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Respiratory System Cheat Sheet by

Physiology: Life Sustaining Systems Source- Principles of Physiology and Anatomy by Tortora

Pulmonary Ventil­ation

Breathing; inhalation and exhalation of air involving exchange of air between the lungs and the atmosp­here.
It occurs due to pressure difference between the lungs and the atmosphere created by the contra­ction and expansion of respir­atory muscles.
Before respir­ation, air pressure inside the lungs is equal to air pressure in the atmosp­here, ie, 760 mmHg.


For inhalation to occur, air pressure in the lungs < air pressure of the atmosp­here.
According to Boyle's law, volume of container is inversely propor­tional to air pressure.
Hence, expansion of lungs must take place for decrease in air pressure within them.
Lung expansion occurs by contra­ction of diaphragm, external interc­ostal muscles and accessory inspir­ation muscles.
Diaphragm: a dome shaped skeletal muscle that forms the floor of the thoracic cavity. It in innervated by the phrenic nerves (emerging from the 3rd, 4th and 5th cervical levels of the spinal cord). The contra­ction of the diaphragm causes it to flatten and increases the vertical diameter of the thoracic cavity. This decreases the air pressure by 2-3 mmHg and causes and inhalation of 500 ml of air.
External Interc­ostals: contra­ction of these muscles causes an elevation of the ribs. They increase the antero­pos­terior and lateral diameters of the chest cavity.
Accessory muscles of inhalation: Serve little purpose during quiet inhala­tion, however they are capable of contra­cting vigorously during forced ventil­ation. They include: sterno­cle­ido­mastoid muscles (elevates the sternum), scalene muscles (elevate the first two ribs) and the pectoralis minor muscle (elevate third through fifth rib)
As the lung volume increases in this way, the air pressure inside the lungs, called alveolar (intra­pul­monic) pressure, drops from 760 to 758 mmHg.
The process of inhalation is said to be active.


For exhalation to occur, air pressure in the lungs> atmosp­heric pressure.
Exhalation results from elastic recoil of the chest wall and lungs, both of which has a tendency to spring back when stretched.
Two inwardly directed forces result in elastic recoil:
1. the recoil of elastic fibres that were stretched during inhala­tion.
2. the inward pull of the surface tension due to film of alveolar fluid.
As the diaphragm relaxes, the dome moves upwards due to its elasti­­city.
As the external interc­­ostals relax, the ribs move downwards.
This results in an increase in air pressure to about 760 mmHg.
The process of exhalation is said to be passive.
During forceful exhala­tion, the internal interc­ostal and the abdominal muscles come into play.
They contract, causing an increase in air pressure within the lungs.
Contra­ction of the abdominal muscles moves the inferior ribs downward and compresses the abdominal viscera, thereby forcing the diaphragm superi­orly.
Contra­ction of the internal interc­ostals, which extend inferiorly and poster­iorly between adjacent ribs, pulls the ribs inferi­orly.

Factors Affecting Pulmonary Ventil­ation

Surface Tension:
Compli­ance: Compliance refers to how much effort is required by the lungs to stretch the lungs and the chest walls. High compliance means that the lungs and chest wall expand easily. In lungs, compliance is related to two princple factors: surface tension and elasti­city.
Airflow Resist­ance:

Exchange of Oxygen and Carbon Dioxide

Exchange of oxygen and carbon dioxide between alveolar air and pulmonary blood occurs by simple diffusion and is governed by the Dalton's Law and the Henry's Law

Dalton's Law

Each gas in a mixture of gases exerts its own pressure as if no other gases as present.
The pressure of a specific gas in a mixture is call partial pressure.
The partial pressures of O2 and CO2 determines their movement between atmosphere and lungs, lung and blood and blood and body cells.
Each gas diffuses across a semi permeable membrane from a region of higher pressure to a region of lower pressure.

Henry's Law

The amount of gas that will dissolve in a liquid is propor­tional to its partial pressure and solubi­lity.
The ability of gas to stay in liquid is higher when their partial pressure is higher and when it has high solubility in water.

External Respir­ation

External respir­ation, or pulmonary gas exchange is the diffusion of )2 from the air in the alveoli to the blood in the pulmonary capill­aries and diffusion of CO2 in the opposite direction.
It is the conversion of deoxyg­enated blood coming from the right side of the heart to oxygenated blood going to the left side of the heart.
O2 diffuses from alveolar air, where its PO2 is 105 mmHg to blood capill­aries, where its PO2 is 40 mmHg.
Diffusion occurs till the PO2 of the blood in the pulmonary capill­aries matches the PO2 of the alveolar air, ie, 105 mmHg.
The PCO2 in the deoxyg­enated blood of the pulmonary capill­aries is 45 mmHg and in the alveolar air is 40 mmHg.
Hence, CO2 diffuses from deoxyg­enated blood into the alveoli till the PCO2 of the blood decreases to 40 mmHg.


