Introduction

In the first place we will analyze the antomophysiology of the breathing system and its relationship with the cardiovascular apparatus.  We will exhaustively develop the concepts of diffusion (gaseous exchange) and blood oxygenation. We will also introduce the concept of pressure as well, including a briefing about the Dalton Law.
In the second place we will in the same way describe the anatomophysiology of the renal system. We will be interested in this case to develop notions of the hydro-saline equilibrium and to make a description of the renine –angiotensine –aldosterona system and its functionality.
 
Breathing System

Concept of Pressure and Dalton Law
 
Pressure is defined as strength on a surface. When we talk about atmospheric pressure we are dealing with a gaseous mass pressure on earth’s atmosphere. The atmosphere is a gaseous mixture composed by Oxygen (21%) and Nitrogen (78%). The 25 left is composed of other gases: Carbon Dioxide, Argon, and Neon. A very important data about the atmosphere is that its steam concentration varies. It will vary regarding its topographic location. In Buenos Aires we are next to the sea. An often heard complaint about our city is the following: “it’s humidity that’s unbearable”. Therefore, as water steam pressure varies, a change is produced in the gas composition.  This is strictly related to Dalton Law which states that  gases’ total pressure is equal to pressure 1 plus pressure 2 plus pressure 3, in other words, to the sum of the individual pressures. If pressure 1 is equal to oxygen pressure, pressure 2 is equal to Nitrogen pressure and pressure 3 is equal to water steam pressure. And if we find ourselves in Buenos Aires and the pressure 3 is the one that rises, the other will lower to maintain the total pressure in equilibrium (atmospheric pressure which is shared all around the globe). So this water steam pressure is variable. Within our body things are different. We will fit the internal media and this will determine a certain water steam value. In such a way we will maintain it homogeneous as we ourselves control the rest of the partial pressures and the water steam pressure itself, fact which does not take place outside ourselves, in the environment as a whole.  So far we have said that barometric pressure is the air column pressure on the earth environment. We have to take into account that this will vary not only because of the water steam pressure. Altitude is also to be taken into account.  When we climb a mountain, we bear fewer gases on top of us. Thus, pressure is lower, and also lower is the exercise of that pressure. Therefore, if I climb a mountain total pressure diminishes, and all partial pressures will also be lower. One of them is that of Oxygen. That is why when you climb a mountain, one of the problems you face is the lack of Oxygen. The organism will adapt itself, it will change, to meet that fewer availableness.  When making any exercise and due to other reasons, oxygen availability also diminishes. Thus, physiological changes which any organism will experience in altitude and during exercise will be the same since in both cases need to adapt itself to smaller oxygen availability.
 
 
Some Data

At sea level, which is the value we have and the usual considered standard, barometric pressure equals 760 mmHg (remember that mmHg is a pressure unity). We had pointed out that 21% of that barometric pressure was Oxygen. Thus oxygen partial share of that pressure is 158. Nitrogen’s share, on the other hand, is 592.
An important element to consider is that if anything changes, the atmosphere does not alter its composition. The share (i.e. 21%) does not change. What might change is the absolute value, the pressure.  In other words, 21% of that barometric pressure always will be made out of oxygen. If a climb a mountain what lowers is the total pressure, it could go down to, let us say, 500. But always, oxygen will have its 21% share of that.
Inside the organism things are similar, as long as there are no gas alterations.  If any other gases were added, then the composition might vary. But if the only thing which is modified is pressure, then the composition remains the same.  In heights the 21 % oxygen share will always remain the same, as we have seen.  However, within any organism, what happens?   Water steam pressure turns constant. Within the organism the water steam pressure is of 47 mmHg. This value is constant, thus percentages may change and this is due to the Dalton Law.
760 mmHg is also 1 atm (which is what the international standards have set). When you are listening to the radio you hear that the normal pressure is of 1013 Hectopascals. It is the same. In a Physiology book you are going to read 1 atm., but in experiments as the ones we will see in this oldclass we will work with mmHg. That is the barometric pressure. Reviewing: sea level barometric pressure equals 760 mmHg. If a person climbs a mountain then the total barometric pressure diminishes but its composition remains the same: 21% oxygen, 78% N. However, the amount of oxygen particles there is in that atmosphere is lower. Thus I reach a smaller amount of oxygen particles and of oxygen availability. And my body will have to adapt to that fact and I will hyper-ventilate, it will open its vessels; it will try to alter some things in order to take the maximum advantage of that oxygen. While making any exercise, and for other reasons, I will also have fewer oxygen availability, and that is why the changes that the body undergoes are similar. They are changes in the total barometric pressure with no alteration of the percentages. Composition does not change. What do change are the vales of the partial pressures.
Now, air gets in the organism and the last will set a water steam pressure value that might probably be different from the environment which surrounds us. Once these values alter, the others will follow, and so will occur with the percentages according to Dalton’s Law: the sum of the partial pressures equals total pressure.
Inside the organism water steam pressure is still: 47 mmHg. That is why in the organism we will refer to it as barometric pressure Z and the rest of the percentages we will take them from there. Pressure Z will be of 760 mmHg minus the figure that belongs to water steam pressure: 713 mmHg and out of this total we will calculate the other values.
 
