Respiratory and Digestive System Flashcards

Regulation of Breathing: Respiratory Centers

Dorsal Respiratory Group (DRG): Located in the medulla; functions as the primary inspiration regulator. It controls the inspiratory muscles, specifically the diaphragm.
Ventral Respiratory Group (VRG): Located in the medulla; primarily involved in forced expiration. It regulates the intercostal and abdominal muscles and controls synergist and fixator muscles.
Pontine Respiratory Group (PRG): Located in the pons; regulates the breathing rate and pattern by adjusting the activities of the DRG and VRG in response to various sensory stimuli.

Modification of Respiratory Center Activity

Mechanoreceptors: * Includes stretch and pressure receptors that respond to changes in lung volume or arterial blood pressure ( $BP$ ). * Inflation Reflex: Over-inflation of the lungs causes mechanoreceptors to fire, which inhibits the inspiratory centers while simultaneously exciting the expiratory centers. * Deflation Reflex: Inhibits the expiratory center.
Chemoreceptors: * Respond to chemical changes in the blood and cerebrospinal fluid ( $CSF$ ). Stimulation increases both the rate and depth of breathing. * Central Chemoreceptors: Located in the medulla; they respond to changes in $Pco_2$ exclusively via changes in $\text{pH}$ . * Peripheral Chemoreceptors: Located in major blood vessels, specifically the carotid and aortic arteries; they monitor $Po_2$ .
Irritant Receptors: * Composed of free nerve endings in the epithelial lining of the airways. * Stimulated by particulate matter, smoke, chemical fumes, cold air, and excess mucus. * These receptors stimulate the DRG via the vagus nerve. The DRG can then initiate coughing, bronchoconstriction, shallow breathing, or breath-holding depending on the nature of the stimulus.
Higher Brain Centers: * Includes the limbic system, hypothalamus, and cerebral cortex. * Controls respiratory modifications during laughing, crying, and voluntary breath-holding.

Functions of the Gastrointestinal (GI) Tract

Ingestion: The intake of food.
Mechanical Processing: Physical breakdown of food.
Digestion: The chemical breakdown of food into absorbable units.
Secretion: The release of water, enzymes, acids, and buffers.
Absorption: The movement of organic and inorganic molecules into the body.
Excretion: Disposal of waste products.
Non-specific Immunity: Defense against pathogens.
Homeostasis: Maintenance of internal balance.

Gross Anatomy of the Digestive System

Mouth: Contains salivary glands. * Paratid Glands: Located in the cheek. * Submandibular Glands: Located in the jaw. * Sublingual Glands: Located under the tongue.
Pharynx: Includes the epiglottis, which prevents food flow to the lungs.
Esophagus: Conduit leading to the stomach.
Stomach: Site of gastric secretions and processing.
Pylorus: The opening from the stomach into the small intestine.
Small Intestine: Approximately $21\,ft$ total length. * Duodenum: Approximately $1\,ft$ long. * Jejunum: Approximately $8\,ft$ long. * Ileum: Approximately $1\,ft$ long; ends at the ileocecal sphincter (valve).
Large Intestine (Colon): * Cecum: Includes the appendix. * Ascending Colon * Transverse Colon * Descending Colon * Sigmoid Colon
Rectum
Anus: Includes internal and external anal sphincters.
Accessory Organs: Pancreas, liver, and gall bladder.

Processing of Food and Swallowing

Oral Cavity, Tongue, and Teeth: * Mastication: Chewing to increase surface area ( $SA$ ). * Sensory Analysis: Evaluation of food. * Lubrication and Secretion: Salivary amylase (also known as ptyalin). * Immune Functions: Initial defense. * Compaction: Formation of food into a bolus.
Swallowing (Deglutition): Consists of voluntary and involuntary responses in three phases: 1. Oral Phase: Includes elevation of the soft palate and movement of the bolus into the nasopharynx. 2. Pharyngeal Phase: Initiates the involuntary swallowing reflex and the closure of the epiglottis. 3. Esophageal Phase: Movements driven by peristalsis.

Movement of Digestive Materials

Peristalsis: * Sequential waves of contraction in the muscularis externa that force food down the GI tract. * Circular muscles behind the bolus contract to move food forward and prevent backflow. * Longitudinal muscles contract to shorten adjacent segments, creating pulses that force the food forward.
Segmentation: * Further fragments food in the small intestine ( $SI$ ). * There is no propulsive force and no specific pattern of contraction. * Increased segmentation is associated with irritable bowel or infection.

