Pulmonary Circulation Oxygen and Carbon Dioxide Transport

Blood Flow to and from the Lungs

1.Deoxygenated blood is supplied to the lungs by the pulmonary artery.

2.Oxygenated blood is received from the lungs by the pulmonary vein.

3.Capillary Network Around Alveoli:

In the lungs, there is a network of capillaries placed around the alveoli. There is an intimate association between the endothelial blood capillaries and the squamous epithelium of the alveoli.

4.This intimate absorption helps in the exchange of O₂ and CO₂ between the blood and the alveoli.

Function of Respiratory Tubes:-

1.Air Conditioning:

The region of the respiratory tube functions as an air conditioner because the incoming air is warmed, moistened, filtered, and sterilized during its passage through the nose.

2.Heat Radiation:

Blood capillaries radiate heat in a manner similar to hot copper pipes. The incoming air passing over them gets warmed up to the body temperature.

3.Moistening Air:

The inspiratory air becomes moistened on coming in contact with the mucus secreted by the glandular epithelium.

4. Cleaning and filtering:-

The nasal hair prevents the entry of dust particles through the incoming site. The ciliated epithelium filters fine dust particles, and mucus secreted from the epithelium adheres to impurities present in the incoming air due to its adhesive nature.

5.Sterilization:

Mucus is antiseptic in nature and kills bacteria in the incoming air.

Transport of O₂ by the Blood

1. In almost all vertebrates and in earthworms, hemoglobin acts as a carrier of oxygen. Hemoglobin consists of a heme (iron-protein part) and a globin (protein part).

2.Hemoglobin contains an iron atom in the ferrous (Fe²⁺) state. In the presence of O₂, the Fe²⁺ ion is converted to the Fe³⁺ ion.

3. The globin consists of four polypeptides, each attached with a heme group. Hence, each molecule of hemoglobin can carry four molecules of O₂ at a time by producing the compound oxyhemoglobin.

4. The formation and dissociation of oxyhemoglobin is a reversible process. Therefore, the reaction is called a reversible oxygenation reaction where O₂ remains bonded.

The chemical reaction for the formation of oxyhemoglobin is:

Hb + 4O₂ → Hb(O₂)₄

The Oxyhemoglobin Formation and its dissociation is reversible in nature

Hence the reaction is called reversible oxygenated Reaction where O2 remains binded with the haemoglobin at its N2 atom.

Causes of Oxyhemoglobin Formation at the Site of the Lungs:-

1.Normal pH of Blood:

The normal pH of blood (7.4) facilitates the formation of oxyhemoglobin.

2.Comparatively Low CO2 Tension:

Lower carbon dioxide (CO₂) tension compared to other parts of the body helps in oxyhemoglobin formation.

3.Lower Temperature:

The temperature in the lungs is relatively lower than in other parts of the body, which aids in the formation of oxyhemoglobin.

4.High O2 Tension:

High oxygen (O₂) tension in the lungs promotes the binding of oxygen to hemoglobin, forming oxyhemoglobin.

Causes of Oxyhemoglobin Dissociation in Tissues:-

1.Slightly Increased Temperature:

An increase in temperature in the tissues causes oxyhemoglobin to release oxygen.

2. High CO2 Tension:

High carbon dioxide tension in the tissues leads to the dissociation of oxyhemoglobin.

3.Low O2 Tension:

Lower oxygen tension in the tissues facilitates the release of oxygen from oxyhemoglobin.

Transport of CO₂ in Blood:-

1.Concentration of CO₂ in Blood:

🪶The concentration of CO₂ in arterial blood is lower than in venous blood.

🪶In arterial blood, its tension is 40 mm Hg, while in venous blood, it is 46 mm Hg.

2.Transport of CO₂ by Physical Solution (Coater):

🪶Under normal conditions of temperature and pressure, about 10% of CO₂ (2.7 ml) is transported by being chemically combined with water in the plasma, forming carbonic acid.

💫CO₂ + H₂O → H₂CO₃

🪶An increase in carbonic acid formation in the blood leads to immediate dissociation into H⁺ and HCO₃⁻.

🪶If all CO₂ were transported this way, the pH of blood would decrease to between 3 and 4.5, which could be fatal.

