CIRCULATORY SYSTEM: FUNCTIONS, PARTS, TYPES, DISEASES - ANATOMY AND PHYSIOLOGY - 2024

The circulatory system comprises a series of organs that orchestrate the passage of blood through all tissues, allowing the transport of various materials such as nutrients, oxygen, carbon dioxide, hormones, among others. It is made up of the heart, veins, arteries, and capillaries.

Its main function lies in the transport of materials, although it also participates in the creation of a stable environment for vital functions in terms of pH and temperature, as well as being related to the immune response and contributing to blood coagulation.

By Lomappmi, from Wikimedia Commons

Circulatory systems can be open - in most invertebrates - consisting of one or more hearts, a space called a hemocele, and a network of blood vessels; or closed - in some invertebrates and in all vertebrates - where blood is limited to a circuit of blood vessels and to the heart.

In the animal kingdom the circulatory systems are very varied and depending on the animal group the relative importance of the organs that compose it changes.

For example, in vertebrates the heart is decisive in the circulation process, while in arthropods and other invertebrates the movements of the extremities are essential.

Features

The circulatory system is primarily responsible for the transport of oxygen and carbon dioxide between the lungs (or gills, depending on the study animal) and body tissues.

Likewise, the circulatory system is responsible for distributing all the nutrients processed by the digestive system to all the tissues of the body.

It also distributes waste materials and toxic components to the kidney and liver, where after a detoxification process, they are eliminated from the individual through the excretion process.

On the other hand, it serves as a transport route for the hormones secreted by the glands, and distributes them to the organs where they must act.

It also participates in: the thermoregulation of organisms, properly adjusting the flow of blood, in regulating the pH of the organism and in maintaining an adequate hydro-electrolyte balance so that the necessary chemical processes can be carried out.

The blood contains structures called platelets that protect the individual from bleeding. Finally, the blood is composed of white blood cells, so it has an important role in defense against foreign bodies and pathogens.

Parts (organs)

The circulatory system is made up of a pump - the heart - and a system of vessels. These structures will be described in detail below:

The heart

Hearts are muscular organs with pump functions, capable of propelling blood through all tissues of the body. In general, they are made up of a series of chambers that are connected in series and are flanked by valves (or sphincters in certain species).

In mammals, the heart has four chambers: two atria and two ventricles. When the heart contracts, blood is expelled into the circulatory system. The multiple chambers of the heart allow pressure to rise as blood moves from the venous to the arterial zone.

The atrial cavity captures blood and its contractions send it to the ventricles, where the contractions send blood throughout the body.

The heart muscle is made up of three types of muscle fibers: the cells of the sinoatrial and atrioventricular node, the cells of the ventricular endocardium, and myocardial fibers.

The former are small and weakly contracting, they are autorhythmic and conduction between cells is low. The second group of cells are larger, weakly contracting but fast conducting. Lastly, the fibers are of intermediate size, with powerful contraction and are an important part of the heart.

Structure of the heart

In humans, the heart is located in the lower anterior region of the mediastinum, supported by the diaphragm and behind the sternum. The shape is conical and reminiscent of a pyramidal structure. The tip of the heart is called the apex and is located in the left region of the body.

A cross-section of the heart would reveal three layers: the endocardium, the myocardium, and the epicardium. The inner region is the endocardium, which is continuous with the blood vessels and is in contact with the blood.

The middle layer is the myocardium and here is the largest amount of cardiac mass. The tissue that forms it is muscular, involuntary contraction and has stretch marks. The structures that connect cardiac cells are the intercalary discs, allowing them to act synchronously.

The outer covering of the heart is called the epicardium and is made up of connective tissue. Finally, the heart is surrounded by an outer membrane called the pericardium, which in turn is divided into two layers: the fibrous and the serous.

The serous pericardium contains the pericardial fluid, whose function is the lubrication and damping of the heart's movements. This membrane is attached to the sternum, spinal column, and diaphragm.

Electrical activity of the heart

The heartbeat consists of the rhythmic phenomena of systoles and diastoles, where the first corresponds to a contraction and the second to the relaxation of muscle mass.

For cell contraction to occur, there must be an action potential associated with them. The electrical activity of the heart begins in an area called a "pacemaker," which spreads to other mated cells through their membranes. Pacemakers are located in the venous sinus (in the heart of vertebrates).

Arteries

Arteries are all the vessels that leave the heart and generally oxygenated blood is found in them, called arterial blood. That is, they can carry oxygenated blood (such as the aorta) or deoxygenated blood (such as the pulmonary artery).

Note that the distinction between veins and arteries does not depend on their content, but on their relationship with the heart and with the capillary network. In other words, the vessels that come out of the heart are the arteries and those that come to it are the veins.

The wall of the arteries are made up of three layers: the innermost is the tunica intima formed by thin endothelium on an elastic membrane; the tunica media formed by fibers of smooth muscle and connective tissue; and finally the tunica externa or adventitia composed of adipose tissue and collagen fibers.

