16.4: Respiratory System - Biology

16.4: Respiratory System - Biology

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The Respiratory System is vital to every human being. During inhalation or exhalation air is pulled towards or away from the lungs, by several cavities, tubes, and openings.

The organs of the respiratory system make sure that oxygen enters our bodies and carbon dioxide leaves our bodies.

The respiratory tract is the path of air from the nose to the lungs. It is divided into two sections: Upper Respiratory Tract and the Lower Respiratory Tract. Included in the upper respiratory tract are the Nostrils, Nasal Cavities, Pharynx, Epiglottis, and the Larynx. The lower respiratory tract consists of the Trachea, Bronchi, Bronchioles, and the Lungs.

As air moves along the respiratory tract it is warmed, moistened and filtered.

Figure 1. Click for a larger image. The major respiratory structures span the nasal cavity to the diaphragm.


There are four processes of respiration. They are:

  1. Breather or ventilation
  2. External Respiration, which is the exchange of gases (oxygen and carbon dioxide) between inhaled air and the blood.
  3. Internal Respiration, which is the exchange of gases between the blood and tissue fluids.
  4. Cellular Respiration

In addition to these main processes, the respiratory system serves for:

  • Regulation of Blood pH, which occurs in coordination with the kidneys,
  • Defense against microbes
  • Control of body temperature due to loss of evaporate during expiration

Respiratory System: Upper and Lower Respiratory Tracts

For the sake of convenience, we will divide the respiratory system in to the upper and lower respiratory tracts:

Upper Respiratory Tract

The upper respiratory tract, can refer to the parts of the respiratory system lying above the sternal angle (outside of the thorax), above the vocal folds, or above the cricoid cartilage. The tract consists of the nasal cavity and paranasal sinuses, the pharynx (nasopharynx, oropharynx and laryngopharynx) and sometimes includes the larynx. Its primary function is to receive the air from the external environment and filter, warm, and humidify it before it reaches the delicate lungs where gas exchange will occur.

Air enters through the nostrils of the nose and is partially filtered by the nose hairs, then flows into the nasal cavity. The nasal cavity is lined with epithelial tissue, containing blood vessels, which help warm the air; and secrete mucous, which further filters the air. The endothelial lining of the nasal cavity also contains tiny hairlike projections, called cilia. The cilia serve to transport dust and other foreign particles, trapped in mucous, to the back of the nasal cavity and to the pharynx. There the mucus is either coughed out, or swallowed and digested by powerful stomach acids. After passing through the nasal cavity, the air flows down the pharynx to the larynx.

Lower Respiratory Tract

The lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory), and lungs (including alveoli). It also sometimes includes the larynx, which we have done here. This is where gas exchange actually takes place.


The larynx (plural larynges), colloquially known as the voice box, is an organ in our neck involved in protection of the trachea and sound production. The larynx houses the vocal cords, and is situated just below where the tract of the pharynx splits into the trachea and the esophagus. The larynx contains two important structures: the epiglottis and the vocal cords.

The epiglottis is a flap of cartilage located at the opening to the larynx. During swallowing, the larynx (at the epiglottis and at the glottis) closes to prevent swallowed material from entering the lungs; the larynx is also pulled upwards to assist this process. Stimulation of the larynx by ingested matter produces a strong cough reflex to protect the lungs. Note: choking occurs when the epiglottis fails to cover the trachea, and food becomes lodged in our windpipe.

The vocal cords consist of two folds of connective tissue that stretch and vibrate when air passes through them, causing vocalization. The length the vocal cords are stretched determines what pitch the sound will have. The strength of expiration from the lungs also contributes to the loudness of the sound. Our ability to have some voluntary control over the respiratory system enables us to sing and to speak. In order for the larynx to function and produce sound, we need air. That is why we can’t talk when we’re swallowing.


Air travels from the larynx to the trachea (Figure 1). The trachea is a tubular structure consisting of dense connective tissue and rings of hyaline cartilage. The trachea is lined with ciliated pseudostratified columnar epithelium with goblet cells. The epithelium moves substances toward the larynx and esophagus for swallowing. The cartilage rings do not completely encircle the trachea but are open posteriorly. The posterior section of the trachea contains a ligament and smooth muscle known as the trachealis muscle. The trachealis muscle can contract and constrict the trachea. The trachea usually ends at about the level of the fifth thoracic segment. The inferior end of the trachea divides into right and left bronchi at an area known as the carina. The carina is the last tracheal cartilage and forms a cartilage division between the two bronchi.

