Circulatory System: Body Fluids

In this lesson we are going to analyse the main body fluids, the exchange of fluids that takes place in our body and the conduction system.

Erythrocyte

Body Fluids

Introduction

Most of our body is a saline liquid. The main dissolvent is water. Water is our main component due to many reasons: life was originated in water, it is a liquid capable of dissolving many different substances (it is called universal solvent), it allows many different chemical reactions and exchanges and it is a good substance to control abrupt temperature changes, because it warms and cools slowly. 

The amount of water is different in different parts of the body. There are tissues, such as bone or adipose, where the water concentration is very low. Some organs, such as the brain, have high amount of water, even higher than 80% of its weight.  The amount of water is also variable depending on the age: older people usually have lower amount of water. Babies' bodies have a water concentration higher than 75%, whereas old people's bodies have a water concentration lower than 70%.

Losses and additions of water

The human body is not an isolated structure, it is constantly losing water that must be replaced to maintain the hydride balance.

The most relevant ways to lose water are:

• Exhalation: when our respiratory system releases air, it has a high amount of water vapour
• Evapotranspiration: water vapour escapes from our body through our skin. It can be divided into two processes:
• Insensible perspiration: it is the water lost by our skin as water vapour. Our body is 37°C, so some liquid water is transformed into water vapour that escapes from our body through our epidermis tissue. It is nearly constant, because our body temperature is also constant.
• Sensible perspiration: it mainly comes from our sweat. Sweat is mainly made up of water with mineral salts. It is released to our skin in order to reduce our body temperature; sweat evaporates cooling our skin. This perspiration is very variable, it depends not only on our body temperature (that can rise or drop due to our physical activity), but also on the external temperature. It can variate from less than half a litre to more than ten litres a day.
• Water in the faeces: it is not a relevant amount of water, lower than half a litre per day. But it can be drastically incremented in some gastric diseases, causing diarrhoea.
• Water in the urine: the excretory system is the main controller of the homeostasis or, in other words, the amount of water and electrolytes in our body. Kidneys can produce different volume of urine with more or less concentration of substances, depending on the hydric state of the body.

As far as the adding of water is concerned:

• Drinking: it is the most relevant way to add water. Our brain makes us feel thirsty when it detects water deficit. Voluntary water comsumption is very variable: some people drink less than half a litre of water per day, whereas others drink more than three litres.
• Water in the food: all the living beings are partially made of water, so when we eat any kind of food, we are also consuming water contained in it. It is variable depending on the kind of food we eat, but it can be higher than one litre per day.
• Metabolic water: our regular metabolic activity produces water. In the basic respiratory reaction, glucose is burnt and transformed into carbon dioxide and water, releasing energy in the process. It can be higher than half a litre per day.

Body Water distribution

Body water can be divided into two groups, according to its distribution:

• Intracellular Water: it is in the interior of our cells. It is the most abundant water in our body, around 70% of the total water in our body (or, in other words, 40% of our total weight).
• Extracellular water: it is in the exterior of our cells. It is around 30% of our total water (or, in other words, 20% of our total wight). It is the place where all the metabolic exchanges between cells or between the organism and the outer media are carried out.

Intracellular water is nearly constant because cells need a precise amount of water to stay alive and small changes in this volume kill them. Each kind of cell has a characteristic volume of inner water.

Extracellular water, however, is more variable. This water can be found in three different spaces:

• Plasmatic space: it is usually called plasma. Plasma is the liquid part of the blood and it is enclosed into the circulatory system. Its function is transporting nutrients from the digestive and respiratory systems to the cells and tissues and waste products from the cells and tissues to the excretory system. This space, however, does not have direct contact with the cells or the exterior.
• Interstitial space: it is located between the cells and it is filled with interstitial liquid. It is the liquid that forms the extracellular matrix that separates the cells. It forms the lymph when it enters the lymphatic vessels. It binds with the capillaries and the cell membranes of the cells.
• Transcellular space: this is the space where different kinds of special liquid can be found. These liquids are built up or confined in several places in order to be eliminated or carry out specific functions. Liquids built up in the gastrointestinal duct or in the urinary system, sweat in the sweat glands, the pleural fluid that surrounds and protect the lungs, the pericardic fluid that surrounds the heart, the cerebrospinal fluid in the central nervous system, the synovial fluid in the synovial joints and the ocular humours are the most typical examples.

