Respiration in Animals

Respiration in Animals Synopsis

Synopsis

Respiration

Food obtained in nutrition is used to release energy required to carry out various metabolic processes.
Respiration is a catabolic process of releasing energy from the simple sugar glucose for carrying out various life processes.
The energy required for all cellular activities is obtained by the oxidation of glucose. If glucose is not available, then the cells may break down proteins and fats to produce glucose. This glucose is then oxidised further to fulfil the respiratory needs of the cell.
The first step towards obtaining energy is six-carbon glucose is broken down into two molecules of three-carbon pyruvate. This process takes place in the cytoplasm.

Characteristics of Respiration

The breakdown of glucose to carbon dioxide and water does not occur in a single step. It involves a series of chemical reactions. It consists of three main phases:

Glycolysis (glucose → pyruvate; occurs in the cytoplasm)
Krebs cycle (pyruvate → CO₂ + H₂O + ATP; occurs in mitochondria)
Electron transport (Energy-rich compounds → ATP; occurs in mitochondria)

Each breakdown step is carried out by a specific enzyme.

Dehydrogenase → Removal of hydrogen
ATP synthase → Synthesis of ATP

A small amount of energy liberated in the breakdown of the glucose molecule is released as heat energy. But a major part of it is converted into chemical energy in the form of Adenosine Triphosphate (ATP). ATP has high energy content. It is also known as energy currency of the cell.

Types of Respiration

Breathing

Breathing is the biological process of inhaling and exhaling of the gases between the cells and the environment.
Breathing involves taking in of oxygen-rich air or inhalation (inspiration) and giving out carbon dioxide-rich air or exhalation (expiration).

Breathing and Respiration

Respiration in Animals

Depending on the level of organisation and habitat, the respiratory organs are different in different animals.
Characteristics of respiratory organs in animals:

They have large surface area to get enough oxygen.
They have thin walls for easy diffusion and exchange of gases.
They have rich blood supply for transport of respiratory gases.

Respiration in Amoeba

Amoeba is a unicellular organism which lives in fresh water. This organism does not have a specialized organ to do the process of respiration.
In amoeba respiration takes place through its cell membrane also called plasma membrane.

Respiration in Earthworm

In earthworms there is no specialized organ for respiration. The respiration process takes place through its moist, thin and highly vascular skin.
The oxygen gas absorbed by the skin diffuses into the blood and transported to all the cells of the body.
In the cells, oxygen is used up for oxidation of food. During this process, carbon dioxide is produced.
Then, this carbon dioxide mixes in the blood and diffused out of the body through its moist skin.

Respiration in Fish

In aquatic animals such as fish respiration takes place through special respiratory organs called gills.
Gills are present on both the sides of the head of fish. The gills are covered by gill covers also called operculum.
The gills of fish are very efficient as they can extract about 80% oxygen dissolved in water.
The gills have an important role in maintaining the right balance of salts in the body.

Respiration in Grasshopper

In grasshopper, exchange oxygen and carbon dioxide between their tissues and the air by a system of air-filled tubes called tracheae. Tracheae open to the outside through small holes called spiracles.
In the grasshopper, the first and third segments of the thorax have a spiracle on each side. Another 8 pairs of spiracles are arranged in a line on either side of the abdomen.
Spiracles open into large tracheal tubes. These, in turn, lead to ever-finer branches. The branches penetrate to every part of the body and terminate at their extreme ends, called tracheoles.
During breathing, oxygen from the air enters the spiracles and reaches each and every part of grasshopper’s body through trachea and tracheoles.
Carbon dioxide produced during respiration is carried back by trachea and tracheoles to the spiracles and is expelled out of the body of insect.