Bronchial Tree

The trachea ends at the carina and divides into two tubular structures called the right and left primary bronchi. The bronchi then divide into smaller branches called secondary or lobar bronchi and then even smaller branches called tertiary or segmental bronchi. The structure of the bronchi is similar to the trachea with incomplete cartilage rings and smooth muscle. As the bronchi get smaller there is less cartilage and more smooth muscle until reaching the tertiary bronchi that consists entirely of smooth muscle. The smooth muscle can constrict the bronchi and impede air passage. The bronchi continue to branch and form small bronchioles which divide to form terminal bronchioles. The terminal bronchioles divide to form respiratory bronchioles that connect with alveolar ducts. The alveolar ducts give rise to alveoli. Alveoli are considered the functional unit of the lung and consist of Dr. Bruce Forciea Page 560 small hollow areas for gas exchange. The alveolar ducts and alveoli are lined with simple squamous epithelium that allows for gas exchange. The cells of the simple squamous epithelium are called Type I pneumocytes. The alveoli also contain other cells known as type II pneumocytes. These cells secrete a substance known as surfactant that helps to decrease the surface tension in the alveoli. The lungs contain about 300 million alveoli.

The Lungs

The lungs are two cone shaped structures residing in the thoracic cavity. The inferior portion of each lung reaches to the diaphragm. The superior portion extends about one inch above each clavicle. The right lung contains three lobes (superior, middle and inferior) and is larger than the left lung which contains two lobes (superior and inferior). The lobes are separated by fissures. The right lung includes a horizontal and oblique fissure while the left lung only contains an oblique fissure. The medial surface of each lung contains an area known as the hilum where vessels enter and exit. The left lung also contains the cardiac notch which is an indentation for the heart. The lungs are surrounded by two pleural membranes. The surface of each lung contains a visceral pleural membrane that closely adheres to the lung’s surface. Lining the interior of the thoracic wall is the parietal pleural membrane. Both are serous membranes. A fluid known as pleural fluid is secreted by each membrane that reduces friction and helps to hold the membranes together.

Learning Objectives

Watch this video to learn more about the respiratory system:

A YouTube element has been excluded from this version of the text. You can view it online here:

Respiratory System in humans – Life Processes, Class 10

Respiration is the exchange of O2 and CO2 between the environment and cells of the body where organic nutrients are broken down enzymatically to release energy. Every cell needs the energy to stay alive. To get energy, every cell needs food and oxygen. Organisms take the food, digest it, and digested food is transported to each and every cell. Similarly, oxygen is taken from the air by a process called breathing and transported to all the cells. Food is oxidized in the cells to produce energy along with carbon dioxide and water. This is called cellular respiration. This reaction takes place in the cytoplasm and mitochondria in the cell.

Glucose + Oxygen à Carbon dioxide + water + energy as ATP

Respiratory System Diagram

When breakdown of glucose occurs with the use of oxygen it is known as aerobic respiration. Sometimes, and in some organisms, respiration takes place in the absence of oxygen. Such respiration is called anaerobic respiration. For example, yeast respires in the absence of oxygen. As a result, a little amount of energy is produced along with ethanol and carbon dioxide. In our body also when energy demand increases and oxygen is in short supply (during heavy exercise, fast running), anaerobic respiration takes place in which lactic acid and carbon dioxide are produced. Lactic acid cause pain and fatigue in the muscles and on getting oxygen, it is converted back to carbon dioxide and water.

Breathing is an act of taking in air, absorbing oxygen from it, releasing carbon dioxide, giving out the air (with less oxygen and more carbon dioxide). Taking in of air is known as inhalation and giving out of the air is known as exhalation. Inhaled air contains about 21 percent oxygen and 0.04 percent carbon dioxide. The exhaled air contains 16.4 percent oxygen and 4.4 percent carbon dioxide.

Types of Respiration

Direct Respiration

Body cells exchange gases with the environment without the aid of any respiratory organ and transportation by blood.

Example: Unicellular organisms, roundworms, flatworms sponges, etc.

Indirect Respiration

There are respiratory surfaces and blood is involved in the transportation of gases between the respiratory surface and body cells. Organs having the respiratory surface is called respiratory organ.

