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Function of Water in the Human Body

Water mainly enters the human body through the food and drink we consume. A small proportion of water is obtained from oxidative metabolism e.g. in respiration. Human beings mainly lose water by excretion in urine and faeces. Water is also lost through evaporation e.g. as sweat (Campbell N. A. et al. 1999, Frederic H.M. 2006)

The kidneys are highly specialised organs of the body and play an important role in homeostasis. The kidney maintains homeostasis by regulating water balance, waste removal and blood composition and pressure. The kidneys dispose of waste by-products of metabolism and hence prevent the build up of toxic products in the body and to regulate the chemical components of the body’s fluids by responding to any imbalances of body fluids. These functions are fulfilled by a process of filtration of blood, which mainly includes the movement of solutes between the internal fluid and external environment. The movement of solutes is normally through a transport epithelium, in the case of the kidney it is in the form of a tubular channel; this tubular channel gives the kidney a large surface area.
The kidneys weigh less than 1% of the human body, they receive approximately 2 % of blood pumped with each heartbeat. Urine exits the kidney through a duct called the ureter. The ureters of both kidneys drain into a common urinary bladder. Urine leaves the body from the urinary bladder to the urethra which empties near the vagina in females or through the penis in males. (Campbell N. A. et al. 1999, Michael F. et al. 2001)
At one end the nephron forms a cup-shaped structure called glomerulus
From the glomerulus a tube runs towards the centre of the kidney first forming a twisted region called the proximal convoluted tubule and then a long hair-pin loop in the medulla, it runs back upwards into the cortex where it forms another twisted region called the distal convoluted tubule, this then joins a collecting duct which leads down the medulla and into the renal pelvis

The functional unit of the kidney is a nephron. Microscopic sections of the kidney show that the kidney is made up of thousands of nephrons. Fig1b shows the location of a nephron and Fig2 shows the detailed structure of a nephron. Each renal capsule is supplied with blood by the afferent arteriole ‘ a branch of the renal artery this splits into many capillaries in the capsule which then rejoin to form the afferent arteriole. The nephrons structure is closely related to its function of regulating solutes
Osmoregulation is maintaining constant levels of water in the body. Cells cannot survive a huge deviation from its osmolality. Hence, cells have a continuous movement of water across their plasma membranes. A net gain of water will cause the cell to swell up and burst, while a net loss of water will cause the cell to shrivel up and die. Water is transported by osmosis around the body. Osmoregulation is accomplished by creating an osmotic gradient; this requires lots of energy and is done by maintaining solute concentrations in the body fluids.
The osmolality of the body is fixed at a mean of 290’5 mosmos/g. The kidney is able to maintain a constant osmolality as it’s able to adjust the rate of water excretion over a wide range. The volume of the extra-cellular fluid is mainly determined by the concentration of sodium ions, hence slight adjustments to the renal excretion rate have a major impact on the extracellular fluid volume. Changes in tubular sodium transport is accompanied by parallel movements of water, this results in no net change in body fluid osmolality (Campbell N. A. et al. 1999, Frederic H.M. 2006, Michael F. et al. 2001)
The loop of Henle creates a longitudinal osmotic gradient across the medulla; this aids the reabsorption of water and other important solutes. Ascending and descending limb are parallel and adjacent to each other with a layer of tissue fluid in between. Fluid enters from the proximal convoluted tubules flows down the descending limb and then up the ascending limb. This is known as a counter-current flow. Thewalls of the descending limb are permeable to water, while the walls of the ascending limb are impermeable to water. The ascending limb of the Loop of Henle is made up of a thick walled tubule which is impermeable to the outward movement of water but not salt. The red arrows on fig3 show the movement of water amd solutes along the loop of Henle and the collecting duct. Also, the walls of the ascending limb contain pumps to remove sodium chloride from the lumen and add it to the surrounding interstitial fluid. Hence sodium and chloride ions are actively transported out of the ascending limb.
