A purebred organism is an organism that contains identical alleles for a particular gene e.g rr or RR. According to this question, in a cross between a purebred dominant organism e.g RR and a purebred recessive organism e.g rr, each parent will produce R and r gametes respectively.
Using the gametes in a punnet square (see attached image), a heterozygous organism (Rr) will be produced in the F1 generation. If these heterozygous organism is self-crossed i.e. Rr × Rr, the following F2 offsprings will result;
RR, Rr, Rr, and rr.
RR, Rr and Rr will exhibit dominant traits while rr will exhibit recessive traits. This means that the F2 generation contains a ratio of approximately 3:1 expressed dominant traits to recessive traits.
The two differences you see when comparing the four chambers of the heart is that the arteries have a thicker wall and veins have thin walls and that the arteries carry oxygenated blood away from the heart which means they get more pressure than veins.