Respiratory System Parts and Functions

There are 8 major organ of Respiratory system:

  • Nasal Cavity
  • Pharynx
  • Epiglottis
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
  • Diaphragm

Nasal Cavity

It is present in the nose and has small hair and mucous which filters the suspended impurities in the air and also warms up the air.

It is an opening into the esophagus and the windpipe. These pipes are covered with epiglottis. Inhaled air enters the pharynx and from there to the windpipe or trachea.

It ensures that air passes into the trachea, and food passes into the esophagus.

Trachea is a tube made of cartilage rings. Trachea is the passage through which air enters the lungs. Its walls are lined with cilia that prevent the suspended particles from being lodged in the lungs if any in the air.

It connects the trachea to the lungs. Air passes through them to a large number of bronchioles.


They are the small narrow tubes that cover the entire area of the lungs. Each bronchiole opens into an alveolus.

Alveoli are hollow, sac-like structures attached to the bronchioles. They have extremely thin walls. Every alveoli is covered by a network of capillaries. The exchange of oxygen and carbon dioxide takes place between blood and air in the alveoli through the walls of the alveoli and blood capillary.

The diaphragm is a muscular organ situated beneath the lungs. It is mainly responsible for the breathing mechanism. When it contracts and moves down, the intercostals muscles also contract, which moves the rib cage up and out. The area inside the lungs increases creating a low-pressure zone forcing air from outside to get in which is called inhalation. The expression of the diaphragm makes it go up, and intercostals muscles expand to bring the rib cage back again. The lungs area is reduced and the extra air inside is pushed out which is called exhalation.

In between inhalation and exhalation, the gaseous exchange takes place between the lungs and the alveoli. Blood flows away from the lungs are rich in oxygen. It is sent through the heart to the cells of different parts of the body for cellular respiration to take place.

Most of the animals breathe through the lungs just like human beings. Cockroaches have spiracles through which air enters the body into a network of the trachea. Every cell gets oxygen from the trachea through diffusion. Earthworms breathe through their skin that is why their skin should always remain moist. Frogs can breathe through their lungs as well as through their skin. Fishes have gills for breathing. They absorb oxygen dissolved in water.

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17.4 Disruptions in the Immune System

A functioning immune system is essential for survival, but even the sophisticated cellular and molecular defenses of the mammalian immune response can be defeated by pathogens at virtually every step. In the competition between immune protection and pathogen evasion, pathogens have the advantage of more rapid evolution because of their shorter generation time, large population sizes and often higher mutation rates. Thus pathogens have evolved a diverse array of immune escape mechanisms. For instance, Streptococcus pneumoniae (the bacterium that causes pneumonia and meningitis) surrounds itself with a capsule that inhibits phagocytes from engulfing it and displaying antigens to the adaptive immune system. Staphylococcus aureus (the bacterium that can cause skin infections, abscesses, and meningitis) synthesizes a toxin called leukocidin that kills phagocytes after they engulf the bacterium. Other pathogens can also hinder the adaptive immune system. HIV infects TH cells using their CD4 surface molecules, gradually depleting the number of TH cells in the body (Figure 17.21) this inhibits the adaptive immune system’s capacity to generate sufficient responses to infection or tumors. As a result, HIV-infected individuals often suffer from infections that would not cause illness in people with healthy immune systems but which can cause devastating illness to immune-compromised individuals.

Inappropriate responses of immune cells and molecules themselves can also disrupt the proper functioning of the entire system, leading to host-cell damage that can become fatal.


Immunodeficiency is a failure, insufficiency, or delay in the response of the immune system, which may be acquired or inherited. Immunodeficiency can allow pathogens or tumor cells to gain a foothold and replicate or proliferate to high enough levels so that the immune system becomes overwhelmed. Immunodeficiency can be acquired as a result of infection with certain pathogens that attack the cells of the immune system itself (such as HIV), chemical exposure (including certain medical treatments such as chemotherapy), malnutrition, or extreme stress. For instance, radiation exposure can destroy populations of lymphocytes and elevate an individual’s susceptibility to infections and cancer. Rarely, primary immunodeficiencies that are present from birth may also occur. For example, severe combined immunodeficiency disease (SCID) is a condition in which children are born without functioning B or T cells.


A maladaptive immune response toward harmless foreign substances or self-antigens that occur after tissue sensitization is termed a hypersensitivity . Types of hypersensitivities include immediate, delayed, and autoimmune. A large proportion of the human population is affected by one or more types of hypersensitivity.