3.Transport of CO₂ as Chemical Compounds:

🪶There are two types of chemical compounds involved: carbamino compounds and bicarbonate.

Carbamino Compounds:--

🪶About 20% of CO₂ (3.7 ml) is transported in the blood by combining with hemoglobin.

🪶The formation of carbaminohemoglobin is rapid and independent of the enzyme carbonic anhydrase.

💫Hb-NH₂ + CO₂ → Hb-NH-COOH

Bicarbonate:-

🪶The majority of CO₂ (about 70%) is transported in the form of bicarbonate (HCO₃⁻).

🪶CO₂ is transported in blood through three main mechanisms: physical solution, carbamino compounds, and bicarbonate.

🪶Each method plays a crucial role in maintaining the pH balance and preventing toxicity in the body.

Carbon Dioxide Transport in Blood:-

1.In Erythrocytes (Red Blood Cells):-

🪶CO₂ from plasma enters red blood cells (RBCs)

🪶CO₂ combines with water to form carbonic acid (H₂CO₃), facilitated by the enzyme carbonic anhydrase.

🪶Carbonic acid dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).

🪶Hydrogen ions bind to hemoglobin, releasing oxygen (O₂) and producing carbaminohemoglobin.

🪶Bicarbonate ions are transported to the lungs, where they are converted back to CO₂ for exhalation.

2.In Blood Plasma:-

🪶CO₂ is transported in plasma by three different means:

🪶 Alkaline phosphates in blood plasma combine with H₂CO₃ to produce NaH₂PO₄ (sodium dihydrogen phosphate) and HCO₃⁻.

🪶Disodium Hydrogen Phosphate: Na₂HPO₄ combines with H⁺ to form NaH₂PO₄.

👉By Plasma Proteins (Protein Buffers):

🪶Protein Buffers: Plasma proteins combine with CO₂ to form sodium bicarbonate (NaHCO₃).

🪶Buffer Action: Proteins react with H₂CO₃ to produce NaHCO₃ and other bicarbonates, facilitating CO₂ transport in the form of bicarbonates in plasma.

👉The Chloride Shift and Its Role in Gas Transport

1.Chloride Shift Explanation:

🪶The chloride shift involves the movement of chloride ions (Cl-) into the red blood cells (RBCs) and bicarbonate ions (HCO3-) out of RBCs through the RBC membrane, which aids in the transport of carbon dioxide (CO2).

🪶The RBC membrane is called the "Donnan Equilibrium Membrane" because it is permeable to negatively charged ions like Cl- and HCO3- but impermeable to positive ions like K+ (potassium) and Na+ (sodium).

2.CO2 Entry and Reaction in RBCs:

🪶At the tissues, when CO2 enters the RBCs, it undergoes a reaction:

🪶 The CO2 reacts with water (H2O) to form carbonic acid (H2CO3), which then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-).

🪶The bicarbonate ions are exchanged with chloride ions (Cl-) to maintain the balance of negative charges between RBCs and plasma.

3.Transport in Plasma:

🪶In the plasma, CO2 and bicarbonate (HCO3-) ions are produced and transported in this form through the blood to the lungs.

4.Conversion at the Lungs:

🪶At the lungs, where the partial pressure of CO2 is higher, bicarbonate (HCO3-) is converted back into CO2 and water (H2O).

🪶The bicarbonate (HCO3-) re-enters the RBCs in exchange for chloride ions (Cl-), and the CO2 diffuses into the lungs to be expelled.

5.Reversible Reactions:

🪶All the reactions involved in the chloride shift are reversible, ensuring efficient transport and release of CO2.

🪶This process can be depicted in the following diagram (provide diagram if needed):

Release of CO₂ at the Respiratory Surface:-

1.Conversion of Compounds in the Lungs:

🪶KHCO₃, Na₂CO₃, Malate, and Hb-NH-COOH are transported to the lungs where they are converted into CO₂ in the presence of various factors.

2.Diffusion and Release of CO₂:

🪶The CO₂ diffuses into the alveoli of the lungs and is then released into the external environment through exhalation.

🪶The following reactions illustrate the formation of CO₂ and its release into the environment:

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