Respiration notes

MDCAT Biology

Respiration involves ventilation, gaseous exchange, and the transport of oxygen and carbon dioxide between the lungs, blood, and body tissues. These notes describe the human respiratory system, the mechanism of breathing, gas exchange in alveoli, transport of respiratory gases, and harmful effects of smoking.

Respiratory System and Its Functions

The human respiratory system brings oxygen from the atmosphere into the body and removes carbon dioxide produced by cells. The movement of air into and out of the lungs is called ventilation. Gaseous exchange occurs when oxygen and carbon dioxide diffuse across respiratory surfaces.

The respiratory passage includes the nose, nasal cavities, pharynx, larynx, trachea, bronchi, bronchioles, and lungs. The alveoli are the actual sites where gases exchange between air and blood.

The main functions of the respiratory system are ventilation, gaseous exchange, oxygen transport, carbon dioxide removal, and contribution to voice production.
The method of bringing oxygenated air into contact with a gas exchange surface is called ventilation.
The voice box is the larynx, and it leads into the trachea.
The trachea is also called the windpipe.
The respiratory surface in humans is formed by the alveoli.
Gaseous exchange in humans occurs between alveolar air and blood in surrounding capillaries.

Nose, Nasal Cavities and Air Passage

Air usually enters the body through the nostrils. It passes into the nasal cavities, where it is filtered, warmed, and moistened before reaching the lungs. Each nasal cavity is divided into three passageways.

The lining of the nasal cavities contains mucus and ciliated epithelium. Mucus traps dust and microorganisms, while cilia move the mucus towards the pharynx, where it can be swallowed.

Each nasal cavity is subdivided into three passageways.
The nasal cavities are lined by a mucous membrane containing ciliated epithelium.
Nasal hairs and mucus help remove dust particles from inhaled air.
The nasal passages warm and moisten the air.
The pharynx is a common passage for food and air.
The epiglottis helps prevent food from entering the trachea during swallowing.

Trachea, Bronchi, Bronchioles and Lungs

The larynx opens into the trachea. The trachea is supported by C-shaped rings of cartilage that prevent its collapse. It divides into two primary bronchi, one entering each lung.

Inside each lung, a bronchus divides and subdivides into progressively smaller tubes called bronchioles. The bronchioles end in clusters of alveoli. This branching system provides a large area for distributing air throughout the lungs.

The trachea divides into two main bronchi.
Each bronchus enters one lung and divides into smaller branches.
The progressively smaller branches are called bronchioles.
Bronchi and larger bronchioles contain cartilage and smooth muscle, while the smallest air passages have little or no cartilage.
The right and left lungs are located in the thoracic cavity.
The lungs are protected by the ribs and separated from the abdominal organs by the diaphragm.
The lungs are covered by pleural membranes.

Pleural Membranes and Mechanism of Breathing

Each lung is covered by a double-layered pleural membrane. A thin layer of pleural fluid lies between the membranes. This fluid reduces friction during breathing and helps the lungs move smoothly with the chest wall.

Breathing consists of inspiration and expiration. Changes in the volume of the thoracic cavity change the pressure inside the lungs. Air moves from a region of higher pressure to a region of lower pressure.

The covering of the lungs is called the pleural membrane, or pleura.
Pleural fluid reduces friction between the pleural layers.
During inspiration, the diaphragm contracts and becomes flatter.
During inspiration, the external intercostal muscles contract and lift the ribs upward and outward.
The volume of the thoracic cavity increases during inspiration.
The pressure inside the lungs decreases, so air enters the lungs.
Inspiration is an active process involving expansion of the lungs.
During expiration, the diaphragm and external intercostal muscles relax, thoracic volume decreases, and air leaves the lungs.

Alveoli as Respiratory Surfaces

The alveoli are tiny air sacs at the ends of bronchioles. They are surrounded by a dense network of blood capillaries. Their large total surface area and thin walls allow rapid diffusion of oxygen and carbon dioxide.

The wall of an alveolus consists of a single layer of squamous epithelium. The capillary wall is also very thin, so the distance between alveolar air and blood is extremely small.

Alveoli are the sites of gaseous exchange in the lungs.
Alveoli are lined by squamous epithelium.
The walls of alveoli are one cell thick.
Alveoli have a large combined surface area.
A moist surface allows respiratory gases to dissolve before diffusion.
A rich capillary supply maintains a concentration gradient for oxygen and carbon dioxide.
Oxygen diffuses from alveolar air into the blood.
Carbon dioxide diffuses from blood into the alveoli and is removed during expiration.
Ventilation continually refreshes alveolar air.

Gaseous Exchange and Transport of Oxygen

The partial pressure of oxygen is higher in alveolar air than in deoxygenated blood arriving at the lungs. Therefore, oxygen diffuses through the alveolar and capillary walls into the blood. Most oxygen then combines reversibly with haemoglobin in red blood cells.

At body tissues, the oxygen concentration is lower than in oxygenated blood. Oxyhaemoglobin releases oxygen, which diffuses into cells for aerobic respiration. The unloading of oxygen is helped by a lower oxygen concentration, higher carbon dioxide concentration, higher temperature, and lower pH in active tissues.

About 97% of oxygen is transported by red blood cells combined with haemoglobin.
The remaining oxygen is transported dissolved in blood plasma.
Haemoglobin combines reversibly with oxygen to form oxyhaemoglobin.
Oxyhaemoglobin is formed mainly in the capillaries surrounding alveoli.
Oxygen is released from oxyhaemoglobin at body tissues.
The diffusion path between alveolar air and blood is short.
Hypoxia is a condition in which the oxygen supply to tissues is inadequate despite adequate blood flow.
A decrease in blood pH reduces the ability of haemoglobin to bind oxygen.