 
 
Environment and our Organism
Which are the changes, the differences, between the body’s atmosphere pressure (alveolar atmosphere) and the environmental one (earth’s)?  It’s great.  Inside us steam water pressure remains constant, outside us is variable. Not only is constant. Its still value is of 47 mmHg. There is also another important difference. We had stated that the atmosphere composition was 21% Oxygen, 78% Nitrogen and the rest made out of Argon, Helio and Carbon Dioxide. Not inside the body. Inside the body Carbon Dioxide has a very important presence. We produce it. Our tissues will produce Carbon Dioxide in such a way that within the alveolus, in the capillary alveolar, we will find an important amount of it. Our body will release it towards the environment. So, difference number two is the great Carbon Dioxide concentration. Its presence is significant.
 

Gaseous Exchange
We have stated that our total pressure inside our body will be called barometric pressure Z. So, we will work from that total, that maximum figure. Starting from that point, calculated oxygen pressure will be of 149 mmHg and what we call oxygen cascade begins to take place. We know that we will be delivering oxygen to other structures, so that the oxygen inspired pressure is what we can find in the respiratory tracts. It reaches the alveolus in a weaker fashion.  It also reaches the arterial blood.  As it goes though the body, the oxygen arterial pressure will be lower.  We shall remember that when we described the aorta’s key (cayado) we stated that its first branches will possess a higher oxygen arterial pressure and that they will reach the organs which need it the most, starting with the brain and the heart. The last vessels, (i.e. toes) will not reach the same arterial pressure.  Thus, inspired oxygen pressure is in its highest point. Inside the alveolus is already somehow lower (100mmHg). When it reaches the blood the oxygen arterial pressure is of 90 mmHg.  Once arteries branch themselves into arterioles, the pressure is obviously lower.  Finally inside the capillary is even lower. The farther we are from the heart, the lower the oxygen pressure is. This is something we have to comprehend conceptually. Why is that so?  It is so because from now on we will abstain to mention it. It is quite difficult to consider an oxygen variable pressure in blood.  So from now on, we will set (according to the author) the oxygen arterial pressure in 90, 100. We should really comprehend that there really exists an oxygen cascade. That its concentration really lowers. And when it reaches the blood is really low but it will be oldstronger than Carbon Dioxide which was produced in our tissues.
Air will get in the body through the nose and the mouse, then will enter the respiratory tracts (pharynges and larynges). The larynges will divide into bronchia (left and right), these will branch into bronchioles. The bronchia have, in the beginning, a very solid, cartilaginous structure.  Bronchioles, on the other, hand are absolutely malleable. The same way the vessels narrow their walls in order to allow the gaseous exchange to take place, something similar goes on with the bronchia, bronchioles.
 
 
 
 
 
However, they are not alone. They are inside a structure called lung. Lung will act as a protection. Thus, everything outside the lung is more solid, more cartilaginous. If we go the other way around, to the inside, everything will become softer and will be inside a soft mass which protects it.  Besides the functional need (exchange) what is inside also get softer because it is protected within an also soft, solid mass.
Lungs possess lobules, three in the right one, two in the left. There is no difference functionally speaking.   They are divided by caesuras but they are not completely apart.
The bronchiole, already weak, is surrounded by structures named alveolus. These are formed by alveolar cells or neumocytes.   Neumo means air. If one wants to be even more schematic, along with this bronchiole branching we will find arteries and bronchiole veins. The lung functionality is destined to allow the exchange of air coming from the bronchia and blood which comes from the organism. Air comes from the bronchioles and blood from the lung capillaries (alveolar) which in turn come from the veins)   and have a vein-capillary-artery structure.  Before we had referred to another vein-capillary-artery, and it was leaving oxygen and taking Dioxide Carbon.  At the lung level, instead, it comes carrying  Carbon Dioxide, or a partial low oxygen pressure, reaches the capillary , the exchange is produced, the capillary delivers the Carbon Dioxide to the alveolus and receives oxygen and then an artery with arterial blood is formed which has a partial high oxygen pressure. This is gaseous exchange. Think about the importance the neumocite will acquire as a barrier against that gaseous exchange.  Any affliction the neoumocite might have could be serious since it will not allow that exchange.  Just think that the structure is the bronchia which contain air. At the alveolus level it possesses the neumocite which will form a gaseous exchange barrier and the blood at a capillary level which will come from a vein carrying Carbon Dioxide or a partial low oxygen pressure, and the artery with a partial high oxygen pressure. We are once again reviewing Dalton’s Law, here the Carbon Dioxide partial pressure is high and will condition the partial oxygen pressure to be low. Why?  It is because the sum of the partial pressures will yield a constant total.  A superior presence on one side provokes an inferior on the other. At any rate, within the body constant values are not to take into account, only relative values are important. Once one of them is higher, the other is lower, no matter what that figure might be. We will now offer the normal values. Inspired Oxygen Pressure equals Alveolar Oxygen pressure which equals arterial oxygen pressure equal to 90-100. Carbon Dioxide Arterial Pressure will be of 40 mmHg. In vein blood that figure could be higher.
 