Changes in partial pressures of oxygen and carbon dioxide (in mmHg) during external and internal respir­ation.

Internal Respir­ation

The exchange of gases between systemic capill­aries and tissue cells is know as Internal Respir­ation or Systemic Gas Exchange.
PO2 of the systemic capill­aries (100 mmHg) is more than PO2 of the tissue cells (40mmHg). Hence, oxygen from the capill­aries diffuses into the tissue cells, where they are used for energy produc­tion.
By the time, blood exits the capill­aries, PO2 is 40 mmHg.
PCO2 of the systemic capill­aries (40 mmHg) is less than PCO2 of the tissue cells (45 mmHg). Hence, CO2, which is constantly produced by the cells, diffuses from the cells into the blood in the systemic capill­aries.
The deoxyg­enated blood is then pumped to the heart and enters another cycle of external respir­ation.

Factors affecting rate of Gas Exchange

1. Partial pressure difference of the gases.
2. Surface area available for gas exchange.
3. Diffusion distance.
4. Molecular weight and solubility of gases.

Transport of Oxygen

1.5% of inhaled O2 dissolves in the blood plasma, while 98.5% of inhaled O2 bind to haemog­lobin in the red blood cells.
Heme portion of haemog­lobin contains 4 iron atoms, each of which binds to an oxygen molecule.
Oxygen and haemog­lobin combine in a reversible reaction to form oxyhem­ogl­obin.
Relati­onship between Oxyhem­oglobin and Partial Pressure of Oxygen:
The higher the PO2, the more oxygen will bind to haemog­lobin. When reduced haemog­lobin is completely converted into oxyhem­ogl­obin, it is said to be fully saturated. When haemog­lobin is a mix of Hb and Hb-O2, it is said to be partially saturated.
Percent Saturation of Haemog­lobin: average saturation of haemog­lobin with oxygen.
The relation between the Percent Saturation and Partial Pressure of Oxygen is illust­rated by Oxygen- Haemog­lobin Dissoc­iation Curve.
When PO2=20 (deoxy­genated blood in contra­cting skeletal muscles), percent saturation = 35%
When PO2=40 (deoxy­genated blood in systemic veins), percent situation = 75%
When PO2=100 (oxyge­nated blood in systemic arteries), percent saturation is near 100.
Factors Influe­ncing Affinity of Haemog­lobin towards Oxygen:
1. Acidity: An increase in acidity, causes affinity of haemog­lobin to O2 to decrease. Hence, curve shifts right. Decreases affinity means oxygen more readily dissoc­iates from the haemog­lobin and is more easily available to tissue cells.
#Bohr's Effect: the effect of pH on the affinity of haemog­lobin towards oxygen. An increase in H+ in blood causes O2 to unload from haemog­lobin and the binding of haemog­lobin to oxygen causes unloading of H+ from haemog­lobin. The explan­ation for the Bohr effect is that hemo- globin can act as a buffer for hydrogen ions (H􏱩). But when H􏱩 ions bind to amino acids in haemog­lobin, they alter its structure slightly, decreasing its oxygen­-ca­rrying capacity.
2. Partial Pressure of CO2: As CO2 enters the blood, much of it is converted into carbonic acid (H2CO3), a reaction catalysed by and enzyme in RBC called carbonic anhydrase. The carbonic acid does formed dissoc­iates into bicarb­onate ions and H+ ions. As H+ ion concen­tration in blood increases, acidity increases, causing more dissoc­iation of oxygen from haemog­lobin. Dissoc­iation curve moves right.
3. Temper­ature: Within limits, an increase in temper­ature promotes unloading of O2 from haemog­lobin. Metabo­lically active cells release acids and heat which in turn promote release of O2 from haemog­lobin to be used by them.
4. 2,3-bi­sph­osp­hog­lyc­erate: decreases affinity of haemog­lobin towards oxygen. Certain hormones such as hGH, thyroxine epinep­hrine, norepi­nep­hrine and testos­terone increase BPG produc­tion. ,
Foetal Haemog­lobin has a much greater affinity to oxygen that adult haemog­lobin. This is very important because the O2 saturation in maternal blood in the placenta is quite low, and the foetus might suffer hypoxia were it not for the greater affinity of foetal haemog­lobin for O2.


