Virueses are considered to be the most abundant biological entity on the planet. What does that statement mean?

Viruses are always going to be around. They affect living creatures on this plant. Virus particles even outnumber cells by an order of magnitude in the ocean. Viruses carry many more uncharacterized genes than cellular life forms.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.