Liquid balance and regulation

The water composition is different in these different spaces and its chemical and physical properties must be constant. Inner fluids permanently flow from one space to another, carrying ions and other components dissolved. On the one hand, the osmotic processes play a relevant role in this process, because they promote the movement of water. On the other hand, many substances must be transported from one place to another without forcing water to move with them. The regulation of the osmotic processes is essential: if water floods any organ, tissue or cell it could cause severe damage. When cells accumulate too much, for instance, water they can explode and die.

The extracellular fluid is richer in potassium and chlorine than extracellular fluid. The extracellular fluid, however, is richer in sodium and phosphates. This balance must allow the intracellular fluid to be much richer in proteins and dissolved organic substances without forcing water to move to the interior of the cell due to osmotic processes. 

Water balance must, at the same time, allow the exchange of substances. On the one hand, the liquid in the plasmatic space has an internal pressure, because it is enclosed in a tube (the blood vessels). It is called hydrostatic pressure, and tends to push the liquid to the exterior of the vessels. On the other hand, plasmatic fluid is rich in some special proteins (called oncotic proteins) that cause a osmotic pressure, called oncotic pressure, that tends to pull water from the exterior to the interior of the tubes. As a result, the oncotic pressure, that is opposite to hydrostatic pressure, prevents water from exiting the tubes excesively.

In the arterial part of the capillaries the hydrostatic pressure is stronger than the oncotic pressure. Due to this, the plasma tends to exit the vessel, flooding the interstitial space. In the venous part of the capillaries, however, the hydrostatic pressure is weaker because of the lower amount of plasma (that has moved to the exterior). The oncotic pressure in this part of the capillaries has risen, because there is less water, so the dissolved oncotic proteins, that cannot exit the capillaries, are more concentrated. As a result, in this zone the fluids tend to move from the interstitial space to the interior of the vessels.

Nevertheless, the amount of water that exits the capillaries in the arterial pole is always bigger than the amount of water that gets back the capillaries in the venous pole. Due to this, the plasmatic fluid tends to be built up in the interstitial space. It will be returned to the plasmatic space by the lymphatic system.

Liquid balance in capillaries.

The lymphatic system drains the interstitial space, preventing it from being flood. When this system does not work properly the plasmatic fluid floods the interstitial space causing edema. 

The Blood

General Characteristics

Blood is a fluid made up of a solid and a liquid part. It moves throughout our body enclosed in the circulatory system.

It is the main system to transport substances. It is also important to send messages between different parts of the body. It is essential in the respiratory, nutritive, excretive, defensive and regulatory functions. It is between 6% and 8% of the total weight of the body. It is a viscous liquid with a pH around 7.4.

The blood has two different parts or phases:

• Liquid part: it is made of a liquid plasma or serum. It is a complex yellow liquid, with many dissolved and suspended components. It is around 55% of the total weight of the blood.
• Solid part: it is made up of cells also called formed elements. It is around 45% of the total weight of the blood.

Liquid part of the blood: plasma

The most abundant component of this yellow liquid is water, around 91%. The rest of the liquid is made up of different components, many of them solid substances. 7% of the total weight is made of proteins.

The most abundant plasmatic proteins is the albumin. It is responsible for controlling the osmotic blood pressure and transporting some substances, above all steroids.

Another abundant protein is the fibrinogen. It is the precursor of the fibrin, that is responsible for the coagulative process of the blood.

In the plasma there can also be found other dissolved components, such as sugars (above all glucose, with a constant concentration of 100 mg per ml), lipids (there is system to transport lipids called apolipoproteins), nutrients, creatine, bilirubin, vitamins, hormones (a large variety at extremely low concentrations, they can move free or linked to transporters), waste products (such as urea or uric acid), gases (carbon dioxide and oxygen, although this one is mainly transported by erythrocytes) and electrolytes (such as Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, PO₄^3-, HCO₃^- and SO₃^2-).

