The Human Respiratory System: Structure, Function, and Physiology

Functions and General Overview of the Respiratory System

The respiratory system is primarily responsible for overseeing the gas exchanges that occur between the blood and the external environment, specifically the exchange of oxygen ($O_2$) and carbon dioxide ($CO_2$). This vital process shares responsibility with the cardiovascular system to ensure that gases are transported to and from the body's tissues. While the system contains various structures, the actual exchange of gases takes place exclusively within the lungs in the alveoli. All other structures in the respiratory system function as passageways that serve to purify, warm, and humidify the incoming air before it reaches the site of gas exchange.

Anatomy of the Upper Respiratory Tract

The upper respiratory tract consists of several interconnected structures beginning at the nose and extending to the larynx. High in the nasal cavity, olfactory receptors are located in the mucosa on the superior surface. The remainder of the nasal cavity is lined with respiratory mucosa, which performs the crucial tasks of moistening the air and trapping incoming foreign particles. The structural boundaries include the cribriform plate of the ethmoid bone, the superior, middle, and inferior conchae, and the hard and soft palates.

Surrounding the nasal cavity are the paranasal sinuses, which are cavities located within the frontal, sphenoid, ethmoid, and maxillary bones. These sinuses serve three major functions: they lighten the weight of the skull, act as resonance chambers for speech, and produce mucus that eventually drains into the nasal cavity.

The Pharynx and Associated Structures

The pharynx, commonly known as the throat, is a muscular passage ranging from the nasal cavity to the larynx. It is divided into three distinct regions based on location and function:

1. Nasopharynx: The superior region located behind the nasal cavity. It contains the pharyngeal tonsils (also known as adenoids) and is the location where the auditory tubes enter.
2. Oropharynx: The middle region located behind the mouth. It houses the palatine tonsils and, along with the laryngopharynx, serves as a common passageway for both air and food.
3. Laryngopharynx: The inferior region that is attached to the larynx. It also serves as a common passageway for air and food.

Additional tonsillar tissue, the lingual tonsils, is located at the base of the tongue. Structural elements such as the uvula and the epiglottis play roles in directing material toward the esophagus or the larynx.

The Larynx and Trachea

The larynx, or voice box, is responsible for routing air and food into their proper channels and plays a central role in speech production. It is constructed from eight rigid hyaline cartilages, including the thyroid cartilage and the cricoid cartilage. It also features a spoon-shaped flap of elastic cartilage known as the epiglottis, which covers the opening of the larynx during swallowing.

The trachea, or windpipe, connects the larynx with the primary bronchi. The walls of the trachea are reinforced with C-shaped hyaline cartilage to maintain an open airway. It is lined with ciliated mucosa; these cilia beat continuously in the opposite direction of incoming air to expel mucus loaded with dust and other debris away from the lungs.

The Bronchial Tree and Lung Anatomy

The lungs occupy most of the thoracic cavity, with the apex located near the clavicle and the base resting on the diaphragm. They are divided into lobes by fissures: the left lung contains two lobes, while the larger right lung contains three lobes. The surfaces of the lungs are covered by the pulmonary (visceral) pleura, while the walls of the thoracic cavity are lined by the parietal pleura. The pleural cavity between these layers is filled with pleural fluid to allow the lungs to glide easily during breathing.

The respiratory tree begins when the trachea divides into the right and left primary bronchi, which enter each lung at a medial depression called the hilus. The right primary bronchus is notably wider, shorter, and straighter than the left. The bronchi then subdivide into a hierarchical structure:

1. Primary bronchi
2. Secondary bronchi
3. Tertiary bronchi
4. Bronchioli
5. Terminal bronchioli

The Respiratory Zone and Membrane

The respiratory zone is the specific site of gas exchange and includes the respiratory bronchioli, alveolar ducts, and the alveoli. The terminal bronchioles lead into these structures. All but the smallest branches of the bronchial tree have reinforcing cartilage to prevent collapse.

