Why lungs have more negative intrapleural pressure at the apex

Biology textbooks rarely use the term gravity to explain mechanisms and Professors don't always take kindly to students using vocabulary or concepts from other disciplines...

The external intercostal muscles are fixed on the superior end of the ribs, the ribcage is pulled upwards upon contraction, which increases negative pressure. Looking at a diagram may help you understand (or take A & P I). It is difficult to understand functions in the body without visual models to show relations, it certainly isn't something you can easily get by only reading text.