Unit 1: Digestion and Respiration

Physiology of Digestion in the Alimentary Canal

Digestion is the process of breaking down complex food substances into simpler, absorbable forms. This occurs both mechanically and chemically within the alimentary canal, a long tube extending from the mouth to the anus.

1. Mouth

Mechanical digestion begins here with chewing (mastication), breaking food into smaller pieces. Chemical digestion begins with saliva, secreted by salivary glands.

• Saliva Contains:
  • Water: Moistens food, facilitates swallowing.
  • Mucin: Lubricates food, forms bolus.
  • Salivary Amylase (Ptyalin): Initiates carbohydrate digestion, breaking down starches into smaller polysaccharides and disaccharides (maltose).
  • Lingual Lipase: Secreted here but becomes active in the acidic environment of the stomach, initiating lipid digestion.
  • Lysozyme: Antibacterial agent.
• Process: Food mixes with saliva to form a bolus, which is then swallowed (deglutition) and moves into the pharynx.

2. Pharynx and Esophagus

These act as passageways, primarily for transport without significant digestion.

• Pharynx: A common passage for food and air. The epiglottis covers the trachea during swallowing to prevent food from entering the respiratory tract.
• Esophagus: A muscular tube connecting the pharynx to the stomach. Food moves through it via peristalsis, rhythmic waves of muscular contraction. The lower esophageal sphincter regulates food entry into the stomach and prevents reflux.

3. Stomach

A J-shaped organ that stores food, mixes it with gastric juices, and initiates protein digestion.

• Gastric Glands Secrete Gastric Juice, Containing:
  • Hydrochloric Acid (HCl): Secreted by parietal cells.
    • Activates pepsinogen to pepsin.
    • Provides an acidic pH (1.5-3.5) for optimal pepsin activity.
    • Denatures proteins.
    • Kills most ingested microorganisms.
  • Pepsinogen: Secreted by chief cells. HCl converts it into Pepsin, a proteolytic enzyme that begins protein digestion by breaking them into proteoses and peptones.
  • Gastric Lipase: Secreted by chief cells. Has a minor role in lipid digestion, particularly in infants.
  • Intrinsic Factor: Secreted by parietal cells. Essential for Vitamin B12 absorption in the small intestine.
  • Mucus: Secreted by goblet cells, protects the stomach lining from HCl and pepsin.
• Mechanical Digestion: Muscular contractions churn the food, mixing it with gastric juice to form a semi-liquid paste called chyme.
• Regulation: Gastrin hormone stimulates HCl and pepsinogen secretion.

4. Small Intestine

The primary site for both chemical digestion and nutrient absorption. It's divided into three parts: duodenum, jejunum, and ileum. Receives secretions from the liver (bile) and pancreas (pancreatic juice).

• Duodenum: Receives chyme from the stomach, bile from the liver, and pancreatic juice from the pancreas.
• Pancreatic Juice (Contains):
  • Bicarbonate: Neutralizes acidic chyme, protecting the intestinal lining and providing an optimal pH for pancreatic enzymes.
  • Pancreatic Amylase: Digests remaining starch into disaccharides.
  • Trypsinogen and Chymotrypsinogen: Inactive proteases activated by enteropeptidase (secreted by intestinal mucosa) into Trypsin and Chymotrypsin, respectively. These further digest proteins into smaller peptides.
  • Carboxypeptidases: Digest peptides from the carboxyl end.
  • Pancreatic Lipase: The primary enzyme for digesting triglycerides into fatty acids and monoglycerides.
  • Nucleases: Digest nucleic acids.
• Bile (Secreted by Liver, Stored in Gallbladder):
  • Bile Salts: Emulsify fats, breaking large fat globules into smaller ones, increasing surface area for lipase action. Bile contains no enzymes.
• Intestinal Juice (Succus Entericus, Secreted by Intestinal Glands):
  • Disaccharidases (Maltase, Sucrase, Lactase): Break down disaccharides into monosaccharides (glucose, fructose, galactose).
  • Dipeptidases and Aminopeptidases: Break down dipeptides and tripeptides into amino acids.
  • Intestinal Lipase: Minor role in lipid digestion.
• Mechanical Digestion: Segmentation contractions mix chyme with digestive juices and bring it into contact with the intestinal wall for absorption. Peristalsis propels the chyme forward.

5. Large Intestine

Primarily involved in water and electrolyte absorption, formation and storage of feces, and houses beneficial gut bacteria.

