*swoops in*
Double-entry accounting man to the rescue!
There are four types of account: assets (what you have), liabilities (what you owe), incomes (what you get), and expenses (what you lose). Net worth is defined as your total assets minus your total liabilities, and when we talk about "saving" rather than "spending", we mean putting income into assets or liabilities (increasing your net worth), rather than into an expense.
Your mortgage is a liability. Suppose you pay $500, and $200 goes to interest. There are two ways to deal with that:
1) Transfer $500 from your current account (asset) to your mortgage (liability). Transfer $200 from "interest" (expense) to your mortgage (liability).
2) Transfer $300 from your current account (asset) to your mortgage (liability). Transfer $200 from you current account (asset) to "interest" (expense).
The net result is the same: $300 saved, $200 spent. Either method is equivalent, but #1 seems more natural to me because interest is something that accrues independently of whether you make a payment.
Double-entry accounting man, away!
*swoops away*