This is the site of reabsorption in the kidney, here fluid from the’enters and the kidney reabsorbs all the useful solutes and water. The permeability of the loop and the collecting duct depends on the osmolality of the blood and is controlled by a negative feedback mechanism by osmoreceptors in the hypothalamus
A high concentration of salt builds up in the medullary tissue, this together with urea retention by these tissues, helps build up a high osmotic pressure in the medullary tissue. This creates a gradient of 200 mosm/g across the tubular wall at any point and causes a loss of water from the descending limb. The loss of water concentrates sodium and chloride ions in the descending limb. Salt concentration in the medullary tissue is highest at the apex of the loop, the tissue in the deeper layers of the medulla contain a very concentrated solution of sodium ions, chloride ions and urea. The fluid leaving the ascending limb is hypo-osmolar as compared to the fluid that enters and has a osmolality of approximately 100 mosm/g .Sodium and chloride ions diffuse out in the lower part of the ascending limb. Fluid passes down the collecting duct through the medullary tissue of increasing salt concentration, water can pass out of it by osmosis. The reabsorbed water is carried away by blood capillaries
(Campbell N. A. et al. 1999, Frederic H.M. 2006, Michael F. et al. 2001)
Control of water regulation
Osmoregulation by the kidney involves a negative feedback mechanism. The osmoreceptors are in the hypothalamus and the effectors are the pituitary gland and the walls of the distal convoluted tubules. Osmoreceptors detect alterations of water levels and send impulses to the pituitary gland which then increase or decrease the production of antidiuretic hormone (ADH). In the case of a low osmolality,when the nerve cells are stimulated by osmoreceptors action potentials travel down them, this causes ADH to be released from their endings into the blood capillaries in the posterior pituitary gland from here it is distributed throughout the body. ADH acts on the plasma membranes of the cells of the collecting ducts. ADH is picked up by a receptor on the plasma membrane which then activates an enzyme. This causes vesicles with water permeable channels to fuse with the plasma membrane hence ADH makes the membrane more permeable to water than usual. Hence more water will be reabsorbed by the collecting duct and more concentrated urine will be produced.
On the other hand, when the blood water content rises the osmoreceptors are no longer stimulated and hence do not lead to the secretion of ADH. Hence, ADH secretion slows down and the collecting duct cells become less permeable to water, so less water is reabsorbed and more diluted urine is produced (Campbell N. A. et al. 1999, Frederic H.M. 2006, Michael F. et al. 2001)
In conclusion, the regulation of water is essential for the survival of human beings and is carried out by the kidneys and monitored by osmoreceptors in the hypothalamus and controlled by the pituitary gland. Each of these plays an equally important role in the regulation of water and without any one of them the body will not be able to function in a normal manner.
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Principles of Acid Base Balance

The purpose of this handout is to educate the student on basic principles of acid base balance. To give a systematic approach to interpretation and understanding of arterial blood gases and appropriate care for the patient who is having a blood gas taken. It is intended that the student will learn from this package but also be encouraged to source other material to broaden their understanding of acid base balance. It is intended that this learning packet will complement their experiences with help of an understanding mentor, who will assist them with questions raised both within themselves and within the book.
An arterial blood gas measures the acidity of the blood, the levels of carbon dioxide and levels of oxygen. The blood is taken from an artery prior to the blood distributing the oxygen from blood cells to the body tissues.
The values the gas will show are:
Partial pressure of oxygen (PaO2) this measures the pressure of oxygen dissolved in the blood Edwards (2009) say this can indicate how good respiratory system is functioning. This can indicate oxygen saturation and how well oxygen can move from the lungs to the blood
Partial pressure of carbon dioxide (PaCO2) this measures how much CO2 is dissolved in the blood and how well it can move from the blood to the lungs (and out of the body). Foxall (2008) explains that co2 mixed with water turns in to carbonic acid that the lung must excrete to prevent an acidosis.
Bicarbonate (HCO3) Bicarbonate is the form in which a large amount of acid is removed from the cells Schilling (2008) says about 70% is removed from tissues and bicarbonate can be measured as either actual or standard bicarbonate. The standard which is the more important value is obtained by using a PCo2 of 5.6 kPa as a reference for the amount of CO2 in the body.