Human Respiratory System

The human respiratory system consists of the following parts:

Nostrils: These are the two openings present at the lower end of the nose just above the upper lips. The nostrils lead to a nasal chamber through the nasal passage.
Nasal Cavity: It is present between the cranium and the palate. It is lined with a thin, moist mucous membrane. Mucus secreted makes the air warm. Bacteria or small dust particles get entangled in the hair of the nasal passage.
Nasopharynx: The nasal cavity opens into the nasopharynx through the internal nares. It opens into the trachea through the glottis of the larynx region.
Larynx: The larynx is a cartilaginous box which is responsible for producing sound. Hence, it is also called the sound box. During swallowing, the glottis is covered by a thin elastic cartilaginous flap called the epiglottis which prevents the entry of food in the larynx.
Trachea: It extends from the larynx in the neck to the bronchi in the thoracic cavity. The walls of the trachea are provided with a hyaline cartilage which prevents the trachea from collapsing.
Bronchi and Bronchioles: The trachea divides into the right and left primary bronchi. Each primary bronchus enters the lungs of its side. After entering the lungs, the bronchi divide into secondary and tertiary bronchi. The secondary and tertiary bronchi branch into the bronchioles—terminal bronchioles and respiratory bronchioles.
Alveoli: Each respiratory bronchiole divides to give rise to alveolar ducts which open into the alveolar sac. Each alveolar sac contains many air sacs called alveoli. They are in proximity with the blood capillaries.
Lungs: The lungs are soft, spongy and elastic organs. They are covered by double-layered pleura. The pleural fluid present between the two pleura reduces friction on the lung surface. The lungs are situated in the thoracic chamber which is an air-tight chamber. Any change in the volume of the thoracic cavity will reflect in the pulmonary or lung cavity

Steps Involved in Respiration

Breathing/pulmonary ventilation by which atmospheric air is drawn in and CO2-rich air is released out.
Diffusion of oxygen and carbon dioxide across the alveolar membrane.
Transport of gases by blood.
Diffusion of oxygen and carbon dioxide between blood and tissues.
Utilisation of oxygen by cells for catabolism and resultant release of carbon dioxide.

Respiratory Volumes and Capacities

Exchange of Gases

Alveoli are the primary sites for the exchange of gases.
The exchange of respiratory gases occurs at two levels:

Between the lungs and blood
Between blood and tissues

Gases are exchanged by simple diffusion.
Diffusion of gases also depends on factors such as

Pressure gradient
Concentration gradient
Solubility of gases
Thickness of membra

Pressure Gradient

Every gas in a mixture of gases exerts a pressure called partial pressure.
A gradient for oxygen is present from alveoli to blood and blood to tissues.
Similarly, a gradient for CO₂ is present in the opposite direction, i.e. from tissues to blood and from blood to alveoli.

Solubility of Gases

The rate of diffusion of respiratory gases also depends on their solubility.

Diffusing Capacity of Respiratory Membrane

The respiratory membrane, i.e. the alveolar membrane is made of three layers — thin squamous epithelium of alveoli, endothelial lining of alveoli capillaries and the basement membrane between them.
Due to it’s less than a millimetre thickness, the easy exchange of gases is possible between alveolar air and blood.

Transport of Gases

Transport of Oxygen

About 97% of oxygen is transported in the blood in the form of oxyhaemoglobin.
Each haemoglobin molecule can combine with four oxygen molecules to form oxyhaemoglobin.
Rest 3% is in the dissolved state in plasma.
Factors such as H⁺ ion concentration, pO₂, pCO₂ and temperature interfere with the binding of oxygen with haemoglobin.

Transport of Carbon Dioxide

CO₂ is carried by blood in three forms:
As a simple solution: About 5–10% of CO₂ of the total blood dissolves in plasma and is carried as a simple physical solution.
As bicarbonate ions: CO₂ diffuses into blood and reacts with water to form carbonic acid. Carbonic acid dissociates into bicarbonate $begin mathsize 12px style HCO subscript 3 superscript minus end style$ and H⁺ ions. These bicarbonate ions are carried by the plasma.
As carbamino-haemoglobin: CO₂ combines loosely with the globin part of the reduced haemoglobin to form carbamino-haemoglobin. When pO₂ is high and pCO₂ is low in alveoli, CO₂ dissociates from carbamino-haemoglobin; hence, CO₂ bound to haemoglobin is released in the alveoli.