Solid part of the blood: formed elements

Introduction

Blood cells can be divided into three groups: erythrocytes, also called red cells, leucocytes, also called white cells and thrombocytes, also called platelets.

Erythrocytes

Haemoglobin

The erythrocytes are, by far, the most abundant cells of the blood: more than 99% of the total blood cells. One cubic millimetre of blood contains around five million erythrocytes.

They are flat and circular, with a shape as a biconcave disc. They don't have nucleus or complex organelles and are just filled with a protein called haemoglobin.

This is a globular protein made of four subunits (two alpha subunits and two beta subunits). Each one has an active centre with a complex organic molecule called heme group, that has iron in its central part (so one haemoglobin molecule has four heme group and each one had one atom of iron). It is responsible for linking oxygen in order to transport it. Summing up, haemoglobin is the protein responsible for transporting oxygen. Ergo erythrocytes are the cells that transport oxygen in the blood. 100 milligrams of blood have between 14 and 20 grams of haemoglobin.

The erythrocytes don't have a nucleus or complex organelles and their metabolic processes are anaerobic (they don't have mitochondrions) to prevent them for consuming the oxygen that are transporting.

They have a complex cytoskeleton that preserve their peculiar cell shape. This shape allows them to move throughout extremely thin blood vessels and capillaries without reducing the blood flow. They can be folded or bent in pronounced curves to move without blocking them.

Due to the lack of organelles erythrocytes can not be repaired, so their life expectancy is very short, around 120 days. Then, they are eliminated by macrophages in the spleen. The heme group must be recycled, the iron is built up forming a molecule called ferritin. The rest of the molecule is transformed into bilirubin, that is released into the faeces by the liver.  

The destroyed erythrocytes must be replaced by new ones to ensure the correct transportation of oxygen. The health disorder caused by low the amount of erythrocytes is called anaemia.

Erythrocyte: shape

Leucocytes

Leucocytes are bigger than erythrocytes and have a nucleus and complex organelles (although they don't have haemoglobin). They are responsible for defending the organism against invaders. They can be divided into several groups:

• Granulocytes: these are leucocytes with lobulated nucleus and grains in the cytoplasm that can be seen using an optic microscope.
• Neutrophils: they are the most abundant granulocytes. Their function is phagocyting invaders matched by antibodies.
• Eosinophils: they are not so abundant and their grains are coloured by acid colorants. They phagocyte invaders and are also related to inflammatory processes.
• Basophils: their grains are coloured by basic colorants. They are related to inflammatory phenomena and they trigger chemical processes that prepare the body to fight against infections when they release the content of their granules. Due to this they are also related to allergies. 
• Agranulocytes: they don't have cytoplasmic grains visible with the optic microscope. They are two types of agranulocytes.
• Monocytes: they are the bigger leucocytes, with a bean-shaped nucleus and a special ability to get out of the blood and become macrophages when they reach any connective tissue. Their main function is phagocyting any unknown external agent, such as bacteria or virus, fragmenting it and showing it to the lymphocytes, that are responsible to produce antibodies. According to this, monocytes and macrophages are the first defensive cells that detect invaders.
• Lymphocytes: they detect invaders that have been phagocyted by monocytes and produce antibodies against these invaders. Antibodies are essential to match the invaders in order to make them easily recognised by other defensive elements, improving the defensive process. There are two types of lymphocytes called T and B. T lymphocytes grow up in the thymus and they produce toxic substances to destroy invaders or even our own cells when they are invaded by virus or have become tumoral. B lymphocytes are responsible for producing antibodies. 

Erythrocyte - Thrombocyte - Leucocyte

Thrombocytes

These are also known as platelets. They are not real cells, but fragments of cells without nucleus, but rich in grains filled with substances that are responsible for triggering coagulative processes. These cells also have a complex cytoskeleton with contractile proteins in  the cytoplasm. Thanks to these proteins, thrombocytes can group, forming compact groups called thrombi that block extravasations when blood vessels are damaged. 

Blood Cells.

Hematopoiesis

Hematopoiesis is defined as the production of blood cells. It is carried out by the stem cells that are located in the bone marrow of larger bones. The stem cells of the bone marrow differentiate to the different blood cells. There are different cell families that can evolve to form specific blood cells. There are three basic families, one to form erythrocytes, another to form leucocytes and finally another one to form thrombocytes.

