Study Notes: The Respiratory and Digestive Systems

Chapter 23: The Respiratory System

Structure and Function of the Respiratory System

The respiratory system is responsible for gas exchange, sound production, and protection of respiratory surfaces. It is divided anatomically and functionally into distinct regions.

• Functions:

  • Provides a large surface area for gas exchange between air and blood.

  • Moves air to and from the lungs along respiratory passages.

  • Protects respiratory surfaces from dehydration, temperature changes, and pathogens.

  • Produces sounds for communication.

  • Detects odors via olfactory receptors in the nasal cavity.

• Anatomical Divisions:

  • Upper respiratory system: nose, nasal cavity, paranasal sinuses, pharynx.

  • Lower respiratory system: larynx, trachea, bronchioles, alveoli.

• Functional Divisions:

  • Conducting zone: nasal cavity to larger bronchioles (no gas exchange).

  • Respiratory zone: smallest bronchioles and alveoli (site of gas exchange).

Defense Mechanisms of the Respiratory System

• Mucus traps particles and pathogens; cilia move mucus toward the pharynx (mucociliary escalator).

• Alveolar macrophages engulf small particles in alveoli.

• Excessive particles (e.g., from smoking) can overwhelm defenses, causing inflammation and mucus plugs.

• Tuberculosis: bacterial lung infection causing cough, chest pain, fever, fatigue, weight loss.

• Cystic fibrosis: genetic defect causing thick mucus, airway blockage, and frequent infections.

Cell Types in the Alveoli

• Pneumocytes type I: simple squamous cells forming the alveolar wall; site of gas diffusion.

• Pneumocytes type II: large cells producing surfactant (reduces surface tension, prevents alveolar collapse).

• Alveolar macrophages: patrol alveolar surfaces, engulfing debris and pathogens.

• Respiratory distress syndrome: inadequate surfactant leads to alveolar collapse after exhalation.

External and Internal Respiration

• External respiration: exchange of O2 and CO2 between lungs and blood.

  1. Pulmonary ventilation (breathing)

  2. Gas diffusion across alveolar and capillary membranes

  3. Transport of gases in blood

• Internal respiration: exchange of O2 and CO2 between blood and tissues.

• Hypoxia: low tissue oxygen levels.

• Anoxia: absence of oxygen in tissues (cell death).

Muscles of Inspiration and Expiration

• Inhalation (active):

  • Diaphragm contracts and flattens.

  • External intercostals elevate ribs.

  • Accessory muscles (sternocleidomastoid, scalene, pectoralis minor, serratus anterior) assist in deep or forceful inhalation.

• Exhalation:

  • Passive (resting): relaxation of diaphragm and external intercostals.

  • Active (forceful): internal intercostals, transversus thoracis, and abdominal muscles contract to depress ribs and compress abdomen.

Intrapulmonary and Intrapleural Pressure

• Intrapulmonary pressure: pressure within alveoli; varies with breathing (e.g., -1 mm Hg during inhalation, +1 mm Hg during exhalation).

• Intrapleural pressure: pressure in pleural cavity; always negative relative to atmospheric pressure (e.g., -4 mm Hg at rest, can decrease to -18 mm Hg during strong inhalation).

• Pressure differences drive airflow; cyclical changes assist venous return to the heart (respiratory pump).

Pressure and Volume Changes During Breathing

• Inhalation: thoracic volume increases, intrapulmonary pressure decreases, air flows in.

• Exhalation: thoracic volume decreases, intrapulmonary pressure increases, air flows out.

• Principle: Air moves from higher to lower pressure (Boyle's Law: $P_1V_1 = P_2V_2$).

Respiratory Rates and Volumes

• Respiratory rate: breaths per minute (normal adult: 12–18 bpm).

• Tidal Volume (TV): air moved per breath (~500 mL).

• Respiratory minute volume (VE): total air moved per minute.

  • Formula: $VE = \text{respiratory rate} \times \text{tidal volume}$

• Alveolar ventilation (VA): air reaching alveoli per minute.

  • Formula: $VA = \text{respiratory rate} \times (\text{tidal volume} - \text{anatomic dead space})$

• Anatomic dead space (VD): air in conducting passages (~150 mL).

• Inspiratory reserve volume (IRV): extra air inhaled after normal inspiration.

• Expiratory reserve volume (ERV): extra air exhaled after normal exhalation.

• Residual volume: air remaining after maximal exhalation.

• Minimal volume: air remaining if lungs collapse.

Factors Increasing Efficiency of Gas Exchange

• Large partial pressure differences for O2 and CO2.

• Short diffusion distance (thin respiratory membrane).

• High lipid solubility of gases.

• Large total surface area of alveoli.

• Good ventilation-perfusion matching (airflow and blood flow coordination).

Oxygen and Carbon Dioxide Transport in Blood

• Oxygen:

  • 1.5% dissolved in plasma.

  • 98.5% bound to hemoglobin (Hb) as oxyhemoglobin (HbO2).

  • Each Hb binds up to 4 O2 molecules; binding is cooperative.

  • Hb saturation depends on PO2, pH, temperature, and 2,3-BPG.

  • Carbon monoxide competes with O2 for Hb binding (dangerous).

• Carbon Dioxide:

  1. 70% as bicarbonate ions (HCO3-):

     • CO2 + H2O $\xrightarrow{\text{carbonic anhydrase}}$ H2CO3 $\rightarrow$ H+ + HCO3-

     • H+ binds Hb; HCO3- exchanged for Cl- (chloride shift).

