT
  • Aiding swallowing
  • Aiding subsequent digestion

Salivation

• Adjustment of the food by saliva
• Salivary glands:
  • Parotid
  • Submandibular
  • Sublingual
  • Buccal
• Secretion of saliva:
  • Basal: 800-1500 ml/day
  • During food intake: increased secretion
• Regulation of salivary secretion:
  • Nervous: parasympathetic and sympathetic
  • Unconditioned reflexes: taste and tactile stimuli
• Composition of saliva:
  • 99.5% water
  • 0.5% substances (organic and inorganic)
• Functions of saliva:
  • Keeps the mouth moist
  • Aids speech
  • Facilitates swallowing
  • Solvent for molecules that stimulate taste buds
  • Solvent for irritating foods
  • Destroys bacteria (thiocyanate ions, proteolytic enzymes)

Swallowing (Deglutition)

• Three stages:
  1. Oral stage: voluntary
  2. Pharyngeal stage: automatic (1 second)
  3. Oesophageal stage
• Control of pharyngeal stage:
  • Swallowing center: in the medulla and lower pons
  • Afferent nerves: Vth, VIIth, IXth, Xth
• Oesophageal stage:
  • Oesophagus: first third striated muscle, last third smooth muscle, middle mixed
  • Innervation: n.vagus, sympathetic nerves, and others
  • Function: transport food from pharynx to stomach by gravity and peristalsis
• Regulation of oesophageal peristalsis:
  • Intrinsic neural circuits
  • Vagal efferent fibers
• Functions of upper and lower oesophageal sphincters:
  • Relaxation and contraction of the sphincters

Stomach Anatomy and Physiology

• Stomach: cardia, fundus, corpus, antrum, pyloric sphincter
• Smooth muscle layers: longitudinal, circular, transversal
• Innervation: myenteric plexus, submucosal plexus
• Vagal and sympathetic control

Gastric Motility

• Functions:
  1. Storage of food
  2. Mixing with gastric secretions
  3. Emptying into the duodenum
• Gastric slow waves: basal electric rhythm
• Velocity: 1-4 cm/s
• Hunger contractions: when the stomach is empty for a long time

Gastric Secretion

• 2.5-3 l of gastric juice daily
• Components:
  • Hydrochloric acid (HCl) from parietal cells
  • Pepsinogens from chief cells
  • Lipase
  • Intrinsic factor from parietal cells
  • Mucus from neck cells
• Regulation of gastric secretion:
  • Local, neural, and humoral mechanisms
  • Phases: cephalic, gastric, and intestinal

Intestinal Motility

• Movement of the small intestine
• Anatomy of the intestinal wall:
  • Serosa
  • Longitudinal muscle layer
  • Circular muscle layer
  • Submucosa
  • Mucosa
• Motility:
  • Local contractions: segmentation, pendular, villous
  • Propulsive: peristaltic waves
• Regulation of intestinal motility:
  • Neural: myenteric reflex, gastroenteric reflex
  • Humoral: acetylcholine, pilocarpine, physostigmine, serotonin, thyroxine, CO2

Ileocecal Valve

• Function: prevention of backflow of fecal contents from the colon into the small intestine
• Receptive relaxation: neural + gastrin
• Feedback control of the sphincter by reflexes from the caecum

Defecation

• Tonic constriction of:
  1. Internal anal sphincter (smooth muscle)
  2. External anal sphincter (striated muscle)
• Distention of the rectum: defecation reflex
• Center S2-S4: activation of parasympathetic nerve fibers
• Voluntary relaxation of the external sphincter
• Deep breath, closing the glottis, contraction of the abdominal wall muscles: expulsion of the fecal content### Metabolic Functions of the Liver
• Storage of glycogen (1-4% of liver weight)
• Gluconeogenesis
• Glycogenesis
• Glucostatic function of the liver
• Metabolism of fats (fatty acid oxidation, formation of ketone bodies, cholesterol, and phospholipids)
• Metabolism of proteins (oxidative deamination, urea formation, manufacture of plasma proteins, and formation of clotting factors)
• Cholesterol metabolism (synthesis from acetate, excretion in bile)
• Hormone metabolism (inactivation of hormones, e.g., angiotensinogen)
• Iron and vitamin metabolism (storage of ferritin, synthesis of vitamins A, B, B12, and 25-hydroxycholecalciferol)

Detoxification Function of the Liver

• Excretion of bilirubin
• Detoxification of ammonia, indole, skatole, alcohol, nicotine, and other substances

