The heart

The internal view of the heart: 

• A closed system = is where blood is pumped through many vessels and it does not leave.
• A open system =  pump blood into a hemocoel with the blood diffusing back to the circulatory system between cells. Blood is pumped by a heart into the body cavities, where tissues are surrounded by the blood. 
• Double circulatory = a circulation in which the blood flows through the heart twice during each complete circulation of the body.
• Single circulatory = a circulation in which the blood flows through the heart once during each circulation of the body.
• A partial double circulatory system has two uncomplete pathways and the deoxygenated blood as well as oxygentated mixes = making it less effective.

The external view of the heart: 

As blood passes through the lungs, oxygen molecules attach to the haemoglobin. As the blood passes through the body's tissue, the haemoglobin releases the oxygen to the cells. The empty hameglobin molecules then bond with the tissue's carbon dioxide or other waste gases, transporting it away

• The deoxygenated blood RBC travels back to the heart through the vena cava.
• It enters the right atrium.
• The right atrium contracts and pushes it through the tricuspid valve and into the right ventricle.
• The right ventricle contracts and pushes it out of the heart through the semi-lunar valve.
• It travels through the pulmonary artery to the lungs. 
• Here it picks up oxygen.
• It travels back to the heart through the pulmonary vein.
• It enters the left atrium.
• The left atrium contracts and pushes it through the semi-lunar valve, out of the heart and into the aorta.
• The RBC travels through the aorta and into our kidneys.
• Then the deoxygenated blood travels up through the vena cava and then it starts again.
• The blood travels around the blood vessels.

At the arterial end of a capillary, the blood is under high pressure due to contractions of the heart (hydrostatic pressure). It will tend to push the blood fluid out of the capillaries. It can leave through tiny gaps in the capillary wall. The fluid consists of plasma with dissolved nutrients and oxygen.

Oxygen:

• Haemoglobin consists of 4 subunits. Each subunit consists of polypeptide and a haem group. The haem group contains one iron ion because the iron ion attracts oxygen. It is said to have an affinity for it. A RBC can hold 4 molecules of oxygen. 

Dissociation curve: 

Haemoglobin can take up oxygen in a way that produces an S-shaped curve. This is called OXYGEN DISSOCIATION CURVE. At a low oxygen tension the haemoglobin does not readily take up oxygen. This is because it is difficult for the O2 molecule to reach the haem group, due to it being in the centre of the blood cell.

When O2 tension rises, the diffusion gradient into the haemoglobin molecule steeply rises. Once 1 molecule of O2 has associated with a haem group, the shape of the haemoglobin molecule slightly changes, making it easier for the 2nd & the 3rd molecules to associate. The change in shape is known as the 'conformational change'

But once the haemoglobin molecule contains 2  O2 molecules, it is difficult for the 4th to associate with the last haem group. This means that it is difficult to achieve 100% saturation, even at high oxygen pressures. A consequence of this is that the curve levels off again, giving an S-shaped graph.

Carbon Dioxide:  5% dissolved in plasma

• 10% combines with haemoglobin to form carbaminohaemoglobin
• 85% transported as hydrogencarbonate ions. As CO2 diffuses into the blood, some of it enters the RBC and combines with water to form carbonic acid - catalysed by carbonic anhydrase. 

This carbonic acid then dissociates to form hydrogen ions and hydrogencarbonate ions.

The hydrogencarbonate ions diffuse out of the RBC. The charge in the RBC maintained by the Chloride Shift; the movement of chloride ions into the cell. Hydrogen ions could cause the contents of the cell to become very acidic so the haemoglobin acts as a buffer. The oxyhaemoglobin dissociates & the hydrogen ions are taken up by the haemoglobin to form haemoglobonic acid.

(M)

• when tissues are respiring more, there will be more CO2 and therefore more hydrogen ions. This means that more O2 will be released from oxyhaemoglobin into the tissues. So, when more CO2 is present, the oxyhaemoglobin dissociatiion curve shifts down to the right.

Different affinities of fetal haemoglobin and adult for O2:

• fetal haemoglobin has a higher affinity for O2 than the haemoglobin of its mother. This is because the fetal haemoglobin must be able to 'pick up' O2 from the haemoglobin from its mother. This reduces the O2 tension within the blood fluid so the maternal blood release O2.
• The oxyhaemglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin