Breathing, respiration & respiratory system

Breathing, respiration & respiratory system

This is a complete, exam-ready explanation of 'Breathing, respiration & respiratory system'.

{{TABLE: title=Breathing vs. Cellular Respiration — The Core Distinction

Untangling the Terms: Breathing and Respiration

Students often use the terms 'breathing' and 'respiration' interchangeably, but in biology, they refer to two distinct, though related, processes. The table above gives you the exam-critical differences at a glance. Breathing is the physical act of moving air into and out of the lungs. Think of it as the delivery service.

Cellular Respiration, on the other hand, is the reason the delivery is needed in the first place. It's a chemical process that happens inside every living cell of your body. Cells use the oxygen delivered by breathing to "burn" glucose (from your food) and release the energy stored within it. This energy is captured in a molecule called ATP (Adenosine Triphosphate), which powers all cellular activities. The carbon dioxide you breathe out is a waste product of this vital chemical reaction.

{{KEY: type=definition | title=Breathing (Ventilation) | text=The mechanical process of moving air into (inhalation) and out of (exhalation) the lungs to facilitate gas exchange with the internal environment, mostly by bringing in oxygen and flushing out carbon dioxide.}}

The Human Respiratory System: Anatomy of an Airway

The human respiratory system is a sophisticated network of organs and tissues responsible for breathing. Its primary function is to supply the blood with oxygen for delivery to all body parts and to eliminate the waste product, carbon dioxide.

{{VISUAL: diagram: overview of the human respiratory system, labelling nostrils, pharynx, larynx, trachea, bronchi, lungs, and diaphragm.}}

The entire system can be divided into two main zones:

1. The Conducting Zone: This includes all the structures that air passes through on its way to the lungs. Its job is to transport, filter, warm, and humidify the incoming air.
2. The Respiratory Zone: This is the actual site of gas exchange, deep within the lungs.

Let's trace the path of a molecule of oxygen as it enters your body.

1. Nasal Cavity and Pharynx

The journey begins at the nostrils (external nares), which open into the nasal cavity. This chamber is lined with mucus and fine hairs (cilia) that trap dust, pollen, and other debris, cleaning the air. The rich blood supply here also warms the air to body temperature.

The air then passes into the pharynx, a muscular tube that serves as a common passageway for both air and food. The pharynx connects the nasal cavity and mouth to the larynx and oesophagus. It's divided into three regions: the nasopharynx, oropharynx, and laryngopharynx.

2. Larynx (The Voice Box)

From the pharynx, air enters the larynx. This is a cartilaginous structure that contains the vocal cords. When air passes over the vocal cords, they vibrate to produce sound. A flap of cartilage called the epiglottis sits atop the larynx. Its crucial job is to cover the opening of the larynx (the glottis) during swallowing, preventing food and drink from entering the airway.

3. Trachea (The Windpipe)

The larynx opens into the trachea, a flexible tube about 12 cm long. It's kept permanently open by C-shaped rings of hyaline cartilage, which prevent it from collapsing. The inner lining of the trachea is also covered with cilia and mucus-producing cells. This "mucociliary escalator" continuously sweeps trapped debris upward, away from the lungs, where it can be swallowed or coughed out.

{{KEY: type=concept | title=The Conducting Zone vs. The Respiratory Zone | text=The Conducting Zone (nose to terminal bronchioles) is the 'airway' that simply moves air; no gas is exchanged here. Its function is to warm, moisten, and filter the air. The Respiratory Zone (respiratory bronchioles, alveolar ducts, and alveoli) is where the magic happens – the actual exchange of O₂ and CO₂ with the blood.}}

4. Bronchi and Bronchioles

At the level of the 5th thoracic vertebra, the trachea divides into two primary bronchi, one entering the right lung and one entering the left. Inside the lungs, these primary bronchi divide into smaller secondary bronchi, then tertiary bronchi, and finally into even smaller tubes called bronchioles. The smallest of these are the terminal bronchioles. This entire branching structure is often called the bronchial tree.

5. Alveoli (The Air Sacs)

The terminal bronchioles lead into the respiratory zone, which ends in millions of tiny, thin-walled air sacs called alveoli. This is the primary site of gas exchange. An adult has about 300-500 million alveoli, providing a massive surface area for diffusion – roughly the size of a tennis court!