It is an extremely controlled process that is activated when the body detects that the amount of some blood cells has decreased. When the body detects, for instance, that the erythrocytes are not able to transport enough oxygen, a hormone called erythropoietin (EPO) is released by the kidneys. As a result, the stem cells of the bone marrow produce erythrocytes.

Homeostasis

Haemostasis is defined as the process responsible for preventing the body from haemorrhages. In other words, hemostasis prevents us from losing a large amount of blood due to extravasations related to damage in blood vessels.

The first reaction that occurs when a vessel suffers a severe damage is the contraction of the muscular wall of the vessel. This contraction is called spasm and is triggered by pain receptors in the vessel wall.

The second reaction is the formation of the thrombus made up of thrombocytes. These cells join and release the substances built up in their grains. The substances promote the aggregation of thrombocytes. The contractile proteins of the cells contract, making the aggregation more compact. The group of joined compacted thrombocytes cover the damaged vessel preventing it from the extravasation.

The third reaction is the production of the red thrombus, formed from the solidification of the liquid part of the blood. It is a chain reaction triggered by some substances released by the cell wall of the vessel when they are damaged. The last reaction transforms fibrinogen, a soluble protein in the plasma, into fibrin that is insoluble. These insoluble proteins solidify the plasma.

There are two coagulative pathways. One of them is called the extrinsic pathway, it is the fastest one and takes place after severe damage. It is triggered by proteins released by surrounding tissues after suffering damage. The second one is called the intrinsic pathway. It is slow and complex and it is triggered when the epithelial cells that cover the inner part of the vessels detect damage.

When the damaged blood vessel is repaired, the thrombus and the fibrillar net must be eliminated. This process is carried out by plasmatic enzymes that destroy the fibrin net.

The Hydrosphere

The Hydrosphere: Definition and Distribution

The hydrosphere is defined as the water that covers the Earth's surface. Water in the Earth can be found in three different states: liquid, solid (called ice) and gas (called water vapour).

70% of total Earth's surface is covered by water. Most of this water is salt water, located in the seas and oceans. In fact, 97% of total water is salt water. The other 3% is fresh water that can be found in different locations and states. 

68.8% of total fresh water is solid water: ice and snow. This kind of water can be found, above all, in the poles, but also in high mountains as snow covering the ground or glaciers (large and erosive rivers made of solid water).

30.1% of fresh water is groundwater, water located in pores of deep rocks, underground currents, aquifers, etc.  

0.45% of total fresh water can be found in the atmosphere, as water vapour or tiny drops of water in the clouds. Other 0.45% can be found in living beings: water is the main component of all the known living things in this planet.

Finally only 0.3% of total fresh water is surface fresh water. Only this percentage is really available for regular human consumption. 

87% of surface water is located in lakes and other similar wetlands. 11% is located in swamps. Only 2% can be found in rivers and other surface currents, such as torrents or streams. 

Water: Properties

Water is a substance with some relevant characteristics and properties that make it essential to understand its role in the planet and living things.

Universal solvent

Water is called universal solvent, because it can dissolve many different substances. It can dissolve nearly any kind of salt and many organic substances. It can also support many chemical reactions, or in other words, many chemical reactions can occur in water.

This is one of the reason why water is the most abundant component of living things. All the chemical reactions essential for in living beings take place in water. Water, besides, dissolves many organic or inorganic substances that make up the living beings.

Specific heat capacity

Water has a high specific heat capacity. This means that water preserves heat, so it warms and cools quite slowly.

This characteristic is related to how seas and oceans prevents from abrupt climatic changes of the atmosphere.

High surface tension

Surface tension is the force of the water surface that makes it extend on a surface as much  as possible. Water has a relevant surface tension. As a result, it has a great force of adhesion and also a great cohesion.

This is related to how water flows, its viscosity and its erosive capacity.

Sea Water

Characteristics

97% of total water on Earth is salt water located in seas and oceans. The main characteristic of this water are:

• Components and salinity.
• Temperature.