Base excess (B.E.) Springhouse (2008) explains that the base excess indicates the amount of excess or lack of bicarbonate in the circulatory system it can be a negative number indicating too much acid or a positive number indicating too much base. It normal range is -2 to 2
Introduction Skinner (2005) and Adam (2009) concur in that arterial blood gas analysis is an essential part of diagnosis and management a patient’s ventilation therapy and their acid base balance. Skinner continues to say the usefulness of this intervention is dependent on the ability of the health professional to analyse and interpret the individual aspects of the gas.
The intention of this learning packet is to introduce the learner to the individual aspects of a blood gas, and there meaning. Additionally it hopes to show how to bring these values together to formulate a decision on the patient’s condition and suggest options for treatment.
Common reasons for blood gas analysis are:
To diagnose and assess existing lung function.
To review treatment for lung disease and evaluate its effectiveness.
To assess if extra oxygen is required for a patient or if further support is required (CPAP, BIPAP or PPV).
To measure the acid base level in patient’s where it is compromised. Patient would include renal patients’, patient with heart failure, severe infected patients’ uncontrolled diabetes or individuals who have taken an overdose.
Preparing the patient. Explain to the patient that they are having a blood test from their artery. It is likely to be taken from a radial artery.
Nettina (2005) Describes a test to assess the puncture site prior to puncture called the Allen’s test procedure. This will evaluate the blood circulation in the hand and whether it is appropriate to use the radial artery for puncture. The site will be cleaned with alcohol and allergy status permitting anaesthetic agents will be applied to reduce discomfort, and increase possibility of success.
Dougherty (2008) suggests that the patient should be encouraged to breathe normally through the procedure and the doctor may ask for cessation of supplementary oxygen prior, to give a better understanding of the patients’ present condition.
After the syringe is full, place gauze over the puncture site and apply pressure until bleeding has stopped. This may be some time if the patient is on blood thinners or has coagulopathy. Once bleeding has stopped apply a dry dressing but monitor for any further bleeding.
After the procedure there is a possibility of bruising although the longer pressure is kept on the puncture site the lower the risk. Some light headedness or nausea may occur during or after the blood draw. On rare occasions the needle may damage the artery or a nerve causing it to become blocked. As a result care must be taken with the wrist once blood draw has taken place.
How it feels Dougherty explains that collecting arterial blood from a patient is a procedure that is often painful. It is more painful than the routine venous phlebotomy your patient may be used to. There are a number of reasons for this, arteries are often deeper than veins and surrounded by nerves.
Ideally the patient is given a local anaesthetic and the patient feels just a sting as the needle punctures the skin. Otherwise there is a sharp pain as the needle enters the artery.
If the procedure becomes protracted either by the practitioner having difficulty finding the artery or the artery is narrow the pain may more than brief. It is important to note that both pain and fear would cause the arteries to narrow so reassurance is important and if the practitioner continues to have difficulty you must advocate on the patients behalf since fear would impact on future successful arterial blood gas collection.
Questions What other sites could a patient have blood gases taken from?
Can only arterial blood be used for blood gases? What values would be markedly different in a venous sample blood gas.
Why would a patient emotional response make blood draw difficult how can we reduce the affects of this to cause a positive outcome
What medications or disease process would make a patients’ bleeding time prolonged after sampling?
Further reading Royal Marsden clinical procedures manual 2008, Dougherty etal
Overview The measurement of a blood gas will show a pH value. PH is a value the can range from 1 to 14 and is a measure of acidity or alkalinity of a substance. Springhouse(2008)explains in the blood stream the pH value is inversely proportional to the number of hydrogen ions in the blood. The fewer ions the higher the number (alkalosis) and vica versa, more ions would mean a lower number (acidosis). A solution with a pH of 1 is acidic and a solution of pH 13 would be alkalotic. A solution of pH 7 is called neutral since it is in the middle, it is neither acidic nor alkalotic, and water has a pH of 7.
