Human Circulatory System

Understanding Human Circulatory System

Each heartbeat follows an electrical sequence that controls the order of contraction. A small group of cells called the sinoatrial node starts a signal near the top of the heart. The signal makes the atria contract first.

It then pauses briefly at the atrioventricular node before spreading through the ventricles. This pause gives the ventricles time to fill. When ventricular pressure rises, the valves between the atria and ventricles close.

The valves leading out of the ventricles open only when pressure becomes high enough. Valves do not pull themselves open. They move because blood pressure changes on each side.

The familiar heart sounds come mainly from valves closing. A disturbed electrical rhythm can make pumping less effective, even when the heart muscle itself is healthy.

Blood vessels have different wall structures because they face different forces. Large arteries have thick, elastic walls that stretch when the heart pushes blood into them. Their recoil helps maintain flow between beats.

Veins carry blood at lower pressure. Many veins in the limbs contain internal valves, while leg muscles squeeze these veins during walking. This muscle action helps return blood upward against gravity.

Capillaries are extremely narrow and have walls only one cell thick. Here, oxygen and dissolved nutrients move into nearby cells by diffusion. Carbon dioxide moves in the opposite direction.

Some fluid leaves capillaries because of pressure, then most of it returns. Extra fluid enters lymph vessels. If this drainage is poor, tissues can swell.

Blood is a living tissue with several specialised parts. Plasma is the liquid portion. It carries water, proteins, hormones, salts, nutrients, and waste substances.

Red blood cells contain haemoglobin, a protein that binds oxygen in the lungs. Their flattened shape gives them a large surface area for gas exchange. Mature red blood cells have no nucleus, leaving more room for haemoglobin.

White blood cells protect the body from pathogens, though different types have different defence roles. Platelets are small cell fragments that begin clotting after a vessel is damaged.

Clotting proteins form fibrin strands that trap cells and seal a wound. A clot is useful at an injury site, but a clot inside an unbroken vessel can block blood flow and cause serious damage.

Students meet circulatory ideas in daily life through pulse, exercise, temperature control, and medical checks. During exercise, the heart beats faster and pushes more blood with each beat. Blood flow increases to working muscles, where cells need more oxygen and glucose.

Flow to the skin can increase too, helping heat leave the body through sweating. Blood pressure is not a measure of fitness by itself. It shows the pressure in arteries during heart contraction and relaxation.

Persistently high pressure can strain vessel walls and make the heart work harder. When learning this topic, follow the direction of blood carefully, but do not use artery or vein as clues for oxygen level.

Those names describe whether blood travels away from or toward the heart. The pulmonary vessels are the important exception to the usual oxygen pattern.