martes, 8 de agosto de 2017

Sensory System

The sensory system is responsible for perceiving stimuli from the environment by the sense organs, sending this information to the central nervous system in order to be interpreted.
The sensory system is made up of five sense with their respective sense organs: sight, hearing, smell, taste and touch.
Sight
Characteristics
This is the sense responsible for perceiving electromagnetic radiation from the environment or, in other words, light. Its sense organ is the eye. We are able to perceive electromagnetic radiation with wave-lengths from 400 nm to 700 nm, known as the visible light spectrum (lower radiation, such as ultraviolet rays, or higher radiations such as infrared rays are not visible).
Sight is, in human beings, the predominant sense.
Eye Anatomy
Eyes are the main organs of sight. Their receptor structure is made up of a layer of cells that perceive light, surrounded and protected by a group of accessory structures.
There are internal and external accessory structures. One of the external structures are the eyelids. These are skin folds that cover the external part of the eye and close when in order to prevent the eye from being damaged by light when it is very intense. Eyelids also protect eyes from being damaged by blows. Finally, they extend lubricating fluids over the eye surface.
There are a group of long and thick hairs on the eyelids edge called eyelashes. They protect the eye from little particles, that are captured by the hairs.
The eyebrows are a group of hairs that form a sort of arc above the eye. Their function is preventing sweat and water that run down from the upper part of the head from falling into the eye.
There are group of glands associated located in the edge of the eyelids, called Meibomian glands (also known as tarsal glands). They lubricate the eye surface and protect them from infections.
Finally, the Lacrimal Glands are located in the upper lateral part of the orbit, and they are responsible for humidifying the eye surface. They produce tears that cover the eye surface and flow to the canals situated in the internal corner of the eye, that are connected to the lacrimal sac to drain the liquid.
The sensitive part of the eye is enclosed in a structure called ocular globe. Its diameter is between two and three centimetres long. And its surface can be divided into three consecutive layers, called fibrous, vascular and nervous tunics.
  • Fibrous tunic: this is the outermost covering of the ocular globe. It has two parts. The anterior one is called the cornea, and the posterior one is called the sclerotic. The cornea is a transparent fibrous tissue that covers the iris. Its shape is curved in order to focus and concentrate the incident light. The sclerotic is a dense layer made up of connective tissue, that covers the posterior or internal part of the ocular globe. It has a hole that is crossed by the optic nerve.
  • Vascular tunic: this is the intermediate layer. It has three parts called the choroid, ciliary body and iris. 
    • Choroid: it is an extremely vascularised layer that covers the posterior part of the ocular globe in order to irrigate the retina.
    • Ciliary Body: it is located in the anterior part of the ocular globe, behind the cornea, covering the entrance of the incident light. It has a muscle called the ciliary muscle that surrounds an internal structure called the crystalline. The crystalline is a transparent structure in the shape of a biconvex lens. It is responsible for projecting and focusing the incident light on the retina. The ciliary muscle changes the curvature of the crystalline so it can focus on the retina the image of different objects, depending on the distance they are.
    • Iris: this is the outermost part of the vascular tunic. It is a disc with a central hole called pupil. Light crosses the pupil to reach the crystalline. This internal hole of the pupil can change its diameter in order to control the amount of light that crosses the iris to reach the retina. When it is dark, the pupil dilates to perceive more light, when it is bright the pupil contracts to prevent the light from causing damage to the retina.
  • Nervous Tunic (Retina): this layer covers the internal surface of the posterior part of the ocular globe. It is responsible for perceiving light, due to the activity of the photoreceptors, called rods and cones.


domingo, 2 de julio de 2017

Endocrine System

The endocrine system is, along with the nervous system, the main controller of bodily functions. Both systems are coordinated to carry out their functions and they control each other: the nervous system controls the production and secretion of hormones and some hormones can control the nervous system.
Both systems have, however, some relevant differences. The nervous system carries out its functions very quickly, in milliseconds. And it controls punctual or not very lasting actions. The endocrine system, on the other hand, carries out its functions more slowly, in seconds or even minutes. And it controls lasting actions, that can take minutes, such as the vasodilation promoted by some hormones, hours, such as the digestive process or even years, such as growth.
The endocrine system is the main regulator of the homeostasis and metabolism (both anabolism and catabolism). Its functionality is based on the secretion of chemical substances called hormones. They are produced and released by endocrine glands.