Functions and Histology of the Stomach

Bolus Entry: Food passes through the lower esophageal sphincter to enter the stomach.
Functions: 1. Mechanical breakdown of food items. 2. Temporary storage of food. 3. Chemical breakdown of proteins. 4. Production of intrinsic factor, which is necessary for Vitamin $B_{12}$ uptake. 5. Immunological destruction of pathogens.
Histology: * Lined with simple columnar epithelial tissue ( $E.T.$ ). * Gastric Pits: Cells produce alkaline mucus to protect the lining. These are exocrine glands, specifically merocrine glands, with a high mitotic rate. * Gastric Glands - Parietal Cells: * Secrete $HCl$ (hydrochloric acid) to lower stomach $\text{pH}$ . Low $\text{pH}$ kills pathogens, breaks down cell walls and connective tissue fibers, and activates proteolytic enzymes. * Secrete Intrinsic Factor for Vitamin $B_{12}$ absorption. * Secrete Ghrelin. * Gastric Glands - Chief Cells: * Secrete Pepsinogen (zymogens), the inactive form of the proteolytic enzyme pepsin. * Secrete gastric lipase.
Stomach Physiology: Chemical and mechanical digestion occur, but there is very little absorption because: 1. A mucus layer covers the epithelium. 2. Epithelial cells lack specialized transport mechanisms. 3. The gastric lining is impermeable to water. 4. Food is only partially digested and not yet ready for absorption.

The Small Intestine and Accessory Organs

Small Intestine Segments: * Duodenum: Receives chyme from the stomach, bile salts from the gall bladder, and pancreatic secretions for digestion and acid neutralization. * Jejunum: The site where most digestion and absorption occur. * Ileum: The final segment that controls release into the colon; stabilized by mesentery.
Neutralization and Digestion: * Stomach acids are neutralized by bicarbonate from the pancreas. * Enzymes: Amylases (carbohydrates), Lipases (lipids), Proteases (proteins, e.g., trypsin), and Nucleases ( $RNA$ and $DNA$ ). * Bile Salts: Originate from the liver and gall bladder to emulsify fat globules. * Absorption: Practically all absorption takes place in the small intestine before molecules enter the blood stream.
Surface Area Enhancement: * Plica, villi, and microvilli increase $SA$ . * A smooth tube would have an $SA$ of $3300\,cm^2$ , but with villi, it is $2\,million\,cm^2$ . * Each villus contains blood vessels, nerves, and lymph vessels. Lymph vessels (lacteals) are necessary for immunity and for transporting materials too large for capillaries (primarily fats).

Functions of Post-Stomach Accessory Organs

Pancreas: 1. Produces sodium bicarbonate to neutralize $HCl$ . 2. Provides the source for most digestive enzymes.
Liver: 1. Source for bile and salts. 2. "Housecleaning": Intestinal blood goes directly to the liver via the Hepatic Portal System. 3. Detoxifies blood and excretes waste (e.g., converting hemoglobin to bilirubin). 4. Excretes excess cholesterol. 5. Hormone production. 6. Production and storage of glucose and glycogen. 7. Storage of vitamins $A$ , $D$ , $K$ , and $E$ . 8. Blood reservoir and red blood cell ( $RBC$ ) recycling. 9. Immunity (complement system) and clotting factors. 10. Synthesis of blood plasma proteins. 11. Regulates osmotic concentration of blood.
Gall Bladder: Concentrates bile salts by a factor of $15\times$ .
Bile Salts Functions: * Emulsify fat into tiny micelles to increase $SA$ for enzyme action. * Aid in transport and absorption of digested fat through the intestinal mucosa to prevent clogging.

Regulation of Secretion and Digestion

Saliva Triggers: Controlled mostly by the parasympathetic nervous system; minor sympathetic influence.
Stomach Triggers: * Parasympathetic stimulation via the Vagus Nerve. * $HCl$ leads to pepsin activation. * Gastrin: Increases $HCl$ , pepsin, and motility. Triggered by the presence of food.
Small Intestine Hormones: * Secretin: Released in response to low $\text{pH}$ . Increases pancreas, liver, and gall bladder activity; decreases stomach motility and secretions. * Cholecystokinin (CCK): Released in response to increased lipids and proteins. Increases pancreas and gall bladder activity. High levels inhibit the CNS (satiety), stomach, pancreas, and gall bladder. * Gastric Inhibitory Peptide (GIP): Released in response to increased insulin levels. Inhibits gastric secretions and motility.

