The Circulatory System

Structure and Components

Central to the circulatory system are the heart, blood vessels, and the blood they carry.
The heart is a muscular organ divided into four chambers: two atria on top and two ventricles at the bottom.
The blood vessels come in three types: arteries, veins, and capillaries.
Arteries carry blood away from the heart. This blood is typically oxygen-rich, except for the pulmonary artery.
Veins transport blood towards the heart. This blood is typically oxygen-poor, except for the pulmonary vein.
Capillaries, the smallest and most numerous of the blood vessels, facilitate the exchange of gases, nutrients, and waste products between the blood and surrounding tissues.
The blood itself is comprised of red and white blood cells, platelets, and plasma, each serving important roles in oxygen transport, immune response, clotting, and carrying nutrients and hormones respectively.

Double Circulatory System

In mammals, including humans, a double circulatory system is present - two distinct circuits in which the blood flows through the heart twice.
The pulmonary circuit carries deoxygenated blood from the right side of the heart to the lungs, where it gets oxygenated, and returns it to the left side of the heart.
The systemic circuit moves oxygenated blood from the left side of the heart to the entire body, delivering oxygen and nutrients to the cells, and returns deoxygenated blood back to the right side of the heart.

Cardiac Cycle

The cardiac cycle is the sequence of events that occurs when the heart beats.
It consists of two phases: systole (contraction) and diastole (relaxation).
During systole, the ventricles contract, forcing blood out of the heart into the arteries - first the atria contract (atrial systole) pushing blood into the ventricles, followed by the ventricles themselves (ventricular systole), pushing blood into the pulmonary artery and the aorta.
During diastole, the heart ventricles are relaxed and the heart fills with blood.

Control of the Heart Rate

The heart rate is controlled by the medulla oblongata, a part of the brain, in response to the body’s needs.
Signals are sent through the autonomic nervous system: the sympathetic system speeds up the heart rate while the parasympathetic system slows it down.
The sinoatrial node is the heart’s natural pacemaker, setting the rhythm of the heartbeat by triggering a wave of electrical signals to both atria, initiating their systole.
These electrical signals are then picked up by the atrioventricular node, which conducts the signals to ventricles, initiating their systole after a short delay that allows the atria to finish their contraction.

Exchange Surfaces and Delivery

Oxygen and nutrients are delivered, and waste products are picked up, at the body’s tissues through thin-walled capillaries.
The rate of this exchange at the capillary beds is regulated by both the body’s needs and by the diameter of arterioles leading to the capillaries.
Oxygen binds to haemoglobin in the red blood cells, forming oxyhaemoglobin, which is carried in the blood stream to cells throughout the body.
At the cellular level, oxygen is used in the mitochondria to produce ATP during cellular respiration.