In a neuron is to sending as is to receiving

In a neuron, axon is to sending as dendrite is to receiving. In addition regarding neurons, they transmit messages in the form of electrical impulses and the fundamental building block of the nervous system. The parts of a neuron are the soma, the axon, the dendrites and the terminal buttons. 

one would be wind another would be water density

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.