  2. 23% bound to Hb (carbaminohemoglobin).

  3. 7% dissolved in plasma.

Effect of pH and Temperature on Oxygen-Hemoglobin Saturation Curve

• Bohr effect: Lower pH (more acidic) decreases Hb affinity for O2 (curve shifts right); higher pH increases affinity (curve shifts left).

• Increased temperature decreases Hb affinity for O2 (curve shifts right); decreased temperature increases affinity (curve shifts left).

Local and Neural Control of Respiration

• Local regulation: Active tissues increase CO2 and decrease O2, enhancing diffusion and blood flow.

• Neural control:

  • Respiratory centers in medulla oblongata and pons regulate rate and depth of breathing.

  • Voluntary control from cerebral cortex can override involuntary centers.

  • Dorsal respiratory group (DRG): controls quiet and forced inspiration.

  • Ventral respiratory group (VRG): controls forced breathing (inspiration and expiration).

  • Apneustic center: stimulates DRG for inspiration.

  • Pneumotaxic center: inhibits apneustic center, triggers exhalation.

Chemoreceptor Response to CO2 Changes

• Hypercapnia: High arterial PCO2 (from hypoventilation) stimulates increased respiratory rate and depth.

• Hypocapnia: Low arterial PCO2 (from hyperventilation) decreases respiratory drive.

Chapter 24: The Digestive System

Structure and Function of the Digestive System

The digestive system processes food, extracts nutrients, and eliminates waste. It consists of the digestive tract and accessory organs.

• Digestive tract: oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, anus.

• Accessory organs: teeth, tongue, salivary glands, pancreas, liver, gallbladder.

• Functions:

  1. Ingestion

  2. Mechanical digestion and propulsion

  3. Chemical digestion

  4. Secretion

  5. Absorption

  6. Defecation

• Nutrients are used for anabolism (building) and catabolism (energy release).

Secretions of the Salivary Glands

Gland	Location	Secretion	Function
Parotid	Inferior to zygomatic arch	Serous, salivary amylase	Starch digestion
Sublingual	Floor of mouth	Mucus	Buffer, lubricant
Submandibular	Inner surface of mandible	Buffers, mucins, amylase	Lubrication, starch digestion

Process of Deglutition (Swallowing)

1. Buccal phase: Voluntary; tongue compresses bolus, soft palate seals nasopharynx, bolus enters oropharynx.

2. Pharyngeal phase: Involuntary; swallowing reflex, epiglottis closes glottis.

3. Esophageal phase: Peristaltic waves move bolus to stomach.

4. Bolus enters stomach as lower esophageal sphincter opens.

Regulation of Gastric Activity: Cephalic, Gastric, and Intestinal Phases

• Cephalic phase: Triggered by sight, smell, or taste of food; CNS increases gastric secretion and motility.

• Gastric phase: Food in stomach stimulates secretion of gastric juice, mixing waves, and gastrin release.

• Intestinal phase: Controls rate of stomach emptying; duodenal stretch and hormones (CCK, GIP, secretin) inhibit gastric activity.

Major Duodenal Hormones

Hormone	Stimulus	Main Actions
Gastrin	Undigested proteins	Increases stomach motility, acid, and enzyme production
Secretin	Chyme in duodenum	Increases pancreatic and bile secretion, decreases gastric activity
GIP	Lipids/carbohydrates	Inhibits gastric activity, stimulates insulin release
CCK	Chyme in duodenum	Stimulates pancreatic enzymes, bile release, inhibits gastric activity
VIP	Chyme in duodenum	Stimulates intestinal glands, dilates capillaries, inhibits stomach acid
Enterocrinin	Chyme in duodenum	Stimulates alkaline mucus production

Motility Reflexes in the Large Intestine

• Gastroileal and gastroenteric reflexes: Move material into cecum.

• Peristaltic waves: Move material along colon.

• Segmentation (haustral churning): Mixes contents of adjacent haustra.

• Mass movements: Powerful contractions move material through colon.

• Defecation reflex:

  • Triggered by rectal stretch receptors.

  • Intrinsic (myenteric) reflex: short, triggers peristalsis and relaxes internal anal sphincter.

  • Parasympathetic reflex: long, increases peristalsis and relaxes internal anal sphincter.

Bile Production and Entry into the Duodenum

• Liver produces bile, which is stored in the gallbladder or secreted into the duodenum.

• Bile flows from hepatocytes into bile canaliculi, then into interlobular ducts, hepatic ducts, and finally the common hepatic duct.

• Bile enters the duodenum via the bile duct (through the duodenal ampulla and papilla) or is stored in the gallbladder via the cystic duct.

• Purpose of bile: Bile salts emulsify lipids, increasing surface area for pancreatic lipase and promoting lipid absorption.

Metabolic Functions of the Liver

• All blood from digestive tract passes through the liver (hepatic portal system).

• Hepatocytes extract and store nutrients, detoxify blood, and correct deficiencies.

• Carbohydrate metabolism: Regulates blood glucose via glycogen synthesis/breakdown and gluconeogenesis.

• Lipid metabolism: Regulates triglycerides, fatty acids, cholesterol.

• Amino acid metabolism: Regulates amino acid levels; converts ammonia to urea.

• Waste removal: Detoxifies drugs, toxins, and metabolic wastes.

• Vitamin storage: Stores fat-soluble vitamins (A, D, E, K) and B12.

• Mineral storage: Stores iron as ferritin.

• Drug inactivation: Breaks down circulating drugs.