miércoles, 31 de mayo de 2017

Locomotor System: Muscles

Muscular System
Muscles in the head
  • Frontal: it moves the scalp forwards. It raises eyebrows.  It wrinkles the forehead skin.
  • Occipital: it moves the scalp backwards. It is joined to the frontal muscle by the aponeurosis that covers the upper part of the cranium. 
  • Nasal: there are two nasal bones. They wrinkles the nose.
  • Buccinator: it inflates the cheeks.
  • Orbicularis oculis: it closes the eyes.
  • Orbicularis oris: it closes and presses the lips together. It can also push them forwards.
  • Risorius: it pulls the lip commissure sideways. Like smiling
  • Supercilii: it pulls the internal part of the eyebrows down. Like frowning.
  • Zygomatic major: it moves the extremes of the lips upwards. Like laughing.
  • Masseter: it closes the mouth raising the mandible.
  • Temporal: it raises and retracts the mandible. If one of them relaxes and the other contracts, the mandible moves laterally.
  • Levator palpebrae superioris: it raises the upper eyelid.
  • Levator lavi superioris: it raises the upper lip.
  • Digastric: it raises the hyoid bone and descends the mandible to open the mouth.
  • Mentalis: it raises the central part of the lower lip. 


lunes, 22 de mayo de 2017

Locomotor System: Muscular Function

Introduction
Muscles are responsible for providing the bodily movements. There are also muscles that do not move bones, but provide involuntary movements of internal organs, such as the peristaltic movements of the intestine and the contraction of blood vessels. Finally, when the muscles contract they produce heat (consuming energy).
There are three different types of muscle: smooth, cardiac and skeletal. When we are talking about the locomotor system, however, we are only referring to skeletal muscles, that provide general movements of the skeleton.
The muscular system is an important part of our body. It is 40 % of our total weight.
Structure of muscular fibres
All the skeletal muscles are surrounded by a layer made up of connective tissue called epimysium. The muscle is divided into fascicles by a connective membrane called perimysium. The fascicles are made up of several cells called muscular fibre. Each muscular fibre is surrounded by a thin connective membrane called endomysium. These three membranes join at the edge of the muscle. After the fusion of these membranes, the connective tissue becomes richer in elastic and cartilaginous fibres, forming the tendon. The tendon firmly connects the muscle to the bone.
Skeletal muscle.
The muscle cells that make up the skeletal muscles, called myocytes, are cylindrical and extremely long. Indeed, they can be more than five centimetres long. They have many nuclei, even more than one hundred nuclei per cell.

domingo, 14 de mayo de 2017

Locomotor System: Joints

Joints (Articulations)
Joints are structures responsible for joining different bones. They support the weight of the body and allow the movement of bones. 
According to their movement, articulations can be classified as:
  • Synarthrosis: they do not allow any movement. The joints of the cranial bones are the most typical examples.
  • Amphiarthrosis: they allow slight movements. The joints of the vertebrae are the most typical examples. 
  • Diarthrosis or synovial joints: they allow complete movements. The bones are linked by ligaments. And the part of the bones in contact with other bones are in covered by cartilage. The space between the cartilaginous pieces that cover the bones in the diarthrosis are filled with a liquid that prevent them from friction and it is called synovial fluid. 

According to the type of movement, the synovial articulation can be classified as:

domingo, 7 de mayo de 2017

Locomotor System: Skeleton

Skeleton
Introduction
The skeleton is the main system to support the body. It forms an internal hard structure that supports other organs, protecting them. Some delicate organs are enclosed in a sort of armour made of bones. The brain, for instance, is enclosed in the cranium and the lungs and heart are protected by the rib cage.
The skeletal muscles are attached to bones and the contraction of different muscles move the bones they are joined to, providing movement to the body.
The bones are, besides, the main reservoir for calcium in human body. When the calcium is required in the blood, it is extracted from the bones.
Finally, in the bone marrow, which can be found in the interior of large bones. The haematopoiesis process is carried out to produce blood cells.
The bones have three parts:
  • Periosteum: it is the outermost layer that surrounds the bone. It is made up of connective tissue and it is related to the growth the bone thickness.
  • Compact bone: it is the hard part of the bone and its main structural component. It is made up of a hard extracellular matrix, rich in collagen and calcium, and cells that maintain this matrix that are called osteocytes. Another less abundant type of cells are the osteoclasts, located in the interior part of the compact bone and responsible for destroying extracellular matrix in order to release calcium into the blood. The extracellular matrix is extremely ordered, forming cylindrical structures called osteons (also known as harversian systems). In the centre of the osteons there is canal where the blood vessels and nerves can be found. Each bone is, in its compact part, made up of thousands of parallel osteons.
  • Spongy bone: it is in the interior of large bones. It is a complex network of tissue that forms a trabecular system, similar to a sponge. The interior of this trabecular system is the place where the stem cells responsible for producing blood cells are nested. Due to this, it is the place where the haematopoietic process occurs.