 
 
 
Let us introduce one more concept. R equals breathing coefficient.  It is the relationship between produced Carbon Dioxide and consumed oxygen, and if we want we might think of it as the performance of oxygen. This is so because the Carbon Dioxide energy which my body produced happened to be generated by oxygen. So R will depend upon our main source of energy: our food. We need it to keep our energy, to maintain our muscles and keep the same and constant internal rhythm. We need then oxygen and food. Thus, if our only food were carbohydrates, R equals 1, if it were proteins R equals 0.5 and in a mixed balance the equation is R equals 0.82.  This will be mainly useful for the alveolar gaseous equation.
Oxygen will help to allow us to use our food as a source of energy.
Alveolar gas equation is a calculus to determine the alveolar oxygen pressure and comprehend the factors which will modify it.  Alveolar oxygen pressure is equal to inspired oxygen pressure minus alveolar Carbon Dioxide pressure divided by a breathing quotient plus a small correction factor usually calculated in 2.  According to this formula my inspired Oxygen pressure is 149 mmHg, my alveolar Carbon Dioxide pressure is 40 (there is no much difference with the alveolar pressure) divided by R (which we said it was 0.82 plus the correction) and this will result in 100 as Alveolar Oxygen Pressure.  We have to comprehend too that in altitude, when atmospheric pressure diminishes, it also lowers the oxygen pressure figure which also means that my inspired pressure will lower and this will provoke an enormous alveolar pressure. That is why high surfaces influence the body since anything that might block respiratory tracts, will provoke a lower inspired oxygen pressure, a lower quantity of oxygen and therefore a smaller quantity of that substance inside the alveolus.  My Carbon Dioxide alveolar pressure also rises when I hyper-ventilate. I favor the entering of a major quantity of CD within the alveolus and my oxygen alveolar pressure will also be affected. The respiratory quotient is moreover a conceptual principle. It is difficult to evaluate the respiratory quotient. It is important to comprehend the relationships.  It is important to notice, for example, that a person’s alimentation influences his capacity to make good use of the oxygen.
 
Diffusion Concept
Blood has two important fractions. A cell fraction carries globules or erythrocytes, white globules or leucocytes, plackets and the so called immune cells (lymphocytes, neutrophyles).  The other one contains plasma.   Plasma is everything in blood which is not cellular.  It has a very important liquid portion. A lot of it is water (99%) and the rest ions (potassium, calcium, and a whole lot of sodium), proteins (among them we can find hemoglobin) and loose gases (oxygen and CD).
Conceptually: level of calcium in blood will be low as compared to the rest. We will find it concentrated in the bones but in the rest of the organism is low. It has a signaling function and that is why its level has to be low. This way it remains vulnerable to any change that might carry a signal. Instead, sodium will be very high. Potassium and bi carbonate are also high. And we will find proteins, all sorts of proteins.  As a concept, proteins are big, heavy, and very difficult to move. Ions instead are small, easily movable, and diffusible. Cells are also big and heavy to move. They contain mitochondria, endoplasm reticule, membrane, a whole large, complex structure. Membranes are not easy to penetrate.  Water is easy moving, since it is a small molecule. And what penetrates that membrane is a gas, thus easily diffusible.
We will find gases in the plasma: Oxygen and Carbon Dioxide. Hemoglobin is a protein which will carry most of the blood’s oxygen. The process by which oxygen passes from the alveolus to the capillary is called diffusion. It is a gas passage. It slides down from a higher to a lower oxygen concentration. And CD also uses that system, from a major CD concentration in the capillary to a lower one in the alveolus. It is no doubt a passive transportation. Then, since oxygen will travel in blood, 97% will do so stuck to hemoglobin and a small percentage will be carried in the plasma.  Carbon Dioxide, on the other hand, will travel the opposite way.

Hemoglobin is a molecule composed by four parts. In the center has an iron atom called tetramerous. Hemoglobin main function is carrying oxygen through the blood.  People that have a low hemoglobin level will not have oxygen availability (or a low one) in their tissues. Blood can not saturate with oxygen, it is not adapted for that function, so its transportation will be low, just because those low hemoglobin levels.  Hemoglobin, then, will capture oxygen through the alveolar capillary, will travel through the arterial blood to the tissues. It will liberate oxygen through the tissue capillary, also incorporate some Carbon Dioxide, but it travels without oxygen until the alveolar capillary in order to be able to charge itself again. Thus, hemoglobin mainly transports oxygen. And some oxygen can be also carried by the plasma.
In a tetra metric structure, with an iron center, anemia means to possess a low hemoglobin level. Hemoglobin’s major quantity is within the erythrocytes, in the red globules. The hg part is dissolved into plasma and part of the red globules.   This means that any treatment or sickness which might provoke a red globules diminution will end up in anemia. Where is hemoglobin? It is dissolved into plasma. Where else is it? It inside the red globules which jeans that it forms part of both blood fractions: cells and plasma. Most of it we will find it inside the red globules. If red globules level diminishes, so does hemoglobin. And we will suffer from anemia as a consequence of a diminution of hemoglobin. Now, I could suffer from anemia without any diminution of red globules. That could happen if a person has any trouble when his initial composition takes place, when protein is formed.  Most of the proteins are synthesized in the liver. During a hepatic sickness which might affect hemoglobin, the last will lower the hemoglobin level in erythrocytes and also in blood.  We could have normal erythrocytes but with a low hemoglobin level.  Hemoglobin will relate to oxygen at the iron level. So, when I mention ferropenic anemia, what am I referring to? I am referring to a low iron level. Without iron there is no hemoglobin. Iron can not be replaced.  If so, other molecule would be formed.  Where does iron come from? It comes from food. Once again we find the importance of alimentation.
 