As the arteries move away from the heart, their composition varies, increasing the proportion of smooth muscle and less elasticity, and they are called muscular arteries.

Blood pressure

Blood pressure can be defined as the force that blood exerts on the walls of the vessels. In humans, standard blood pressure ranges from 120 mm Hg in systole to 80 mm Hg in diastole, and is usually denoted by the digits 120/80.

The presence of elastic tissue allows the arteries to pulse while blood flows through the structure, thus helping to maintain high blood pressure. Artery walls must be extremely thick to prevent them from collapsing when blood pressure drops.

Veins

Veins are blood vessels responsible for transporting blood from the capillary network system to the heart. Compared with arteries, veins are much more abundant and have a thinner wall, they are less elastic and their diameter is greater.

Like arteries, they are made up of three histological layers: the inner, the middle, and the outer. The pressure in the veins is very low - in the order of 10 mm Hg - therefore they must be assisted with valves.

Capillaries

Capillaries were discovered by the Italian researcher Marcello Malpighi in 1661, studying them in the lungs of amphibians. They are very abundant structures that form extensive networks near almost all tissues.

Its walls are composed of fine endothelial cells, connected by fibers of connective tissue. It is necessary that the walls are thin so that the exchange of gases and metabolic substances occurs easily.

They are very narrow tubes, in mammals they have an approximate diameter of 8 µm, large enough for blood cells to pass through.

They are structures that are permeable to small ions, nutrients and water. When exposed to blood pressure, fluids are forced out into the interstitial space.

Fluids can pass through clefts in endothelial cells or through vesicles. In contrast, substances of a lipid nature can easily diffuse through endothelial cell membranes.

Blood

Blood is a thick and viscous fluid responsible for the transport of elements, it is generally found at a temperature of 38 ° C and makes up 8% of the total weight of an average individual.

In the case of very simple animals, such as a planarian, it is not possible to speak of "blood", since they only have a clear, watery substance composed of cells and some proteins.

With regard to invertebrate animals, which have a closed circulatory system, blood is generally known by the term hemolymph. Finally, in vertebrates, blood is a highly complex liquid tissue and its main components are plasma, erythrocytes, leukocytes, and platelets.

Plasma

Plasma constitutes the liquid potion of blood and corresponds to 55% of its total composition. Its main function is the transport of substances and the regulation of blood volume.

Some proteins are dissolved in plasma, such as albumin (main component, more than 60% of total proteins), globulins, enzymes and fibrinogen, in addition to electrolytes (Na ⁺, Cl ^-, K ⁺), glucose, amino acids, waste metabolism, among others.

It also contains a series of dissolved gases, such as oxygen, nitrogen and carbon dioxide, the residue produced in the respiration process and must be eliminated from the body.

Solid components

Blood has cellular components that correspond to the remaining 45% of blood. These elements correspond to the red blood cells, the white blood cells and the cells related to the clotting process.

Red blood cells, also called erythrocytes, are biconcave discs and are responsible for transporting oxygen thanks to the presence of a protein called hemoglobin. A curious fact about these cells is that, in mammals, mature erythrocytes lack a nucleus.

They are very abundant cells, in a milliliter of blood there are 5.4 million red blood cells. The half-life of a circulating erythrocyte is about 4 months, in which it can travel more than 11,000 kilometers.

White blood cells or leukocytes are related to the immune response and are found in a lower proportion than red blood cells, in the order of 50,000 to 100,000 per milliliter of blood.

There are several types of white blood cells, including neutrophils, basophils, and eosinophils, grouped under the category of granulocytes; and agranulocytes corresponding to lymphocytes and monocytes.

Finally, there are the cell fragments called platelets - or thrombocytes in other vertebrates -, which participate in the coagulation process, preventing bleeding.

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Types of circulatory systems

Small animals - less than 1 mm in diameter - are capable of transporting materials in their bodies by simple diffusion processes.

However, with the increase in body size comes the need to have specialized organs for the distribution of materials, such as hormones, salts or waste, to the different regions of the body.

In larger animals, there is a variety of circulatory systems that effectively fulfill the function of transporting materials.

All circulatory systems must have the following elements: a main body in charge of pumping fluids; a system of arteries capable of distributing blood and storing pressure; a capillary system that allows the transfer of materials from the blood to the tissues and finally a venous system.

The set of arteries, veins and capillaries form what is known as “peripheral circulation”.

In this way, the set of forces carried out by the previously mentioned organs (the rhythmic beats of the heart, the elastic recoil of the arteries and the contractions of the muscles that surround the blood vessels) make possible the movement of blood in the body.

Open circulatory systems

Open circulation is present in different groups of invertebrate animals, such as crustaceans, insects, spiders and different mollusks. It consists of a system of blood that is pumped by the heart and reaches a cavity called hemocele. In addition, they have one or more hearts and blood vessels.

