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The P Wave - What Is Happening, Which Muscles Contract, and What Events Occur

What Is the P Wave?

"The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins."

• Guyton and Hall Textbook of Medical Physiology

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Which Muscles Contract During the P Wave?

• Right atrium is activated first (early part of the P wave), because the SA node sits at the junction of the right atrium and the superior vena cava
• Left atrium is activated slightly later (latter part of the P wave), as the impulse spreads across both atria via internodal pathways and Bachmann's bundle
• The ventricular muscle does NOT contract during the P wave - this is the key distinction. Ventricular contraction is triggered by the QRS complex, which follows later

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Sequence of Events During the P Wave

1. SA Node Fires (Pacemaker Discharge)

2. Electrical Impulse Spreads Across Both Atria

• Depolarization begins at the SA node and spreads in all directions across the atria through gap junctions and specialized internodal pathways
• The direction of spread is generally from the upper right atrium (SA node region) downward and to the left, toward the AV node
• Because this direction aligns with the positive axes of standard limb leads I, II, and III, the P wave is normally upright (positive) in leads I, II, and III

3. Atrial Muscle Depolarizes (= P Wave on ECG)

• The normal resting negative membrane potential inside the atrial cells reverses (inside becomes positive, outside becomes negative)
• This electrical event is what the ECG records as the P wave

"Depolarization of the atria begins in the sinus node and spreads in all directions over the atria. This record of atrial depolarization is known as the atrial P wave."

• Guyton and Hall Textbook of Medical Physiology, p. 155

4. Atrial Systole (Atrial Contraction) - "Atrial Kick"

• Immediately after depolarization sweeps through the atrial muscle, atrial contraction (systole) follows
• Both atria contract, squeezing blood through the open AV valves (mitral and tricuspid) into the ventricles
• This final push of blood into the ventricles accounts for roughly 20-25% of the total ventricular filling (the "atrial kick")
• Left atrial contraction causes a rise in left atrial pressure, which appears as the "a wave" on the jugular venous pulse (JVP) trace

"Atrial systole is atrial contraction. It is preceded by the P wave on the ECG, which marks depolarization of the atria. Contraction of the left atrium causes an increase in left atrial pressure."

• Costanzo Physiology, 7th Edition

5. The PR Interval (Delay at AV Node)

• After the P wave ends, there is the PR interval - a brief pause (~0.12-0.20 sec)
• The electrical impulse arrives at the AV node and is deliberately delayed there, giving the atria time to finish contracting and fill the ventricles before ventricular contraction begins
• This delay is NOT reflected in the P wave itself, but in the flat PR segment that follows it

Summary Table

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Diagram: Atrial Depolarization and the P Wave

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Key Points to Remember

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PQRST Waves of the ECG - Complete Guide

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P Wave

"The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins."

• Guyton and Hall Textbook of Medical Physiology

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PR Interval (not a wave, but important)

• Includes the delay at the AV node - this pause allows atria to finish contracting before ventricles begin
• Normal: 120-200 ms (0.12 - 0.20 sec)
• Prolonged PR = heart block; short PR = pre-excitation (e.g., WPW syndrome)

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Q Wave

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R Wave

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S Wave

"The QRS complex is caused by potentials generated when the ventricles depolarize before contraction - that is, as the depolarization wave spreads through the ventricles."

• Guyton and Hall Textbook of Medical Physiology

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ST Segment (between S and T)

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T Wave

"The T wave is caused by potentials generated as the ventricles recover from depolarization... The T wave is known as a repolarization wave."

• Guyton and Hall Textbook of Medical Physiology

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U Wave (bonus - sometimes seen)

• Follows the T wave; small and not always visible
• Prominent U waves are seen in hypokalemia
• May represent repolarization of the Purkinje fibers or mid-myocardial (M) cells

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Master Summary Table

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What Happens to Atrial Repolarization?

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Anatomy of Digestion

Overview

1. GI tract - esophagus, stomach, small intestine, large intestine
2. Liver and gallbladder - reside within the peritoneum
3. Pancreas - primarily retroperitoneal (except its tail)
4. Accessory structures - salivary glands, teeth, tongue

"The digestive system consists of four components: the GI tract, the liver and gallbladder that reside within the peritoneum, as well as the pancreas, that primarily resides in the retroperitoneal space."