• No significant digestion or absorption of nutrients occurs here.
• Bacterial Action: Gut bacteria synthesize some vitamins (e.g., Vitamin K, B vitamins) and ferment undigested carbohydrates, producing gases.
• Feces Formation: Undigested food residues, dead cells, and bacteria are compacted into feces.

Absorption of Carbohydrates, Proteins & Lipids

Absorption mainly occurs in the small intestine, specifically the jejunum and ileum, through the highly folded inner surface (villi and microvilli) which dramatically increases surface area.

1. Carbohydrate Absorption

• Digestion: Carbohydrates are digested into monosaccharides: glucose, galactose, and fructose.
• Absorption Mechanism:
  • Glucose and Galactose: Absorbed into enterocytes (intestinal cells) by secondary active transport via the SGLT1 co-transporter (sodium-glucose cotransporter). This process requires sodium ions and energy. They then exit the enterocyte into the bloodstream via facilitated diffusion using the GLUT2 transporter.
  • Fructose: Absorbed into enterocytes by facilitated diffusion via the GLUT5 transporter. It also exits into the bloodstream via GLUT2.
• Transport: Once in the capillaries of the villi, monosaccharides are transported via the hepatic portal vein to the liver.

2. Protein Absorption

• Digestion: Proteins are digested into amino acids, dipeptides, and tripeptides.
• Absorption Mechanism:
  • Amino Acids: Absorbed into enterocytes by secondary active transport (Na+-dependent co-transporters). Different transporters exist for different types of amino acids. They exit the enterocyte into the bloodstream via facilitated diffusion.
  • Dipeptides and Tripeptides: Absorbed into enterocytes by secondary active transport via H+-dependent co-transporters (e.g., PEPT1). Once inside the enterocyte, most are hydrolyzed into amino acids by intracellular peptidases. The resulting amino acids then enter the bloodstream.
• Transport: Amino acids are transported via the hepatic portal vein to the liver.

3. Lipid Absorption

• Digestion: Lipids (triglycerides) are digested into monoglycerides and free fatty acids by pancreatic lipase, after emulsification by bile salts.
• Absorption Mechanism:
  • Micelle Formation: Monoglycerides and fatty acids, being hydrophobic, combine with bile salts and phospholipids to form water-soluble spheres called micelles. Micelles transport these lipid breakdown products to the brush border of enterocytes.
  • Diffusion: At the enterocyte surface, monoglycerides and fatty acids diffuse out of the micelles and across the cell membrane into the enterocyte. Bile salts remain in the lumen and are reabsorbed in the ileum.
  • Re-esterification: Inside the enterocyte, monoglycerides and fatty acids are re-esterified back into triglycerides.
  • Chylomicron Formation: These triglycerides, along with cholesterol and phospholipids, are packaged with proteins to form larger lipoprotein particles called chylomicrons.
• Transport: Chylomicrons are too large to enter blood capillaries directly. They are absorbed into the lacteals (lymphatic capillaries) within the villi. From the lacteals, they enter the lymphatic system and eventually drain into the bloodstream via the thoracic duct, bypassing the liver initially.

Pulmonary Ventilation

Pulmonary ventilation, or breathing, is the process of air movement into and out of the lungs. It involves two phases: inspiration (inhalation) and expiration (exhalation). This process relies on pressure changes within the thoracic cavity.

1. Mechanics of Inspiration (Inhalation)

Inspiration is an active process that increases the volume of the thoracic cavity, leading to a decrease in intra-pulmonary pressure, causing air to flow into the lungs.

• Diaphragm Contraction: The diaphragm, a dome-shaped muscle, contracts and flattens, moving downwards. This significantly increases the vertical dimension of the thoracic cavity.
• External Intercostal Muscles Contraction: These muscles contract, pulling the rib cage upwards and outwards. This increases the anteroposterior and lateral dimensions of the thoracic cavity.
• Thoracic Cavity Volume Increase: The combined actions of the diaphragm and external intercostals lead to a substantial increase in the overall volume of the thoracic cavity.
• Intra-pulmonary Pressure Decrease: Due to the increased thoracic volume, the pressure inside the lungs (intra-pulmonary pressure) drops below atmospheric pressure.
• Air Inflow: Air flows from the higher atmospheric pressure into the lower intra-pulmonary pressure until the pressures equalize.
• Accessory Muscles: During forced inspiration (e.g., during exercise), accessory muscles like the sternocleidomastoids and scalenes also contract to further elevate the rib cage and increase thoracic volume.

