Blood moves into and then out of a heart chamber because 1. the veins and arteries constrict and dilate to propel and attract blood. 2. it moves along its pressure gradient, and that gradient depends on contraction and relaxation during the cardiac cycle. 3. it is under constant pressure, but its movement is dictated by the control of valve openings and closures. 4. All of the choices are correct.

I honk number 4 because Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. ... As the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.

Ithink its because more have died recently than in the past.

Na+ loss through sweet plus the dilution has lowered na+ concentration outside of neurons, altering the na+ equilibrium potential. sodium exists more outside the cells than inside. when na+ is lost by sweat plus the dilution( because they drink a lot of water) changes the concentration outside the cell, forcing the water to enter the cell and attempt to balance the concentration inside/outside.the problem is that the abundance of water will cause the cells to swell. most cells can adapt, however, the brain cells can't that's why the runner has confusion and can't control the muscles.  if not treated and this occurs in less than 48 hours, it can be immediately fatal .