Comprehensive Notes on the Respiratory System of the Frog

General Overview of Frog Respiration

Definition of Respiration: Respiration is a complex process involving the intake of oxygen and the oxidation of food with oxygen to release energy. The byproduct of this process is carbon dioxide.
Chemical Equation: $C_6H_{12}O_6 \rightarrow 6CO_2 + 6H_2O + \text{energy}$
Larval Stage Respiration: During the tadpole or larval stage, respiration is Branchial (Gill respiration). This typically involves $4-5$ pairs of gills.
Amphibious Adaptation: Due to their amphibious lifestyle, adult frogs utilize three distinct modes of respiration:
- Cutaneous respiration: Respiration through the skin.
- Bucco-pharyngeal respiration: Respiration through the lining of the mouth and pharynx.
- Pulmonary respiration: Respiration through the lungs.

Cutaneous Respiration

Definition: Respiration that occurs through the moist skin of the frog.
Operational Context: This mode of respiration takes place constantly, regardless of whether the frog is in water or on land.
Critical Necessity: Cutaneous respiration is the exclusive mode of respiration when the frog is under water or in hibernation.
Maintenance of Moisture: Frogs must stay near water to keep their skin moist. Mucus glands in the skin secrete mucus to prevent the skin from drying out when the frog is on land.
Mechanical Movement: Unlike other forms of respiration, movement (like pumping) is not required for cutaneous respiration because the skin is directly exposed to air or water.
Physiological Mechanism:
- The skin has a rich supply of blood capillaries and is high permeable to gases.
- Oxygen ( $O_2$ ) first dissolves in the moist skin and then diffuses into the blood.
- Within the blood, oxygen combines with hemoglobin and is transported to various parts of the body to release energy and produce carbon dioxide ( $CO_2$ ).
- Waste $CO_2$ combines with hemoglobin to form carboxy-hemoglobin, which is transported to the veins of the skin and then passed outside via diffusion.
Asphyxia Risk: If the skin dries out completely, cutaneous respiration becomes impossible, leading to the frog's death by asphyxia.

Bucco-pharyngeal Respiration

Definition: Respiration that occurs through the buccopharyngeal cavity.
Operational Context: This occurs when the frog is on land and requires additional oxygen.
Structural Features: The mucous epithelial lining of the buccal cavity is moist and richly supplied with blood capillaries to absorb oxygen from the air.
Key Conditions: During this process, the mouth and glottis remain permanently closed while the nostrils (external nares) remain open.
Muscular Action: The floor of the buccal cavity is alternately raised and lowered using two specific pairs of muscles:
- Sternohyals: Located below the mouth cavity; they connect the hyoid to the sternum. When they contract, the mouth cavity floor is lowered.
- Petrohyals: Located outside the mouth cavity; they connect the squamosal bone to the hyoid bone. When they contract, the mouth cavity floor is pushed upward.
Mechanism of Action:
1. Lowering: Sternohyal muscles contract, lowering the floor of the cavity. This increases the area/volume of the buccal cavity and decreases air pressure, causing air to enter through the nostrils.
2. Gaseous Exchange: Oxygen diffuses into the blood capillaries of the mucus membrane, and carbon dioxide diffuses out into the cavity.
3. Expulsion: Petrohyal muscles contract, raising the floor of the cavity. This decreases the area/volume and increases air pressure, forcing air out through the nostrils.
Exclusion of Lungs: The lungs do not participate in bucco-pharyngeal respiration.

