The veins in the feet and legs are used to send deoxygenated blood towards the lungs via the heart. Because humans walk and stand on two legs, the blood has to flow back a long way against gravity. Sitting doesn’t help, because one  of the main veins in the back of the knee becomes restricted to flow. Slow moving blood in veins risks swollen ankles and blood clots forming, known as deep vein thromboses (DVT) and these can be very dangerous. Any longer periods of inactivity flying/driving is a problem, and the risk increases with age.