Components and salinity

Apart from water molecules (H₂O), sea water has other components. The most relevant ones are salts and electrolytes, that are related to the salinity. But sea water has other components, such as gases and organic molecules.

Sea water has different dissolved gases. Oxygen (O₂) is one of the most abundant dissolved gases and it is essential for living beings that need this molecule for respiration. Nitrogen and carbon dioxide are quite abundant too.

The organic molecules dissolved in sea water come from the vital activity of living beings or from several human activities (in this case, we talk about contaminants).

Salinity is defined as the concentration of salts in the water. Salts are inorganic substances. The most abundant one is, by far, sodium chloride (NaCl). Sea water has higher concentration of salts than surface water or groundwater. Due to this, sea water is also called salt water.

The concentration of salts, above all of sodium chloride, is one of the most important characteristics of sea water. Different oceans have different salinity. We can say, in general, that the colder the sea is, the lower amount of salts it has. Due to this, The Artic Sea is characterised by its low salinity. The Dead Sea is  the sea with highest salinity.

The average salinity of sea water is around 35 grams of salt per litre of water. Although the essential components of salt are, as we have just studied, sodium and chlorine, there are other components, such as magnesium, calcium, sulphur or potassium.

Temperature

The temperature of sea water depends on the latitude (distance from the equator) and the depth.

As far as latitude is concerned, sea water temperature decreases with latitude. In other words, it is hotter near equator and it is colder in the poles. This fact is related to some water movements called sea currents.

As far as depth is concerned, the water temperature decreases with depth. At 1000 meters deep, the temperature is lower to -2°C. In fact, it is still liquid because pressure and salinity prevents it from freezing.

Sea water movements

The most relevant sea movements are:

• Waves.
• Ocean Currents.
• Tides.

Waves

Waves are surface movements of water in seas or oceans. They mainly result from wind wind flowing over the seas. Waves can move from thousands kilometres before reaching the coast. 

Waves can have different sizes depending on the strength and direction of the wind. Wave size also depends on the coast features.

Waves are relevant erosive factors. They are related to the formation of cliffs and beaches and how they evolve or change.

Waves are, besides, very important because they oxygenate the water.

Ocean Currents

Ocean currents are continuous directed movements of seawater. They are large masses of water that move like rivers through the oceans.

These currents are mainly produced by the winds and differences in the temperature and salinity of the water. These differences cause variation in the density of the water and this variable density is the main reason for its movement.

The Coriolis effect is another relevant factor. This effect comes from the centrifugal forces caused by the Earth's rotation. The centrifugal forces affect, above all, the fluids, such as atmospheric gases and seawater. It affects the direction and trajectory of the current.

There are also vertical currents. These are vertical movements of seawater. There are two different currents, the surface current that involves the first four hundred meters, and the deep current that involves water deeper than four hundred meters. These two layers of seawater are nearly independent and water does not tend to move from one layer to other. In fact, some water properties, such as salinity and temperature, change abruptly causing a sort of boundary between these two regions. 

Tides

Tides are periodical rises and falls of seas levels. There are one or two high and low tides per day that are caused by gravitational forces exerted by the Moon and the Sun. The Moon's gravitational effect is much stronger than the Sun's one.

The amplitude of the tides depend on two factors: the relative position of the Moon and the Sun and the characteristics of the shore.

Tides are higher when the Moon and the Sun are aligned with the Earth, and are lower when  they are not in a line.

The structure of the shore is also a relevant factor. When the shore directly connects to an large ocean, tides tend to be higher. Tides in the Cantabrian Sea, for instance, are higher than tides in the Mediterranean Sea because the Cantabrian Sea is directly connected to the Atlantic Ocean, that has much more water mass than the Mediterranean Sea and as a result, the effect of the Moon and the Sun is also bigger.

Fresh Water

It is called continental water, and its main characteristic is the low concentration of dissolved salts. There are several types of fresh water:

• Solid water (ice).
• Groundwater.
• Lakes.
• Wetlands.
• Streams, torrents and rivers.

Solid Water

It is the most abundant type of fresh water. It can be found in glaciers, caps, ice sheets or icebergs.