Cellular Respiration

Cellular respiration is a set of metabolic reactions occurring inside the cells to convert biochemical energy obtained from the food into a chemical compound called adenosine triphosphate (ATP).
The three stages of cellular respiration are glycolysis, Krebs cycle and oxidative phosphorylation by the electron transport system (ETS). The energy is stored in the form of ATP as respiratory substrates such as glucose are oxidised.

Glycolysis

Glycolysis also called EMP pathway, occurs in the cytoplasm of cell.
In this cycle, glucose is converted into pyruvic acid in the presence of many enzymes and co-enzymes.
Oxygen is not required during glycolysis.

1 molecule of glucose gives rise to 2 molecules of pyruvic acid.
4 molecules of ATP are formed of which 2 ATP molecules are utilized.
Net gain of ATP is two molecules.
2 NADP molecules are reduced to 2 NADPH₂, which later produces 6 ATP molecules.
Overall production of ATP in glycolysis is 2 ATP + 6 ATP = 8 ATP
There is no production of CO₂ during this process.

After glycolysis, pyruvic acid is converted into acetyl Co-A with the release of CO2 and the process is called as ‘oxidative decarboxylation’. It occurs in mitochondria of the cell. Besides, 6 ATP are also formed during this step.

Krebs Cycle

It occurs in the mitochondria of the cell and is also called aerobic oxidation.
It was discovered by Sir Hans Kreb and is also called TCA cycle (tricarboxylic acid cycle) or citric acid cycle.
It brings about the conversion of pyruvic acid, fatty acids, fats and amino acids into CO₂ and water by oxidation.
It is the common path for oxidation of carbohydrates, fats, proteins.
It accounts for 24 ATP molecules.
It starts with acetyl Co-A which is then converted into several intermediate compounds with the release of NADPH₂, FADH₂, ATP, hydrogen atoms and then Acetyl Co-A is regenerated back.

Electron Transport System or ETS

In this hydrogen atoms produced during oxidation of various intermediates during Krebs cycle are first broken into protons and electrons.
These protons and electrons after passing through a series of coenzymes and cytochromes combine with oxygen to form water molecules.
During these series of events 1 NADPH₂ releases 3 ATP molecules and 1 FADH₂ gives 2 ATP molecules which were produced during Krebs cycle and Glycolysis.
The net gain of ATP molecules during respiration is 38 ATP molecules of which, 8 ATP are from glycolysis 6 ATP are from conversion of pyruvic acid into acetyl CO and 24 ATP are from Krebs cycle. Besides, CO₂ and H₂O are also released.

Regulation of Respiration

The respiratory rhythm centre present in the medulla oblongata is responsible for the regulation of the respiratory rhythm.
The pneumotaxic centre present in the pons regulates or moderates the functions of the respiratory rhythm centre.
A chemosensitive area is located adjacent to the rhythm centre which is highly sensitive to CO₂ and hydrogen ions.
Increase in CO₂ and hydrogen ions activates the chemosensitive area. This in turn signals the respiratory rhythm centre to make adjustments by which CO₂ and hydrogen ions are eliminated.
Receptors present in the aortic arch recognise changes in CO₂ and H⁺ ion concentration and send signals to the rhythm centre for remedial actions.
The role of oxygen in the regulation of respiratory rhythm is insignificant.

Disorders of Respiratory System

Asthma

It is a chronic inflammatory disease of bronchi and bronchioles.
An asthmatic patient experiences difficulty in breathing which causes wheezing.

Emphysema

Terminal bronchioles get obstructed which reduces the ventilation of alveoli.
Alveolar walls are damaged and many alveoli collapse to form large chambers.
This reduces the alveolar surface for the respiratory gases.

Occupational Respiratory Disorders

Occupational respiratory disorders are common in workers in industries which involve processes such as grinding and stone breaking or the manufacture of asbestos.
Because of excessive dust, the defence mechanism of the body is sometimes unable to fully cope.
This results in inflammation leading to fibrosis which causes serious lung damage.

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Respiration in Animals