lunes, 1 de mayo de 2017

Locomotor System: Planes and Body Regions

Introduction
The locomotor system is responsible for sculpting our corporal structure, shaping our body, promoting the movements and protecting some delicate organs.
It is made up of two big parts: skeletal and muscular systems.
Planes and body regions
The different parts of the body
Just before studying the skeletal and muscular systems, we must describe the different body regions and the body planes. It makes finding or locating different anatomic parts, bones and regions easier.
The body planes are layers that cross the body. There are different planes according to how they transect the body. The three most relevant are:

  • Frontal plane: it longitudinally crosses the body, dividing it into anterior and posterior parts.
  • Sagittal plane: it is perpendicular to the frontal plane and divides the body into right and left parts.
  • Transversal plane: it crosses the body transversally and divides the body into upper and lower parts. It is transversal to the sagittal and frontal planes. 

martes, 11 de abril de 2017

The Lymphatic System

It is made up of a fluid called lymph, the vessels that transport this liquid, called lymphatic vessels and a group of structures and organs related to the lymph.
The lymphatic system has three essential functions:
  • Draining the interstitial liquid.
  • Transporting fats from the digestive system to the blood.
  • Defending our body. Lymph transport many leucocytes and some lymphatic organs and structures are related to the immune system.
There are two types of lymphatic vessels.
  • Lymphatic capillaries: they are small, thin vessels, made of endothelial cells with bottle-like shape. This shape is related to the junction between successive cells, forming a valve that makes the lymph move only in one direction. These junctions, besides, can open and close to allow the interstitial liquids, the substances and the defensive cells to enter the lymphatic capillaries.
  • Lymphatic vessels: they are vessels with thin walls and many valves that prevent lymph from flowing in the opposite direction. The lymphatic system doesn't have any organ to pump the lymph, so preventing the reflux is essential. The lymphatic vessels are formed when the capillaries join and, just like the blood vessels, they have anastomoses. The biggest lymph vessels are called lymphatic trunks. Through these vessels our body can transport between two and four litres of lymph. The lymph moves due to the peristaltic contractions of the muscular wall that can be found in the lymphatic vessels. Indeed, they can contract several times per minute. 
The big vessels, besides, run between skeletal muscles that press the vessels when they contract, draining the lymph and forcing it to move. This process is known as milking. All the blood vessels end up in to big trunks that are connected to the circulatory system in two points: the angle that form the left internal jugular vein and the left subclavian vein, and the angle that form the right internal jugular vein and the right subclavian vein.
Regarding the lymphatic organs, the most relevant ones are these:

  • Lymphatic ganglions: they are oval in structure, with variable size, from one to twenty five centimetres in diameter, that are located along the lymphatic vessels. The human body has between six hundred and seven hundred ganglions, isolated or forming groups. Sometimes these groups are divided into superficial and deep groups. They are made up of an external capsule and an internal trabecular system. The capsule is made of dense connective tissue. The interior of the ganglion builds up lymphocytes and macrophages. The ganglions have two main functions. They can be used as reservoirs for lymph. The most relevant function, however, is the filtration of lymph, detecting invaders, such as viruses and bacteria that are trapped in the interior of the ganglion and attacked by the defensive cells. 
  • Tonsils: they are an aggregation of lymphatic ganglions forming a ring that surrounds the pharynx. They protect the body against inhaled or ingested invaders.
  • Spleen: this is an oval organ, around twelve centimetres long, located in the upper left part of the abdomen, between the diaphragm and the stomach. It is the place where lymphocytes B grow. It is also related to the destruction of damaged erythrocytes and lymphocytes. Blood can also be built up in this organ, being released it to the circulatory system when it is required.
  • Thymus: bilobulated organ located in the upper part of the mediastinum. It is the place where the lymphocytes T grow. It is really active in children, becoming inactive in many adults.

domingo, 2 de abril de 2017

Circulatory System: Components

Components of the Circulatory System
Introduction
The circulatory system has two components: the organ that pumps the blood, called the heart, and the tubes that transport blood throughout the body, called blood vessels.