2. Mechanics of Expiration (Exhalation)

Expiration is typically a passive process during quiet breathing, driven by the elastic recoil of the lungs and thoracic wall.

• Diaphragm Relaxation: The diaphragm relaxes and moves upwards, returning to its dome shape.
• External Intercostal Muscles Relaxation: These muscles relax, allowing the rib cage to move downwards and inwards due to gravity and elastic recoil.
• Thoracic Cavity Volume Decrease: The relaxation of these muscles leads to a decrease in the overall volume of the thoracic cavity.
• Intra-pulmonary Pressure Increase: As the thoracic volume decreases, the pressure inside the lungs (intra-pulmonary pressure) increases above atmospheric pressure.
• Air Outflow: Air flows from the higher intra-pulmonary pressure out into the lower atmospheric pressure until the pressures equalize.
• Forced Expiration: During forced expiration (e.g., coughing, exercise), it becomes an active process. The internal intercostal muscles contract, pulling the ribs down and in more forcefully, and the abdominal muscles contract, pushing the diaphragm upwards, further decreasing thoracic volume and expelling more air.

Respiratory Volumes and Capacities

Respiratory volumes are measurements of air movement during breathing, while respiratory capacities are combinations of two or more volumes. These are typically measured using a spirometer.

Clinical Significance: These measurements are crucial for diagnosing various respiratory conditions such as asthma, emphysema, and restrictive lung diseases.

Transport of Oxygen and Carbon Dioxide in Blood

The blood acts as a transport medium for respiratory gases between the lungs and body tissues.

1. Transport of Oxygen (O₂)

Oxygen is transported in the blood in two main forms:

• 1. Dissolved in Plasma (approx. 3%): A small amount of oxygen dissolves directly in the plasma. This dissolved oxygen creates the partial pressure of oxygen (PO₂), which drives oxygen into the tissues.
• 2. Bound to Hemoglobin (approx. 97%): The vast majority of oxygen is reversibly bound to the iron-containing pigment, hemoglobin (Hb), found in red blood cells.
  • Each hemoglobin molecule can bind up to four oxygen molecules, forming oxyhemoglobin (HbO₂).
  • The binding of oxygen to hemoglobin is influenced by several factors, including PO₂, pH, PCO₂, and temperature.

Oxygen-Hemoglobin Dissociation Curve: This S-shaped curve illustrates the relationship between the partial pressure of oxygen (PO₂) and the percentage of hemoglobin saturation with oxygen. A rightward shift of the curve indicates a decreased affinity of hemoglobin for oxygen (releasing more O₂ to tissues), while a leftward shift indicates an increased affinity.

2. Transport of Carbon Dioxide (CO₂)

Carbon dioxide is transported in the blood in three main forms:

• 1. Dissolved in Plasma (approx. 7-10%): A small fraction of CO₂ dissolves directly in the plasma and is transported.
• 2. Bound to Hemoglobin (Carbaminohemoglobin, approx. 20-23%): CO₂ binds to the amino groups of hemoglobin (not the heme iron) to form carbaminohemoglobin (HbCO₂). This binding is more favored when hemoglobin is deoxygenated.
• 3. As Bicarbonate Ions (HCO₃⁻, approx. 70%): This is the most significant mechanism for CO₂ transport.
  • CO₂ diffuses into red blood cells from the tissues.
  • Inside the red blood cell, CO₂ rapidly combines with water (H₂O) in the presence of the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃).
  • H₂CO₃ then quickly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
  • The bicarbonate ions diffuse out of the red blood cell into the plasma. To maintain electrical neutrality, chloride ions (Cl⁻) move into the red blood cell from the plasma; this is known as the chloride shift.
  • The hydrogen ions (H⁺) produced are buffered by hemoglobin, preventing a significant drop in blood pH.

Bohr Effect: Increased CO₂ (or decreased pH, increased H⁺) and increased temperature in the tissues decrease hemoglobin's affinity for oxygen, causing it to release more oxygen to the metabolically active tissues. Conversely, in the lungs, where PCO₂ is low and pH is higher, hemoglobin's affinity for O₂ increases, facilitating oxygen uptake.

Haldane Effect: Oxygenation of blood in the lungs displaces CO₂ from hemoglobin (reduces hemoglobin's affinity for CO₂ and H⁺), promoting CO₂ release from the blood. Conversely, deoxygenated blood in the tissues has a higher capacity to carry CO₂. This effect is significant in facilitating both oxygen loading and carbon dioxide unloading.