THE RESPIRATORY SYSTEM

Agriscience 332 Animal Science #8646-B TEKS: (c)(2)(A)

Introduction

Respiration is the process of inhaling and exhaling air, including oxygen and carbon dioxide.

Oxygen is the most critical requirement of life support for an animal, which can only survive a few minutes without it.

Major functions of the respiratory system: • Providing oxygen to tissues and cells; • Removing carbon dioxide from the body; • Controlling body temperature; • Eliminating water (as vapor); and • Aiding in voice production.

The respiratory system consists of the nostrils, nasal cavity (chamber), pharynx, larynx, trachea, bronchial tubes, and lungs.

Anatomy of the Respiratory System

The organs of the respiratory system are divided into two parts: • Upper respiratory tract - extends from the nasal opening to the pharynx, and • Lower respiratory tract - extends from the larynx to the lungs.

Upper Respiratory Tract The upper respiratory tract includes the nostrils, nasal cavity, and pharynx.

The muzzle, which holds the nostrils, is made up of the nose and lips on most domestic animals.

The nostrils are the external openings of the respiratory tract through which air passes during the breathing process.

The horse’s muzzle and nostrils are soft and expandable, allowing for large amounts of air to pass when needed.

It is also very sensitive and contains many oil glands and sweat glands.

Photo by M. Jasek.

Swine snouts are more rigid and do not contain any oil glands.

Photo by M. Jasek.

Cattle and sheep muzzles do not contain oil glands, but do contain sweat glands.

Cattle with dry muzzles are often feverish.

Photo by M. Jasek.

Photo by M. Jasek.

A hard palate and a soft palate separate the nasal cavity from the mouth.

The nasal cavity is divided into two halves by cartilage and connects the nostrils to the pharynx.

The nasal passages are lined with a membrane of epithelial cells which are covered by thousands of cilia.

Mucous coats the epithelial cells and cilia to create an air-filtering system that also moistens and warms the air to protect the other respiratory structures.

The nasal passages contain olfactory receptors in the turbinate bones.

These olfactory receptors are involved with the sense of smell.

Sinuses, which are air-filled cavities in the forehead bones, are connected to the nasal cavity.

The frontal sinuses extend to the horn cores in cattle and may become exposed to the atmosphere when mature cattle are dehorned.

If foreign materials fall into these openings, sinus infections may occur.

Air flows from the nasal cavity to the pharynx, which is a short, funnel-shaped tube.

The nasal cavity, mouth, eustachian tubes (from middle ear), esophagus and larynx empty into the pharynx, which is lined with a mucous membrane and ciliated cells.

Both food and air pass through the pharynx, but the epiglottis keeps them from passing through at the same time.

The epiglottis is a valve-like flap of tissue, above the trachea, that closes the air passage when the animal swallows feed or water to prevent them from entering the trachea and lungs.

Lower Respiratory Tract The lower respiratory tract includes the larynx, trachea, bronchial tubes, and lungs.

The larynx, commonly known as the “voice box,” is responsible for voice production, control of breathing, and preventing inhalation of foreign objects into the lungs.

The larynx is composed of five cartilage structures.

The parts of the larynx are: • Thyroid cartilage – commonly called the “Adam’s Apple,” • Arytenoid cartilages – two cartilages that assist in closing the epiglottis and control the pitch of the voice by tightening or loosening the vocal chords, and

• Cricoid cartilage – helps maintain shape of the larynx and is a site of muscle attachment.

The trachea (windpipe) is a tube composed of a series of adjacent cartilage rings, which are rigid to prevent collapsing of the trachea.

As a single tube, the trachea goes from the larynx to a level just above the base of the heart.

The trachea divides into two branches called the primary bronchi.

Each bronchi passes into a lung, where they branch out even further into bronchioles.

The trachea, bronchi, and the first few bronchioles, lined with mucous membranes and ciliated cells, contribute to cleansing the passing air.

