6.1 Why Do We Respire?

1. Cellular Energy Requirement: All living organisms, including humans, are made up of tiny units called cells. Each cell performs various functions such as obtaining nutrition, transporting materials, excreting waste, and reproducing. To carry out these functions, cells require energy. This energy comes from the food we eat. Thus, we need to eat regularly to fuel our cells and keep our body functioning properly.

2. Role of Oxygen: Breathing, a part of the respiration process, involves inhaling oxygen-rich air and exhaling carbon dioxide-rich air. Oxygen from the air we breathe is transported to all parts of the body and reaches every cell. Within the cells, oxygen helps break down food to release energy. This process is known as cellular respiration, and it is essential for the survival of all living organisms.

3. Aerobic vs. Anaerobic Respiration: Cellular respiration can occur with or without oxygen. When oxygen is used, it is called aerobic respiration, which breaks down food into carbon dioxide and water, releasing energy. In the absence of oxygen, some organisms, like yeast, perform anaerobic respiration, which produces alcohol and carbon dioxide. Human muscle cells can also undergo anaerobic respiration temporarily during intense exercise, producing lactic acid and causing muscle cramps.

6.2 Breathing

1. Breathing Process: Breathing involves inhaling air rich in oxygen and exhaling air rich in carbon dioxide. This process is continuous and essential for life. When we inhale, air enters our body through the nose or mouth, travels through the windpipe, and reaches the lungs. Exhaling releases carbon dioxide and other waste gases from the lungs.

2. Breathing Rate: The breathing rate, or the number of breaths taken per minute, varies depending on activity levels. At rest, an adult typically breathes 15-18 times per minute. During exercise, the body requires more oxygen, so the breathing rate increases. For instance, after running or brisk walking, breathing becomes faster to supply more oxygen to the muscles.

3. Factors Affecting Breathing: Various activities and conditions affect the breathing rate. Physical activities like running or exercising increase the demand for oxygen, thus increasing the breathing rate. Conversely, when resting or feeling drowsy, the breathing rate slows down. Breathing can be consciously controlled to some extent, but it is primarily an involuntary process regulated by the brain.

6.3 How Do We Breathe?

1. Inhalation and Exhalation: Breathing involves the coordinated movement of the diaphragm and rib cage. During inhalation, the diaphragm contracts and moves downward while the rib cage expands, creating more space in the chest cavity and allowing air to fill the lungs. During exhalation, the diaphragm relaxes and moves upward while the rib cage contracts, pushing air out of the lungs.

2. Chest Cavity and Lungs: The lungs are located in the chest cavity, protected by the rib cage. The diaphragm, a large muscle at the bottom of the chest cavity, plays a crucial role in breathing. When the diaphragm and rib muscles move, they alter the size of the chest cavity, facilitating the movement of air in and out of the lungs.

3. Breathing Mechanism Model: A simple model to understand breathing involves using a plastic bottle to represent the chest cavity and balloons to represent the lungs. By manipulating a rubber sheet at the base (diaphragm), one can observe how changing the volume of the chest cavity causes the balloons to inflate and deflate, mimicking the breathing process.

6.4 What Do We Breathe Out?

1. Composition of Exhaled Air: When we exhale, the air contains more carbon dioxide and less oxygen compared to the air we inhale. This is because oxygen is used by the body’s cells during respiration, and carbon dioxide is produced as a waste product. Exhaled air also contains water vapor, which is why a mirror fogs up when we breathe on it.

2. Experiment with Lime Water: Blowing into lime water through a straw shows the presence of carbon dioxide in exhaled air. The lime water turns milky, indicating a chemical reaction between carbon dioxide and the lime water. This simple experiment helps visualize the gases involved in respiration.

3. Importance of Oxygen: Oxygen is vital for cellular respiration, the process by which cells generate energy. The body’s cells continuously use oxygen to break down glucose into carbon dioxide and water, releasing energy. This is why maintaining a constant supply of oxygen through breathing is crucial for survival.

6.5 Breathing in Other Animals

1. Breathing Mechanisms: Different animals have adapted various respiratory mechanisms. For instance, insects like cockroaches use spiracles and tracheae for breathing. Air enters through spiracles (small openings) and travels through a network of tubes (tracheae) to reach the cells directly. This system is unique to insects.

2. Aquatic Respiration: Fish use gills to extract oxygen from water. Gills are specialized structures with numerous blood vessels, allowing efficient gas exchange. Water passes over the gills, and oxygen is absorbed into the bloodstream while carbon dioxide is expelled.

3. Respiration in Earthworms and Frogs: Earthworms breathe through their moist skin, which allows gases to diffuse directly into their bloodstream. Frogs have lungs for breathing air, but they can also absorb oxygen through their moist skin, especially when underwater. This dual respiratory system helps frogs survive in both aquatic and terrestrial environments.