• Miller's Anesthesia, 10th Edition

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The Wall of the GI Tract - 4 Layers (Inside to Outside)

1. Mucosa (innermost layer)

• Epithelium - lines the lumen directly; absorbs nutrients, secrets enzymes, senses contents
• Lamina propria - loose connective tissue containing blood vessels, lymphatics, nerve endings, immune cells
• Muscularis mucosae - thin smooth muscle layer; moves villi and folds the mucosa

2. Submucosa

• Dense connective tissue with larger blood vessels and lymphatics
• Contains Meissner's plexus (submucosal plexus) - transmits sensory information from the epithelium to the enteric and central nervous systems
• May contain glands (e.g., Brunner's glands in the duodenum) and lymphoid tissue

3. Muscularis (Muscularis Externa)

• Two smooth muscle sublayers:
  • Inner circular layer - contracts to narrow the lumen
  • Outer longitudinal layer - contracts to shorten the segment
• Auerbach's myenteric plexus sits between these two layers - it generates and coordinates peristalsis and mixing contractions

4. Serosa (outermost layer)

• Thin smooth membrane of connective tissue covered by mesothelium (simple squamous epithelium)
• Secretes serous fluid to reduce friction
• Continuous with the mesentery (suspends the intestines in the abdomen and carries blood vessels, nerves, and lymphatics)
• Regions not suspended in the abdomen (e.g., esophagus) have an adventitia instead of serosa

"All regions of the GI tract have certain structural features in common. A hollow tube with a variable diameter lumen, the tract's wall has four main layers: the mucosa, submucosa, muscularis, and serosa."

• Junqueira's Basic Histology, 17th Edition

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Organs of the Digestive Tract - Structure & Function

1. Mouth (Oral Cavity)

• Teeth - mechanical breakdown of food (mastication)
• Tongue - manipulates food, contains taste buds
• Salivary glands (parotid, submandibular, sublingual) - produce saliva containing:
  • Salivary amylase - begins starch digestion
  • Mucus - lubricates the food bolus
• Food is formed into a bolus and swallowed

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2. Pharynx & Esophagus

• Esophagus - muscular tube ~18-25 cm long, connecting pharynx to stomach
• Extends from C6 (hypopharynx) to T11 (gastroesophageal junction)
• Three regions:
  • Cervical esophagus (~4-5 cm) - surrounded by trachea anteriorly, vertebrae posteriorly
  • Thoracic esophagus - runs posterior to trachea, deviates right at aortic arch
  • Abdominal esophagus - from diaphragmatic hiatus to stomach cardia
• Upper 1/3 = striated muscle, lower 2/3 = smooth muscle
• Lower esophageal sphincter (LES) - relaxes to allow food into the stomach
• Function: propels food to the stomach via peristalsis - no digestion occurs here

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3. Stomach

• J-shaped muscular organ with four regions:
  • Cardia - where esophagus connects
  • Fundus - upper dome-shaped region (stores gas)
  • Body (corpus) - main secretory region
  • Pylorus/Antrum - lower region; mixes food and controls gastric emptying via pyloric sphincter
• Rugae - large mucosal folds that allow expansion
• Gastric wall has an extra oblique muscle layer (3 layers total) for churning
• Key secretions:
  • Parietal cells → HCl (acid; denatures proteins, kills bacteria) + Intrinsic factor (needed for Vitamin B12 absorption)
  • Chief cells → Pepsinogen (converted to pepsin by acid; digests proteins)
  • Mucous cells → Mucus (protects stomach lining from acid)
  • G cells → Gastrin (hormone that stimulates acid secretion)
• Function: mechanical churning + chemical digestion of proteins; produces chyme

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4. Small Intestine (~6-7 meters long)

1. Plicae circulares (circular folds) - permanent folds of mucosa and submucosa
2. Villi - finger-like projections of mucosa (~0.5-1.5 mm tall); contain a central lacteal (lymphatic) and capillary network
3. Microvilli (brush border) - tiny projections on each enterocyte; contain digestive enzymes (lactase, sucrase, maltase, peptidases)

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5. Large Intestine (~1.5 meters long)

• Cecum + Appendix - blind pouch at the junction with ileum
• Ascending colon - right side, travels upward
• Transverse colon - crosses the abdomen
• Descending colon - left side, travels downward
• Sigmoid colon - S-shaped, leads to rectum
• Rectum - stores feces before defecation
• Anal canal - internal sphincter (smooth, involuntary) + external sphincter (skeletal, voluntary)

• Haustra - sac-like pouches caused by the teniae coli (3 longitudinal muscle bands)
• Teniae coli - three bands of outer longitudinal muscle (instead of a complete layer)
• No villi (no major absorption of nutrients)
• Absorbs water, sodium, and potassium; forms and stores feces
• Houses gut microbiome

