Transport of Gases Flashcards
Air
Earth is surrounded by a thin envelope of air (the atmosphere).
As altitude increases, the concentration or density of air molecules decreases.
As density decreases, so does air pressure.
Air pressure is measured in pascals (Pa). One pascal is the force of one newton exerted on an area of one square metre. Pascals are very small, so kilopascals (kPa).
At sea level air pressure is 101.3kPa, on top of Mount Everest it is 31 kPa.
Partial Pressures
The composition of air does not change with the level of atmosphere.
Partial pressures- the total air pressure is a sum of the mixture of the component gases.
At sea level, the partial pressure of oxygen is 20.9% of the atmosphere, or 21.17kPa.
Partial pressures are written as Px or PO2
Partial pressures of gases affect gas exchange. Gasses diffuse from areas of high pressure to low pressure until equal.
The greater the pressure gradient the greater the rate of diffusion.
Oxygen Transport
Oxygen moves from the air in the alveoli (13.3 kPa) to the bloodstream (5.33 kPa).
This significant pressure gradient causes the diffusion from air in the alveoli to the liquid component of blood called plasma.
The circulatory system transports oxygen to the body cells in two ways: attached to hemoglobin within red blood cells (98.5%) and dissolved in blood plasma (1.5%)
Hemoglobin
An iron-containing protein in red blood cells that bind with molecules of oxygen to form oxyhemoglobin (gives blood its bright red colour).
Deoxygenated blood is dark red.
Blood without hemoglobin carries only about 0.3mL of oxygen per 100mL of blood. Blood with hemoglobin carries about 20mL of oxygen per 100mL of blood.
Oxygen rich blood reaches the cell and diffuses into the tissue fluid then into the tissue cell. This diffusion reduces the PO2 of blood plasma. The oxygen attached to the hemoglobin is released and diffuses into the blood plasma, into the tissue fluid then into the tissue cells.
The supply of oxygen in hemoglobin is not ever depleted. The veins still carry oxygen. This way you can hold your breath for a short time and not die.
Carbon Dioxide Transport
Carbon dioxide is the by-product of cellular respiration and must be removed.
The PCO2 of tissue fluid is 5.60 kPa and in the capillaries it is 5.33 kPa.
Carbon dioxide is transported in 3 ways: dissolved in plasma (7%), attached to hemoglobin to form carbaminohemoglobin (20%), and reacts with water in plasma to form carbonic acid (73%).
Carbonic acid quickly separates into bicarbonate ions and hydrogen ions.
As hemoglobin releases oxygen to the tissue cells, the hydrogen ions attach to the hemoglobin which prevents the dangerous accumulation of these ions in the blood.
Hydrogen Ions
In the lungs, the hydrogen ions separate from the hemoglobin and diffuse into the blood plasma. There they react with the bicarbonate ions and reform carbon dioxide and water.
Now, the PCO2 is 5.60 kPa in the capillaries and 5.33 kPa in the air. Carbon dioxide diffuses out from the blood into the air and is exhaled.
Altitude
As we go up in altitude, the pressure decreases, eventually to a point that we cannot survive.
Even though oxygen still constitutes 20.9% of the air, the density and partial pressure of oxygen are too low. There is not enough oxygen in a given volume of air.
The partial pressure is now much lower, so the gradient between the air and blood is reduced so there is decreased diffusion of oxygen to the body.
Altitude sickness can result: shortness of breath, headache, dizziness, tiredness and nausea.
High Altitude
Over time, when oxygen supply is reduced, the kidneys increase secretion of erythropoietin (EPO).
EPO is a hormone that stimulates the production of red blood cells. This increases the amount of oxygen that can be absorbed from the air and delivered to cells.
Training at high altitudes can provide athletes with a legal competitive edge because they increase the number of red blood cells and therefore have an extra reservoir of oxygen.
Control of Breathing
Breathing is involuntary and is controlled by the nervous system and the circulatory system.
Normal rhythmic movements of inhalation/exhalation are controlled from the respiratory centre in the brain stem.
The brain sends signals that cause the diaphragm and external intercostal muscles to contract causing inhalation. Stretch receptors in the lungs send signals back to the brain so it stops sending signals to the diaphragm. It then relaxes and causes exhalation.
We can consciously override these signals for a short time. Ex) talking, singing, holding your breath.
Oxygen & Carbon Dioxide Levels
Breathing rate is determined by the demand for oxygen or the need to eliminate carbon dioxide.
The most significant effect is the level of carbon dioxide. Receptors in the brain detect the decrease in pH of blood because of accumulation of carbon dioxide.
There are also pH receptors in the arteries of the neck and heart. A decrease in pH sends a signal to the respiratory centre in the brain which increases breathing rate and volume of inhalation.
The heart rate also increases so that oxygen can be delivered and carbon dioxide can be removed quickly.
Oxygen levels are monitored via receptors in the arteries and in the blood leaving the heart and going to the brain. A breathing rate response will not be triggered until oxygen levels are significantly below normal.