Adams (2009) explains that the normal PH of the body ranges from 7.35 to 7.45. In order for normal metabolism to take place the body must maintain this fine balance at all times. He clarifies that if the pH level rises the blood is said to be alkalotic or acidic if it drops below 7. Hall (2009) says the ability of the body to function normally is impaired if the pH moves from these parameters. Hall also concludes that in acidosis the body’s response to medication is muted, cardiac function is impaired since contractility and vascular response to catecholamine’s is reduced. If the patients pH is raised then oxygenation is effected which interferes with neurological and muscle function. Adams points out that severe changes in pH that is above 7.8 or below 6.8 will interfere with basic cell function and respiration and if not corrected will result in death.
Below is a discussion on how the body regulates this delicate balance. We will elaborate on the processes the renal and respiratory systems use to buffer the body’s processes to keep this fine balance.
The respiratory buffer system
Hinds (2008) explain that carbon dioxide (CO2) is a normal by product of cellular metabolism. Carbon dioxide is carried in the blood to the lungs where excess CO2 combines with water (H2O) to form carbonic acid (H2CO2) in the blood. The blood pH will change according to the level of this acid in the blood. This fluctuation triggers either a rise or fall in respiration until the level of CO2 is returned to the patients’ base line. Hinds explain that this system is fairly rapid and can be triggered in a short space of time a few minutes in most cases.
The renal buffer system
Henessey (2007) simplifies the metabolic system explaining that the kidneys also maintain acid base balance by the excretion or retention of bicarbonate (HCO3). As the pH rises HCO3 is excreted and in return as the pH decreases HCO3 is retained. Although an effective system the renal system is slow to respond to imbalances, requiring hours or days to attend altered pH.
Questions If neutral pH is 7 why does the body require a mean of 7.4 a slightly alkalotic environment to operate?
The notes above indicate the body’s response to catecholamine’s is muted what are these and why are they important?
Normal values pH
7.35 to 7.45
11 to 13.3 kPa
4.8 to 6.0 kPa
21 to 28 mmol/l
Acid Base Disorders Respiratory acidosis.
Henessy (2007) discussion on respiratory acidosis is defined as a pH less than 7.35 with a Pco2 greater than 6.0 kPa. This type acidosis is caused by a build up of CO2 which combines with water in the body to produce carbonic acid thus lowering the pH of blood. Driscoll (1997) says any condition that results in a reduction in ventilation can cause this type of acidosis.
Head trauma, which has inflicted damage to the respiratory centre leading to respiratory depression.
Sedatives, narcotics, neuromuscular blocking agents or anaesthesia, which can cause central nervous system depression.
Impaired respiratory muscle function related to spinal cord injury or neuromuscular disease.
Poor lung function such pneumothorax, pneumonia, atelectasis or bronchial obstruction.
Hypo inflation due to pain chest injury or abdominal distension.
Hasan (2009) simplifies the presentation of the signs and symptoms of respiratory acidosis are centred within the respiratory, cardiovascular and nervous systems. These symptoms can range from shallow breathing or dyspnoea to headaches or altered consciousness and irritability. If left unchecked these symptoms deteriorate towards drowsiness and coma.
Increasing ventilation support will correct this type of acidosis. The specifics of how this will be done is dependant on the mode of insult to the respiratory system. Edwards (2009) suggests ventilator support could be oxygen via a face mask, non invasive ventilation (N.I.V.) or positive pressure ventilation (P.P.V.). If medications are inhibiting respiratory function then reversal agents can be deployed whilst supporting the patients’ respiratory needs. Pneumothorax and pain are problems that can be reversed promptly once the patients’ condition allows. Marino (1997) say that if the patients symptoms or condition, cannot easily be resolved then it may be appropriate to ventilate the patient mechanically. Commonly patient’s with respiratory acidosis are hypo ventilating, as a result they will benefit from supplemental oxygen but this only improves the quality of respiration; it does not in fact remedy the problem.
Respiratory Alkalosis
Respiratory alkalosis is defined as a pH greater than 7.45 with a PaCO2 less than 4.8 kPa. Any condition that causes hyper inflation can result in respiratory alkalosis. These conditions include,
Anxiety fear or panic
Medications which stimulate the respiratory system
Lesions in the brain affecting the respiratory centre
Increased metabolic demands such as fever sepsis or pregnancy.