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6. Liver

• Largest solid organ; adult weight 600-1800 g (2-2.5% of body weight)
• Divided into 8 functional segments (Couinaud system) based on portal vein and hepatic vein distribution
• Right and left hemi-livers divided at the middle hepatic vein
• Dual blood supply:
  • Portal vein (75%) - nutrient-rich blood from GI tract
  • Hepatic artery (25%) - oxygenated blood
• Key digestive function: produces bile (stored in gallbladder)
• Bile contains bile salts that emulsify fats in the small intestine, enabling lipid absorption

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7. Gallbladder

• Small pear-shaped sac on the undersurface of the liver
• Stores and concentrates bile (concentrates ~5-fold) between meals
• Releases bile into the duodenum via the common bile duct when stimulated by CCK (cholecystokinin) after a fatty meal

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8. Pancreas

• Primarily retroperitoneal (except the tail which touches the spleen)
• Parts: head (in C-curve of duodenum), neck, body, tail
• Exocrine function (95% of tissue): acinar cells secrete pancreatic juice into the duodenum via the pancreatic duct:
  • Proteases (trypsinogen, chymotrypsinogen) - protein digestion
  • Lipase - fat digestion
  • Amylase - carbohydrate digestion
  • Bicarbonate - neutralizes stomach acid in duodenum
• Endocrine function (islets of Langerhans): insulin, glucagon, somatostatin

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Blood Supply Summary

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Digestion and Absorption Summary

"Digestion is the chemical breakdown of ingested foods into absorbable molecules... Absorption is the movement of nutrients, water, and electrolytes from the lumen of the intestine into the blood."

• Costanzo Physiology, 7th Edition

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Anatomy of the Pharynx

Overview

• Auditory (Eustachian) tubes
• Nasal cavity
• Oral cavity
• Larynx

1. Directs air into the laryngeal inlet (respiration)
2. Directs food into the esophagus (deglutition/swallowing)
3. Assists in pressure equalization in the middle ear (via auditory tube)
4. Aids in vocalization and speech

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Subdivisions of the Pharynx

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1. Nasopharynx

• Superior/Roof: Body of sphenoid + basilar occipital bone (basisphenoid)
• Anterior: Choanae (posterior nasal apertures), separated by the vomer
• Lateral walls: Superior constrictor muscle + pharyngotympanic (Eustachian) tube orifice
• Floor: Soft palate
• Inferior: Communicates with oropharynx via the pharyngeal isthmus (behind the soft palate)

• Walls are rigid and bony (except the soft palate floor) - always patent for unhindered airflow
• Contains the pharyngotympanic (Eustachian) tube opening - ~1-1.5 cm from the posterior end of the inferior turbinate; connects the middle ear to the nasopharynx to equalize pressure
• Fossa of Rosenmüller (pharyngeal recess) - posterior to the Eustachian tube opening; common site of nasopharyngeal carcinoma
• Nasopharyngeal tonsil (adenoids) - lymphoid tissue in the roof/posterior wall; largest in childhood, involutes after puberty
• Lined by pseudostratified ciliated (respiratory) epithelium

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2. Oropharynx

• Superior: Junction of hard and soft palate
• Inferior: Valleculae (plane of the hyoid bone)
• Anterior: Communicates with oral cavity through the oropharyngeal isthmus (between the palatoglossal arches)
• Lateral: Faucial pillars (palatoglossal arch anteriorly, palatopharyngeal arch posteriorly)

• Palatine tonsils - sit in the tonsillar fossae between the two faucial pillars (anterior = palatoglossus, posterior = palatopharyngeus); part of Waldeyer's ring
• Base of tongue (posterior 1/3) - slopes down from the sulcus terminalis; contains lingual tonsils (completing Waldeyer's ring)
• Valleculae - shallow depressions between the tongue base and epiglottis, divided by the median glossoepiglottic fold
• Soft palate and uvula - acts as a "flutter valve" to separate nasopharynx from oropharynx during swallowing
• Lined by non-keratinized stratified squamous epithelium

Waldeyer's Ring (Lymphoid Ring of the Pharynx)

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3. Hypopharynx (Laryngopharynx)

• Superior: Level of hyoid bone (pharyngoepiglottic folds)
• Inferior: Lower border of cricoid cartilage (continues as esophagus)
• Shaped like a funnel with its anterior segment missing

• Pyriform sinuses (piriform fossae) - paired recesses on either side of the laryngeal inlet; bordered medially by aryepiglottic folds and laterally by thyroid cartilage; common site of food trapping and foreign bodies; also common site of hypopharyngeal carcinoma
• Posterior pharyngeal wall - flat muscular wall behind the larynx
• Postcricoid region - immediately behind the arytenoids/cricoid; connects to the esophagus
• Cricopharyngeus muscle - the sphincter of the pharynx; normally closed, opens only during swallowing; weak point above it (Killian's dehiscence) is where a Zenker's diverticulum can form

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Wall of the Pharynx - Layers (Outside to Inside)

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Muscles of the Pharynx

Constrictor Muscles (Circular - compress the lumen for swallowing)

Clinical note: The weak area between thyropharyngeus and cricopharyngeus (Killian's dehiscence) is where a Zenker's diverticulum forms as a mucosal herniation.

