Circulation and Cardiac Cycles

Introduction to the circulatory system

What is interstitial fluid?

Interactive animation of the Cardiac Cycle and how the pressure graphs and electrocardio grams match up.

An animation describing the changes the heart goes through in one cardiac cycle.

Here is a youtube clip showing the electric circuits in the heart that cause it to pump.

Oxygen carriage Dissociation curves.

Oxygen carriage

Oxygen does dissolve in plasma but the solubility is low and decreases further if the temperature increases. The amount that could be carried by the plasma therefore would be completely insufficient to supply all cells.

There is a protein in the blood however that will carry 4 molecules of oxygen. The protein is called haemoglobin (Hb) and is made up of 4 polypeptide chains, each with a haem group. Each haem group can pick up 1 molecule of O₂. 

What is The Partial Pressure of Oxygen?

During this topic you will come across the term of partial pressure of oxygen. It does not mean the pressure of the blood itself. Essentially it is a measure of the concentration of oxygen. It is written in shorthand as pO₂ and is measured in kilopascals (kPa).

Inhaled air in the alveoli has a pO₂ = 14kPa. The pO₂ of resting tissue = 5.3kPa

(lower pO₂ = lower O₂ concentration due to respiration) and the pO₂ of active tissues = 2.7kPa.

In either case, blood arriving at the lungs has a lower pO₂ than that in the lungs.

There is therefore a diffusion gradient and oxygen will move from the alveoli into the blood. The O₂ is then loaded onto the Hb until the blood is about 96% saturated with oxygen. The Hb is now called oxyhaemoglobin (HbO₂).

Hb + 4O₂ → HbO₈ 

The blood is then taken to tissues where the cells are respiring all the time, using oxygen.

The pO₂ in the cells of the respiring tissue will be low. As the red blood cell enters this region, the Hb (with it’s high concentration of Oxygen) will start to unload the O₂, which will diffuse into the tissues and be used for further respiration.

As the blood passes through the tissues, since much of the Hb will have already unloaded the O₂, a much lower percentage of the blood will be saturated with O₂ as the blood leaves the tissues. But this Hb still needs to release O₂ in this area so this tissue does not miss out on oxygen.

A graph of the percentage saturation of blood with O₂, i.e. the amount of HbO₂ as opposed to Hb at different pO₂ is shown below. It is called an oxygen dissociation curve:

Put simply, we can see that if blood is in an area of high partial pressure of Oxygen, there will be more Oxygen in the blood.


A short video describing this

A longer video describing this. 

Why is the dissociation curve an S shape??

It is S-shaped because of the behaviour of the Hb in different pO₂. Basically, haemoglobin has difficulty picking up the first oxygen molecule. The partial pressure of oxygen is higher than expected when the blood is 20% saturated. Once one Oxygen molecule has been picked up, it becomes easier for the Hb to pick up oxygen so the next 3 O₂ molecules are picked up at lower partial pressures than you’d expect.

The first molecule of O₂ combines with an Hb and slightly distorts it. The joining of the first is quite slow (the flatter part of the graph at the beginning) but after the Hb has changed shape a little, it becomes easier and easier for the second and third O₂ to join. This is shown by the curve becoming steeper. It flattens off at the top because joining the fourth O₂ is more difficult.

Overall, it shows that at the higher and lower end of the partial pressures, there isn't a great deal of change in the saturation of the Hb, but in the middle range, a small change in the pO₂ can result in a large change in the percentage saturation of the blood.

Why are they called dissociation curves?

If we think about these curves in a converse way, rather than thinking about Hb picking up oxygen, we can think of it dropping off Oxygen.

Ie If Haemoglobin was in a tissue with a partial pressure of oxygen of 4Kpa, then we can see that if that Hb had been fully saturated (100%) when it left the heart, it is now only 50% saturated, so it has ‘let go’ of 50% of it’s oxygen and the oxygen has gone into the tissues. These graphs show us the partial pressures of oxygen needed in tissues in order for them to be low enough for the Hb to release Oxygen.

Foetal Haemoglobin and Myoglobin curves.

Foetal Haemoglobin

A oxygen curve that is further to the left of a normal oxygen dissociation curve shows that at the same ppO2, the curve to the left has a higher affinity for oxygen. 

Here we can see that at 6 ppO2, Foetal Hb has a higher affinity for oxygen than the adult hb. 

Foetal Hb needs to have a higher affinity for oxygen when it is at the same partial pressure as the mothers (adult) Hb. 

        

        Foetal has two different chains to adult haemoglobin. This difference allows foetal haemoglobin to combine more readily than adult haemoglobin.

The foetus is able to gain oxygen from the blood of the mother.