Transport of Carbon Dioxide and Chloride Shift

Carbon dioxide is produced by body cells during respiration. It diffuses from cells into tissue fluid and blood. It is transported from tissues to the respiratory surface by plasma and erythrocytes.

Most carbon dioxide is carried in the form of bicarbonate ions. Some combines with the amino groups of haemoglobin to form carbaminohaemoglobin, while a small amount remains dissolved in plasma.

Carbon dioxide is transported from tissues to the respiratory surface by plasma and erythrocytes.
About 70% of carbon dioxide is transported as bicarbonate ions in plasma.
About 20% to 30% is transported as carbaminohaemoglobin, depending on the reference used.
Carbaminohaemoglobin forms when carbon dioxide combines with the amino groups of haemoglobin.
A small amount of carbon dioxide is transported dissolved directly in plasma.
In red blood cells, carbonic anhydrase helps convert carbon dioxide and water into carbonic acid.
Carbonic acid dissociates into hydrogen ions and bicarbonate ions.
The chloride shift maintains electrical neutrality when bicarbonate ions move out of red blood cells.
In the lungs, bicarbonate ions re-enter red blood cells and carbon dioxide is formed and exhaled.

Effects of pH and Smoking on Respiration

Carbon dioxide affects the acidity of blood. When carbon dioxide combines with water, carbonic acid forms and releases hydrogen ions. An increase in hydrogen ions lowers blood pH. Hydrogen ions bind with the protein part of haemoglobin and reduce its ability to bind oxygen. This helps oxygen leave haemoglobin in active tissues.

Tobacco smoke contains nicotine, carbon monoxide, tar, and other harmful substances. These substances damage the respiratory system and reduce the efficiency of gaseous exchange. Tar contains many carcinogenic compounds, including more than 10 compounds involved in causing cancer.

As blood pH decreases, hydrogen ion concentration increases.
Hydrogen ions combine with the protein part of haemoglobin and reduce its ability to bind oxygen.
Carbon dioxide combines with water to form carbonic acid in the blood.
Carbon monoxide combines strongly with haemoglobin and reduces oxygen transport.
Tar damages ciliated epithelium and causes mucus to accumulate in air passages.
Smoking increases the risk of chronic bronchitis and emphysema.
Smoking damages alveolar walls and reduces the surface area available for gaseous exchange.
Smoking is associated with lung cancer because tobacco smoke contains carcinogenic compounds.
Tuberculosis is caused by bacteria of the genus Mycobacterium, especially Mycobacterium tuberculosis.
Tuberculosis spreads more easily in poor living conditions and in people affected by malnutrition.
Malaria is caused by a parasite and is not caused by smoking.

Gas Exchange in Other Organisms and Plants

Gas exchange surfaces differ among organisms, but they share useful features such as a large surface area, a thin barrier, moisture, and a mechanism for maintaining a concentration gradient. In insects, air reaches tissues through a tracheal system rather than through blood.

Plants exchange gases through stomata, young stems, and other surfaces. Xerophytes reduce water loss while still allowing gas exchange. Their structural adaptations are especially useful in dry habitats.

Cockroaches have 20 spiracles for the entry and exit of air.
In insects, air passes through tracheae and tracheoles directly to body tissues.
Insects do not use blood to transport oxygen to their cells in the same way as humans.
Stomata are pores in the epidermis of leaves that regulate gaseous exchange and water loss.
Xerophytes have small, thick leaves to limit water loss by reducing surface area.
Other xerophytic adaptations include a thick cuticle, sunken stomata, and reduced number of stomata.
A respiratory surface should generally be thin, moist, and large in area.

Key terms

Respiration: The cellular process in which energy is released from organic substances, usually with the use of oxygen in aerobic respiration.
Ventilation: The movement of air into and out of a respiratory surface.
Inspiration: The active process in which air enters the lungs because thoracic volume increases and lung pressure decreases.
Expiration: The process in which air leaves the lungs when thoracic volume decreases and lung pressure increases.
Trachea: The windpipe that carries air from the larynx to the bronchi.
Bronchus: One of the two main air tubes formed when the trachea divides.
Bronchiole: A small air passage formed by repeated branching of a bronchus.
Alveolus: A microscopic air sac that acts as a site of gaseous exchange in the lungs.
Squamous epithelium: A thin layer of flattened cells that lines the alveoli and provides a short diffusion pathway.
Pleural membrane: The double-layered membrane covering each lung.
Oxyhaemoglobin: The reversible compound formed when oxygen combines with haemoglobin.
Carbaminohaemoglobin: The compound formed when carbon dioxide combines with the amino groups of haemoglobin.
Bicarbonate ion: The main form in which carbon dioxide is transported in the blood.
Chloride shift: The movement of chloride ions into red blood cells as bicarbonate leaves, maintaining electrical neutrality.
Hypoxia: A condition in which tissues receive an inadequate supply of oxygen despite adequate blood flow.
Carbon monoxide: A poisonous gas in tobacco smoke that combines strongly with haemoglobin and reduces oxygen transport.
Tar: A mixture of harmful substances in tobacco smoke that damages respiratory tissues and contains carcinogens.
Spiracle: An external opening of the tracheal system in insects.

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