 
 
So then, oxygen joins hemoglobin at iron level. We had already expressed that proteins are very complex, large, special structures. Hemoglobin possesses a system denominated cooperation system. This is because hemoglobin saturation curve is a sigmoid curve. Hemoglobin will have a great job to capture oxygen’s first molecule.  It will have to alter its conformation, its spatial disposition.  The second one will be an easier task. And so on. This is what we called cooperativeness. As oxygen saturation rises easier will be to catch it. The same process occurs with their liberation. Once it liberates the first, the easier it goes.  It is a concept similar to inertia. Just think that oxygen it’s intimately linked to hemoglobin.  Why is this useful? It is in order to comprehend the diffusion concept. In altitude, the body’s response will be hyper-ventilate.  In a quasi involuntary fashion the thorax muscles will raise ventilation. Almost instinctively a body starts to hyper-ventilate be it in altitudes as in exercising. That is why oxygen availability diminishes.  We had expressed that Carbon Dioxide was dissolved in blood, which allows its exit; it is available to mechanisms such as diffusion.  Oxygen, however, is essentially linked to hemoglobin. And hemoglobin does not have an easy task in liberating oxygen. It will liberate what normally it is used to. But when facing an alveolar ventilation level rise (amount of gases carried from alveolus to blood per minute) oxygen will not be able to respond that easily. It will not be liberated in the same way that CD does. If I hyper-ventilate I am liberating much CD –this substance diminishes in blood-, and I am seeking for oxygen. However, the amount of oxygen that I might be able to incorporate will not be much since its transportation system is linked to hemoglobin and its amount is not as available as Carbon Dioxide is.  Thus, oxygen rise in blood in not significant, and the operation will not be successful. Therefore, hyperventilation as a reflex act is not very successful. Moreover, other methods are applied and hyperventilation can even work the other way around and could provoke fainting, unbalancing.     Carbon Dioxide is the main brain vessel dilatator. Vessel dilatation will help carrying blood. Thus if I diminish CD in blood I am also diminishing my brain vessel dilatation. On the contrary, there will be vessel constriction, with less oxygen available and fainting –in the best of cases is inevitable.   The worst could be coma, death. Once our brain lacks any oxygen coma and death are its cruel destiny.

 
Conceptual Formulas
 
Alveolar ventilation: amount of gases which are to be exchanged between the alveolus and the capillary blood. This will be equal to current volume per respiratory frequency. Respiratory frequency is the amount of breathing per minute, and its usual amount is 12. Current volume is the amount of air exchanged through inspiration and expiration. When I perform a quiet inspiration, 500 ml of air enter the body. And the same goes on with a normal expiration. Current volume is the volume normally exchanged between inspiration and expiration.
 
.
 
What is dead volume? Not all the air incorporated will reach the alveolus, not all will be perform the alveolar ventilation.  Therefore, out of the current volume some will be lost; it will not be used in alveolar ventilation, other part will. Then current volume equals alveolar ventilation plus dead volume. Dead volume is the air a person incorporates which will not perform the ventilation process; it will not make any gaseous exchange. This dead volume possesses two portions: an anatomic fraction given by the conduction tracts, the air that reaches the trachea or the larynges. If I have an obstruction, let us say a tumor in the trachea or in the larynges then the dead volume will increase.  The second fraction of the dead volume is functional. It will be the air remaining in the apical alveolus which might be poorly per-fused. Lung has a base and a vertex or apex. In both portions we will find alveolus, and they are respectively denominated basal and apical.  They are not anatomically equal nor are the pressures they deal with due to a matter of gravity.
In order to analyze the, therefore, we will introduce two concepts.
Q equals perfusion and perfusion deals with the reached blood amount.
D equals ventilation and ventilation deals with the amount of air which reaches the alveolus. 
Gaseous exchange requires these two things, oxygen from the alveolus and oxygenated blood. Due to gravity the apical alveolus are larger, they are more distended. The basal alveoli are smaller. The apical will be better ventilated. They are larger then air reaches them much easier. Basal ones, instead, are worse ventilated.  On the other hand, due to the heart location and the vessel distribution (lung arteries) it will be harder for blood to irrigate lung’s superior portions. And due to gravity, basal portions of the lung will be irrigated more easily.  Then the basal alveolus will be better per-fused and the apical worse per-fused.   Perfusion is more significant at a basal level. Blood which comes from the heart through the lung artery will reach basal alveolus rather than apical ones. If we are to make the integration with the cardiovascular: venous blood reaches the right auricle, will pass the tricuspid valve to the right ventricle, and from there to the lung artery. Lung artery branches into smaller ones. Most of them will reach the basal alveolus. A minority will reach the apical alveolus.  Gaseous exchange is produced there at the lung capillary level. Carbon Dioxide is liberated and oxygen incorporated. That oxygen will travel in the lung veins to the left auricle, from there reaches the left ventricle through the mitra valve.  Goes out the left ventricle in ejection phase, in the systole phase comes out in direction to the aorta. The aorta branches into other arteries. It will reach the tissue capillary where diffusion is produced.  Diffusion takes place once again; oxygen and CD do it through a membrane. Oxygen is left and CD incorporated which is carried in a dissolved fashion.  Oxygen, to a lesser extent, also travels dissolved. It does it together with hemoglobin and this will impede any change. It returns through the veins towards the right auricle. The Superior and Inferior Cave Veins ended in the right auricle. We see once again a sort of holistic vision of the body. Body organs do not work separately from one another. They just need to be analyzed that way.
 