Thermoregulatory Function of the Liver

• Heat production

The Bile

• Product of the liver, modified by the gallbladder
• Composition: bile pigments (biliverdin and bilirubin), bile salts (cholic acid, deoxycholic acid, and lithocholic acid), cholesterol
• Functions: reduction of surface tension, emulsifying function, formation of micelles, and facilitation of fat absorption

Regulation of Biliary Secretion

• Neural regulation (parasympathetic and sympathetic nerves)
• Humoral regulation (cholecystokinin and secretin)

Functions of the Bile

• Neutralization of gastric HCl
• Facilitation of fat digestion and absorption
• Excretory function (excretion of bilirubin, cholesterol, and other substances)

Regulation of Food Intake

• Motivation: subjective feeling of hunger
• Centers: lateral hypothalamus (feeding center) and ventromedial hypothalamus (satiety center)
• Stimuli: glucostatic cells, afferentation from the GIT, ambient temperature, blood temperature, metabolic condition, emotions, and hormones

Control of Water Balance

• Motivation: subjective feeling of thirst
• Center: lateral hypothalamus
• Stimuli: osmotic receptors, volumoreceptors, and periphery (dry mucosa in oral cavity)

Nutritional Aspects in Special Groups

• Newborns and sucklings (high caloric requirements, high fat intake, iron and vitamin D supplements)
• Children (high caloric requirements, proportion of fat and carbohydrates in diet)
• Pregnant women (increased caloric requirements, folic acid and iron supplements)
• Lactating women (increased caloric requirements, high carbohydrate and protein intake)
• Elderly people (decreased caloric requirements, difficulty with chewing and swallowing)
• Sportsmen and physically active people (increased energy expenditure, carbohydrate loading)

Metabolism

• Breakdown of organic molecules to produce energy for various biological work
• Energy expenditure: heat production and work (external and internal)
• Basal metabolic rate (BMR): minimum energy expenditure to maintain vital functions
• Factors influencing BMR: age, gender, body size, body temperature, ambient temperature, emotions, physical activity, food intake, and hormones
• Measurement of BMR: direct and indirect calorimetry, respiratory quotient, and energetic equivalent

Ontogeny of the Digestive System

• Prenatal period: histotrophic and hemotrophic nutrition, swallowing of amniotic fluid
• Postnatal period: lactotrophic nutrition, sucking reflex, salivation, and swallowing reflex

Gastrointestinal System in Elderly People

• Oral cavity: hyposalivation, mastication disorders, and swallowing disorders
• Stomach: decreased motility and HCl secretion, atrophic gastritis
• Small intestine: decreased surface area for absorption, increased fibrotic tissue
• Large intestine: decreased motility, constipation, and irritable colon
• Liver: decreased blood flow, slowed metabolic rate, and decreased elimination of medications

Thermoregulation

• Maintenance of body temperature ((homeothermy) through heat production and heat loss
• Heat production: metabolism, muscle contraction, and shivering
• Heat loss: radiation, conduction, convection, and vaporization
• Thermoregulatory mechanisms: neural and humoral mechanisms

Hormonal Changes in Cold and Hot Environments

• Cold environment: increased thyroid hormone, noradrenaline, and vasopressin
• Hot environment: decreased metabolic rate, reduced muscular activity, and increased sweat rate### Prevention of Cold Stress and Hypothermia in Neonatal Care
• Exposure to a cool environment can result in cold stress and pathophysiological changes
• Lowered body temperatures are inversely correlated with survival
• Neutral Thermal Environment: a range of ambient temperatures within which the metabolic rate is minimal and thermoregulation is achieved by basal physical processes alone
• In adults, the Neutral Thermal Environment is 25-30°C, but in newborns, it is at a higher temperature
• Prevention of heat loss is crucial, and incubators can be used to achieve this

Physiology of Fever

• Fever: an increase in body temperature due to immunologic reactions, by increasing the set point of the central thermostat with a defensive role
• Hyperthermia: a condition where heat production exceeds heat dissipation
• Mechanisms of Fever:
  • Pyrogens (substances initiating fever): microorganisms, viruses, protozoa, toxins, necrotic cells, and cancer cells
  • Fever effects: hematologic-immunologic, metabolic, and effector mechanisms
  • Role of fever: defensive mechanism, activation of the immune system, and inhibition of growth of some microorganisms

