Lesson 1, Topic 1
In Progress

Circulatory system

The circulatory system consists of the heart and blood vessels. This system is a closed circuit that has the function of transporting blood. This, in turn, has as main function to bring oxygen and nutrients to all cells of the body and collect from them carbon dioxide and other products resulting from metabolism.


The heart is a hollow, muscular organ that is found in the chest, in a cavity called mediastinum, located between the lungs, behind the sternum. It is surrounded by a thin film or membrane, the pericardium, which allows you to beat continuously without friction problems. The heart is situated obliquely, about a third on the right and two thirds to the left of the sternum.

The heart, is a powerful muscle pump that, in adulthood, beats about 70 times a minute and propels to the body about 60ml of blood in each contraction. During physical exercise, this flow (cardiac output) doubles or triples, depending on the intensity of the exercise.

From the point of view of functionality, the heart consists of two separate pumps, one on the left side and one on the right side. Each of these pumps has two chambers and works based on muscle contractions, ejecting blood out of the chambers.

The upper chambers, the auricles , receive blood from the body (venous blood) and lungs (arterial blood) and the lower chambers, the ventricles , receive blood from the auricles and pump it out of the heart . The left ventricle pumps arterial blood (rich in oxygen) to the entire body through a path called large circulation and the right ventriculi pump venous blood (low in oxygen) into the lungs, through a path called pulmonary circulation or small circulation.


An adult has on average about 6,000ml of blood, that is, about 1/12 of his body weight. It is a viscous liquid consisting of a solid part and a liquid part. The solid part constitutes about 42% of the total blood in men and 38% in women, and consists exclusively of cells. The liquid part that carries these suspended cells is plasma.

The cells that make up the solid part are distinguished structurally and functionally in red blood cells, white blood cells and platelets.

Red blood cells are more than 99% of blood cells. These red blood cells contain a protein, hemoglobin, with several iron atoms where O2 is connected. In people living in places with low oxygen concentration (high altitude), the number of red blood cells increases.

White blood cells (leukocytes) play an important role in defending the body against disease-bearing vectors (bacteria) and in maintaining our immunity. There are several types, with different functions.

Platelets are important in blood clotting. When a blood vessel is damaged the platelets add up forming a buffer (clot) that prevents blood from spilling outwards.


Blood coming out of the left ventricle circulates through the aorta artery, which is the largest in the body. This has a thick muscle wall that contracts during ventricular relaxation to maintain blood pressure during the filling of the ventricles. The aorta branches to the entire body.

Blood coming out of the right ventricle passes through the pulmonary trunks, which divide almost immediately into the right and left pulmonary arteries. This system takes blood to the lungs.

Blood vessels constitute a tree with the progressive branching of the branches, the arteries, in smaller and smaller ones from which the capillaries depart, which are the blood vessels of smaller caliber.

Capillaries are the last part of the entire cardiovascular system. It is through its thin walls that gas and nutrient sexchanges take place. The heart and large vessels are just a pumping and distribution system for capillaries.

From the capillaries the blood is collected by a few veins of very thin walls, the venulas, and returned to the auricles through the veins, larger. Most veins control the direction of blood circulation through one-way valves, which prevent blood from flowing in the wrong direction.