Longitudinal Muscles (Elevators/Dilators - shorten and widen the pharynx)

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Nerve Supply

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Blood Supply

Clinical note: The internal carotid artery lies only 2.5 cm posterolateral to the palatine tonsils - an important surgical danger zone during tonsillectomy.

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Lymphatic Drainage

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Summary Table

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The Esophagus - Anatomy

Overview

• Length: 18-26 cm (resting); shortens significantly during swallowing
• Diameter: Can distend up to 2 cm anteroposteriorly and 3 cm laterally
• No serosa - unlike the rest of the GI tract, the esophagus has an adventitia (loose connective tissue) instead of serosa. This allows easier spread of esophageal cancers and perforations.

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Three Regions

1. Cervical Esophagus (~4-5 cm)

• From the pharyngoesophageal junction (C5-C6) to the suprasternal notch
• Relations:
  • Anterior: Trachea
  • Posterior: Vertebral column (C5-T1)
  • Lateral: Carotid sheaths and thyroid gland

2. Thoracic Esophagus (~16 cm)

• From the suprasternal notch to the diaphragmatic hiatus (T10)
• Runs just posterior to the trachea in the posterior mediastinum
• At T4 (aortic arch level) - deviates right to accommodate the aortic arch
• Posterior to the tracheal bifurcation (carina) and left main bronchus
• At T8 - turns left and crosses anterior to the aorta
• At T10 - passes through the diaphragmatic hiatus

3. Abdominal Esophagus (0.5-2.5 cm)

• From the diaphragmatic hiatus to the cardia of the stomach
• Relations:
  • Anterior: Left lobe of liver
  • Right: Caudate lobe of liver
  • Left: Fundus of stomach
  • Posterior: Right crus of diaphragm and aorta
• Compressed by increased intraabdominal pressure (acts as anti-reflux mechanism)
• Enters the stomach at an acute angle - the angle of His - which helps prevent gastric reflux

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Narrowings of the Esophagus (4 physiological constrictions)

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Sphincters

Upper Esophageal Sphincter (UES)

• A 2-3 cm zone of elevated resting pressure at the pharyngoesophageal junction
• Primarily formed by cricopharyngeus muscle (C-shaped, attaches to cricoid cartilage at C5-6), plus contributions from the inferior constrictor and thyropharyngeus
• Normally closed - opens during swallowing, belching, and vomiting
• Prevents air entering the esophagus during breathing

Lower Esophageal Sphincter (LES)

• Located at the esophagogastric junction (T10)
• Has two components:
  • Intrinsic: Smooth muscle of the LES itself (generates myogenic tone)
  • Extrinsic: Crural diaphragm contractions
• Marked externally by the Z-line (squamocolumnar junction) - the abrupt color change from white-tan squamous epithelium to reddish gastric mucosa
• Normally closed - relaxes to allow food into the stomach
• Dysfunction → Gastroesophageal reflux disease (GERD) or achalasia

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Wall Layers (Inside → Outside)

"The esophagus does not have a serosa but only a loose connective tissue that makes up the adventitia. The absence of a serosal layer allows esophageal perforations and malignancies to disseminate more readily."

• Yamada's Textbook of Gastroenterology, 7th Edition

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Muscle Composition by Region

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Blood Supply

• Cervical: → Inferior thyroid vein → brachiocephalic vein → superior vena cava
• Thoracic: → Azygos vein (right), hemiazygos vein (left) → superior vena cava
• Abdominal/lower: → Left gastric vein → portal vein

Clinical note - Esophageal varices: The lower esophagus is a portal-systemic anastomosis site. In portal hypertension (e.g., liver cirrhosis), blood backs up through the left gastric vein into the esophageal veins, dilating them into varices which can rupture and cause life-threatening hemorrhage.