The bronchioles divide many more times into smaller branches called intralobular bronchioles, terminal bronchioles, and respiratory bronchioles.

The respiratory bronchioles end with the smallest and final air passageways of the respiratory system, the alveoli.

Alveoli are tiny air sacs surrounded by capillaries (tiny blood vessels of the circulatory system).

Oxygen and carbon dioxide move through the walls of the alveoli and capillaries via a process called diffusion.

Diffusion is also responsible for the exchange of oxygen and carbon dioxide between the body cells and capillaries.

Diffusion is a process of passive transport, whereby particles or molecules move from areas of high concentration to areas of low concentration.

Inhaled air has a high concentration of oxygen (O high CO 2 2 ) and a lower concentration of carbon dioxide (CO and low O 2 2 ), while the blood in the capillaries around the alveoli has concentrations.

O 2 diffuses from the alveoli into the capillaries and attaches to the hemoglobin in red blood cells.

CO 2 diffuses from the blood into the alveoli.

Illustration by Patrick Lynch courtesy of Wikipedia.

Mammals’ lungs are made of elastic, spongy material that greatly expands when filled with air.

Lungs are cone-shaped and incompletely divided into lobes.

The bronchi, pulmonary artery, pulmonary vein, nerves, and lymph vessels connect to the lungs at the same location.

When lungs are expanded to total capacity, they completely fill the thoracic (chest) cavity.

Once an animal takes its first breath, the lungs will never completely collapse and will float in water.

If a newborn’s lungs sink, it was born dead.

The lungs of birds are relatively non-expandable.

Birds have accessory air sacs that aid in respiration and add to their ability to float.

Chickens have nine accessory air sacs and perforations in their long bones.

Physiology of the Respiratory System

The primary function of the lungs is the exchange of gases, O 2 and CO 2 .

The exchange of gases between the alveoli and capillaries is called external respiration because it occurs outside the animal’s body.

The exchange of gases between the capillaries and the body cells is called internal respiration because it occurs inside the animal’s body.

Inspiration is the inhaling of air.

When the diaphragm contracts and the thoracic cavity enlarges, a vacuum is created that expands the lungs and draws in air.

Quiet respiration, which is also called abdominal or diaphragmatic respiration, occurs mainly as a result of the diaphragm contracting to pull in air.

Labored respiration involves the contraction of the external intercostal (rib) muscles, which increases the capacity of the thorax.

When carbon monoxide is inhaled, it bonds with the iron in hemoglobin and prevents the transport of oxygen.

This carbon monoxide “poisoning” results in death, caused by the lack of oxygen.

Nitrates, chlorates, cyanide, and prussic acid are other chemicals that interfere with respiration.

Artificial respiration might be helpful, if breathing stops, especially in cases of newborns, animals struck by lightening, or animals overdosed with anesthetics or tranquilizers.

Artificial respiration can be performed by applying rhythmic pressure on the chest cavity.

Expiration, or exhaling of air, is accomplished by the relaxation of the diaphragm muscles and contraction of the internal intercostal muscles.

The upward movement of the viscera (due to diaphragm relaxing) and the down and inward movement of the ribs (due to intercostals contracting) reduces the size of the thoracic cavity and forces air out of the lungs.

Nerve cells in the medulla control respiratory rates.

The inspiratory nerves stimulate muscle contraction for inspiration or inhaling.

The expiratory nerves stimulates relaxation of the muscles for expiration or exhaling.

The pneumotaxic nerves are stimulated by the inspiratory center during inspiration and, in turn, stimulate the expiratory center to cause expiration.

Several factors influence the rate at which the brain stimulates breathing, including: • Carbon dioxide content of the blood, • Body temperature, and • Messages from other parts of the brain.

An increase in the concentrations of carbon dioxide increases the acidity of the blood, which causes the respiration rate to increase.

An increase in body temperature triggers the respiration rate to increase.

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2007