As the concentration of molecules outside a cell increases, more molecules will enter the cell because--

of diffusion

Explanation:

Molecules move down a concentration gradient without the need to use energy. This means they move from a high concentration to a low concentration. This occurs across a semi-permeable membrane, like the cell membrane, by simple diffusion.

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.