Binding of Oxygen to Haemog­lobin to form Oxyhem­ogl­obin.


Oxygen Dissoc­iation Curve. Factors increasing affinity of haemog­lobin towards oxygen move the graph towards the left, and factors decreasing the affinity of haemog­lobin towards oxygen move the graph towards the right.

Transport of Carbon Dioxide

1. Dissolved CO2: The smallest percen­tag­e—about 7%—is dissolved in blood plasma.
2. Carbamino compounds: about 23%, combines with the amino groups of amino acids and proteins in blood to form carbamino compounds. most of the CO2 transp­orted in this manner is bound to hemogl­obin. The main CO2 binding sites are the termi- nal amino acids in the two alpha and two beta globin chains. Heamog­lobin that has bound CO2 is termed carbam­ino­hem­oglo- bin (Hb–CO2)
3. Bicarb­onate ions: icarbonate ions. The greatest percentage of CO2—about 70%—is transp­orted in blood plasma as bicarb­onate ions (HCO3􏳠). As CO2 diffuses into systemic capill­aries and enters red blood cells, it reacts with water in the presence of the enzyme carbonic anhydrase (CA) to form carbonic acid, which dissoc­iates into H+ and HCO3-.
#Haldane Effect: The lower the amount of oxyhem­oglobin (Hb–O2), the higher the CO2 carrying capacity of the blood, a relati­onship known as the Haldane effect.
Some HCO3- moves out into the blood plasma, down its concen­tration gradient. In exchange, chloride ions (Cl-) move from plasma into the RBCs. This exchange of negative ions, which maintains the electrical balance between blood plasma and RBC cytosol, is known as the chloride shift


Formation of carbonic acid by carbonic anhydrase and its dissoc­iation to form bicarb­onate ions.


Summary of chemical reactions that occur during gas exchange.


Transport of oxygen (O2) and carbon dioxide (CO2) in the blood.

Summary of chemical reactions

(a) As carbon dioxide (CO2) is exhaled, haemog­lobin (Hb) inside red blood cells in pulmonary capill­aries unloads CO2 and picks up O2 from alveolar air. Binding of O2 to Hb-H releases hydrogen ions (H+). Bicarb­onate ions (HCO3-) pass into the RBC and bind to released H+, forming carbonic acid (H2CO3). The H2CO3 dissoc­iates into water (H2O) and CO2, and the CO2 diffuses from blood into alveolar air. To maintain electrical balance, a chloride ion (Cl-) exits the RBC for each HCO3- that enters (reverse chloride shift). (b) CO2 diffuses out of tissue cells that produce it and enters red blood cells, where some of it binds to haemog­lobin, forming carbam­ino­hem­oglobin (Hb–CO2). This reaction causes O2 to dissociate from oxyhem­oglobin (Hb–O2). Other molecules of CO2 combine with water to produce bicarb­onate ions (HCO3-) and hydrogen ions (H+). As Hb buffers H+, the Hb releases O2 (Bohr effect). To maintain electrical balance, a chloride ion (Cl-) enters the RBC for each HCO3+ that exits (chloride shift).

Control of Respir­ation

Respir­atory Centres: Clusters of neurons in the medulla oblongata and pons in the brain stem that send nerve impulses to the respir­atory muscles, stimul­ating them to contract.
The respir­atory centre can be divided into three areas on the basis of their function:
1) Medullary Rhythm­icity Area: controls the basic rhythm of respir­ation and is present in the medullary oblongata. Can be classified into inspir­atory and expiratory areas:
Inspir­atory Area: Nerve impulses generated from the inspir­atory area set the basic rhythm of breathing. Nerve impulses are generated for 2 seconds. The nerve impulse travels to the external interc­ostals through the interc­ostal nerves and the diaphragm through the phrenic nerves. This causes the diaphragm and the muscles to contract. At the end of the 2 seconds, the muscles must relax for 3 seconds, allowing for passive elastic recoil of the muscles for exhala­tion.
Expiratory Area: Remains inactive during quiet breathing. During forceful expira­tion, the inspir­atory centre activates the expiratory centre to send nerve impulses to the abdominal and internal interc­ostals muscles, causing them to contract and forcing air out of the lungs.
2) Pneumo­taxic Area: Present in the upper pons. Transmit inhibitory impulses to the inspir­atory area, preventing the lungs from becoming too full of air.
3) Apneustic Area: Present in the lower pons. This area sends stimul­atory impulses to the inspir­atory area that activate it and prolong inhala­tion.


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