Each alveolus is a hollow cavity surrounded by a dense network of capillaries (tiny blood vessels). The wall of the alveolus and the wall of the capillary are each only one cell thick. This incredibly thin barrier (the respiratory membrane) allows for rapid and efficient diffusion of gases between the air in the lungs and the blood.

{{VISUAL: diagram: detailed structure of the alveoli showing the alveolar sac, type I and II pneumocytes, alveolar macrophage, and the surrounding capillary network with red blood cells.}}

The alveolar walls contain two main types of cells:

• Type I pneumocytes: Extremely thin squamous epithelial cells that form the main structure and are the site of gas diffusion.
• Type II pneumocytes: Secrete a substance called surfactant, a complex lipoprotein that reduces the surface tension of the alveolar fluid, preventing the alveoli from collapsing during exhalation.

The Mechanism of Breathing

Breathing is a mechanical process driven by changes in pressure within the thoracic cavity (chest). It relies on the coordinated action of the diaphragm (a large, dome-shaped muscle at the base of the chest cavity) and the intercostal muscles (muscles between the ribs).

The guiding principle is simple: Air flows from an area of higher pressure to an area of lower pressure.

{{VISUAL: diagram: mechanism of breathing, showing two panels for inspiration and expiration. Labels should include diaphragm (contracted/relaxed), intercostal muscles, movement of ribs and sternum, and changes in thoracic volume.}}

{{TABLE: title=Inhalation vs. Exhalation

• Inhalation: To breathe in, the diaphragm contracts and flattens, while the external intercostal muscles contract, lifting the rib cage up and out. Both actions increase the volume of the thoracic cavity. This expansion causes the pressure inside the lungs (intrapulmonary pressure) to drop below the atmospheric pressure outside the body. As a result, air rushes into the lungs to equalise the pressure.
• Exhalation: Quiet exhalation is a passive process. The diaphragm and external intercostal muscles relax, causing the rib cage to move down and inward and the diaphragm to rise. This decreases the volume of the thoracic cavity, which increases the pressure inside the lungs to a level higher than atmospheric pressure, forcing air out. Forceful exhalation (like during exercise) becomes an active process, involving the contraction of internal intercostal muscles and abdominal muscles.

Gas Exchange and Transport

Once oxygen is in the alveoli, it must get into the blood. At the same time, carbon dioxide must move from the blood into the alveoli to be exhaled. This movement happens via diffusion, driven by differences in partial pressures.

{{KEY: type=exam | title=Partial Pressure is Key | text=Exam questions on gas exchange almost always relate to partial pressure gradients. Remember: gases always diffuse from a region of higher partial pressure to a region of lower partial pressure. Master the values for O₂ and CO₂ in different parts of the system.}}

Partial pressure (P) is the pressure exerted by a single gas in a mixture of gases. For example, Pₒ₂ is the partial pressure of oxygen.

Gas Exchange in the Lungs (External Respiration)

• Blood arriving at the lungs in the pulmonary arteries is deoxygenated. It has a low Pₒ₂ (around 40 mmHg) and a high Pₑₒ₂ (around 45 mmHg).
• The air in the alveoli has a high Pₒ₂ (around 104 mmHg) and a low Pₑₒ₂ (around 40 mmHg).
• Due to these pressure gradients:
  • Oxygen diffuses from the alveoli into the capillary blood.
  • Carbon dioxide diffuses from the capillary blood into the alveoli.

{{VISUAL: diagram: summary of gaseous exchange at the alveoli and tissues, showing the partial pressures of O2 and CO2 in inspired air, alveolar air, deoxygenated blood, oxygenated blood, and tissues.}}

Gas Exchange in the Tissues (Internal Respiration)

• The oxygenated blood, now with a high Pₒ₂ (around 95 mmHg), travels to the body tissues.
• The body cells are constantly using oxygen for cellular respiration, so the Pₒ₂ in the tissues is low (around 40 mmHg).
• The cells are also constantly producing CO₂, so the Pₑₒ₂ in the tissues is high (around 45 mmHg).
• Due to these new pressure gradients:
  • Oxygen diffuses from the blood into the tissue cells.
  • Carbon dioxide diffuses from the tissue cells into the blood.

The deoxygenated blood then returns to the lungs, and the cycle continues.

