Breathing System

Anatomy: Defined as the study of the structure of an organism or its individual parts.
Physiology: Defined as the study of the functions and biological processes of an organism or its individual parts.

Thoracic Cavity: The pair of lungs are located within the thoracic cavity (the chest cavity).
Physical Boundaries:
- Surroundings: The cavity is surrounded and protected by the ribs and the intercostal muscles.
- Base: The base of the thoracic cavity is formed by a large muscle known as the diaphragm.
Muscular Action: Air is moved into and out of the lungs through the coordinated contraction and relaxation of the diaphragm and intercostal muscles.
Component Checklist:
- Nose
- Pharynx
- Epiglottis
- Larynx
- Rings of cartilage
- Intercostal muscles
- Diaphragm
- Ribs
- Trachea
- Bronchus
- Bronchiole
- Alveolus

The Nose: Breathing through the nose is preferred over the mouth due to three primary benefits:
- Filtration: Air is filtered (cleaned) of debris as it passes through the nostrils by hairs and mucus.
- Moistening: Air is moistened as it travels through the nasal passages.
- Warming: Air is warmed via diffusion as it passes through the nasal passages, preparing it for the lungs.
Pharynx (Throat): A passage that consists of and leads to the epiglottis and larynx.
Epiglottis: A flap of tissue that closes over the trachea (windpipe) during the act of swallowing. This mechanism prevents food or liquid from entering the trachea and reaching the lungs.
Larynx (Voice Box): An organ that produces vibrations to create sound.

Structural Support: The trachea, bronchi, and bronchioles are reinforced with rings of cartilage and muscle. These cartilaginous rings are essential because they prevent these airways from collapsing inwards.
Branching Pathway:
- The trachea (windpipe) divides into two main branches called bronchi.
- The bronchi further branch into smaller tubes within the lungs known as bronchioles.
Protective Lining: The trachea, bronchi, and bronchioles are lined with mucus and cilia. This system serves to trap dust, bacteria, or viruses, preventing them from reaching the deep lung tissue.

Lungs:
- They are spongy structures capable of expanding and contracting in size.
- Pleural Membranes: The lungs are enclosed by a double membrane known as the pair of pleural membranes.
- Pleural Cavity: The space between these membranes contains a specific liquid that lubricates the membranes. This lubrication reduces friction as the lungs move during the breathing process.
Alveolus (Plural: Alveoli):
- Location: Found at the terminus (end) of each bronchiole.
- Quantity: There are approximately $300 \times 10^6$ (300 million) alveoli in a set of lungs.
- Primary Function: To facilitate gas exchange between the lungs and the blood.
- Gas Movement:
  - Oxygen ( $O_2$ ) diffuses from the alveoli into the blood.
  - Carbon dioxide ( $CO_2$ ) diffuses from the blood into the alveoli.
- Blood Supply: The capillaries surrounding the alveoli arise from the pulmonary artery.
Alveolar Adaptations for Efficient Gas Exchange:
- Large Surface Area: Created by the massive number of alveoli.
- Thin Walls: Allows for easier and faster gas exchange across the membrane.
- Moist Surfaces: Moisture helps the gases dissolve, which facilitates easier diffusion.
- Rich Blood Supply: Surrounded by an extensive network of capillaries to maintain a steep concentration gradient.

Cellular Level: Carbon dioxide ( $CO_2$ ) and water ( $H_2O$ ) diffuse out of body cells and into the blood.
Transport to Lungs:
- Carbon dioxide and water then diffuse out of the blood plasma and into the alveoli of the lungs.
- Oxygen ( $O_2$ ) diffuses from the alveoli into the blood to be transported to body cells.
Transport Mechanisms:
- Carbon Dioxide: Transported primarily within the blood plasma.
- Oxygen: Mainly transported by combining with haemoglobin found in red blood cells.

Neurological Control: The medulla oblongata in the brain monitors the levels of carbon dioxide ( $CO_2$ ) in the blood.
Muscular Trigger: The brain sends a signal to the diaphragm causing it to contract, which makes it move downward.
Rib Action: The intercostal muscles situated between the ribs contract, pulling the ribs upward and outward.
Volume and Pressure Changes:
- The volume of the chest cavity (thorax) increases.
- Consequently, the air pressure inside the chest falls.
Air Movement: Because internal pressure is lower than external pressure, air is forced into the lungs.
Energy Requirement: Inhalation is an active process because it involves the contraction of muscles, which requires energy.

Diaphragm Action: The diaphragm relaxes and moves upward.
Rib Action: The intercostal muscles relax, causing the ribs to move downward and inward.
Volume and Pressure Changes:
- The volume of the chest cavity (thorax) decreases.
- Consequently, the air pressure inside the chest increases.
Air Movement: Air is pushed out of the lungs to equalize pressure.
Energy Requirement: Exhalation is a passive process because it occurs when the intercostal muscles and diaphragm relax.

Comparison of Inhaled vs. Exhaled Air:
- Oxygen: Exhaled air contains less oxygen because body cells utilize oxygen for cellular respiration.
- Carbon Dioxide: Exhaled air contains more carbon dioxide because it is produced as a waste product of cellular respiration.
Rate of Breathing Control:
- Primary Stimulus: The concentration of carbon dioxide ( $CO_2$ ) in the blood is the gas that controls the rate of breathing.
- Physiological Response: High levels of carbon dioxide trigger an increase in the breathing rate (more breaths per minute).
Chemical Mechanism:
- Carbon dioxide is an acidic gas.
- Increased concentrations of $CO_2$ decrease blood pH (Blood is normally near $pH\,7$ (neutral); an increase in $CO_2$ can drop it to approximately $pH\,6$ ).
- The medulla oblongata detects this drop in pH and signals the diaphragm and intercostal muscles to breathe faster and deeper, particularly during physical exercise.