8.1 Types of Respiratory Surfaces
To maximize the rate of gaseous exchange across respiratory surfaces, living organisms possess specialized respiratory structures adapted to their environment.
Common Characteristics of Respiratory Surfaces
- Large Ratio of Surface Area to Volume (SA/V): Maximizes the rate of gas diffusion.
- Thin Respiratory Structure: Usually one cell thick, providing a short diffusion distance.
- Moist Surface: Allows respiratory gases (oxygen and carbon dioxide) to dissolve prior to diffusion.
- Dense Network of Blood Capillaries: (Except in insects) Maintains a steep concentration gradient for efficient gas transport.
Respiratory Structures across Different Organisms
- Insects (Tracheolar System):
- Consists of spiracles (valved openings), tracheae (reinforced with chitin rings), and fine tracheoles.
- Tracheoles contain fluid where oxygen dissolves directly into body cells without requiring a circulatory system.
- Frogs (Skin and Lungs):
- Cutaneous Respiration (Skin): Highly vascularized, moist, and thin skin used for gas exchange, especially under water or during rest.
- Pulmonary Respiration (Lungs): Folded internal membranes used during active states on land.
- Fish (Gills):
- Consists of gill arches, gill filaments, and microscopic lamellae.
- Employs a countercurrent exchange mechanism: Blood flows through lamellae in the direction opposite to water flow, maintaining a continuous concentration gradient for maximum oxygen absorption.
- Humans (Lungs and Alveoli):
- Air travels via trachea, bronchi, bronchioles, and terminates in millions of tiny sacs called alveoli.
- Alveoli provide a vast surface area surrounded by a dense capillary network.
8.2 Mechanisms of Breathing
Breathing involves mechanical inhalation (inspiration) and exhalation (expiration) driven by volume and pressure changes within the thoracic cavity.
Breathing Mechanism in Humans
- Inhalation:
- External intercostal muscles contract; internal intercostal muscles relax.
- Ribcage moves upwards and outwards.
- Diaphragm muscle contracts and flattens downwards.
- Thoracic volume increases, causing thoracic pressure to decrease below atmospheric pressure.
- Air is forced into the lungs.
- Exhalation:
- Internal intercostal muscles contract; external intercostal muscles relax.
- Ribcage moves downwards and inwards.
- Diaphragm muscle relaxes and curves upwards (dome-shaped).
- Thoracic volume decreases, causing thoracic pressure to increase above atmospheric pressure.
- Air is forced out of the lungs.
Comparing Breathing Mechanisms in Other Animals
- Insects: Abdominal muscles contract and relax to alter body cavity volume, drawing air in/out through spiracles.
- Frogs: Positive pressure breathing. Floor of the mouth (buccal cavity) lowers and raises to force air into lungs through the glottis.
- Fish: Opening/closing of the mouth coordinated with the movement of the operculum creates pressure differences, forcing water across gill lamellae.
8.3 Gaseous Exchange in Humans
Gaseous exchange between alveolar air and blood capillaries occurs by simple diffusion down partial pressure gradients ($P_{\text{O}_2}$ and $P_{\text{CO}_2}$).
Transport of Oxygen
- Oxygen diffuses from alveolar air into blood capillaries and binds reversibly to hemoglobin in red blood cells to form oxyhemoglobin:
$$\text{Hemoglobin} + \text{O}_2 \rightleftharpoons \text{Oxyhemoglobin}$$
- In body tissues with low partial pressure of oxygen ($P_{\text{O}_2}$), oxyhemoglobin dissociates to release oxygen into tissue cells.
Transport of Carbon Dioxide
Carbon dioxide produced by cellular respiration is transported in blood in three forms:
- Bicarbonate Ions ($\text{HCO}_3^-$): Approx. 70% of $\text{CO}_2$ diffuses into red blood cells and reacts with water catalyzed by carbonic anhydrase to form carbonic acid ($\text{H}_2\text{CO}_3$), which dissociates into $\text{H}^+$ and $\text{HCO}_3^-$. Bicarbonate ions diffuse into blood plasma.
- Carbaminohemoglobin: Approx. 23% binds directly to the amino groups of hemoglobin.
- Dissolved $\text{CO}_2$: Approx. 7% dissolves directly in blood plasma.
8.4 Health Issues Related to the Human Respiratory System
- Chronic Obstructive Pulmonary Disease (COPD): Includes chronic bronchitis and emphysema.
- Emphysema: Alveolar walls break down, reducing the surface area for gaseous exchange, causing shortness of breath.
- Chronic Bronchitis: Inflammation of bronchioles and excessive mucus secretion blocking airways.
- Asthma: Inflammation and narrowing of airways (bronchioles) triggered by allergens (dust, pollen), causing wheezing and breathing difficulty.
- Lung Cancer: Uncontrolled cell division in lung tissue often caused by carcinogens in tobacco smoke (e.g., tar).