The respiratory membrane, or the air-blood barrier, consists of a thin layer of squamous epithelial cells lining the alveolar walls. The external surfaces of these alveoli are covered by a dense network of pulmonary capillaries. Gas exchange occurs through this membrane via diffusion—oxygen enters the blood, and carbon dioxide enters the alveoli. The membrane also contains surfactant-secreting cells, which produce surfactant to coat the alveolar surfaces and lower surface tension, and macrophages to provide immune protection against pathogens.

The Four Events of Respiration

Respiration involves four distinct physiological processes:

1. Pulmonary ventilation: This is the mechanical moving of air into and out of the lungs (breathing).
2. External respiration: Gas exchange occurring between the pulmonary blood and the air within the alveoli.
3. Respiratory gas transport: The transport of oxygen and carbon dioxide through the bloodstream.
4. Internal respiration: Gas exchange occurring between the blood in systemic capillaries and the actual tissue cells.

Biochemistry of External and Internal Respiration

The chemical processes during gas exchange involve the following reactions:

External Respiration (Lungs):

• Oxygen loading into the blood: $Hb + O_2 \rightarrow HbO_2$ (forming oxyhemoglobin).
• Carbon dioxide unloading from the blood: $HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O$ (bicarbonate ions and hydrogen ions form carbonic acid, which breaks down into water and carbon dioxide).

Internal Respiration (Tissues):

• Oxygen unloading into tissue cells: $HbO_2 \rightarrow Hb + O_2$.
• Carbon dioxide loading into the blood: $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$ (carbon dioxide and water form carbonic acid, which dissociates into hydrogen ions and bicarbonate ions).

Mechanics of Pulmonary Ventilation

Pulmonary ventilation is a mechanical process driven by volume changes in the thoracic cavity, which lead to pressure changes that cause gas to flow to equalize pressure. It consists of two phases:

Inspiration occurs when the diaphragm and external intercostal muscles contract. This causes the size of the thoracic cavity to increase (the diaphragm moves inferiorly and the ribs are elevated). As intrapulmonary volume increases, internal pressure decreases, pulling external air into the lungs.

Exhalation is largely a passive process dependent on the natural elasticity of the lungs. As the inspiratory muscles relax, the diaphragm moves superiorly, and the ribs are depressed, pushing air out. Forced expiration can be achieved by contracting the internal intercostal muscles to further depress the rib cage.

Respiratory Volumes and Regulatory Factors

Normal quiet breathing moves approximately $500\,ml$ of air with each breath, a measurement known as the tidal volume ($TV$). After a complete exhalation, approximately $1200\,ml$ of air remains in the lungs, known as the residual volume. Respiratory capacity is influenced by size, sex, age, and physical condition.

The rate and depth of respiration are controlled by several factors:

• Physical Factors: Increased body temperature, exercise, talking, and coughing.
• Volition: Conscious, voluntary control of breathing.
• Emotional Factors: Fear, excitement, or stress.
• Chemical Factors: This is the primary regulator. The level of carbon dioxide in the blood acts directly on the medulla oblongata; increased $CO_2$ results in increased respiration. Changes in oxygen concentration are detected by chemoreceptors in the aorta and carotid artery, which then signal the medulla oblongata.

Developmental and Aging Aspects

In a fetus, the lungs are filled with fluid. They do not fully inflate with air until approximately two weeks after birth. Surfactant, which is necessary to lower alveolar surface tension, is not present until late in fetal development, which can cause respiratory issues for premature infants. Throughout life, the respiratory rate changes as follows:

• Newborns: $40$ to $80$ respirations per minute.
• Infants: $30$ respirations per minute.
• Age 5: $25$ respirations per minute.
• Adults: $12$ to $18$ respirations per minute.

As the body ages, the elasticity of the lungs decreases, leading to a decrease in vital capacity and blood oxygen levels. The stimulating effects of carbon dioxide also decrease, and there is an increased risk of respiratory tract infections. The respiratory rate often increases slightly in old age.