Human Respiratory System: Anatomy, Function, and TB

Introduction to the Respiratory System

The drive to breathe is involuntary and generally automatic, although one can change breathing patterns, and they change when we sleep or engage in different activities. The lungs and respiratory system function to move air 24 hours a day, 7 days a week, because the body cannot store the oxygen it needs for cellular respiration and energy production. Thus, air is constantly flowing in and out of the lungs (Healthline Editorial Team).

Respiration, in terms of human physiology, has more than one definition. It includes cellular respiration, a process ongoing within the mitochondria, where glucose is broken down to ultimately produce ATP (adenosine triphosphate), providing energy to the body. The body is also capable of brief periods of internal anaerobic respiration, which produces lactate in muscles, for example. However, the focus of this paper is on respiration as the process through which air moves into and out of the lungs — that is, inhalation and exhalation. Respiration using the physiological respiratory system is a process whereby gases are transferred from the lungs to the blood vessels and from the blood vessels back to the lungs, with blood vessels carrying gases to the various bodily tissues (The Respiratory System).

The majority of the respiratory system functions for the intake and output of air, particularly oxygen and carbon dioxide; the smaller alveoli and alveolar conduits are the sites where gas exchange occurs. In addition to air exchange, the respiratory system directs, warms, and humidifies incoming air. Organs in the respiratory system also function in smell and speech. The respiratory system is further involved in physiological homeostasis of the body's internal environment; indeed, it can be argued that the respiratory system is one of the most important for the body's proper functioning (Healthline Editorial Team).

The respiratory system includes the following: the left and right lungs, both housed within the thoracic cavity. The right lung has three lobes, whereas the left lung has only two lobes and is partially compressed because the heart is positioned on the left side of the body. Air enters and exits the lungs through a complex system of upper airways. The respiratory system also involves the diaphragm, and is supported by the muscles and bones that make up the thoracic cavity (DiGiovanna).

The remainder of this paper focuses on the human respiratory system, addresses illnesses that affect respiratory function and overall well-being, and concludes with preventive measures.

Anatomy of the Human Respiratory System

The respiratory system includes the respiratory tract, the pulmonary circulation, other components such as the central nervous system (CNS), and the chest wall. The entire framework of the respiratory system is housed in the thorax, head, and neck. The upper respiratory tract is found in the head and neck, while the lower respiratory tract begins at the trachea and extends through the lungs (Person and Mintz). The anatomy of the respiratory tract includes the naso-oropharynx, the alveoli, the respiratory bronchioles, and the conducting airways. The lungs can also be considered as comprising both conducting airways and respiratory units. The trachea, bronchi, and bronchioles bring air in from the outside world and convey it to the alveoli, which serve as the respiratory units. Gas exchange occurs at the level of the alveoli, delivering oxygen to the blood to be transported throughout the body for normal cellular functions (Person and Mintz).

Air enters through the nose and/or mouth and then moves through the pharynx, larynx, and trachea into the lungs. Within the lungs, air moves into the primary bronchi, then the bronchioles, and finally the alveoli at the respiratory membrane. These organs can be subdivided into the upper respiratory tract (nose, pharynx, larynx) and the lower respiratory tract (thoracic respiratory organs).

Nose

Air in the nasal cavity enters through the nose's two nares (nostrils). On the superior side are the nasal bones, and plates of hyaline cartilage sit toward the end of the nose and are responsible for shaping its structure.

Nasal Cavity

The nasal cavity is divided into left and right sides by the nasal septum. The septum is formed by an inferior vomer bone, a superior ethmoid bone, and a septal cartilage positioned anteriorly. Inside the nose, a sticky mucous membrane lines the nasal cavities and serves to trap dust particles. Small hairs called cilia then help move that dust toward the nose to be expelled; these also function to prevent debris from entering the respiratory system (Healthline Editorial Team).

Pharynx

The pharynx, commonly called the throat, is anatomically divided into three regions based on their location in the body. These three segments are called the nasopharynx, the oropharynx, and the laryngopharynx.

The naso-oropharynx begins at the nostrils and includes the nasal passage, glottis, and sinuses, extending to the trachea. This airway is lined by specialized ciliated pseudostratified columnar epithelial tissue. The cilia function to move bodily fluids and trapped debris away from the lungs. One function of the naso-oropharynx is to warm and humidify air before it enters the lungs; another is to trap and filter out large particles (Person and Mintz).