The immune reaction that results from immediate hypersensitivities in which an antibody-mediated immune response occurs within minutes of exposure to a usually harmless antigen is called an allergy . In the United States, 20 percent of the population exhibits symptoms of allergy or asthma, whereas 55 percent test positive against one or more allergens. On initial exposure to a potential allergen, an allergic individual synthesizes antibodies through the typical process of APCs presenting processed antigen to TH cells that stimulate B cells to produce the antibodies. The antibody molecules interact with mast cells embedded in connective tissues. This process primes, or sensitizes, the tissue. On subsequent exposure to the same allergen, antibody molecules on mast cells bind the antigen and stimulate the mast cell to release histamine and other inflammatory chemicals these chemical mediators then recruit eosinophils (a type of white blood cell), which also appear to be adapted to responding to parasitic worms (Figure 17.22). Eosinophils release factors that enhance the inflammatory response and the secretions of mast cells. The effects of an allergic reaction range from mild symptoms like sneezing and itchy, watery eyes to more severe or even life-threatening reactions involving intensely itchy welts or hives, airway constriction with severe respiratory distress, and plummeting blood pressure caused by dilating blood vessels and fluid loss from the circulatory system. This extreme reaction, typically in response to an allergen introduced to the circulatory system, is known as anaphylactic shock. Antihistamines are an insufficient counter to anaphylactic shock and if not treated with epinephrine to counter the blood pressure and breathing effects, this condition can be fatal.

Delayed hypersensitivity is a cell-mediated immune response that takes approximately one to two days after secondary exposure for a maximal reaction. This type of hypersensitivity involves the TH1 cytokine-mediated inflammatory response and may cause local tissue lesions or contact dermatitis (rash or skin irritation). Delayed hypersensitivity occurs in some individuals in response to contact with certain types of jewelry or cosmetics. Delayed hypersensitivity facilitates the immune response to poison ivy and is also the reason why the skin test for tuberculosis results in a small region of inflammation on individuals who were previously exposed to Mycobacterium tuberculosis, the organism that causes tuberculosis.

Concepts in Action

Try your hand at diagnosing an allergic reaction by selecting one of the interactive case studies at the World Allergy Organization website.


Autoimmunity is a type of hypersensitivity to self-antigens that affects approximately five percent of the population. Most types of autoimmunity involve the humoral immune response. An antibody that inappropriately marks self-components as foreign is termed an autoantibody . In patients with myasthenia gravis, an autoimmune disease, muscle-cell receptors that induce contraction in response to acetylcholine are targeted by antibodies. The result is muscle weakness that may include marked difficultly with fine or gross motor functions. In systemic lupus erythematosus, a diffuse autoantibody response to the individual’s own DNA and proteins results in various systemic diseases (Figure 17.23). Systemic lupus erythematosus may affect the heart, joints, lungs, skin, kidneys, central nervous system, or other tissues, causing tissue damage through antibody binding, complement recruitment, lysis, and inflammation.

Autoimmunity can develop with time and its causes may be rooted in molecular mimicry, a situation in which one molecule is similar enough in shape to another molecule that it binds the same immune receptors. Antibodies and T-cell receptors may bind self-antigens that are structurally similar to pathogen antigens. As an example, infection with Streptococcus pyogenes (the bacterium that causes strep throat) may generate antibodies or T cells that react with heart muscle, which has a similar structure to the surface of S. pyogenes. These antibodies can damage heart muscle with autoimmune attacks, leading to rheumatic fever. Insulin-dependent (Type 1) diabetes mellitus arises from a destructive inflammatory TH1 response against insulin-producing cells of the pancreas. Patients with this autoimmunity must be treated with regular insulin injections.


  • Moore, K.L., Persaud, T.V.N. & Torchia, M.G. (2015). The developing human: clinically oriented embryology (10th ed.). Philadelphia: Saunders. Chapter 10 Respiratory System
  • Schoenwolf, G.C., Bleyl, S.B., Brauer, P.R., Francis-West, P.H. & Philippa H. (2015). Larsen's human embryology (5th ed.). New York Edinburgh: Churchill Livingstone. Chapter 11 Development of the Respiratory System and Body Cavities
  • Before We Are Born (5th ed.) Moore and Persaud Chapter 13 p255-287
  • Essentials of Human Embryology Larson Chapter 9 p123-146
  • Human Embryology Fitzgerald and Fitzgerald Chapter 19,20 p119-123
  • Anatomy of the Human Body 1918 Henry Gray The Respiratory Apparatus