Exercise Physiology

• Quantification of exercise intensity:
  • Energy consumption: mild (120-450 kcal/h), heavy (450-600 kcal/h), and super heavy (>600 kcal/h)
  • Oxygen consumption: at rest (250 ml/min), maximum (up to 3000 ml/min)
• Regulation of physiological function in exercise:
  • Neural regulation: autonomic nervous system (ANS), changes in ANS before exercise
  • Humoral regulation: adrenal medulla, hypophysis, and pancreas
• Effects of exercise on the cardiovascular system:
  • Heart rate: rapid-onset increase during exercise, recovery time dependent on intensity
  • Stroke volume and cardiac output: increase during exercise
  • Blood pressure: systemic and pulmonary, distribution of blood flow to muscles and other organs
• Ventilation and metabolism:
  • Ventilation: increase during exercise, proportionate to oxygen intake
  • Oxygen consumption: aerobic capacity, oxygen debt, and anaerobic threshold

Effects of Training on Physiological Parameters

• Effects of training on the cardiovascular system:
  • Increase in stroke volume and cardiac output
  • Decrease in heart rate at rest and during exercise
  • Increase in aerobic capacity
• Effects of training on the respiratory system:
  • Increase in lung volumes and capacities
  • Increase in maximum oxygen intake
• Effects of training on the skeletal muscle system:
  • Increase in muscle strength and endurance
  • Changes in muscle fiber type and metabolism

Physiology of Skeletal Muscle

• Anatomy and histology of skeletal muscle fibers
• Biochemical characteristics of skeletal muscle:
  • Myosin and actin filaments
  • Mechanisms of excitation and contraction
• Mechanisms of excitation:
  • Neuromuscular junction and neurotransmission
  • Action potential of skeletal muscle
• Mechanisms of contraction:
  • Excitation-contraction coupling
  • Role of calcium ions in contraction

Gastrointestinal Tract (GIT) Physiology

• Functions:
  • Digestion of food
  • Absorption of nutrients, electrolytes, vitamins, and water

Mouth Physiology

• Mechanical and chemical digestion of food
• Source of unconditioned reflexes
• Control of physical and chemical properties of food
• Mechanical activity:
  • Mastication (chewing)
  • Anterior teeth: cutting action
  • Posterior teeth: grinding action
  • Maximal closing force: 15 kg (incissors), 50 kg (molars)
  • Innervations: 5th, 8th, 12th cranial nerves
  • Centers: near brain stem and cerebral cortex centers for taste
• Act of mastication:
  • Movement of the lower jaw down: contraction of muscles
  • Movement up: stretching of the muscles
  • Rebound of antagonists: inhibition of the jaw drops
  • Compression of the bolus of food against the linings of the mouth
• Functions of mastication:
  • Grinding the food
  • Mixing with saliva
  • Preventing excoriation of the GIT
  • Aiding swallowing
  • Aiding subsequent digestion

Salivation

• Adjustment of the food by saliva
• Salivary glands:
  • Parotid
  • Submandibular
  • Sublingual
  • Buccal
• Secretion of saliva:
  • Basal: 800-1500 ml/day
  • During food intake: increased secretion
• Regulation of salivary secretion:
  • Nervous: parasympathetic and sympathetic
  • Unconditioned reflexes: taste and tactile stimuli
• Composition of saliva:
  • 99.5% water
  • 0.5% substances (organic and inorganic)
• Functions of saliva:
  • Keeps the mouth moist
  • Aids speech
  • Facilitates swallowing
  • Solvent for molecules that stimulate taste buds
  • Solvent for irritating foods
  • Destroys bacteria (thiocyanate ions, proteolytic enzymes)

Swallowing (Deglutition)

• Three stages:
  1. Oral stage: voluntary
  2. Pharyngeal stage: automatic (1 second)
  3. Oesophageal stage
• Control of pharyngeal stage:
  • Swallowing center: in the medulla and lower pons
  • Afferent nerves: Vth, VIIth, IXth, Xth
• Oesophageal stage:
  • Oesophagus: first third striated muscle, last third smooth muscle, middle mixed
  • Innervation: n.vagus, sympathetic nerves, and others
  • Function: transport food from pharynx to stomach by gravity and peristalsis
• Regulation of oesophageal peristalsis:
  • Intrinsic neural circuits
  • Vagal efferent fibers
• Functions of upper and lower oesophageal sphincters:
  • Relaxation and contraction of the sphincters

Stomach Anatomy and Physiology

• Stomach: cardia, fundus, corpus, antrum, pyloric sphincter
• Smooth muscle layers: longitudinal, circular, transversal
• Innervation: myenteric plexus, submucosal plexus
• Vagal and sympathetic control