 
 
 
Apical Alveolus: richly ventilated, poorly per fused
Basal Alveolus: richly per fused, poorly ventilated.
Why? It is because of gravity.
Then we start to comprehend why the apical alveolus have a dead volume. They receive much air, little blood. Blood that might pass through there will oxygenate but they will receive much oxygen with little use which will form part of this dead volume, since that air will not perform any gaseous exchange.
The current volume will be equal to the useful air which will be ventilated at alveolus level and that air which will not have utility because it might remain in the conductive tracts or that might end in the apical alveolus which in turn does not receive enough blood to use it during a gaseous exchange.
Lung Volume is the amount of air that reaches the lung equal to current volume against respiratory frequency, a simple calculus. Current volume is 500 ml against 12 respiratory frequencies per minute, or 6 liters. This is the air that reaches the lung and comes out from it. However, we also do know that not every bit is useful. Only some parts receive alveolar ventilation. And current volume will include both.
 
 
 

RENAL SYSTEM
 
Generalities
 
Which is the kidney’s main function? Is it filtering? What else? What does the kidney produce? It is obviously urine. How is this produced? It starts in blood. The kidney will just make some clearing, some sort of purification. It will regulate the salt exit (this last substance entering will not be regulated; generally in water and food both entering and going out are regulated, thirst, hunger, urination, defecation)
When we analyze the renal system wee talk again about liquids, ions, we are referring once again to blood, to plasma.
The kidney will regulate the hydro saline (water and salts) equilibrium. Water in men is about 60%, and in women some 50%. Women naturally have more fat than men. Fat is a low liquid tissue. Other tissues, however, contain a lot of water, blood for example. It is a fluid tissue (99% water) but a tissue at last. This liquid is distributed in a certain way: intracellular liquid represents two thirds of the total corporal water, a 40% of the total body weight. The extra-cellular liquid represents a third of the body’s total liquids and a 20% of the body’s total weight (within the blood, for example, it represents a 5% of the total weight; within the interstice (the space between cells) we might find a 15%.
 
Renal Anatomy and Blood Filtration

The kidney’s function is to filter blood. Plasmatic fraction is 99% water and besides it constitutes a 5% of the body’s total weight. That water will be filtered by the kidney. At this level blood will pass through a artery-capillary-vein system.  It will be filtered by one part of the kidney thus producing urine.  This filtration will be produced through a membrane, very similar to membranes we have seen so far. The alveolar-capillary diffusion membrane and the diffusion membrane at a tissue level (capillary tissue) will have to be very thin in order to allow gas passage.  This membrane will also be very selective, thin. It will allow the passage of water, ions, not cells, proteins. What it is filtered is water, ions, small molecules. Filtration criterion is size and weight.
Anatomic scheme of the kidney
Suprarenal glands or adrenals are nervous glands.  The cellular origin of these glands is the same as in the brain. They are located over the kidney. They are made of nervous tissue. They will liberate neuronal hormones.
The kidney is formed by a fat capsule which protects them, a cortex and a medulla. Cortex and medulla will form triangular structures called Malpighi pyramids, with a medullar center, surrounded by cortex columns. These columns constitute a tubular system which contains a nefron, the functional unity of the kidney. This fraction will filter the blood until it turns it into urine.
 
 
 