Solid water is exclusive to cold places. The atmospheric temperature depends on the altitude and the latitude and the coldest locations are at high altitude or high latitude. Due to this, ice and snow can merely be found in covering high mountains or near the poles. In fact, mountains above four thousand meters and polar regions have permanent snow or ice. 

Glaciers are characteristic structures made of a permanent mass of ice in continuous movement. The ice in the glaciers moves, flows like the water in a river, but very slowly. Although it is not a fast movement, it has an extreme erosive power. They characteristically wear deep and wide U shaped valleys.

Groundwater

Groundwater is defined as the water that can be found beneath the Earth's surface. It is stored in soil pores, fractures of rocks or in underground rivers or lakes.

Groundwater can sometimes form underground caves and galleries.

When it emerges to the surface, it can form springs, seeps, wells or wetlands. It usually feeds rivers, streams and lakes.

The line that separates the lower parts of the soil filled with water to the upper parts of the soil with no water in the pores is called phreatic level. 

Lakes

Lakes are large masses of water surrounded by land. They can be fed by rivers, glaciers or groundwaters. Rivers and groundwaters can also drain lakes.

They are always located in basins or depressions and they can have variable size and depth, from few square metres to many square kilometres. The largest lake of the world is the Caspian Sea, that is 371,000 Km2. 

Wetlands

Wetlands are areas permanently or seasonally saturated with water. There are many different types of wetlands:

• Swamps: forested wetlands.
• Marshes: wetlands covered by herbaceous plants.
• Bogs: wetlands rich in discomposed plants and other decaying materials.
• Fens: wetlands dominated by grass and sedges.

Rivers and Torrents

Natural water courses of freshwater. Rivers are permanent water courses. Torrents and streams are intermittent or discontinuous water courses with variable courses.

They are responsible for surface runoff, they transport water from the land to the sea. They are also a relevant erosive factor, not as strong as glaciers but much more frequents. They wear V shaped valleys in he upper course and U shaped valleys in the lower course.  

Water Cycle

The water cycle includes all the process that promotes changes in water and its transformations from one type of water to another.

• Evaporation: the Sun warms water, transforming it into water vapour. This process occurs, above all, in seawater, because it is extremely abundant and covers 75% of Earth's surface. But it also takes place in freshwater.
• Evapotranspiration: the water from living beings also evaporates. It is a low amount of water.
• Condensation: the atmospheric water vapour is condensed, transforming the vapour into tiny drops of water that made the clouds.
• Transport: the wind moves the clouds. This is specially relevant in the sea, so clouds formed above the sea due to the evaporation and condensation can move from the sea to the land.
• Precipitation: when the tiny drops of water of the clouds cool and condense, forming bigger drops that fall. This process can occur in many different places, but is very usual in higher zones of the atmosphere and when the clouds shock with the mountains. The water falls forming rain, snow or hail.
• Surface Runoff: this is the movement of the surface water from the land to the sea. Surface runoff forms rivers, streams and torrents.
• Infiltration: the infiltration is the movement of water from the Earth's surface to deeper parts of the ground, forming groundwater.
• Deep Runoff: this is the movement of groundwater to the sea. Due to this, groundwater returns from the land to the oceans.

Función nerviosa, tres segundos y un gol.

No solemos ser conscientes de todo lo que rodea nuestra realidad, de la cantidad de decisiones que nuestro cuerpo debe tomar en fracciones de segundo. De cómo funciona el sistema nervioso y mantiene al cuerpo anclado al mundo, cómo analiza los estímulos, cómo ordena respuestas de una forma precisa y preciosa.

Partido de futbol. Minuto cincuenta y dos, veinticuatro segundos. Empate a cero.

Nuestro delantero cruza la medular en dirección a la frontal del área contraria. A su cerebro llega multitud de información procedente de sus sentidos, su piel envía información sobre la temperatura ambiente y la ligera brisa que la cubre, sus odios perciben los gritos del público e incluso podría notar el olor de la hierba cortada a través de sus fosas nasales, o percibir la humedad del aire en la cara interna de las aletas de su nariz .

