Oxygen dissociation curves

What is an oxygen dissociation curve?

It shows the relationship between the oxygen tension in the surrounding environment (the partial pressure of oxygen) and the oxygen saturation of haemoglobin (through the percentage of oxygen)

What is partial pressure of oxygen?

It is a measure of the oxygen concentration - the greater the concentration of oxygen, the higher the partial pressure

What is normal atmospheric pressure?

100 kiloPascals (kPa)

What is the normal partial pressure of oxygen?

21kPa as oxygen makes up 21% of the atmosphere

What shape are oxygen dissociation curves?

Sigmoidal - S-shaped, because the first O2 joins slowly, but this causes a conformational shift so the whole haemoglobin slightly changes shape. As a result, the 2nd and 3rd molecules bind more easily, giving the steep part of the curve. It is harder for the last molecule to join, making the curve flatten

How is oxygen loaded?

When blood enters the lung capillaries from the pulmonary arteries, it has low oxygen and high carbon dioxide levels. Oxygen diffuses into the red blood cells and binds to the haemoglobin inside

How is oxygen unloaded?

The oxygenated blood returns to the heart via the pulmonary veins and is pumped into the systemic circulation. No gas exchange happens until the blood reaches the capillaries of the body tissues. Here, the haemoglobin unloads oxygen to be used in respiration (dissociation)

What is loading and unloading dependent on?

The partial pressure of oxygen surrounding the capillaries

What happens if a tissue has a high rate of aerobic respiration?

It will have a low partial pressure of oxygen and haemoglobin will unload more oxygen than it would with tissues with a lower rate of aerobic respiration

What does a left shifted curve mean?

Organisms which are adapted to live in low oxygen environments have haemoglobin that fully saturates at lower partial pressures of oxygen than adult human haemoglobin

Examples of organisms which are left shifted

Lugworms, llama, foetal haemoglobin

What is a disadvantage of left shifted curves?

Oxyhaemoglobin does not dissociate as easily

How is a foetus’ haemoglobin left shifted?

A foetus gains its oxygen from its mother’s blood across the placenta. It can absorb oxygen from the mother’s blood at all partial pressures of oxygen

What is myoglobin?

A respiratory pigment found in muscle cells which can only bind one molecule of oxygen. It loads at much lower partial pressures of oxygen and acts as a store of oxygen, only releasing oxygen at very low partial pressures when muscles are respiring rapidly to delay the onset of anaerobic respiration

How is haemoglobin adapted in the blood of llamas?

They live in mountainous areas of high altitudes (5000m above sea level). At these high altitudes, there is a drop in atmospheric pressure so ppO2 is less than at lower altitudes. Llama haemoglobin has an increased affinity to bind with oxygen so their oxygen dissociation curve is shifted to the left. They also have large numbers of erythrocytes to increase oxygen uptake

How is haemoglobin adapted in the blood of lugworms?

They are found on the lower shore of a beach in burrows in the sand. The ppO2 of the water is the burrow can be very low. Lugworms pump seawater through the burrows to increase the concentration of dissolved oxygen. Their haemoglobin is also left shifted so that it has an unusually high affinity for oxygen. They also have a low metabolic rate so their oxygen needs are lower