 
The nefron is composed by a glomerular portion and a tubular portion. Where is the nefron? It is inside the Malpighi pyramids. In the middle there is an interstice, a contention structure.  Nefrons present a glomerular and a tubular portion (scheme).
The last part of the tubular portion of the nefron end in what are called calyxes. Every calyx ends in the renal pelvis. And what comes out from there is the urethral which ends in the bladder, out of the bladder comes the urethra.  Blood then comes through the renal artery and enters the kidney, covering the renal pelvis. At a pelvis level it divides into branches, forming arches until it reaches the glomerulus.  Inside the glomerulus filtration is produced using the artery-capillary-vein scheme.  Water, ions and small molecules will pass through.  These substances will pass through a tubular portion of the kidney and there two processes will take place: secretion and re-absorption.
There are then three steps: filtration secretion and re-absorption.
Filtration: It is produced in one glomerular part of the nefron.  It is the passage of the glomerular capillary (blood) to the glomerulus.
Re-absorption:  it takes place in the tubular portion of the nefron. It is the passage of the tubule to the per-tubular artery. Within the kidney’s tubular portion, a part of the filtered elements “regrets” the process. It re-absorbs and returns to the body.  Re-absorption is what passes from the tubule to the organism or from the tubule to the per-tubular space; it returns to the blood, it returns to the body. It will return to some per-tubular arteries. It is always an exchange between the nefron and the blood.
Secretion: inside the tubular portion of the nefron. This is what I want to eliminate once again. It is the passage from the pre-tubular artery to the tubule.
It sounds ridiculous, doesn’t it? I filter substances, sodium for example.  It filters an enormous amount of sodium, it reabsorbs during the passage through the tubules and sodium is secreted once and once again in that tract. Why does exist so many comes and goes during this process? 
Inside the body this happens all the time, very often in every system. Why is so much energy spent in making and unmaking all the time? These are very important processes within the body. Since they are important they have to be regulated. Sodium’s cc is so important that the body spends lots of energy filtering, re-absorbing and returning to secrete that sodium.
What is left in the tubule will direct to the calyxes, and then into the pelvis.  At this level, what finally reached the sector is urine, already formed. It passes into the ureter, bladder (a contention, storage unit). When certain levels are reached the bladder signals the brain “you have to pee”, and when the bladder valve opens, that liquid enters the urethra. 
Let us analyze the glomerulus in detail.  We had said that arteries that reach the glomerulus will be branches of the renal arteries. At this level it will form once again an artery-capillary-vein system which will receive a special denomination. This artery is called afferent arteriole. This capillary is called glomerullar capillary since it borders the glomerulus.  And the vein is not called vein. It is denominated efferent arteriole. Is it a vein or not?
 
In reality it is not called vein since it does not allow any gaseous exchange. Therefore since it will not contain any venous blood, there will not be any oxygen exchange. However, the concept is still the same. An artery arrives, there is a capillary, and some different element, without a former substance just comes out. A filtration process then takes place (water, ions, small molecules), what is left?  Cells and proteins are the only non-filtered elements since they are large and complex.
 
Renal Blood Flow
It is the amount of blood that reaches the kidney. It constitutes some 20% of net volume (let us also remember the amount of blood that the heart expels per minute).
The Kidney is a highly irrigated tissue since filtering of the body will take place there.
Renal Plasmatic Flow: It is a value which might attract much more our interest since is a part of the renal blood flow. Another value is the amount of plasma which reaches the kidney per second. Up to now we are only considering the plasma.
How much it enters to produce filtration. Glomerular Filtration Volume is how much blood filtrates into the glomerulus; and the filtration fraction. It is a relationship, equal to the glomerular filtration volume and to the renal plasmatic flow. It is 20%, which means that this is the percentage of the plasma which is filtered.
The same way that before we put water, oxygen and blood in contact, this time we are putting blood into contact with the glomerulus and limiting the amount of substance that passes through. This is due to a process denominated filtration. Filtration fraction is the glomerular filtration volume divided by renal plasmatic volume. In other words, how much of the incoming substance is filtered (20%).
Proteins are not filtered. We have then to consider two types of pressure: hydrostatic and oncotic. The first one is given by the water column, and the second by the proteins, it is given by the attraction produced by proteins.  At the afferent arteriole level, concentration of proteins will be higher. We shall remember that we are always speaking of relative values. Up to his point we do not find any synthesized proteins, but water happened to be extracted, thus raising the protein concentration. Their influence is oldstronger and the oncotic pressure will also be higher as opposed to hydrostatic pressure which will lower as a consequence of the loss of water. 
If we analyze these forces within the capillary they are the ones which will carry the filtration process, since this process is the result between these two forces’ relationship. No proteins reach the glomerulus, therefore the oncotic pressure within the glomerulus equals zero. On the opposite hand, there will be a lot of water and that is why the glomerulus’ hydrostatic pressure is enormous.
Since at this level we find no proteins, we always speak of relative pressures; one will be compensated by the other, and both make one.
   
 
 
Glomerular hydrostatic pressure is high, and this will allow the water passage. And will not allow proteins to go through. This relationship between the hydrostatic and oncotic pressure is the one which will allow filtration to take place.
 
Juxtaglomerular Apparatus
Next to the glomerulus exists an apparatus denominated “Juxtaglomerular”. It will have several components, being the granular cells the most important.  They are called that way because they contain some granules and these contain a protein called Renine. It is a protease, or a protein which breaks other proteins. This rupture process takes place with water, it is a hydrolysis.