Alkalosis will present cardiovascular or central nervous system disorder. Springhouse (2008) illustrates that presentations can be dysrhythmias and palpitations to numbness and confusion. Additional symptoms are dry mouth, blurred vision and titanic spasms of the arms and legs.
To resolve the alkalosis the cause of the hyper ventilation must be attended to. These patients are at risk of suddenly deteriorating, they have tachypnea and must be supported to reduce fatigue. If they become tired their own ability to ventilate adequately will be impaired leading to respiratory failure.
Questions What would be the signs and symptoms of a patient with a respiratory caused imbalance?
Which kind of medications can cause an acidotic condition and what would be the reversal agents?
In respiratory alkalosis why do patients suffer with tetany?
What are the signs and symptoms of respiratory failure?
Metabolic acidosis
Metabolic acidosis is defined as a bicarbonate level less than 21mEq/L with a pH of less than 7.35. Schilling (2008) explains metabolic acidosis is caused either by a deficit of base in the blood stream or an excess of acids other than CO2. Excessive bowel action such as diarrhoea and intestinal fistulas may cause decreased levels of base. Increased acids can be caused by a number of factors such as:
Renal failure
Diabetic ketoacidosis
Anaerobic Metabolism
Salicylate intoxication
Hall (2009) Signs and symptoms of metabolic acidosis are varied affecting numerous systems. The nervous system presents with headaches, dizziness leading to confusion or later coma. Dysrhythmias are common as conduction pathways are affected and low blood pressure due to desensitivity to catecholamines such as epinephrine. Marino (1997) elaborates to say the respiratory system will attempt to correct imbalances by breathing out more CO2. Kussmaul respirations these are deep and laboured breaths. In the gastro intestinal tract nausea and vomiting is noted as well as warm flushed skin.
The Hinds (2008) says treatment of the metabolic acidosis is to resolve the cause, this invariably means an initial review of body systems and their function. By assessing each function and its efficiency, underperfused or hypoxic tissue beds can be identified. Hypoxemia can lead to generalised anaerobic metabolism, but hypoxia of a specific tissue bed will produce metabolic acids even if oxygenation (PaO2) is normal. To reverse this acidosis perfusion must be restored which in turn will cease the anaerobic metabolism. Hinds warns that other causes of metabolic acidosis should be addressed after the possibility of hypoxia and poorly perfused tissue beds have been resolved or ruled out.
Metabolic alkalosis
Metabolic alkalosis is defined as a bicarbonate level of 28mEq/L with pH greater than 7.45. Metabolic alkalosis obviously is the reverse of the previous condition deriving from an excess of base or a deficit of acid. Adam (2009) suggests that excessive base comes from ingestion of antacids, excess use of bicarbonate or lactate in dialysis. Low amounts of acid come from overuse of diuretics, gastric suction or protracted vomiting.
It presents through neurological signs and symptoms varying from light headedness to seizures and coma or musculoskeletal symptoms of weakness, muscle cramps and tetany. Other associated signs might be nausea and vomiting and respiratory depression. This is a relatively uncommon presentation and presents a challenge in treatment. Bicarbonate can be stimulated thought the kidneys by drugs such as Acetazolamide but it is a protracted therapy. Severe cases I.V. administration of acids may be used
Questions Which other value is closely linked with the metabolic state of the body? What does it signify?
What signs and symptoms would a patient show who presented with a metabolic acidosis?
In a very severe alkalosis state what I.V. acids could be administered?
Steps to Arterial Blood gas interpretation There are simply 3 steps to interpreting a blood gas result and each must be done in order to prevent confusion and misdiagnosing your patient. The components are pH PaCO2 and HCO3 below are three steps and following are examples to assist you in interpreting them.
Step One
Review the pH initially is this normal or abnormal? If the pH is above 7.45 it is alkalotic if it is below 7.35 then it is acidotic.
Step Two
If the blood sample pH is altered then we must consider how this is being affected. Initially assess the PaCO2 this value will move in the opposite direction to the pH when there is a insult to the respiratory system. That is as the pH falls out of normal values the PaCO2 rises from its normal limits. The reverse is true if the PaCO2 falls then the pH will rise.