Pulmonary Respiration Anatomy

Definition: Respiration taking place through the lungs on land, also known as lung respiration.
Context: This occurs during periods of high activity or locomotion when the demand for oxygen increases.
Respiratory Organs and Passages:
1. External Nares (Nostrils): Two openings on the tip of the snout dorsally which allow for inhalation and exhalation.
2. Nasal Chambers: Small chambers that connect external nares to internal nares.
3. Internal Nares: Openings that lead from the nasal chambers into the bucco-pharyngeal cavity.
4. Buccopharyngeal Cavity: Large wide space between the upper and lower jaws.
5. Glottis: A median vertical slit-like structure on the floor of the pharynx that opens into the larynx.
6. Laryngo-tracheal Chamber (Larynx): A thin-walled chamber in the throat that serves as both the larynx and trachea (as frogs lack a neck). It is often called the "voice box."
7. Bronchi: The larynx opens posteriorly into a pair of very small tubes (bronchi) that lead to each lung.
8. Lungs: Ovoid, thin-walled, elastic sacs located in the anterior part of the body cavity, flanking the heart.

Detailed Structures of the Respiratory Tract

Laryngo-tracheal Chamber Supports: The chamber is supported by two arytenoid cartilages and one cricoid cartilage.
Vocal Apparatus:
- The internal lining forms a pair of elastic horizontal bands known as vocal cords.
- Croaking is produced by vibrations when air from the lungs is forced through the glottis.
- Male frogs possess vocal sacs to amplify the croaking sound.
Lung Structure:
- Peritoneum: The external covering of the lungs.
- Septa (Trabeculae): Irregular folds on the inner surface that divide the lung into many small chambers.
- Alveoli: Small chambers where gas exchange occurs. They surround a large central cavity (air sac).
- Epithelium: The inner surface of the alveoli is covered with thin, flattened epithelial cells. At the edges of the septa, these cells are ciliated and columnar.
Hydrostatic Function: Beyond respiration, the lungs also act as hydrostatic organs to help the frog maintain balance or buoyancy.

Mechanism of Pulmonary Respiration (Breathing)

Ventilation Process: Ventilation involves two steps: Inspiration (intake of air) and Expiration (removal of air).
Inspiration (Inhalation):
1. Aspiration: The floor of the buccal cavity is lowered by the contraction of the sternohyal muscles. The glottis is closed and nostrils are open. Fresh air enters the bucco-pharyngeal cavity.
2. Sealing: Submental muscles contract, pushing the mento-meckelian bones of the lower jaw forward, which in turn pushes the premaxillae upward, closing the external nares.
3. Compression: Petrohyal muscles contract, raising the floor of the cavity. This decreases volume and increases pressure.
4. Lung Entry: With nostrils and gullet closed, the glottis opens, and air is forced into the lungs and alveoli for gas exchange.
Expiration (Exhalation):
1. Lung Contraction: Lungs and abdominal muscles contract, increasing pressure within the lungs.
2. Collection: Sternohyals contract to lower the buccal floor, decreasing pressure in the cavity so air rushes from the lungs into the buccal cavity through the glottis.
3. Opening Nares: Submental muscles relax, the premaxillae return to their original position, and external nares open.
4. Expulsion: Petrohyals contract, raising the buccal floor and closing the glottis. Air is forced out through the external nares.

Physiology of Respiration

Respiratory Pigment: Frog blood contains Red Blood Cells (erythrocytes) with hemoglobin (Hb). Hemoglobin binds to $O_2$ and $CO_2$ temporarily, depending on the partial pressure of the gases.
Oxygen Transport:
- In the alveoli, $O_2$ concentration is high. It diffuses into the blood and combines with hemoglobin to form oxyhaemoglobin.
- In the tissues, $O_2$ concentration is low. Oxyhaemoglobin dissociates, releasing oxygen to the tissues.
Cellular Oxidation:
- The absorbed oxygen is used to oxidize food, releasing energy according to the formula: $C_6H_{12}O_6 \rightarrow 6CO_2 + 6H_2O + \text{energy}$ .
Carbon Dioxide Transport:
- The resulting $CO_2$ is in high concentration in tissues. It diffuses into the blood (where $CO_2$ concentration is lower).
- The blood carries the $CO_2$ back to the respiratory surfaces (skin or lungs) to be released from the body.
Energy Utilization: The released energy is used to power various life activities of the frog.