The hemocele can occupy in some organisms up to 40% of the total body volume and is located between the ectoderm and the endoderm, remembering that triblastic animals (also known as triploblastics) have three embryonic leaves: the endoderm, mesoderm and ectoderm.

For example, in some species of crab the blood volume corresponds to 30% of the body volume.

The liquid substance that enters the hemocele is called hemolymph or blood. In these types of systems, there is no distribution of blood through capillaries to the tissues, but the organs are bathed directly by the hemolymph.

When the heart contracts, the valves close and blood is forced to travel to the hemocele.

The pressures of closed circulatory systems are quite low, between 0.6 and 1.3 kilopascals, although contractions produced by the heart and other muscles can raise blood pressures. These animals are limited in the speed and distribution of blood flow.

Closed circulatory systems

In closed circulatory systems, blood travels in a circuit made up of tubes and follows the path from the arteries to the veins, passing through the capillaries.

This type of circulatory system is present in all vertebrate animals (fish, amphibians, reptiles, birds and mammals) and in some invertebrates such as the earthworm and cephalopods.

Closed systems are characterized by presenting a clear separation of functions in each of the organs that compose it.

Blood volume occupies a much smaller proportion than in open systems. Approximately 5 to 10% of the individual's total body volume.

The heart is the most important organ and is responsible for pumping blood within the arterial system, thus maintaining high blood pressure.

The arterial system is in charge of storing the pressure that forces the blood to pass through the capillaries. Therefore, animals with closed circulation can transport oxygen rapidly.

Capillaries, being so thin, allow the exchange of materials between blood and tissues, mediating simple diffusion, transport or filtration processes. Pressure enables ultrafiltration processes in the kidneys.

Evolution of the circulatory system

Throughout the evolution of vertebrate animals, the heart has notably increased in complexity. One of the most momentous innovations is the gradual increase in the separation of oxygenated and deoxygenated blood.

Fishes

In the most primitive vertebrates, fish, the heart consists of a series of contractile cavities, with only one atrium and one ventricle. In the circulatory system of fish, blood is pumped from the single ventricle, through capillaries in the gills, where oxygen uptake occurs and carbon dioxide is expelled.

The blood continues its journey through the rest of the body and the oxygen supply to the cells occurs in the capillaries.

Amphibians and reptiles

When the lineage of amphibians originated and then that of reptiles, a new chamber appears in the heart, now exhibiting three chambers: two atria and one ventricle.

With this innovation, the deoxygenated blood reaches the right atrium and the blood coming from the lungs reaches the left atrium, communicated by the right ventricle.

In this system, the deoxygenated blood remains in the right part of the ventricle and the oxygenated in the left, although there is some mixing.

In the case of reptiles, the separation is more noticeable since there is a physical structure that partially divides the left and right regions.

Birds and mammals

In these lineages, endothermy ("warm-blooded" animals) leads to higher demands on the supply of oxygen to the tissues.

A heart with four chambers is able to meet these high requirements, where the right and left ventricles separate the oxygenated blood from the deoxygenated. Thus, the oxygen content reaching the tissues is the highest possible.

There is no communication between the left and right chambers of the heart, as they are separated by a thick septum or septum.

The cavities located in the upper portion are the atria, separated by the interatrial septum, and are responsible for receiving blood. The superior and inferior vena cava are connected with the right atrium, while the four pulmonary veins reach the left atrium, two coming from each lung.

The ventricles are located in the lower region of the heart and are connected to the atria through the atrioventricular valves: the tricuspid, found on the right side, and the mitral or bicuspid on the left.

Common diseases

Cardiovascular diseases, also known as coronary or heart disease, comprise a series of pathologies associated with the incorrect functioning of the heart or blood vessels.

According to surveys carried out, cardiovascular diseases are the leading cause of death in the United States and in certain European countries. Risk factors include a sedentary lifestyle, high-fat diets, and smoking. Among the most common pathologies are:

Arterial hypertension

Hypertension consists of elevated values ​​of systolic pressure, greater than 140 mm Hg and diastolic pressure greater than 90 mm Hg. This leads to an abnormal flow of blood throughout the circulatory system.

Arrhythmias

The term arrhythmia refers to the modification of the heart rate, the product of an uncontrolled rhythm - tachycardia - or bradycardia.

The causes of arrhythmias are varied, ranging from unhealthy lifestyles to genetic inheritance.

Murmurs in the heart

Murmurs consist of abnormal heart sounds that are detected through the auscultation process. This sound is associated with increased blood flow due to valve problems.

Not all murmurs are equally serious, it depends on the duration of the sound and the region and intensity of the noise.

Atherosclerosis

It consists of the hardening and accumulation of fat in the arteries, mainly due to unbalanced diets.

This condition makes it difficult for blood to pass, increasing the likelihood of other cardiovascular problems, such as strokes.

Heart failure

Heart failure refers to the inefficient pumping of blood to the rest of the body, causing symptoms of tachycardia and breathing problems.

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