The Heart
The heart is a muscular organ responsible for pumping blood throughout the blood vessels. It is a hollow, conic-shaped and weighs around 300 grams in an adult human. It lies over the diaphragm, in a cavity in the thorax called mediastinum. It is in the left part of the thorax, with its lower vertex towards the left.
The heart is surrounded by a sac called pericardium that is made of three layers. The outer layer is made of fibrillar connective tissue and it is called fibrous pericardium. This is a hard and inelastic layer responsible for preventing the heart form over-distension. The second layer is the serous pericardium, a thin and delicate membrane. The inner layer is a visceral membrane called epicardium. Between the serous pericardium and the epicardium there is a liquid called pericardial fluid, that is responsible for lubricating the layers, improving the movement of the heart.
The heart is divided into two nearly symmetrical parts, the right and the left part. They are not exactly symmetrical because the left part has a thicker wall. Each part is divided into two cavities, the upper chambers are called atrium and the lower chambers are called ventricles. 
Heart Anatomy
The right part of the heart sends blood to the pulmonary circuit, that transports blood to the lungs in order to oxygenate it and to remove the carbon dioxide. The left part of the heart is responsible for sending the oxygenated blood to the rest of the body. This second circuit, called systemic circuit, is longer than the first one. As a result, the muscle of the left part of the heart is thicker. These two circuits form the double circulation. 
Circulation
The right atrium opens into the right ventricle through the tricuspid valves. The left atrium opens into the left ventricle through the mitral valve. These valves, called atrioventricular valves, prevent blood from flowing back from the ventricles to the atriums, when the ventricles contract to pump the blood.
There are also valves between the ventricles and the arteries to prevent blood from flowing back from the arteries to the ventricles after the contraction. They are called semilunar or sigmoid valves.
The heart has its own small circulatory system, that sends blood to the muscular system, ensuring the distribution of oxygen and nutrients and the elimination of carbon dioxide and waste products. These veins and arteries that surround the heart are called coronaries.
The heart rhythmically contracts, on average, between 60 and 80 times per minute. The heart contraction is called systole and this movement pumps the blood from the heart. The heart relaxation or expansion is called diastole and this movement fills the heart of blood.
Diastole and systole are controlled by the central nervous system and the sympathetic and parasympathetic pathways of the autonomous nervous system. When the body needs more blood irrigation the heart beat rate rises and when the body needs less blood irrigation the heart beat rate decreases. There is also an autonomous system to ensure a basic rhythm that is controlled by a group of nerves and nodes located in the heart surface. The nodes send rhythmical impulses that are transmitted by the nerves to different parts of the heart to control and coordinate the contraction. All the muscular fibres must contract at the precise moment and the nodes control the process. When the heart beat rate must be increased, the central nervous system and the autonomous nervous system send impulses to the nodes to increase the rhythm without losing the coordination. 
Cardiac Cycle.
After the contraction, the ventricles are contracted and emptied. In this moment, the atriums are filled with blood. The sigmoid valves are open. Then, the ventricles relax. The expansion causes negative pressure in the ventricles that open the atrioventricular valves and close the sigmoid valves. The blood falls from the atrium to the ventricle (because the valves are open).
Then the atriums contract in order to pump all the blood from these chambers to the ventricles. After the contraction, the atriums relax, causing negative pressure in the atriums. This negative pressure close the atrioventricular valves.
At this moment, the atriums are empty, the ventricles filled with blood and the mitral and tricuspid valves closed. Then the ventricles abruptly contract. The blood can not flow towards the atriums, because the atrioventricular valves are closed, so it is pumped towards the arteries and the sigmoid valves open during the ventricular contraction.
Just after the ventricular contraction the atriums relax, suctioning blood from the veins. And this is again the initial situation. When the ventricles relax, the negative pressure closes the sigmoid glands preventing the blood from flowing back from the arteries to the ventricles. And open the atrioventricular valves, so the blood falls from the atrium to the ventricles.
 Thanks to this cycle the heart can pump from five to six litres of blood per minute in normal conditions.
The blood enters the right atrium from the inferior vena cava and the inferior vena cava. And it is pumped from the right ventricle to the pulmonary trunk, that divides into the right and left arteries that transport blood to the right and left lung respectively.
The blood enters the left atrium from two left pulmonary veins and two right pulmonary veins. And it is pumped form the left ventricle to the aorta, that is the main artery of the systemic circulation. 
Cardiac cycle