Gastric Motility

• Functions:
  1. Storage of food
  2. Mixing with gastric secretions
  3. Emptying into the duodenum
• Gastric slow waves: basal electric rhythm
• Velocity: 1-4 cm/s
• Hunger contractions: when the stomach is empty for a long time

Gastric Secretion

• 2.5-3 l of gastric juice daily
• Components:
  • Hydrochloric acid (HCl) from parietal cells
  • Pepsinogens from chief cells
  • Lipase
  • Intrinsic factor from parietal cells
  • Mucus from neck cells
• Regulation of gastric secretion:
  • Local, neural, and humoral mechanisms
  • Phases: cephalic, gastric, and intestinal

Intestinal Motility

• Movement of the small intestine
• Anatomy of the intestinal wall:
  • Serosa
  • Longitudinal muscle layer
  • Circular muscle layer
  • Submucosa
  • Mucosa
• Motility:
  • Local contractions: segmentation, pendular, villous
  • Propulsive: peristaltic waves
• Regulation of intestinal motility:
  • Neural: myenteric reflex, gastroenteric reflex
  • Humoral: acetylcholine, pilocarpine, physostigmine, serotonin, thyroxine, CO2

Ileocecal Valve

• Function: prevention of backflow of fecal contents from the colon into the small intestine
• Receptive relaxation: neural + gastrin
• Feedback control of the sphincter by reflexes from the caecum

Defecation

• Tonic constriction of:
  1. Internal anal sphincter (smooth muscle)
  2. External anal sphincter (striated muscle)
• Distention of the rectum: defecation reflex
• Center S2-S4: activation of parasympathetic nerve fibers
• Voluntary relaxation of the external sphincter
• Deep breath, closing the glottis, contraction of the abdominal wall muscles: expulsion of the fecal content### Metabolic Functions of the Liver
• Storage of glycogen (1-4% of liver weight)
• Gluconeogenesis
• Glycogenesis
• Glucostatic function of the liver
• Metabolism of fats (fatty acid oxidation, formation of ketone bodies, cholesterol, and phospholipids)
• Metabolism of proteins (oxidative deamination, urea formation, manufacture of plasma proteins, and formation of clotting factors)
• Cholesterol metabolism (synthesis from acetate, excretion in bile)
• Hormone metabolism (inactivation of hormones, e.g., angiotensinogen)
• Iron and vitamin metabolism (storage of ferritin, synthesis of vitamins A, B, B12, and 25-hydroxycholecalciferol)

Detoxification Function of the Liver

• Excretion of bilirubin
• Detoxification of ammonia, indole, skatole, alcohol, nicotine, and other substances

Thermoregulatory Function of the Liver

• Heat production

The Bile

• Product of the liver, modified by the gallbladder
• Composition: bile pigments (biliverdin and bilirubin), bile salts (cholic acid, deoxycholic acid, and lithocholic acid), cholesterol
• Functions: reduction of surface tension, emulsifying function, formation of micelles, and facilitation of fat absorption

Regulation of Biliary Secretion

• Neural regulation (parasympathetic and sympathetic nerves)
• Humoral regulation (cholecystokinin and secretin)

Functions of the Bile

• Neutralization of gastric HCl
• Facilitation of fat digestion and absorption
• Excretory function (excretion of bilirubin, cholesterol, and other substances)

Regulation of Food Intake

• Motivation: subjective feeling of hunger
• Centers: lateral hypothalamus (feeding center) and ventromedial hypothalamus (satiety center)
• Stimuli: glucostatic cells, afferentation from the GIT, ambient temperature, blood temperature, metabolic condition, emotions, and hormones

Control of Water Balance

• Motivation: subjective feeling of thirst
• Center: lateral hypothalamus
• Stimuli: osmotic receptors, volumoreceptors, and periphery (dry mucosa in oral cavity)

Nutritional Aspects in Special Groups

• Newborns and sucklings (high caloric requirements, high fat intake, iron and vitamin D supplements)
• Children (high caloric requirements, proportion of fat and carbohydrates in diet)
• Pregnant women (increased caloric requirements, folic acid and iron supplements)
• Lactating women (increased caloric requirements, high carbohydrate and protein intake)
• Elderly people (decreased caloric requirements, difficulty with chewing and swallowing)
• Sportsmen and physically active people (increased energy expenditure, carbohydrate loading)