Other element of the juxtaglomerular apparatus is what we call dense Macula, a perception system, a censor. This will test all characteristics which blood might carry. Blood comes through the afferent arteriole. It will trigger a signal at the dense macula’s level. If it comes very charged with ions, then some substances will say “let us secrete”. And if the opposite occurs, they will just send the opposite message.
When we analyze blood we have to take into account two concepts: volemia (blood volume) which will mainly be given by the amount of water blood possesses. And osmolarity which is amount of particles that blood carries, mainly ions like sodium, a very influential substance.
A hyper tense person will have hyper osmolarity, or much sodium in blood. Instead, a person with little sodium (little osmolarity) will have high volemia since both are strictly related to one another.  We have stated that the body always deals with proportions, volume as water, and osmolarity as particles, as ions.
A hyper volemia will come along with a hyper osmolarity. When volemia rises, blood’s water portion will be higher than its particles.
 
Complementary Concepts
Blood comes through the afferent arteriole which is a branch of the renal artery. A certain plasmatic flow will also arrive, a certain renal plasmatic flow with its own different characteristics.  In the case of a hyper tense person, the juxtaglomerular apparatus which is analyzing the incoming blood (dense macula is the censor), censes a high osmolarity level and it will send a signal in order to expel these incoming ions. It will emphasize filtering and secretion, thus diminishing re-absorption.
These are the regulating points.
Given their characteristics, sodium and water come along with other substances. Even though this concept sounds contradictory, taking diuretics and raising urine’s amounts a person also raises the eliminated sodium.  
 
   
Renine-Angiotensine-Aldosterone System: the hydro saline equilibrium
 
A person with a low osmolarity level, hypo tense (a teenager that might faint in hot summer day, for example), suffers a rise of volemia (volume as compared to osmolarity) and the recommendation is to add salt in his food. That is the manner to raise osmolarity. Blood which carries   these characteristics arrives, the dense macula censes it. It perceives a hyper volemia, and a signal triggers: “re-absorb a small quantity”, and inside the tubule it will say “let it out, let it out, do not re-absorb it”. At this dense macula stage, volemia and osmolarity levels can be modified. This yuxta-glomerular apparatus has two parts: a censor which perceives volemia and osmolarity qualities carried by blood, and a system composed by granulated cells which makes and executes -through all this protease and renina- a system of signals which will modify what goes on at the nefron’s level, how that urine will be handled in order to compensate blood characteristics. This is denominated renine-angiotensine-aldosterone system. And when we refer to the body’s hydro saline equilibrium, to a great extent, we are referring to this system.  

This is how osmolarity and volemia are regulated in blood. Granullar cells are to be found in the juxtaglomerular apparatus, next to the glomerulus, and belong to the nefron which is inside the Malpighi Pyramid also inside the kidney.
Once the dense macula sends a signal, renine will enter blood. Blood has a protein called angiotensinogen, synthesized in the liver. Renine, which is a protease, will break that angiotensinogen and turn it into angiotensine I, angiotensinogen’s active state. As a matter of fact we should state that it possesses a greater activity since angiotensine I is an intermediary activity state.  It is angiotensine II, however, the one which will show a maximum activity. Renine (which transformed angiotensinogen into angiotensine) keeps on circulating inside the blood, and this blood eventually reaches the heart. Mainly in the heart (though eventually we might also find it in other tissues) there is an enzyme called angiotensine converter enzyme (ace). An enzyme is a protein which will help to provoke certain reactions. This is how angiotensine I becomes angiotensine II, and the last one really, happens to be the one which shows a very active state.  It is a vessel constrictor and it will obviously obstruct the blood passage thus also raising blood’s pressure. If I raise my blood’s osmolarity, I raise the amount of particles and the pressure too.  Some way of raising the blood’s pressure is allowing sodium to get in (I raise osmolarity). Another manner is through vessel constriction, given by angiotensine II.  Volemia will diminish. However, this is not the only course of action; we will see that it possesses a much more direct action.   ACE will have a double action. On the one hand it will turn angiotensine I into angiotensine II, thus having a vessel constrictor power, on the other ECA deactivates brad-quinine, a vessel dilator.  Therefore we see that ECA possesses a double vessel constrictor power (it activates a vessel constrictor and deactivates a vessel dilator). Most of hyper tension medicines are ECA inhibitors.    Angiotensine II besides its own action (being a Vessel constrictor) will also enter the suprarenal gland, which is a triangular shaped nervous structure which secretes lots of different neuronal hormones. Angiotensine, however, will stimulate only the release of one of them:  aldosterone. Aldosterone will act in the kidney’s tubule, where re-absorption and secretion is produced.   Aldosterone will stimulate sodium re-absorption and potassium and hydrogen secretion.
Angiotensine does not only raise arterial pressure, it does it getting right to the point: stimulates aldosterone liberation, this reaches the renal tubule, stimulates sodium re-absorption, raising directly that osmolarity, it diminishes volemia and it will also raise potassium and hydrogen secretion.
The problem with this is that if a certain amount of aldosterone is released, a person might suffer from hyper potasemia, and a decomposition of the base acid will take place since these protons or hydrogen regulates our body’s major acidity level. 
The renine angiotensine aldosterone system is then a vessel constriction one, triggered by the dense macula and performed by the granulated cell.
Besides, this system also stimulates another mechanism in the heart through the NFA (Natri-ureic factor –sodium-, Atrial –since we find it in the heart’s auricles). It stimulates sodium secretion. 
In spite of the fact that it comes from the heart we can also find it in other tissues as well.
Sodium excretion will produce hypo tension. Angiotensine II will centrally stimulate an SNC structure (called hypothalamus) where the anti-diuretic hormone (ADH) will be produced.
It stimulates the sodium re-absorption. NFA will act as hypo tensor, aldosterone and ADH as hyper tensors. Renine, angiotensine are hyper tensors.
The sympathetic system will produce vessel constriction.
Example: during any acute dehydration, volemia will be diminished, thus the baro receptors (we find them in auricles, inside the carotid and aortic key) will be diminished and the osmo receptors (within the hypothalamus) magnified. 
As a whole, these actions will provoke thirst, a neuronal signal which stimulates water search and ADH secretion (just as the angiotensine II). This raises re-absorption of sodium inside the collector tubule, diminishing water excretion. Thirst will stimulate search for water, and water will be drank.   This way the body raises the volume and diminishes plasmatic osmolarity taking it to normal levels. This is the normal process.

