D.6 Transport of respiratory gases Flashcards
Define gaseous exchange
The diffusion of gases between the alveoli and capillaries in the lungs.
What is the main purpose of the alveoli?
Gas exchange
The cells forming the alveolus are called ___
Pneumocytes
What are the two types of pneumocytes?
Type I and type II
Describe the structure and function of type I pneumocytes
- They are flat with a great surface area (approximately 90–95% of the alveolar surface).
- They are involved in the process of gas exchange between the alveoli and blood.
Describe the structure and function of type II pneumocytes
- They have a cubic shape and cover a small fraction of the alveolar surface area (5%).
- They secrete pulmonary surfactant, a fluid that decreases the surface tension within the alveoli.
- They are also capable of cellular division, giving rise to more type I pneumocytes when the lung tissue is damaged.
Diagram and micrograph of the structure of the alveolus (images to the left), light micrograph of lung ×100 (top right) and ×400 (bottom right)
Identifying structures in an alveolus for the exam
You should be able to identify pneumocytes, capillary endothelium cells and blood cells in light micrographs and electronmicrographs of lung tissue.
What does the rate of gaseous exchange depend on?
The pH of blood
Explain how the rate of gaseous exchange depends on the pH of the blood
- It is regulated to stay within the narrow range of 7.35 to 7.45. Within blood plasma and tissue fluids, hydrogen carbonate, proteins, and ions (such as phosphate) act as buffers to maintain the pH close to neutral (slightly alkaline).
- Carbon dioxide combines with water, producing carbonic acid that lowers the pH.
- The carbonic acid dissociates into alkaline hydrogen carbonate, increasing the pH, plus a hydrogen ion that acidifies the medium, decreasing the pH.
- This is called the hydrogen carbonate buffering system
Equation for the hydrogen carbonate buffering system
What is the range that the pH of blood is regulated to stay within?
7.35 to 7.45
What is a difference between Type I and Type II pneumocytes?
Type I have a larger surface area than Type II pneumocytes (to allow for gaseous exchange between the alveoli and blood).
How does the hydrogencarbonate buffering system maintain the pH of blood within limits?
Dissolved carbonic acid dissociates into hydrogen carbonate and hydrogen ions.
What is hemoglobin and what is it made up of?
- Haemoglobin is a protein composed of four peptide chains, two alpha and two beta chains, each with a ring-like heme group containing an iron atom.
- Oxygen binds reversibly to these iron atoms.
Describe the structure of adult hemoglobin
This structure is oxyhaemoglobin, with four oxygen molecules.
Diagram of a molecule of adult oxyhemoglobin
Describe the transport of respiratory gases between the mother and fetus during pregnancy
- During pregnancy, the mother must deliver O2 to the fetus and remove CO2 through the placenta.
- Mother and fetal blood never mix, so capillaries from both must come in close proximity for exchange to happen.
Describe the structure of fetal hemoglobin and the reason for it
- In order to get O2 into fetal blood, the hemoglobin in fetuses is slightly different from the adult hemoglobin.
- Instead of having two alpha and two beta peptides, the fetal hemoglobin has two alpha and two gamma peptides.
Diagram of fetal deoxyhemoglobin
Fetal hemoglobin has more ___ than adult haemoglobin.
Affinity for oxygen
Picture of the protein sequence alignment obtained using pBLAST for the gamma peptide of fetal haemoglobin and beta peptide of adult haemoglobin
Describe the pBLAST for fetal (Query 1) and adult hemoglobin (Sbjct 1)
- As you can see in this sequence alignment, there are some similarities between the sequences of beta and gamma hemoglobin peptides, but there are also some amino acids that are different.
- This allows fetal haemoglobin to bind O2 with a greater affinity, therefore extracting it from the mother’s blood.
What is myoglobin?
The protein used to bind oxygen in muscles.
Describe the structure and function of myoglobin
- It consists of only one peptide chain and a heme group containing iron.
- There is no sequence similarity in haemoglobin and myoglobin chains.
- Myoglobin can only bind one oxygen molecule, but this binding is stronger than that in haemoglobin.
- Therefore myoglobin can take the oxygen from haemoglobin in respiring muscle cells.
Diagram of myoglobin
In what way are hemoglobin and myoglobin the same?
They are oxygen binding proteins as both contain iron molecules.
In what way is fetal haemoglobin different to adult haemoglobin?
It has a greater affinity for oxygen.
In order to get O2 into fetal blood, the haemoglobin in fetuses is slightly different to the adult haemoglobin, making it take in oxygen more readily.
How is oxygen transported in humans?
Oxygen is transported mainly bound to haemoglobin as oxyhemoglobin (98.5%) and dissolved in plasma (1.5%).
Explain how oxygen is bound to hemoglobin
- Oxygen is bound to haemoglobin in the lungs.
- Once the first heme binds to oxygen, there is a small change in the protein structure of haemoglobin, making the heme of another chain join oxygen more easily.
- Cooperative oxygen binding by haemoglobin causes conformational changes in an individual peptide that are propagated to the other peptides.
- The joining of the third and fourth oxygen molecules becomes easier due to this allosteric change in the haemoglobin molecule, leading to an S-shape or sigmoid curve.
How does oxygen binding to hemoglobin change in the different tissues of the body?
- Oxygen remains tightly bound to hemoglobin in the lungs, but it will be progressively released as partial oxygen pressure drops in the different tissues of the body.
- The release of a second, and even more so the third oxygen molecule, require smaller drops in pressure.
What does the oxygen dissociation curve show?
How the partial pressure of oxygen (pO2) in the tissues determines the percentage of haemoglobin that contains oxygen at pH 7.4 and 38°C.
Diagram of the oxygen dissociation curve for adult haemoglobin
Define partial pressure
The individual pressure exerted independently by a particular gas within a mixture of gases.
How is blood pumped throughout the body?
- The pulmonary vein brings oxygenated blood to the left side of the heart, which pumps it via the aorta to all parts of the body.
- When this blood returns to the right side of the heart, it is pumped along the pulmonary artery.
Blood cells in the capillaries surrounding the alveoli carry ___
100% of the hemoglobin as oxyhemoglobin (with four molecules of oxygen bound).
Where do the capillaries surrounding the alveoli lead to?
The pulmonary vein, which carries blood to the heart.
What happens as the blood leaves the heart?
- The partial pressure of oxygen decreases (to about 40 mmHg in the pulmonary arteries) and the hemoglobin molecule releases one oxygen molecule.
- This causes an allosteric change in the hemoglobin molecule that makes the further release of the other oxygen molecules easier, as less energy is required.
- This means that a smaller drop in partial pressure is required to liberate a molecule of oxygen, leading to an S-shape or sigmoid curve.
- In this way, hemoglobin attaches the largest possible amount of oxygen in the lungs and delivers all of it where and when it is needed.
Fetal hemoglobin- oxygen dissasociation curve (check/reword)
- Fetal haemoglobin binds O2 with a greater affinity, therefore extracting it from the mother’s blood in the placenta.
- This means that at lower partial pressures of O2, the fetal hemoglobin loads O2 easier than adult hemoglobin.
- This would cause a shift to the left in the oxygen dissociation curve.
Affinity for oxygen in myoglobin vs. hemoglobin
- Myoglobin has a stronger affinity for oxygen than haemoglobin.
- Because myoglobin is formed by only one peptide, there is no allosteric effect in the molecule.