Introduction

Every living cell needs a steady supply of nutrients, oxygen and other substances, and needs its carbon dioxide and other wastes carried away just as steadily. In simple, small-bodied animals such as sponges and coelenterates, no dedicated transport system exists at all - water from the surroundings enters the body cavity and lets cells exchange materials directly by diffusion. Larger, more active animals cannot rely on this: locomotion itself is energy-expensive, coordinating it through the nervous system takes even more energy, and diffusion alone is far too slow to keep every cell supplied. These animals instead evolved specialised body fluids and organised transport systems that move materials in bulk, using pumps - a process fundamental to respiration, digestion and excretion alike, and dramatically faster than diffusion (blood can travel from the human heart to the feet and back in about a minute, a journey that would take diffusion many years).

Inside the human body, this bulk transport does several jobs at once: oxygen and carbon dioxide are exchanged at the lungs and in the tissues, nutrients absorbed from digestion are carried to the liver, cellular wastes are carried to the kidneys for removal, and hormones are delivered to their target organs. The circulatory system also helps hold body fluid levels and body temperature steady. Its own regulation matters too - the cardiovascular system works to keep blood flowing steadily to the heart and brain, and when that regulation briefly fails, as in vasovagal syncope, a sudden drop in blood pressure can starve the brain of oxygen and cause fainting. This chapter looks at how the heart and blood vessels work together to keep this system running smoothly, most of the time without incident.