Digestion and Absorption Sites by Nutrient Type

Vitamins: Organic molecules serving as cofactors or coenzymes for chemical reactions. Cofactors must bind to an enzyme before it can bind to a substrate.
Carbohydrates (Starches): * Mouth: Less than $5\%$ via ptyalin. * Stomach: $30\text{-}40\%$ via active ptyalin for up to an hour. * Small Intestine: Broken into maltose via pancreatic amylase. * Enterocytes of Brush Border: Maltase, sucrase, and lactase break down maltose, sucrose, and lactose into glucose, fructose, and galactose. * $80\%$ of carbs are broken into glucose. They are water-soluble and transported into the blood (Hepatic Portal System).
Lipids (Triglycerides, Phospholipids, Cholesterol Esters): * Mouth: Small amount via lingual lipase. * Stomach: Negligible. * Small Intestine: Most digestion occurs via pancreatic lipase, bile salts, lecithin, enteric lipase, phospholipase $A_2$ , and cholesterol ester hydrolase. * Broken into free fatty acids, monoglycerides, phosphate groups, and cholesterol. * Lipid-soluble; transported into lymph vessels via chylomicrons (specially coated fats).
Proteins: * Mouth: Negligible. * Stomach: $10\text{-}20\%$ via pepsin. * Small Intestine: Digestion via trypsin, chymotrypsin, carboxypolypeptidase, and elastase. * Enterocytes of Brush Border: Aminopolypeptidase, dipeptidase, and other peptidases. * $99\%$ of proteins pass into the blood as individual amino acids.

Functions of the Large Intestine (Colon)

Reabsorption of water.
Compaction and storage of feces.
Absorption of vitamins generated by bacteria (Vitamin $K$ for clotting, Biotin for glucose metabolism, and Vitamin $B_5$ for steroid hormones/neurotransmitters).
Reabsorption of bile salts for transport back to the liver.
Bilirubin Breakdown: Metabolized by bacteria; some to urine, some to feces (giving color). Bacteria also break down toxins for disposal in the liver.
Defecation Reflex: Controlled by the presence of material, pressure, irritation, and chemical fluidity in the colon.

Energy Homeostasis and Cellular Metabolism

Energy Balance: $\text{Energy Input} + \text{Conversion} = \text{Utilization} + \text{Output}$ . The body converts energy rather than producing it.
ATP Usage: Required even at rest for maintaining structure, composition of components, growth, secretion, and movement.
Energy Sources: * Carbohydrates: $4.2\,kcal/g$ (Preferred source). * Lipids: $9.4\,kcal/g$ (Readily used; preferred by resting muscles). * Proteins: $4.2\,kcal/g$ (Used as a last resort). * Nucleic Acids: Rarely used for energy.
Cellular Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ADP + P \rightarrow 36\,ATP - 38\,ATP$ . * Involves Glycolysis, the Citric Acid Cycle (TCA), and the Electron Transport Chain (using $NADH$ and $FADH_2$ ).
Metabolic Pathways: * Glycogenesis: Converting glucose to glycogen for storage (liver and skeletal muscles). * Glycogenolysis: Converting glycogen back to glucose. * Gluconeogenesis: Producing glucose from non-carbohydrate molecules.
Energy Output: Released in two forms: 1. Heat: Approximately $60\%$ . 2. Work: Approximately $40\%$ (Mechanical, chemical, and transport work using $ATP$ ).

States of Metabolism

Absorptive State: * Occurs during and up to $3\text{-}4$ hours after ingestion. * Nutrients in blood are plentiful; glucose is the primary energy source ( $80\text{-}90\,mg/dL$ ). * Anabolic State: Nutrients converted to macromolecules for storage (e.g., Glycogenesis).
Post-Absorptive State: * Occurs between absorption periods. * Rate of energy expenditure > absorption. * Catabolic State: Energy stores are mobilized via Glycogenolysis and Gluconeogenesis. * Glucose is prioritized for the CNS; other cells use fatty acids or glycogen.

Hormonal Regulation of Metabolism

Basal Metabolic Rate (BMR): Set by Thyroid Hormones ( $T_3$ and $T_4$ ), which increase cell respiration and protein synthesis.
Insulin: Secreted by pancreatic Beta cells. * Promotes energy storage synthesis during the absorptive state. * Stimulates glucose uptake and inhibits gluconeogenesis and glycogenolysis. * Stimulated by: increased plasma glucose, increased plasma amino acids, increased GIP, and increased parasympathetic activity. * Inhibited by: Sympathetic activity or Epinephrine.
Glucagon: Secreted by pancreatic Alpha cells. * Promotes mobilization of energy molecules during the post-absorptive state. * Increases gluconeogenesis and glycogenolysis. * Stimulated by: decreased plasma glucose, increased amino acids, and increased sympathetic activity/epinephrine.

Pathologies and Questions

Diabetes Mellitus: * Type I (IDDM): Juvenile onset; genetic; $10\text{-}15\%$ of cases. Insulin secretion is reduced or absent due to lost Beta cells. * Type II (NIDDM): Adult onset; reduced target cell responsiveness to insulin (receptor down-regulation or beta cell "burn out").

Questions & Discussion

Q: Tests show that glucose is absorbed by a patient if sucrose, lactose, or glucose is ingested but not if complex starches are ingested. Where is the likely digestion defect? Explain. * A: The likely defect is in the production or activity of amylases (specifically pancreatic amylase). Since the patient can absorb the resulting monosaccharides from ingested disaccharides (sucrose/lactose) or pure glucose, the problem must be the initial step of breaking down complex polysaccharides into the smaller units like maltose which occurs before the brush border enzymes act.