Transport of Gases in the Blood

1. Oxygen Transport Oxygen is not very soluble in water, so only about 1.5-3% of it is transported dissolved in the blood plasma. The vast majority (97-98.5%) is transported by binding to haemoglobin (Hb), a protein found in red blood cells.

• Hb + O₂ ⇌ HbO₂ (Oxyhaemoglobin)

This binding is reversible. In the lungs, where Pₒ₂ is high, haemoglobin readily binds with oxygen. In the tissues, where Pₒ₂ is low, haemoglobin releases its oxygen.

The relationship between Pₒ₂ and the percentage of haemoglobin saturated with oxygen is described by the Oxygen-Haemoglobin Dissociation Curve, which has a characteristic sigmoid (S-shape).

{{VISUAL: chart: oxygen-haemoglobin dissociation curve, a sigmoid curve with partial pressure of oxygen on the x-axis and percentage saturation of haemoglobin on the y-axis. Show the effect of increased PCO2 (Bohr effect).}}

Several factors can shift this curve, affecting haemoglobin's affinity for oxygen:

• Increased Pₑₒ₂, increased temperature, and decreased pH (more acidic) all shift the curve to the right. This means haemoglobin has a lower affinity for oxygen and releases it more readily. This is called the Bohr effect, and it's highly efficient, as active tissues (which need more O₂) have higher CO₂, higher temperature, and lower pH.

2. Carbon Dioxide Transport Carbon dioxide is transported in the blood in three forms:

{{KEY: type=points | title=Transport of Carbon Dioxide | text=- As bicarbonate ions (HCO₃⁻): This is the main method, accounting for about 70%. CO₂ combines with water in red blood cells (catalysed by the enzyme carbonic anhydrase) to form carbonic acid (H₂CO₃), which then dissociates into H⁺ and HCO₃⁻.

• Bound to haemoglobin: About 20-25% of CO₂ binds to the amino groups of haemoglobin to form carbaminohaemoglobin (HbCO₂). This is a different binding site from oxygen.
• Dissolved in plasma: About 5-10% is transported simply dissolved in the blood plasma.}}

Regulation of Respiration

Breathing is largely an involuntary process controlled by respiratory centres in the brainstem.

• Neural Control: The primary control centre is the respiratory rhythm centre located in the medulla oblongata. It generates the basic rhythm of breathing (inspiration and expiration). Another centre in the pons, the pneumotaxic centre, can moderate the function of the rhythm centre, primarily to fine-tune the breathing rate.
• Chemical Control: The body also monitors the chemical composition of the blood. Chemoreceptors (in the aorta and carotid arteries, and near the medulla) are highly sensitive to changes in Pₑₒ₂ and H⁺ concentration (pH). An increase in Pₑₒ₂ or H⁺ is the most powerful stimulus to breathe. It signals the respiratory centre to increase the rate and depth of breathing to blow off the excess CO₂ and restore normal blood pH. The role of Pₒ₂ is less significant under normal conditions.

Respiratory Volumes and Capacities

Spirometry is a technique used to measure the volume of air moved during breathing. Understanding these volumes is crucial for assessing lung function.

• Tidal Volume (TV): Volume of air inspired or expired during a normal breath (~500 mL).
• Inspiratory Reserve Volume (IRV): Additional volume of air that can be forcibly inhaled after a normal inspiration (~2500-3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume of air that can be forcibly exhaled after a normal expiration (~1000-1100 mL).
• Residual Volume (RV): Volume of air remaining in the lungs even after a forceful expiration (~1100-1200 mL). This air prevents the lungs from collapsing.

These volumes can be combined to calculate capacities:

• Inspiratory Capacity (IC): TV + IRV
• Functional Residual Capacity (FRC): ERV + RV
• Vital Capacity (VC): The maximum volume of air a person can breathe out after a forced inspiration. (ERV + TV + IRV).
• Total Lung Capacity (TLC): Maximum volume of air the lungs can hold. (RV + ERV + TV + IRV or VC + RV).

{{FLASHCARD: q=What is the main difference between Vital Capacity (VC) and Total Lung Capacity (TLC)? | a=Vital Capacity is the maximum exchangeable air in the lungs (what you can move in and out). Total Lung Capacity includes the Vital Capacity plus the Residual Volume (the air that you can't exhale).}}