Human Respiratory System G12 Short Notes

Chapter # 3 The Human Respiratory System and Its Disorders - NBF Short Notes

---

Properties of a Respiratory Surface

• Moist and Permeable: Must be moist and thin for gases to diffuse through effectively.
• Thin: Diffusion is efficient only over a distance of 1 mm or less.
• Large Surface Area: Needed for sufficient gas exchange.
• Good Blood Supply: Required to transport gases.
• Good Ventilation: Maintains a steep diffusion gradient.

The Respiratory System of Man

The respiratory system is responsible for gas exchange between the body and the external environment. It's divided into two main regions: the upper respiratory tract and the lower respiratory tract.

Upper Respiratory Tract

Includes the nostrils, nasal cavity, and pharynx.

• Nose: The external part of the respiratory system, composed of bones, cartilage, and fatty tissues.
• Nostrils: The external openings of the nose.
• Nasal Cavities: The internal hollow spaces, divided by the nasal septum. The anterior parts of the nasal cavities are called vestibules, which contain hair. The nostrils and nasal cavities are lined by ciliated mucous membranes.
• Nose, Mucus, and Cilia: Act as a defense mechanism against pathogens and particulate matter. Mucus traps harmful particles and microorganisms, while cilia move trapped substances out of the respiratory system. Mucus also helps in moistening the air, and blood capillaries in the underlying membrane warm the air.
• Pharynx: A cone-shaped passageway leading from the oral and nasal cavities. It is divided into three sections: nasopharynx, oropharynx, and laryngopharynx.

Lower Respiratory Tract

• Larynx (Voice Box): An enlargement in the airway at the top of the trachea. Composed of muscles and cartilages. The **epiglottis** is a cartilage that prevents food and liquids from entering the air passages by covering the glottis during swallowing. The larynx contains **vocal cords** responsible for vocalization.
• Trachea (Windpipe): A membranous tube reinforced with 15-20 C-shaped pieces of cartilage.
• Bronchi and Bronchioles: The trachea divides into two primary **bronchi**. Primary bronchi divide into secondary bronchi, which then subdivide into even smaller terminal **bronchioles**. C-shaped cartilage plates are replaced with cartilage plates in the bronchioles, and terminal branches have no cartilage structures.
• Alveolar Ducts and Alveoli: The terminal bronchioles lead to alveolar ducts, which end in tiny air-filled chambers called **alveoli**. Alveoli are the sites of gas exchange, with over 700 million in the lungs. A film of lipoprotein called **alveoli surfactant** lines the alveoli, preventing them from collapsing.
• Alveoli Structure: The wall of each alveolus is only 0.1 µm thick. A dense network of blood capillaries surrounds the outside of each alveolus. The alveoli are lined with a moist squamous epithelium and contain collagen and elastin, allowing them to expand and recoil during breathing.

External Structure of Lungs

The lungs are the principal organs of respiration, conical in shape. They rest on the diaphragm, with their apex extending just above the clavicle. The right lung has three lobes (superior, middle, and inferior), and the left lung has two lobes (superior and inferior). A triangular depression on the heart side called the **hilum** is where blood vessels and airways pass through.

Mechanism of Ventilation (Breathing)

Ventilation is the process by which lungs draw in and push out air, involving the diaphragm and intercostal muscles.

• Inspiration (Inhaling): An active phase. The diaphragm contracts and flattens, and external intercostal muscles contract, causing the rib cage to move up and forward. This increases the internal space of the thoracic cavity, reducing pressure and drawing air into the lungs.
• Expiration (Exhaling): A passive phase. The diaphragm relaxes and returns to its dome shape, and the external intercostal muscles relax, causing the rib cage to move down and backward. This decreases the internal space, increasing pressure and forcing air out of the lungs.

Control of Breathing (Ventilation)

Breathing is an involuntary process controlled by a **breathing centre** in the **medulla** of the brain. It can be consciously controlled to a certain extent by the cerebral cortex. The **inspiratory centre** increases the rate and depth of inspiration, while the **expiratory centre** inhibits inspiration and stimulates expiration. Emotions from the limbic system can also affect the respiratory centre.

Transport of Gases

Respiratory gases (oxygen and carbon dioxide) are transported throughout the body by the blood. Blood transports oxygen from the lungs to the tissues and carbon dioxide from the tissues to the lungs.

Transport of Oxygen in Blood

• Haemoglobin: Approximately 97% of oxygen is carried by red blood cells as **oxyhaemoglobin**, as oxygen binds to haemoglobin molecules at high partial pressure.
• Plasma: The remaining 3% of oxygen is transported as dissolved oxygen in the blood plasma.