 
Sodium Excretion Regulation Mechanisms
 
There are three mechanisms for sodium excretion. We shall remember that its entrance is not regulated, only its way out.  
 
The main modifiers are: glomerular filtration volume, aldosterone and a third factor which does not depend upon the former ones.  Changes in plasmatic volume will act as a trigger of these regulating mechanisms.
1. VFG which depends upon the RPA (renal plasmatic flow), the main regulator, and the FNP (filtration net pressure). The amount of blood which reaches the glomerulus will be the main regulator of how much is filtered.  If I ingest more sodium, which is what usually happens with hyper tense people, plasma and osmo receptors’ osmolarity will also rise. This triggers thirst and water drinking.  ADH is also produced and water excretion diminishes. These mechanisms as a whole produce the raise of the plasmatic volume. Plasma’s oncotic pressure thus lowers and so does in blood. Then when blood reaches the capillary, pressure will be lower. Oncotic pressure in the glomerular capillary lowers.  NFP then raises, VFG too and the same happens with sodium excretion.
Plasmatic volume, on the other hand, stimulates baro receptors (volume receptors) consequently diminishing the renal sympathetic tone since the sympathetic provokes vessel constriction.  This is what we know as retro alimentation system.  Pathology appears when it stops functioning.  Renal sympathetic tone will diminish as the afferent arteriole dilates thus allowing the rise of the renal plasmatic flow, also rising the filtration net pressure and also the VFG and sodium excretion.  Due to all these reasons, VFG main regulator will then be the FPR.
2. Aldsoterone will be stimulated by: High potassium levels (we should remember that aldosterone stimulates potassium excretion).
Angiotensine:
ACTH Neuronal signal which will independently stimulate aldoesrone secretion.
FNA: this factor will stimulate sodium secretion. Therefore: FNA diminution will stimulate aldosterone secretion. That was sodium secretion will not vanish. 
Sodium Pressure Diminution in Blood.
Renine triggers are: rise of the renal sympathetic (vessel constrictor)
Volemia’s change. Tubular liquid composition change.
Hydroestatic glomerular pressure rise.
Baro receptor diminution.
 
Then, a rise in the sodium levels (i.e. a hyper tense person) raises plasma osmopolarity and stimulates the osmo receptors. On one hand this stimulates thirst mechanisms and provokes water ingestion.
 
 
 
 
On the other stimulates DHA which in turn diminishes water excretion since stimulates re-absorption.
Due to these two reasons plasma’s volume rises and baro receptors are stimulated, and renal sympathetic tone diminishes due to the retro alimentation system. This diminution produces the afferent arteriole dilation due to its vessel constriction action diminution and renal perfusion raises thus provoking a large blood income. Renine secretion diminishes slowing the renine-angiotensine-aldosterone system, and sodium excretion is stimulated.
3.      As sodium ingestion raises, appears what we know as “third factor effect” which will produce sodium excretion rise.
These are five mechanisms.
Renal sympathetic diminishes.
Angiotensine II also diminishes, and so does aldosterone, consequently raising sodium excretion.
FNA rises and sodium excretion is produced (natri-uresis)
Capillary hydrostatic pressure rises and oncotic pressure in capillary diminishes.
Glomerular hydrostatic pressure diminishes.
 
Therefore a large rise of sodium ingestion will produce the growth of plasma’s osmopolarity and the subsequent stimulation of the osmo receptors. This stimulates thirst and water ingestion.
On the other hand stimulates DHA which in turn diminishes water excretion since the last stimulates re-absorption.
 
Plasma volume rises due to these two reasons.
Therefore plasma’s oncotic pressure diminishes. Per tubular capillary oncotic pressure diminishes and then sodium re-absorption in the proximal tubular lowers and sodium excretion rises. On the other hand FNA also stimulates sodium excretion.
On the other hand, plasmatic volume rise stimulates the baro receptors, thus diminishing the renal sympathetic tone (due to the retro alimentation system. This lowering of the renal sympathetic tone produces dilation of the afferent arteriole due to the diminution of its vessel constriction action and raises the renal perfusion. Re-absorption of the capillary tubular also rises. Therefore, sodium diminishes re-absorption in the proximal tubular and excretion raises.