Chapter Overview: Respiratory System

In this chapter, you will learn about the respiratory system — the system that exchanges gases (such as oxygen and carbon dioxide) between the body and the outside air. Specifically, you will learn about:

  • The process of respiration, in which oxygen moves from the outside air into the body and carbon dioxide and other waste gases move from inside the body into the outside air.
  • The organs of the respiratory system, including the lungs, bronchial tubes, and the rest of the respiratory tract.
  • How the respiratory tract protects itself from pathogens and other potentially harmful substances in the air.
  • How the rate of breathing is regulated to maintain homeostasis of blood gases and pH.
  • How ventilation, or breathing, allows us to inhale air into the body and exhale air out of the body.
  • The conscious and unconscious control of breathing.
  • Nasal breathing compared to mouth breathing.
  • What happens when a person is drowning.
  • How gas exchange occurs between the air and blood in the alveoli of the lungs, and between the blood and cells throughout the body.
  • Disorders of the respiratory system, including asthma, pneumonia, chronic obstructive pulmonary disease (COPD), and lung cancer.
  • The negative health effects of smoking.

As you read the chapter, think about the following questions:

  1. Where are the bronchial tubes? What is their function?
  2. What is the function of mucus? Why can too much mucus be a bad thing?
  3. Why did Dr. Choo check Erica’s blood oxygen level?
  4. Why do you think Dr. Choo warned Erica to avoid cough suppressant medications?
  5. How does acute bronchitis compare to chronic bronchitis? How do they both relate to smoking?

16.4: Respiratory System - Biology

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An Example of Phylum Mollusca: Neopilina

In this article we will discuss about of Neopilina:- 1. Discovery of Neopilina 2. Habit and Habitat of of Neopilina 3. External Structure 4. Coelom 5. Digestive System 6. Locomotion 7. Respiratory System 8. Circulatory System 9. Excretory System 10. Nervous System and Sense Organs 11. Reproductive System 12. Relationships with other Molluscs.

  1. Discovery of Neopilina
  2. Habit and Habitat of of Neopilina
  3. External Structure of Neopilina
  4. Coelom of Neopilina
  5. Digestive System of Neopilina
  6. Locomotion of Neopilina
  7. Respiratory System of Neopilina
  8. Circulatory System of Neopilina
  9. Excretory System of Neopilina
  10. Nervous System and Sense Organs of Neopilina
  11. Reproductive System of Neopilina
  12. Relationships of Neopilina with other Molluscs

1. Discovery of Neopilina:

Neopilina galatheae is a living representa­tive of the class Monoplacophora. This newly discovered species possesses peculiar ad­mixture of molluscan and annelidan fea­tures. Neopilina is a very primitive member amongst the molluscs and represents a sort of connecting bridge between the annelids and molluscs.

There are many fossil relatives of this genus which were quite abundant in Cambrian to Devonian strata. The genus, Neopilina, was collected from the western coast of Mexico. This discovery has added new dimension as regards the phylogenetic relationship of the molluscs as a whole with the annelids.

The Danish Zoologist, Lemche first collected the species in 1952 from the assorted molluscs collected in Galathea ex­pedition. Another species was discovered during the voyage of American Research Vessel, Vema in 1958. The anatomy of Neopilina galatheae was exhaustively worked out by Lemche and Wingstrand (1959).

2. Habit and Habitat of of Neopilina:

Neopilina is a deep-sea variety and was collected from the Pacific Ocean at depths from 2500 to 5000 m. Neopilina lives in mud and feeds mainly on foraminifera. It also takes radiolarians and diatoms as evidenced by their remains in its stomach.

3. External Structures of Neopilina:

The body of Neopilina is more or less bilaterally symmetrical and exhibits metameric segmentations like annelids. Neopilina galatheae is about 3.7 cm long, 3.3 cm wide and 1.4 cm high. The dorsal side of the body is covered by a thin shell. The shell is circu­lar in outline. The apex of the shell is drawn anteriorly and is slightly bent (Fig. 16.2A).

The shell is composed of a thick calcareous middle prismatic and innermost calcareous nacreous layer, covered over by periostracum. These three layers of the shell are formed from the margin of the mantle.

The soft parts of the body are seen only from the ventral side. There is a large flat foot like that of Chiton. Surrounding the foot and head there is a deep pallial groove contain­ing five pairs of gills.

