Biology · Chapter 17
Study notes aligned to the official NEB syllabus.
Respiration is defined as the metabolic process in which the living cells of an organism obtain energy in the form of ATP, by taking in O2 and liberating CO2 from the oxidation of complex organic substances.
Respiration that involves the use of molecular O2 and the release of CO2 simultaneously is called aerobic respiration. Organisms which carry out aerobic respiration are termed aerobes. It is of two types:
a) Direct respiration
This is the exchange of environmental O2 with the CO2 of body cells without special respiratory organs and without the aid of blood.
Example: Protista, sponges, aerobic bacteria, flatworms, coelenterates.
b) Indirect respiration
This involves special respiratory organs, such as skin, buccopharyngeal lining, and lungs, and needs the help of blood.
Organs for respiration:
| Organ | Type of respiration | Example |
|---|---|---|
| Skin | Cutaneous respiration | Earthworm, frog |
| Trachea | Tracheal respiration | Insects (cockroach) |
| Gills | Branchial respiration | Fishes, prawn (arthropods) |
| Lungs | Pulmonary respiration | Higher vertebrates |
| Book lungs | (not named in the source) | Scorpions and spiders |
| Book gills | (not named in the source) | Limulus (living fossil, often called the horseshoe crab) |
| Pulmonary sac (modified mantle cavity) | (not named in the source) | Molluscs |
Indirect respiration has two phases, external and internal respiration, which are preceded by a preliminary phase called breathing or ventilation.
a) Breathing
The process of exchange of O2 from the atmosphere with CO2 produced by the cell is called breathing. It occurs in two stages, inspiration and expiration. During inspiration, air enters the lungs from the atmosphere, and during expiration, air leaves the lungs.
Ventilation is necessary because gas exchange depends on diffusion, and the greater the concentration gradient of gases across the respiratory surface, the faster the gas will diffuse between the respiratory medium and the blood.
External respiration: this is the intake of O2 by the blood from water or air in the respiratory organs, and the elimination of CO2.
Internal respiration: it involves four processes:
The process of releasing energy without the use of oxygen is called anaerobic respiration. It is also termed fermentation.
Example: yeast oxidizes glucose:
$$C_6H_{12}O_6 \rightarrow 2\ C_2H_5OH + 2CO_2 + 2\ ATP$$
(ethanol)
Whereas in our body, due to excessive exercise:
$$C_6H_{12}O_6 \rightarrow 2\ C_2H_3OH.COOH + 2\ ATP$$
(the product is lactic acid)
Fermentation yields only 5% of the energy available in glucose, thus it is a wasteful process. The organisms which carry out this process are called anaerobes.
Example: bacteria, parasitic worms (further examples in the source not fully legible).
The human respiratory system may be divided into two major components: the respiratory organs and the respiratory tract.
This serves as a passageway for fresh air to flow from outside to the lungs, and for foul air to return from the lungs to the exterior. Gas exchange does not occur here.
Parts of the respiratory tract:
1) External nose
The holes of the nose are called nostrils. These are paired openings that open into the two nasal chambers.
2) Nasal chamber
The two nasal chambers are separated from each other by a thin, cartilaginous, median vertical partition called the nasal septum. Each chamber shows three regions: lower vestibular, middle respiratory, and upper olfactory.
Nasal conchae: arising from the nasal chamber, there are three shallow bony ridges called nasal conchae, i.e. superior, middle (lateral) and inferior nasal conchae. They are covered with mucus membrane and greatly increase the surface area of the nasal chamber.
3) Internal nares
These are the posterior openings of the nasal cavity that lead into the nasopharynx.
4) Pharynx
The pharynx is a short, vertical tube, about 12 cm long, behind the buccal cavity. Food and air pass across here. Its upper part is called the nasopharynx, the middle part the oropharynx, and the lower part the laryngopharynx. The front part leads to the wind pipe (trachea), and the back part leads to the oesophagus. The pharynx is lined with non-keratinized stratified squamous epithelium, which is ciliated in the nasopharynx.
5) Larynx
It is situated (position not fully legible in the source) and communicates with the anterior part of the trachea. It is protected by a stiff cartilage called (name not fully legible in the source, appears to reference the epiglottis and associated cartilage). The larynx contains a pair of vocal cords and the epiglottis. The vocal cords vibrate when air passes into the larynx, which sets up sound.
6) Trachea
It is a long, ring-thin-walled tube about 21 cm long and 2.5 cm wide. It is also called the wind pipe. It extends downward through the neck. It is supported by C-shaped elastic cartilaginous rings to prevent collapsing. It is lined internally with mucus membrane to trap dust particles, bacteria and other foreign bodies. It also warms the air.
7) Bronchi
Inside the thorax, the trachea bifurcates into two bronchi, each of which enters one lung. In each lung, the bronchus undergoes repeated division to form secondary and tertiary bronchi, which further sub-divide into bronchioles. These branches further re-divide as they end, to form respiratory bronchioles.
8) Lungs
There are two large, bag-like, spongy structures, which are the main respiratory organs. Each is enclosed by a double pleural membrane. The right lung consists of three lobes, and the left consists of two lobes.
