The Spanish word 'riñón' (meaning 'kidney') comes from the Vulgar Latin word 'renio', which itself derived from the Classical Latin word 'ren', also meaning 'kidney'. The evolution from Latin to Spanish shows typical sound changes, where the 'e' became 'i', and the development of the Spanish 'ñ' sound from the Latin 'ni' combination.

The word 'renal' in Spanish, though more technical, is a related term that comes from the same Latin root 'ren'. This is commonly used in medical contexts, just as in English.

English speakers might recognize the connection to words like 'renal' (relating to the kidneys) and 'nephritis' (inflammation of the kidneys). While 'renal' comes from the same Latin root 'ren' as 'riñón', it entered English through scientific Latin rather than through natural evolution of the word.