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Nerve Supply

• Upper esophagus/UES: cell bodies in nucleus ambiguus
• Lower esophagus/LES: from dorsal motor nucleus
• Vagal fibers form the esophageal plexus around the thoracic esophagus
• Below the diaphragm: form anterior vagal trunk (left vagus → anterior gastric + hepatic branches) and posterior vagal trunk (right vagus → posterior gastric + celiac plexus)
• Cervical esophagus: recurrent laryngeal nerves (branches of vagus)

• Superior cervical ganglion, sympathetic chain, major splanchnic nerve, celiac ganglion
• Mediates pain (nociception) from the esophagus

• Auerbach's (myenteric) plexus - coordinates peristalsis of the smooth muscle portion

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Lymphatic Drainage

"This arrangement accounts for the frequent wide intramural and mediastinal lymphatic spread of esophageal carcinoma."

• Yamada's Textbook of Gastroenterology, 7th Edition

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Summary: The Esophagus at a Glance

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Why Food Does NOT Enter the Larynx During Swallowing

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The 5 Protective Mechanisms - All Happen Simultaneously

1. Soft Palate Rises - Blocks the Nose

• The soft palate is pulled upward to seal off the posterior nares (choanae)
• This prevents food from refluxing into the nasal cavity
• Muscles responsible: levator veli palatini, tensor veli palatini

2. Palatopharyngeal Folds Narrow - Size Filter

• The palatopharyngeal folds on each side are pulled medially, forming a narrow sagittal slit
• This acts as a size filter - only well-chewed food can pass through
• Prevents large, unchewed pieces from proceeding further

3. Larynx Elevates and Moves Forward - Most Important Step

• The larynx is pulled upward and anteriorly by the suprahyoid and thyrohyoid neck muscles
• The hyoid bone is also displaced anteriorly
• This upward movement does two things simultaneously:
  • Causes the epiglottis to flip backward (like a trap door) over the laryngeal inlet
  • Pulls open the upper esophageal sphincter to receive the food bolus

4. Epiglottis Covers the Laryngeal Opening - Physical Lid

• As the larynx moves up and forward, ligaments prevent the epiglottis from moving upward
• So it is forced to swing backward and downward, covering the laryngeal inlet like a lid
• The epiglottis directs food sideways into the pyriform sinuses (on either side of the larynx) and then into the esophagus

"The backward tilt of the epiglottis covers the opening of the larynx to prevent aspiration."

• Schwartz's Principles of Surgery, 11th Edition

5. Vocal Cords Clamp Shut - The Last Line of Defense

• The vocal cords (true vocal folds) are strongly approximated (adducted) by:
  • Adduction of the arytenoid cartilages
  • Contraction of the lateral cricoarytenoid muscles
  • Contraction of the false vocal folds (ventricular folds) above
• The glottis is completely closed for about 2.3 seconds during swallowing
• This is the most essential mechanism - if the vocal cords fail to close, food can get past the epiglottis and into the airway

"Most essential is the tight approximation of the vocal cords, but the epiglottis helps to prevent food from ever getting as far as the vocal cords. Destruction of the vocal cords or of the muscles that approximate them increases the risk of airway obstruction and choking while swallowing food or drink."

• Guyton and Hall Textbook of Medical Physiology

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Summary Diagram of Events

Food reaches back of mouth/pharynx
        ↓
Swallowing receptors (tonsillar pillars, pharyngeal wall) stimulated
        ↓
Signals sent via CN V (trigeminal) + CN IX (glossopharyngeal) → Medulla
        ↓
SWALLOWING CENTER (medulla + lower pons) activated
        ↓ (all 5 events fire simultaneously)

1. Soft palate → UP           (blocks nose)
2. Palatopharyngeal folds → IN  (size filter)
3. Larynx → UP + FORWARD      (opens esophagus, pulls epiglottis down)
4. Epiglottis → BACKWARD       (covers larynx opening)
5. Vocal cords → CLOSE         (seals the airway completely)

        ↓
Pharyngeal constrictors contract top to bottom
        ↓
Food forced into upper esophageal sphincter (which relaxes to receive it)
        ↓
Peristalsis carries food to stomach in 8-10 seconds

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Breathing is Also Paused

"The swallowing center specifically inhibits the respiratory center of the medulla during this time, halting respiration at any point in its cycle to allow swallowing to proceed."

• Guyton and Hall Textbook of Medical Physiology

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What Happens If These Mechanisms Fail?

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The Nerve Control

• Scott-Brown's Otorhinolaryngology Head & Neck Surgery
• Guyton and Hall Textbook of Medical Physiology (p. 788-789)
• Schwartz's Principles of Surgery, 11th Edition