Inside the lungs, the respiratory bronchioles give rise to alveolar ducts, alveolar sacs, and finally the smaller alveoli. Each lung contains millions of alveoli.
Respiration is defined as the metabolic process in which the living cells of an organism obtain energy in the form of ATP, by taking in O2 and liberating CO2 from the oxidation of complex organic substances.
Respiration that involves the use of molecular O2 and the release of CO2 simultaneously is called aerobic respiration. Organisms which carry out aerobic respiration are termed aerobes. It is of two types:
a) Direct respiration
This is the exchange of environmental O2 with the CO2 of body cells without special respiratory organs and without the aid of blood.
Example: Protista, sponges, aerobic bacteria, flatworms, coelenterates.
b) Indirect respiration
This involves special respiratory organs, such as skin, buccopharyngeal lining, and lungs, and needs the help of blood.
Organs for respiration:
| Organ | Type of respiration | Example |
|---|---|---|
| Skin | Cutaneous respiration | Earthworm, frog |
| Trachea | Tracheal respiration | Insects (cockroach) |
| Gills | Branchial respiration | Fishes, prawn (arthropods) |
| Lungs | Pulmonary respiration | Higher vertebrates |
| Book lungs | (not named in the source) | Scorpions and spiders |
| Book gills | (not named in the source) | Limulus (living fossil, often called the horseshoe crab) |
| Pulmonary sac (modified mantle cavity) | (not named in the source) | Molluscs |
Indirect respiration has two phases, external and internal respiration, which are preceded by a preliminary phase called breathing or ventilation.
a) Breathing
The process of exchange of O2 from the atmosphere with CO2 produced by the cell is called breathing. It occurs in two stages, inspiration and expiration. During inspiration, air enters the lungs from the atmosphere, and during expiration, air leaves the lungs.
Ventilation is necessary because gas exchange depends on diffusion, and the greater the concentration gradient of gases across the respiratory surface, the faster the gas will diffuse between the respiratory medium and the blood.
External respiration: this is the intake of O2 by the blood from water or air in the respiratory organs, and the elimination of CO2.
Internal respiration: it involves four processes:
The process of releasing energy without the use of oxygen is called anaerobic respiration. It is also termed fermentation.
Example: yeast oxidizes glucose:
(ethanol)
Whereas in our body, due to excessive exercise:
(the product is lactic acid)
Fermentation yields only 5% of the energy available in glucose, thus it is a wasteful process. The organisms which carry out this process are called anaerobes.
Example: bacteria, parasitic worms (further examples in the source not fully legible).
The human respiratory system may be divided into two major components: the respiratory organs and the respiratory tract.
This serves as a passageway for fresh air to flow from outside to the lungs, and for foul air to return from the lungs to the exterior. Gas exchange does not occur here.
Parts of the respiratory tract:
1) External nose
The holes of the nose are called nostrils. These are paired openings that open into the two nasal chambers.
2) Nasal chamber
The two nasal chambers are separated from each other by a thin, cartilaginous, median vertical partition called the nasal septum. Each chamber shows three regions: lower vestibular, middle respiratory, and upper olfactory.
Nasal conchae: arising from the nasal chamber, there are three shallow bony ridges called nasal conchae, i.e. superior, middle (lateral) and inferior nasal conchae. They are covered with mucus membrane and greatly increase the surface area of the nasal chamber.
3) Internal nares
These are the posterior openings of the nasal cavity that lead into the nasopharynx.
4) Pharynx
The pharynx is a short, vertical tube, about 12 cm long, behind the buccal cavity. Food and air pass across here. Its upper part is called the nasopharynx, the middle part the oropharynx, and the lower part the laryngopharynx. The front part leads to the wind pipe (trachea), and the back part leads to the oesophagus. The pharynx is lined with non-keratinized stratified squamous epithelium, which is ciliated in the nasopharynx.
5) Larynx
It is situated (position not fully legible in the source) and communicates with the anterior part of the trachea. It is protected by a stiff cartilage called (name not fully legible in the source, appears to reference the epiglottis and associated cartilage). The larynx contains a pair of vocal cords and the epiglottis. The vocal cords vibrate when air passes into the larynx, which sets up sound.
6) Trachea
It is a long, ring-thin-walled tube about 21 cm long and 2.5 cm wide. It is also called the wind pipe. It extends downward through the neck. It is supported by C-shaped elastic cartilaginous rings to prevent collapsing. It is lined internally with mucus membrane to trap dust particles, bacteria and other foreign bodies. It also warms the air.
7) Bronchi
Inside the thorax, the trachea bifurcates into two bronchi, each of which enters one lung. In each lung, the bronchus undergoes repeated division to form secondary and tertiary bronchi, which further sub-divide into bronchioles. These branches further re-divide as they end, to form respiratory bronchioles.
8) Lungs
There are two large, bag-like, spongy structures, which are the main respiratory organs. Each is enclosed by a double pleural membrane. The right lung consists of three lobes, and the left consists of two lobes.
Inside the lungs, the respiratory bronchioles give rise to alveolar ducts, alveolar sacs, and finally the smaller alveoli. Each lung contains millions of alveoli.