Metabolism

• Breakdown of organic molecules to produce energy for various biological work
• Energy expenditure: heat production and work (external and internal)
• Basal metabolic rate (BMR): minimum energy expenditure to maintain vital functions
• Factors influencing BMR: age, gender, body size, body temperature, ambient temperature, emotions, physical activity, food intake, and hormones
• Measurement of BMR: direct and indirect calorimetry, respiratory quotient, and energetic equivalent

Ontogeny of the Digestive System

• Prenatal period: histotrophic and hemotrophic nutrition, swallowing of amniotic fluid
• Postnatal period: lactotrophic nutrition, sucking reflex, salivation, and swallowing reflex

Gastrointestinal System in Elderly People

• Oral cavity: hyposalivation, mastication disorders, and swallowing disorders
• Stomach: decreased motility and HCl secretion, atrophic gastritis
• Small intestine: decreased surface area for absorption, increased fibrotic tissue
• Large intestine: decreased motility, constipation, and irritable colon
• Liver: decreased blood flow, slowed metabolic rate, and decreased elimination of medications

Thermoregulation

• Maintenance of body temperature ((homeothermy) through heat production and heat loss
• Heat production: metabolism, muscle contraction, and shivering
• Heat loss: radiation, conduction, convection, and vaporization
• Thermoregulatory mechanisms: neural and humoral mechanisms

Hormonal Changes in Cold and Hot Environments

• Cold environment: increased thyroid hormone, noradrenaline, and vasopressin
• Hot environment: decreased metabolic rate, reduced muscular activity, and increased sweat rate### Prevention of Cold Stress and Hypothermia in Neonatal Care
• Exposure to a cool environment can result in cold stress and pathophysiological changes
• Lowered body temperatures are inversely correlated with survival
• Neutral Thermal Environment: a range of ambient temperatures within which the metabolic rate is minimal and thermoregulation is achieved by basal physical processes alone
• In adults, the Neutral Thermal Environment is 25-30°C, but in newborns, it is at a higher temperature
• Prevention of heat loss is crucial, and incubators can be used to achieve this

Physiology of Fever

• Fever: an increase in body temperature due to immunologic reactions, by increasing the set point of the central thermostat with a defensive role
• Hyperthermia: a condition where heat production exceeds heat dissipation
• Mechanisms of Fever:
  • Pyrogens (substances initiating fever): microorganisms, viruses, protozoa, toxins, necrotic cells, and cancer cells
  • Fever effects: hematologic-immunologic, metabolic, and effector mechanisms
  • Role of fever: defensive mechanism, activation of the immune system, and inhibition of growth of some microorganisms

Exercise Physiology

• Quantification of exercise intensity:
  • Energy consumption: mild (120-450 kcal/h), heavy (450-600 kcal/h), and super heavy (>600 kcal/h)
  • Oxygen consumption: at rest (250 ml/min), maximum (up to 3000 ml/min)
• Regulation of physiological function in exercise:
  • Neural regulation: autonomic nervous system (ANS), changes in ANS before exercise
  • Humoral regulation: adrenal medulla, hypophysis, and pancreas
• Effects of exercise on the cardiovascular system:
  • Heart rate: rapid-onset increase during exercise, recovery time dependent on intensity
  • Stroke volume and cardiac output: increase during exercise
  • Blood pressure: systemic and pulmonary, distribution of blood flow to muscles and other organs
• Ventilation and metabolism:
  • Ventilation: increase during exercise, proportionate to oxygen intake
  • Oxygen consumption: aerobic capacity, oxygen debt, and anaerobic threshold

Effects of Training on Physiological Parameters

• Effects of training on the cardiovascular system:
  • Increase in stroke volume and cardiac output
  • Decrease in heart rate at rest and during exercise
  • Increase in aerobic capacity
• Effects of training on the respiratory system:
  • Increase in lung volumes and capacities
  • Increase in maximum oxygen intake
• Effects of training on the skeletal muscle system:
  • Increase in muscle strength and endurance
  • Changes in muscle fiber type and metabolism

Physiology of Skeletal Muscle

• Anatomy and histology of skeletal muscle fibers
• Biochemical characteristics of skeletal muscle:
  • Myosin and actin filaments
  • Mechanisms of excitation and contraction
• Mechanisms of excitation:
  • Neuromuscular junction and neurotransmission
  • Action potential of skeletal muscle
• Mechanisms of contraction:
  • Excitation-contraction coupling
  • Role of calcium ions in contraction