Transport of Carbon Dioxide in Blood

Carbon dioxide ($CO_2$) is transported in three main ways:

1. Carbaminohemoglobin: About 23% of $CO_2$ binds with the globin part of haemoglobin.
2. Dissolved in Plasma: Only about 7% of $CO_2$ is transported this way.
3. Bicarbonate Ions ($HCO_3^−$): This is the main method, carrying approximately 70% of the $CO_2$.
   • $CO_2$ diffuses into red blood cells (RBCs) and combines with water ($H_2O$) to form carbonic acid ($H_2CO_3$), a reaction catalyzed by the enzyme **carbonic anhydrase**.
   • The $H_2CO_3$ then dissociates into a hydrogen ion ($H^+$) and a bicarbonate ion ($HCO_3^−$).
   • The accumulation of $H^+$ ions in the RBCs is buffered by haemoglobin, which binds to them. This binding causes oxyhaemoglobin to release its oxygen.
   • The bicarbonate ions ($HCO_3^−$) diffuse out of the RBCs into the plasma. To maintain the electrical balance, **chloride ions ($Cl^−$)** move into the RBCs in the opposite direction. This exchange is known as the **chloride shift** or **Hamburger phenomenon**.

The transport of $CO_2$ depends on its partial pressure. In tissues, where the partial pressure of $CO_2$ is high, it diffuses into the blood. The process then reverses in the lungs to release $CO_2$.

Blood pH Maintenance

The pH of blood is maintained at approximately **7.4** by a buffer mechanism. Chloride ions entering the red blood cells combine with potassium ions ($K^+$) to form potassium chloride ($KCl$). Bicarbonate ions in the blood plasma combine with sodium ions ($Na^+$) to form sodium bicarbonate ($NaHCO_3$).

Role of Respiratory Pigments

Respiratory pigments are colored molecules that act as oxygen carriers. The two main pigments are haemoglobin and myoglobin.

• Haemoglobin: Consists of four polypeptide chains with an iron-containing haem group. Found in red blood cells. It has a lower affinity for oxygen than myoglobin and releases oxygen when the partial pressure of oxygen ($PO_2$) is 60 mmHg.
• Myoglobin: Consists of one polypeptide chain with an iron-containing haem group. Found primarily in skeletal muscles. It stores oxygen and has a higher affinity for oxygen, releasing it when the partial pressure of oxygen ($PO_2$) is 20 mmHg.

Carbon Monoxide (CO) Poisoning

Caused by the incomplete combustion of gas heaters in enclosed environments. Carbon monoxide (CO) binds to haemoglobin with an affinity **249 times greater** than that of oxygen, preventing oxygen transport and impairing the release of oxygen already bound to haemoglobin. Symptoms include nausea, vomiting, headache, and cherry-red lips.

Respiratory Disorders

Diseases affecting the lungs and other parts of the respiratory system.

Upper Respiratory Tract Infection

• Sinusitis: Inflammation of the nasal sinuses. Causes include cold, pollution, smoke, and viral infections. Treated with antibiotics or nebulization.
• Otitis Media (Middle Ear Infection): Caused by a malfunction of the Eustachian tube. Symptoms include ear pain, fluid drainage, and difficulty hearing. Treated with antibiotics or pain medication.

Lower Respiratory Tract Infection

• Pneumonia: Infection that inflames the alveoli, causing them to fill with fluid. Caused by bacteria, viruses, and fungi. Treatment depends on the cause.
• Pulmonary Tuberculosis (TB): A highly contagious bacterial infection of the lungs caused by *Mycobacterium tuberculosis*. Treated with medication for 9 months. Prevention includes ventilation, hygiene, and isolation.

Disorders of the Lungs

• Emphysema: Damages the walls of the alveoli, leading to blockages and reduced oxygen absorption. The main cause is **smoking**.
• Chronic Obstructive Pulmonary Disease (COPD): A group of conditions (including emphysema) caused by long-term exposure to irritants, primarily **smoking**. Symptoms include shortness of breath and a persistent cough.

Effects of Smoking

• Cigarette smoking causes about **87% of all lung cancer cases**.
• It is also a major cause of mouth, larynx, and esophagus cancers.
• Causes chronic diseases like **chronic bronchitis**, **emphysema**, and **asthma**.
• Smoke damages or destroys **cilia**, impairing the lungs' protective function.
• Makes breathing difficult and can worsen allergies.

Cardiopulmonary Resuscitation (CPR)

A technique for artificial respiration.

1. Preparation: Stretch the victim on their back, loosen tight clothing, and clear their mouth and throat.
2. Positioning: Lift the chin and tilt the head back to prevent the tongue from blocking the throat.
3. Start Breathing: Begin artificial respiration by pinching the nostrils and blowing into the victim's mouth.
4. Check for Response: Watch the chest to see if it rises.
5. Continuation: If the patient is revived, keep them warm and do not move them until a doctor arrives.

Stay Updated!

For more educational content and updates, join our WhatsApp Channel.

Join WhatsApp Channel