Learn Icelandic: Góð líkamsstaða bætir blóðrásina. - Good posture improves the blood circulation.

When joining nouns to make a compound word, Icelandic usually uses the genitive (possessive) case for the first word. The genitive of the weak noun líkami is normally líkama, but over time, an -s was tacked onto the root to form compounds. This is why you see the prefix líkams- in words like líkamsstaða (posture) and líkamsrækt (fitness/gym).

We are learning about root nouns changing vowels in the plural via i-umlaut. How does that connect to this sentence?

The verb bætir (improves) gives you a great look at i-umlaut in action! It comes from the root noun bót (a remedy or improvement). Just like the root noun bók changes to bækur in the plural, the historical suffix that turned bót into the verb bæta caused the exact same vowel shift from ó to æ.

The English translation says "improves blood circulation", but blóðrásina has the definite article attached (-ina). Why?

Icelandic uses the definite article much more frequently than English when referring to abstract ideas, general concepts, or bodily systems. While English leaves it off, Icelandic literally says "improves the blood circulation" because it is a specific, known system within the body.

Why is blóðrásina in the accusative case here?