The oropharynx is the section of the pharynx located just behind the nasopharynx. It serves not only as part of the respiratory system but also as the route through which food and liquids enter the body as part of the digestive system. Because of the wide variety of substances that enter through the mouth, the oropharynx is lined with a tougher variety of cells called stratified squamous epithelial cells (The Respiratory System).

The laryngopharynx is defined as the region from the epiglottis to the very beginning of the esophagus. Like the oropharynx, its cellular tissue is made up of stratified squamous epithelial tissue, and it handles not only air intake but also the passage of fluids and foods as part of the digestive system. Solids and fluids move from the laryngopharynx into the esophagus. However, once the breathing reflex is stimulated, air travels into the larynx through an opening called the glottis (The Respiratory System).

Larynx

The larynx is the fundamental anatomical structure involved in speech. The epiglottis remains in a vertical position that separates the path of air — which flows from the laryngopharynx to the larynx and trachea — from the path of food and fluids into the esophagus (The Respiratory System).

Trachea

Once air reaches the larynx, it flows into the trachea — a tube that is relatively rigid, being comprised of approximately 18 to 20 C-shaped hyaline cartilage supports. These cartilage rings function to hold the trachea open. Smooth muscle is also present at the posterior aspect of the trachea. This smooth muscle tissue allows for expansion of the upper esophagus during swallowing. The trachea is the primary airway to the lungs and is located just below the larynx (Healthline Editorial Team).

The trachea is lined with ciliated pseudostratified columnar epithelial cells and tissue, as is the nasopharynx and the nasal cavity. Some of these cells also secrete mucus. Although invisible to the naked eye, the air we routinely breathe contains a large number of particles such as smoke, pollen, and dust. The cilia lining the trachea work continuously — even during sleep — to move mucus and entrapped particles up to the pharynx for removal. The trachea then divides into the right and left primary bronchi, each of which enters its respective lung (The Respiratory System).

The Lungs

The lungs function to absorb oxygen and deliver it to the blood vessels, and to eliminate carbon dioxide through exhalation (Healthline Editorial Team). The right lung is comprised of three lobes — upper, middle, and lower — each further divided into sub-sections with their own airways. The left lung has only two lobes, as the heart occupies a portion of the left side of the thoracic cavity (Person and Mintz).

Each of the two primary bronchi enters its respective lung at a location called the hilum, which is also where other vessels enter and exit the lung. The left bronchus is slightly flatter than the right because of the space occupied by the heart. The main bronchi and the majority of their branches form the bronchial tree. Bronchi that serve separate lobes of the lung are called lobar bronchi and arise from branches of the main bronchus — there are three lobar bronchi in the right lung and two in the left (The Respiratory System).

Cartilage plates support all of the bronchi, holding them open for continuous airflow. Bronchioles are the smallest branches of the bronchi and split into even smaller tubes. Unlike bronchi, bronchioles have no cartilage; instead, they are lined with smooth muscle. This smooth muscle allows the airways to dilate and constrict as part of the breathing reflex. Air passes from the bronchioles into the alveoli (Person and Mintz).

Alveoli

The alveoli are tiny air sacs at the end of each bronchiole, often described as resembling small clusters of grapes. They are covered by a network of capillaries, which is where gaseous exchange takes place — oxygen diffuses into the blood vessels while carbon dioxide diffuses out. Estimates indicate there are as many as 150 million alveoli within each lung. The walls of the alveoli are composed primarily of very thin simple squamous epithelial cells that permit the exchange of gas molecules. The alveoli also contain alveolar cells and numerous immune system cells, including macrophages, neutrophils, and eosinophils. The alveolar cell wall creates an air-blood interface with the capillary system (Person and Mintz).

Diaphragm

The diaphragm is the primary respiratory muscle that enables breathing. When it contracts and flattens, it increases the volume of the chest cavity, facilitating inhalation.

Physiological Functions: How Breathing Works

Airflow in the body is neither pushed nor forced mechanically; rather, it is driven by changes in pressure. Within the chest, a variety of muscles act to expand the rib cage — these include the pectoralis minor, external intercostal, and sternocleidomastoid muscles, which work together to draw air in as the rib cage expands. When the diaphragm contracts and flattens, the volume of the chest cavity is further increased (The Respiratory System). Inhalation then occurs as air rushes in through the nose and mouth to equalize the pressure. When these muscles relax, exhalation occurs naturally as the cavity returns to a smaller volume.

Under normal conditions, breathing is passive and requires no deliberate effort. However, when the abdominal wall and intercostal muscles are actively engaged, forced exhalation takes place. This action compresses the air cavity more rapidly, driving air out at a greater rate (The Respiratory System).