As a result of the non-selectivity of the kidney's filtration of small molecules, see concept 44.4 (page 986) as a result of the non-selectivity of the kidney's filtration of small molecules, see concept 44.4 (page 986) the kidneys have little control over body fluid composition useful substances must be selectively reabsorbed the proportions of all the substances in the blood are the same as in the urine many useful substances are lost in the urine urine is always much less concentrated than blood

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.