The mouth is situated medially in front of the foot and the anus is located posterior to the foot (Fig. 16.2B). In front of the mouth there is a transverse labial swelling which prolongs laterally into two ciliated lobes, one on each side.

These ciliated lobes are comparable to the anterior labial palps of Unio. Anterior to the transverse swelling there is a pair of small pre-oral ten­tacles. There is a swollen labium which bears a number of postoral tentacles on the anterior surface.

There are eight pairs of dorso-ventral muscles in Neopilina (Fig. 16.3A). These muscles originate from the shell and extend to the median wall of the pallial groove. The dorso-ventral muscles correspond to the columellar muscles of the gastropods.

4. Coelom of Neopilina:

The coelom is represented by a caudal pericardium and two pairs of spacious gonocoels.

5. Digestive System of Neopilina:

The mouth leads into the buccal cavity which is covered by a cuticular plate. The pharynx extends to the dorsal side and bears a radular sac with a radula.

The pharynx pro­duces a pair of long, thin-walled diverticula called the foregut gland. The stomach contains a crystalline style in its median diverticu­lum. The stomach bears a pair of large folded midgut glands. The stomach continues as a coiled midgut (Fig. 16.3B) to end in the anus.

6. Locomotion of Neopilina:

There is a large flat ventral foot which occu­pies almost whole of the ventral side of the body. It is modified for creeping movement.

7. Respiratory System of Neopilina:

The respiratory organs of Neopilina are the gills attached to the dorsal side of the pallial groove. There are five pairs of gills in Neopilina galatheae while in Neopilina ewingi there are six pairs. The posterior gills have lamellae usually on one side, while the anterior ones have lamellae on both sides. The gills are supplied with blood by the afferent gill sinus to be returned by efferent gill sinus (Fig. 16.4).

8. Circulatory System of Neopilina:

The heart consists of two pairs of auricles and two ventricles. The ventricles are located on the lateral side of the hindgut and continue as a single aorta. The aorta supplies blood to the different parts of the body and is collected in different sinuses like other molluscs.

9. Excretory System of Neopilina:

The excretory organs are six pairs of nephridia. The nephridia are designated as the metanephridia which lie at the bottom of the pallial groove. A metanephridium has the appearance of a folded sac which opens to the exterior by nephridiopore near the gill. The metanephridia are modified coelomoducts.

10. Nervous System and Sense Organs of Neopilina:

The nervous system is primitively built and corresponds to the ladder-like arrange­ment of Chiton. Two ill-developed cerebral ganglia, one on each side of the pharynx are present. The cerebral ganglia are connected by cerebral commissure which encircles the mouth. The cerebral ganglion and its com­missure innervate the preoral tentacles, labrum, oral lobes and subradular organ.

The pedal cords innervate the foot. The pleural cords send nerves to the mantle, gills and nephropores. The sense organs of Neopilina include a pair of statocysts, a subradular organ and pre-oral tentacles. The pedal and pleural cords are connected posteriorly to form loops with many cross-connections between them. Such an arrangement in the nervous system speaks of primitive organisation.

11. Reproductive System of Neopilina:

Neopilina is a dioecious animal. The gonads are paired ventral organs (Fig. 16.4) which open into the third and fourth metanephridia. These metanephridia act as the gonoducts. The sperms and ova are discharged through nephropores. Accessory reproductive glands are lacking.

12. Relationships of Neopilina with other Molluscs:

Neopilina possesses many peculiar fea­tures which make the precise relationship of it debatable. The metameric arrangement of dorso-ventral muscles, gills, nephridia and gonads establish its phylogenetic relation­ship with the annelids. As regards its intraphylar position, it shows many similari­ties with Chiton, Nautilus and many gastro­pods. The Ideation of the body in the shell corresponds closely to that of Nautilus.

Fea­tures common in Neopilina and Chiton are the shape of body, the flat ventral foot, the shell covering the head and mantle many dorso-vental muscles, auricles, numerous gills, etc. But Neopilina differs from Chiton by having a single shell, statocysts and the nephropores functioning as the gonoducts.

It is claimed that the gastropods have evolved from Neopilina via Nautilus-like form which has a symmetrical shell. In gastropods, tor­sion of the visceral mass has resulted the asymmetrical configuration, reduction of dorso-ventral muscles into a single columellar muscle, reduction in the number of gills and metanephridia and many other features.