Step Three
The third step is to assess the HCO3 value. If there is an altered metabolic function the HCO3 will alter in a similar direction to the pH. As the HCO3 value rises so will the pH and as one decreases so will the other.

Examples: Using the table above and your knowledge you have gained try and diagnose the problems below.
Example 1
Mr Brown is a 72 year old man admitted with recent chest infection to the assessment unit. He is quite short of breath and has a strong cough his blood gas show the following information
Patient: John Brown
PH – 7.30
PaCO2 – 8
HCO3 – 25
Step one, assess the pH is it normal? It is not, it is low therefore it is acidotic.
Step two, assess the PaCO2 is it normal? It is not, it is raised which is the opposite direction of the movement of the pH.
Step three, assess the HCO3, is that normal? Yes it is within its normal range.
Reviewing the grid it can be seen the pH being low, the PaCO2 raised and the HCO3 normal shows a respiratory acidosis.
Example 2
Maria 29, who has a long history indigestion and reflux, has come to the drop in clinic with vomiting unresponsive to her usual medications and cramp in her hands. A routine blood gas shows the information below.
Patient: Maria Goode
D.O.B.: 01:01:1981
pH – 7.51
PaCO2 – 5.5
HCO3 – 35
Assess the pH, is it normal? It is high indicating alkalosis
Assess the PaCO2 is it normal? It is normal
Assess the HCO3 is it normal? It is raised, moving in the same direction as the pH.
Looking at the chart above a raised pH and a raised HCO3 would indicate a metabolic alkalosis state
Discussion on compensation So far we have only looked at a simple blood gas scenarios, with only one system failing. As Hasan (2009) indicates that often if one system fails or falls out of normal range altering the pH the second system will activate and work harder to compensate to bring the pH back in to normal limits. This activity is called compensation.
Foxall (2008) describes that when a patient develops an imbalance over a period of time the body will naturally attempt to compensate. The lungs and the kidneys are the primary response mechanisms and so the body will try to resolve any metabolic or respiratory imbalance to return the pH to normal
There are varying degrees of compensation initially uncompensated, an altered pH with only one value out of normal range. Partially compensated blood gas, an altered pH value with both values out of normal range. Compensated blood gas, a normal pH value with possibly both values out of range.
Previous examples we looked at were simple uncompensated blood gases. Now let’s look at more advanced gases such as partial compensation.
To review these gases as before break the interpretation down in to three simple steps
Assess the pH, is the gas acidotic or alkalotic
Assess the PaCO2; is the PaCO2 a normal value? As reviewed before respiratory imbalances will move the pH in the opposite direction to which the PaCO2 moves when causing a primary imbalance. If the PaCO2 is moving in the same direction that is either increasing, or decreasing in value, then this would be a compensatory behaviour and it would indicate the primary insult is coming from the kidneys (metabolic). In a compensatory environment a decreasing PaCO2 would show the lungs are buffering by excreting excess acid by blowing off Co2 in order to equalise the balance of acids and return the pH to normal. Conversely a raised pH and raised PaCO2 would indicate a buffering response by the lungs which would reduce acid excretion in an effort to return to homeostasis. In summary, if there is evidence of compensation, but the pH has not yet arrived back into normal limits then it is only partial respiratory compensation.
Assess the HCO3. In our original uncompensated examples the pH and the HCO3 moved in the same direction when the primary insult was metabolic. Following our discussion above in compensatory behaviours the values will work counter to their normal presentation. So if the pH is decreasing when the HCO3 is increasing or decreasing when the pH is increasing this is a compensatory action therefore the primary insult is a respiratory one. The kidneys will hold on to or release HCO3 in response to the abnormal pH to equalise the acid in the body to return the body’s pH to normal

The essential difference between these two states is that they are on a journey towards normal from possibly uncompensated , to partially compensated, to fully compensated environment (normal pH). The body is always trying to correct the imbalance however successful, but the body will never over compensate. As can be seen from the above table the pH in fully compensated states is normal. Knowing which side of 7.40 will help in determine the original imbalance that is now compensated.
More technical questions Example 1 A patient enters the A