The Respiratory System

Behind the nasal cavity is the pharynx. Both food and air pass through this long tube. Just below the pharynx is the larynx, or voice box. It channels air to the trachea and pushes food past the epiglottis, which covers the trachea during swallowing to prevent food from entering the lungs. Once food passes the epiglottis, it moves toward the esophagus. When air reaches the trachea, or windpipe, it travels down a long tube that branches into bronchi. The bronchi enter the lungs. As shown in the image, the bronchi branch into bronchioles before reaching tiny air sacs in the lung called alveoli. Gas exchange occurs in the alveolar region.

The diaphragm is a muscle that plays a large role in breathing. It is found at the base of the lungs and spreads across the bottom of the rib cage, forming the chest cavity. The human body has two lungs that vary in size and weight. The right lung, which is larger and heavier, has three lobes. The left lung has two lobes.

Pulmonary surfactant consists of chemical compounds that lines the alveolar surfaces of the lungs. They are lipopolysaccharides which have both a hydrophobic and hydrophilic layer to help keep the lungs inflated or prevent them from collapsing. On the outside of the lungs are serous membranes that cover each lung, called pleura. They form a two-layered membrane that cushions the lungs while reducing friction between the lungs and chest cavity or rib cage.

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Did You Know?

The total surface area of the alveoli in the lungs is roughly the size of a tennis court. Such a large surface area is needed to facilitate gas exchange and ensure the body is oxygenated at all times.

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Respiratory Functions and Breathing Mechanics

Recall that the primary function of the respiratory system is to provide oxygen to and remove carbon dioxide from the body. In addition to gas exchange, the respiratory system enables a person to breathe. Breathing, or inhalation, is essential to life. It is the mechanism that provides oxygen to the body. Without oxygen, cells are unable to perform their functions necessary to keep the body alive.

The primary muscle of inspiration is the diaphragm. Known as the chest cavity, this dome-shaped structure flattens when it contracts. The rib cage moves outward, allowing outside air to be drawn into the lungs. During relaxation, the diaphragm returns to its dome shape and the rib cage moves back to its natural position. This causes the chest cavity to push air out of the lungs.

The respiratory system can be functionally divided into two parts:

• Air-conducting portion: Air is delivered to the lungs. This region consists of the upper and lower respiratory tract—specifically, the larynx, trachea, bronchi, and bronchioles.
• Gas exchange portion: Gas exchange takes place between the air and the blood. This portion includes the lungs, alveoli, and capillaries.

Oxygen from the air enters the body through the respiratory system. But the cardiovascular system circulates oxygen throughout the body via the blood. As shown in the image, alveoli are surrounded by a capillary bed in the lung.

This anatomical structure allows blood to absorb oxygen and transport it through a network of blood vessels to cells in various tissues throughout the body. During the process of gas exchange, the blood system absorbs carbon dioxide from cells and carries it to the respiratory system, where it is exhaled from the body.

Be Careful!

When regulating blood gas and pH homeostasis levels, carbon dioxide, not oxygen, must be closely monitored.

The respiratory system works closely with both the cardiovascular and nervous systems to maintain blood gas and pH homeostasis. The body must regulate blood pH levels. When there is too much carbon dioxide in the blood, it is acidic (that is, its pH value is too low). If there is not enough carbon dioxide in the blood, it will be too alkaline (its pH value will be too high).

The Mechanics of Respiration

As shown in the image, the process of gas exchange between the outside air and the body is called respiration. It occurs on two levels: internal and external.

• External respiration occurs between the lungs and blood. When a person inhales, alveoli fill with oxygen through diffusion. Oxygen content is much higher than carbon dioxide levels. While in the alveoli region, blood becomes oxygen-rich. Once oxygenated, the blood leaves the lungs and travels through the left side of the heart, where it is pumped into circulation.

• Internal respiration occurs between the blood and tissues. Once blood enters circulation, it reaches the capillaries. Oxygen diffuses through the capillaries into the cells. Carbon dioxide diffuses from the cells into the capillaries. Because carbon dioxide content is higher than oxygen content in blood at this point, it is called oxygen-poor blood. This oxygen-poor blood travels to the right side of the heart. It moves through the pulmonary circuit, where external respiration begins.

Let’s Review

• The respiratory system supplies oxygen to the body and removes carbon dioxide.
• Blood pH levels are regulated by the respiratory, cardiovascular, and nervous systems.
• Respiratory organs are anatomically divided into the upper and lower tract.
• Breathing is a mechanical process that provides oxygen, which is essential to all living things.
• Internal respiration involves gas exchange between blood and body tissues.
• External respiration is a gas exchange that happens between blood and the lungs.

Respiratory Overview

Respiratory System Flashcards