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17. Hughes M and West JB. (2008). Gravity is the major factor determining the distribution of blood flow in the human lung. Journal of Applied Physiology, 104, 1531–3. Find this resource:

18. Glenny R. (2008). Gravity is not the major factor determining the distribution of blood flow in the human lung. Journal of Applied Physiology, 104: 1533–6. Find this resource:

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Heritability and Differential Susceptibility (to Mental Disorders)


Individual differences in parasympathetic functioning, such as the heart rate variability measure respiratory sinus arrhythmia (RSA), may correlate with depression risk ( Beauchaine, 2012 ). High baseline RSA is observed in individuals who are able to self-regulate emotions and cope with potential threats, while depressed individuals exhibit chronically low RSA levels. Depressed individuals with maltreatment history have also shown flattening of naturally occurring daytime cortisol production, but increased stress reactivity ( Cicchetti and Rogosch, 2012 Dennis et al., 2012 Gunnar and Adam, 2012 ). Lastly, sensitivity to criticism among depressed individuals might be explained by more pronounced amygdala activation in response to unpleasant social stimuli and a limited ability to down-regulate this response ( Reiss and Leve, 2007 ). Therefore, genetic pathways influencing RSA, emotion processing, and cortisol system functioning may provide valuable insights into depression risk.

16.4: Respiratory System - Biology

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The respiratory system consists of a series of passageways and ducts that bring air from the outside of the body to the lungs inside.

Air first enters through the nostrils and into the nasal cavity where particulate matter, potentially harmful to breathe like bacteria and dirt, is filtered out by hairs and mucus and cleaner air is warmed and humidified.

Air can also flow through the mouth, reaching the pharynx and larynx before being funneled into the main airway, the trachea.

As the trachea leads into the thoracic cavity, the ends bifurcate into two bronchi, corresponding to the left and right lungs, which are surrounded by pleural membranes and bound inferiorly by the diaphragm.

Within each lung a bronchus splits into increasingly smaller branches called bronchioles. At the smallest tips are alveolar ducts subdivided into tiny sacs of alveoli, thin layers of parenchymal cells.

Such organization provides a large surface area for gas exchange to occur with the circulatory system.

22.1: The Respiratory System

The respiratory system is comprised of the organs that enable breathing. Air enters the nostrils and mouth, followed by the pharynx (throat) and larynx (voice box), which lead to the trachea (windpipe). In the thoracic cavity, the trachea splits into two bronchi that allow air to enter the lungs. The bronchi split into progressively smaller bronchioles and terminate in small groups of tiny sacs in the lungs called alveoli, where gas exchange occurs.

Removal of Debris

Air is cleansed in the nasal cavity, but anything that passes those defenses or enters through the mouth can be caught in the lungs. The lungs produce mucus that traps foreign particles, and the bronchi and bronchioles are lined with cilia that beat mucus and debris upward toward the throat for disposal (i.e., swallowing). Smoking damages the cilia, making removal of the excess mucus produced by smoking more difficult. This is one of the reasons smokers are more susceptible to respiratory infections.


The trachea is a 10-12 cm long tube located in front of the esophagus that allows air to enter and exit the lungs. Its C-shaped hyaline cartilage keeps the trachea open. When the smooth muscle of the trachea contracts, the diameter of the trachea decreases and exhaled air is pushed out with great force (e.g., coughing). In cases of damage to the throat or mouth that blocks breathing, a tracheostomy, a surgically-created hole in the trachea, can allow air into and out of the lungs.


A single alveolar duct at the end of a bronchiole divides into approximately 100 alveolar sacs, which each resembling a bunch of grapes on a stem. Each alveolar sac has 20-30 alveoli, which look like little bubbles, or grapes. Alveoli are the sites of gas exchange and have direct contact with capillaries. This structure is crucial to the function of the lungs&mdashproviding the body with oxygen and removing carbon dioxide&mdashbecause it maximizes the surface area of the lungs. The lungs contain 75 m 2 of alveoli surface area, the size of a small apartment!

Hogan, Brigid L. M., Christina E. Barkauskas, Harold A. Chapman, Jonathan A. Epstein, Rajan Jain, Connie C. W. Hsia, Laura Niklason, et al. &ldquoRepair and Regeneration of the Respiratory System: Complexity, Plasticity, and Mechanisms of Lung Stem Cell Function.&rdquo Cell Stem Cell 15, no. 2 (August 7, 2014): 123&ndash38. [Source]

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