3.1: Heart Structure and Blood Flow
The heart is a hollow, muscular organ located in the thoracic cavity that functions as a specialised double pump. To understand sports science, analysing the heart's anatomy and the precise path blood takes through its chambers is essential. The heart is divided into a left and right side by a thick muscular wall called the septum. This structural barrier ensures that oxygenated and deoxygenated blood never mix, preserving the concentration gradient required for efficient gas exchange. Each side of the heart contains two distinct chambers: an upper atrium that receives incoming blood and a lower, thicker ventricle that forcefully ejects blood out of the organ.
Deoxygenated blood returning from the body’s working tissues enters the right side of the heart through two large veins. Blood from the upper body travels through the superior vena cava, while blood from the lower body arrives via the inferior vena cava. Both empty into the right atrium. As the right atrium fills, pressure increases, forcing the tricuspid valve, a one-way atrioventricular valve, open. Blood flows down into the right ventricle, which then contracts during ventricular systole. This contraction increases internal pressure, closing the tricuspid valve to prevent backflow and forcing open the pulmonary semilunar valve. Blood is then driven into the pulmonary artery, which carries deoxygenated blood away from the heart to the lungs, where carbon dioxide is released, and fresh oxygen is absorbed.
Once oxygenated in the lungs, the blood must return to the heart to be pumped throughout the body. It travels through the pulmonary veins and enters the left atrium. As the left atrium fills and contracts, it forces the bicuspid, or mitral, valve open, allowing blood to flow into the left ventricle. The left ventricle is the most muscular chamber of the heart because it must generate enough force to pump blood throughout the entire systemic circulation loop, whereas the right ventricle only needs to pump blood a short distance to the lungs.
When the left ventricle contracts, the resulting high pressure snaps the bicuspid valve shut and forces the aortic semilunar valve open. Blood is forcefully propelled into the aorta, the body’s largest artery, which branches into smaller arteries to deliver oxygen-rich blood to the brain, vital organs, and actively contracting skeletal muscles.
The entire process relies on the rhythmic coordination of the heart valves. These valves open and close passively in response to pressure changes within the chambers, ensuring that blood flows in a strict, one-way direction. During physical exercise, the entire cycle accelerates. The cardiac conduction system increases the rate of these contractions, while the ventricular muscle walls contract with greater force, ensuring a rapid, continuous delivery of oxygen to meet the athlete's elevated metabolic demands.