Respiratory System Cheat Sheet by Ajita

Breathing; inhalation and exhalation of air involving exchange of air between the lungs and the atmosp­here.

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.

Hence, expansion of lungs must take place for decrease in air pressure within them.

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.

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)

The process of inhalation is said to be active.

For exhalation to occur, air pressure in the lungs> atmosp­heric pressure.

Two inwardly directed forces result in elastic recoil:

2. the inward pull of the surface tension due to film of alveolar fluid.

As the external interc­­ostals relax, the ribs move downwards.

The process of exhalation is said to be passive.

They contract, causing an increase in air pressure within the lungs.

Contra­ction of the internal interc­ostals, which extend inferiorly and poster­iorly between adjacent ribs, pulls the ribs inferi­orly.

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

The partial pressures of O2 and CO2 determines their movement between atmosphere and lungs, lung and blood and blood and body cells.

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.

The exchange of gases between systemic capill­aries and tissue cells is know as Internal Respir­ation or Systemic Gas Exchange.

By the time, blood exits the capill­aries, PO2 is 40 mmHg.

The deoxyg­enated blood is then pumped to the heart and enters another cycle of external respir­ation.

1. Partial pressure difference of the gases.

3. Diffusion distance.

1.5% of inhaled O2 dissolves in the blood plasma, while 98.5% of inhaled O2 bind to haemog­lobin in the red blood cells.

Oxygen and haemog­lobin combine in a reversible reaction to form oxyhem­ogl­obin.

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.

The relation between the Percent Saturation and Partial Pressure of Oxygen is illust­rated by Oxygen- Haemog­lobin Dissoc­iation Curve.

When PO2=40 (deoxy­genated blood in systemic veins), percent situation = 75%

#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.

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.

1. Dissolved CO2: The smallest percen­tag­e—about 7%—is dissolved in blood plasma.

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^-.

(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).

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.

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:

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.

3) Apneustic Area: Present in the lower pons. This area sends stimul­atory impulses to the inspir­atory area that activate it and prolong inhala­tion.