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# Mini MCAT passage: The effects of high blood pressure on the heart

## Problem

A heart attack occurs most often when a blood clot blocks normal flow of blood through the coronary arteries, which supply the tissues of the heart. As a result, parts of the muscle of the heart die because they do not have access to oxygen. Similarly, a stroke occurs most often when a blood clot blocks arteries supplying blood to the brain. There are two ways for clots to appear at the site of injury. First, they can form at the site of injury itself. Second, they can form elsewhere in circulation, then become detached from their site of origin, get swept along through the blood and stuck in a thinner blood vessel at the site of injury.
A study assesses the association between high systolic blood pressure and heart attacks, and between high blood pressure and strokes. The study enrolls $1250$ male participants, and measures their blood pressure on day $1$ of the study. Blood pressures are taken to be the average of a $24$-hour continuous at-home blood pressure monitor. Then the five-year risk of heart attacks and strokes are measured for three different classes of blood pressure: ($1$) systolic < $120$, ($2$) $120$ ≤ systolic ≤ $139$, ($3$) systolic ≥ $140$.
Systolic blood pressure at baseline (number of subjects)Risk of heart attack over $5$ years ($95\mathrm{%}$ confidence interval)Risk of stroke over $5$ years ($95\mathrm{%}$ confidence interval)
<$120$mmHg ($530$)$0.013$ ($0.011$-$0.015$)$0.028$ ($0.026$-$0.030$)
$120$-$139$mmHg ($420$)$0.019$ ($0.017$-$0.022$)$0.034$ ($0.031$-$0.036$)
$\ge$ $140$mmHg ($300$)$0.025$ ($0.022$-$0.028$)$0.039$ ($0.037$-$0.042$)
Where does blood entering the right atrium come from?