Solo la parte autónoma de su cerebro reacciona, produciendo sudor o erizando el pelo de su piel. La parte consciente de su cerebro, sin embargo, obviará toda esta información y aunque los impulsos llegan desde los receptores al cerebro, éste la ignorará mediante un fenómeno conocido como inhibición lateral, para centrar toda la atención en lo que realmente importa en ese momento: el balón, la portería, el portero, el gol.

Mira al balón y la luz que choca contra el esférico, el césped, el guardameta, rebota en la materia, traspasa el aire para llegar a su córnea y pasar el iris, tras el cual el cristalino la concentra sobre la córnea y la información visual es transformada en un impulso eléctrico que sale del ojo a través del nervio óptico. (1b)

Los nervios ópticos salen de los ojos para dirigirse al cerebro. Antes de alcanzarlo, se cruzarán de derecha a izquierda: el nervio del ojo izquierdo pasa al hemisferio derecho del cerebro y el nervio del ojo derecho al hemisferio izquierdo.

La información visual atraviesa el cerebro, llega al quiasma óptico y se dirige hacia una zona de corteza situada en la parte posterior, también llamada occipital, del cerebro conocida como corteza visual. Allí se interpreta la información que llega de los ojos, se conforman las imágenes.

Y el delantero ve al portero, la portería y centra su atención, su vista, en el balón para ejecutar el lanzamiento y hacer al esférico volar hacia la escuadra derecha de la meta.

Mira el balón, decide qué zona del mismo debe golpear. Y cómo.

Al cerebro llega información sobre la posición corporal, el grado de contracción de todo el sistema muscular, cómo se encuentra la pierna, la rodilla y el tobillo en relación al resto del cuerpo. (1)

Los tendones envían señales sobre su estado de contracción.  Éstas salen del tendón por un nervio que se dirige hacia la médula espinal y que se conoce como nervio aferente. (2)

El nervio aferente llega a la médula espinal. En la médula podemos distinguir dos zonas, una zona interna, con forma de mariposa, de color grisáceo oscuro y compuesta de cuerpos o somas neuronales. Hay infinidad de neuronas con conexiones internas entre ellas. Es la zona de integración, donde se toman decisiones.

Si en esta zona se interpretase que existe alguna posibilidad de daño severo, se ordenaría el movimiento muscular para evitarlo, sin que exista interacción del cerebro. Esto se conoce como reflejo medular y es el que se desencadena, por ejemplo, cuando tocamos algo muy caliente y retiramos la mano sin pensar, o cuando nos golpean con un objeto el tendón rotuliano y extendemos la pierna.

En nuestro caso no hay reflejo medular que desencadenar, la información debe ser enviada al cerebro. De esto se encargan los cordones nerviosos que ascienden y descienden por las zonas blancas periféricas de la médula. (3)

El estado de contracción de los músculo, por lo tanto, asciende por estas zonas periféricas de la médula en dirección al encéfalo. Atraviesa el bulbo y llega a la protuberancia. Aquí sucede algo parecido a lo que ocurre con los ojos: la información de la parte derecha del cuerpo pasa al hemisferio izquierdo y la información de la derecha pasa al derecho. La zona donde se cruzan los haces nerviosos se denomina decusación de las pirámides. (4)

De la protuberancia la información pasa al tálamo. Y del tálamo, viaja a zonas de la corteza cerebral, repleta de cuerpos neuronales y conexiones entre neuronas que conforman la materia gris. En la corteza cerebral existe un mapeo extremadamente preciso del cuerpo, un esquema que nos hace saber con mucha precisión la posición de nuestro cuerpo, así como las sensaciones que nos llegan por la piel. Se denomina corteza sensitiva. (5)

Si en lugar de información sobre posición, llegase información sobre dolor, esta se quedaría fundamentalmente en el tálamo. En el tálamo el mapeo no es tan preciso y por esa razón el dolor está mucho más deslocalizado que las sensaciones de tacto o contracciones musculares habituales. La patada que nuestro jugador recibió hace unos minutos no le duele en una zona precisa de su pie izquierdo, se duele literalmente en todo el tobillo.