Blood vessels.
The blood vessels transport blood throughout the bloody. A human being has around one hundred thousand kilometres of blood vessels.
There are three types of blood vessels.
  • Arteries: they transport blood from the heart to the different tissues of the body
  • Veins: they transport blood from the different tissues to the heart
  • Capillaries: they are microscopic vessels where the exchange of substances between the blood and the tissues takes place.
Arteries, veins and capillaries
They are also subdivision. The big arteries divide into small arteries and these divide into arterioles. The capillaries that irrigate the different tissues of the organs join forming venules. Venules join to form bigger veins that join to form the large veins in our body.
Arteries and arterioles have higher blood pressure than venules and veins, because they transport blood that has been directly pumped from the heart. Due to this, their walls have more elastic fibres and a muscular layer that allows arteries to change their diameter.  
Veins and venules have, however, a thinner wall that it is not so elastic. The muscular layer of the wall is also very thin, so veins and venules can not nearly change their diameter. The veins of some parts of the body, above all of the legs, have valves to prevent blood from flowing backwards.
The blood pressure is strong in arteries and its intensity gets less with distance. The further from the heart the vessel is, the lower the pressure of the blood in the vessel. This pressure is lower in the veins, above all the veins in lower parts of the body. There are some mechanisms that help the blood to return to the heart.
  • Many veins have valves to prevent the blood from flowing backwards.
  • The muscular layer of the veins contracts to move the blood. This muscular layer causes small peristaltic movements that pump the blood.
  • Veins and arteries lies side by side. Due to this, when the a artery dilates due to the movement of blood, the adjacent vein contracts pumping the blood.
  • The movement in the diaphragm changes the rib cage volume, causing suction.
  • When the atriums relax, they also cause suction in the veins. It is called siphoning effect.
  • Many veins ascend between the muscles. When the muscles contract, they press these  veins pushing the blood from one valve to the upper one. This is the reason why walking is better than standing still to improve the circulation.
Veins and arteries have communication vessels at many points. They are called anastomoses and they are very useful to ensure the distribution of blood when one blood vessel is damaged or blocked, because there are always alternative pathways. They can usually open and close. They can be found communicating veins and arteries, venules and arterioles or capillaries.
Main arteries and veins
Blood Pressure.
Blood pressure is the force of the blood against the walls of the vessels. The arterial pressure is higher than the venous pressure. Venous pressure, besides, is constant: 10 mm of Hg, more or less. Arterial pressure, however, changes depending on the movement of blood. The arterial pressure is lower when the heart is relaxing. It is called diastolic pressure and it is around  80 mm of Hg. It is the pressure when the artery is at rest. The arterial pressure is higher when the heart contracts, because the movement of the pumped blood through the vessel puts pressure on the wall of the vessel. It is called systolic pressure and it is around 120 mm of Hg.
Arterial pressure changes with the heart rate and the contraction of the arteries. It can also change depending on the hydric state of our body. It is controlled by the central nervous system, the autonomous nervous system and the endocrine system, mainly by the secretion of antidiuretic hormone (ADH) and Aldosterone (Ald).
Microcirculation.
Microcirculation is defined as the blood circulation inside the organs, where the exchange of substances between the blood and the tissues takes place. The nutrients and oxygen transported by the blood exit the blood vessel to reach the different cells of the tissue, whereas the waste products and carbon dioxide enter the blood vessels to be released to the exterior.
The exchange of products occurs in the capillaries. These blood vessels have a thin wall that allows the movement of molecules through them. Arterioles and venules do not allow any kind of exchange, although they are a part of the microcirculation because they are always inside the organs.
The endothelial cells that cover the inner face of the capillaries and the basal membrane that support these cells are essential to the exchange process. There are different types of capillaries that have different permeability. Fenestrated capillaries, for instance, have real holes in their walls, so their permeability is really low and they can release large amounts of plasma and bigger molecules to the interstitial space.

The microcirculatory system is controlled by small sphincters that can open and close different microcirculatory paths, increasing or decreasing the circulation of different parts of the organs. The hydrostatic and oncotic pressures are also very important to control the exchange of products.

domingo, 26 de marzo de 2017

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-, PO43-, HCO3- and SO32-).
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.