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Organisms exchange substances with their environment (AQA AS
Biology) PART 4 of 5 TOPICS
Mass transport in animals:
Haemoglobin is a protein which has a quaternary structure.
Each haemoglobin molecule can carry up to 4 oxygen molecules. Oxyhaemoglobin is formed when
oxygen attaches to haemoglobin. The binding of haemoglobin with oxygen is a reversible reaction
showing that oxygen can easily detach which is vital for respiring cells.
At high partial pressures of oxygen (in the lungs), haemoglobin has a high affinity for oxygen so the
oxygen binds to the haemoglobin to become oxyhaemoglobin. At low partial pressures of oxygen (in
tissues such as muscle), the haemoglobin had a low affinity for oxygen and so the oxyhaemoglobin
will release the oxygen to become haemoglobin. This is represented by a sigma shaped graph below.
Notice that the start at the start it is a curve rather than a straight line; this is because the first oxygen
molecule is hard to bind to the haemoglobin and so when the attachment has been done, the other
oxygen molecules are relatively easier to bind to haemoglobin compared to the first therefore
creating a steeper line after the curve.
CO2 causes the Bohr's shift where the graph is moved to the right because the blood's pH has lowered.
The following veins and arteries of these organs must be known:
Veins - Vena Cava and Pulmonary Vein
Arteries - Aorta and Pulmonary artery
Veins - Pulmonary Vein
Arteries - Pulmonary Artery
Veins - Renal Vein
Arteries - Renal Artery
The changes in pressure and volume of atrium and ventricles are required (NB: The action potentials
do not need to be known, only for A2 Biology). Ventricular contraction causes the atrioventricular
valves (AV valves) to close preventing backflow of blood into the atrium and begins the ventricular
systole; semi-lunar valves (SL valves) remain close. Continued pressure causes a high pressure and so
the semi-lunar valves open. As the ventricles are relaxing the semi-lunar vales shut preventing
backflow of blood into the ventricles. This the ventricular diastole and the AV valves remain shut.
When the pressure in the ventricles becomes lower than the pressure in the atrium, the AV valves
open and blood flows into the ventricles. The atria contracts to complete the ventricular filling.
The following structures need to be known for the exam:
Arteries: These have a smooth muscle tissue which controls the pressure of the blood flow.
The elastic layer stretches and recoils to maintain the blood pressure. Smooth endothelium is
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Arteries carry blood away from the heart to respiring cells. NB: Not
all arteries carry oxygenated blood as illustrated in the heart diagram.
Veins: Blood in the veins are under low pressure therefore the walls of the veins are thin and
also they have thin muscle layers and elastic fibres. This means that their lumen is bigger than
arteries. As the blood is under low pressure they have valves to keep the blood flowing in one