En las zonas de integración se tomas decisiones. Aquí llega información de la corteza sensorial para saber la posición del cuerpo, así como información de la corteza visual para decidir la dirección del movimiento, de forma que la patada impacte en el punto exacto del balón. También se compara con la información recibida con anterioridad, con la memoria, con la experiencia. Y se decide qué tipo de movimiento es el más adecuado para esta situación, dependiendo de la posición del balón, del portero, o dónde está la portería. (6)

Una vez se ha decidido el movimiento, la zona de integración envía la señal sobre qué quiere el futbolista hacer hacia la corteza motora, que es la encargada de ordenar el movimiento de los músculos. Al igual que sucede con la corteza sensorial, en la corteza motora hay un mapa muy preciso de todos los músculos del cuerpo, de forma que el cerebro puede actuar sobre todos ellos de manera muy ajustada, decidiendo el movimiento exacto que quiere llevarse a cabo.

Pero existe, además, un sistema adicional para concretar los movimientos. La zona integradora, además de enviar información a los músculos sobre qué movimiento se quiere hacer, también la envía a una zona del encéfalo denominada cerebelo. Al cerebelo también llega información de la corteza sensorial que le indica cuál es la posición exacta de nuestros cuerpo, del sistema muscular. (7)

En resumen, al cerebelo llega información sobre qué quiere hacer nuestro delantero y sobre lo que realmente está haciendo. Si existen imprecisiones, si el movimiento muscular no se ajusta a lo que la corteza motora ha ordenado, el cerebelo envía información a la corteza motora para corregir el movimiento y ajustarlo a lo que la zona integradora había ordenado.

Cuanto más preciso sea el cerebelo, más preciso es el movimiento. Por ese motivo es más fácil hacer un movimiento preciso moviéndose lentamente que hacerlo rápido, pues se le da al cerebelo más tiempo para actuar y corregir las imprecisiones.

Nuestro delantero ha chutado a puerta tal multitud de veces, ha entrenado tantas veces el movimiento de golpeo, que el cerebelo y las zonas integradoras ya tienen el movimiento muy mecanizado, por lo cual será más fácil para todo su encéfalo precisar exactamente el movimiento de la pierna.

El cerebelo también colabora en mantener el equilibrio y mandará información a la corteza motora para que brazos, abdominales y lumbares ayuden a que la posición global del cuerpo sea la adecuada.

La orden de contracción muscular parte de la corteza motora y de dirige a la médula. Antes de entrar en ésta, se cruza de derecha a izquierda y de izquierda a derecha de nuevo en la decusación de las pirámides. Desciende por los cordones blancos de la periferia y llegan al nervio eferente, que llevará la información desde la médula a los órganos efectores, en este caso los músculos de la pierna. (9)

El nervio eferente parte de la médula y llega al músculo, donde provocará la descarga neuromotora que hará que el músculo se contraiga de manera precisa. (10)

Entonces el cuádriceps se contrae y la articulación de la rodilla se extiende a la vez que la parte superior de la pierna, impulsada por los músculos pectíneo, psoas y abdominal impulsa hacia delante toda la pierna. El pie se coloca ligeramente en lateral para que el impacto con el balón tenga lugar con el empeine. (11)

Minuto cincuenta y dos, veinticinco segundos.

Con el choque, la energía muscular se transmite de la pierna al balón. Entonces el balón vuela, desplazándose con un movimiento acelerado.

El corazón del delantero late a doscientas pulsaciones por minuto, sus pulmones toman aire en profundidad con un ritmo de unas veinte inspiraciones por minuto. Sus músculos necesitan obtener energía mediante la combustión de glucosa, por eso su cuerpo requiere una gran cantidad de oxígeno.

El ritmo cardiaco y la frecuencia respiratoria son controladas por una zona del encéfalo llamada bulbo raquídeo, justo al final de la médula. Adecúa estos ritmos a las necesidades del cuerpo.  Es una zona esencial del encéfalo y cualquier daño en esta zona sería fatal. (12)

Tras el esfuerzo, viene un momento de relax, todo se para mientras el esférico traza su trayectoria ligeramente curvada, por encima de la mano del portero, pasa ligeramente por debajo del larguero y se envuelve con violencia en la red de la portería, que lo frena y lo devuelve al suelo con cierta dulzura.

Minuto cincuenta y dos, veintisiete segundos. Uno a cero.