ACID BASE BALANCE & BODY FLUID. Ani Retno Prijanti Renal and Body Fluids Module Juni 2008

Transcription

1 ACID BASE BALANCE & BODY FLUID Ani Retno Prijanti Renal and Body Fluids Module Juni

2 Continuous Mixing of Body Fluids 2

3 Water Balance and ECF Osmolality To remain properly hydrated, water intake must equal water output Water intake sources Ingested fluid (60%) and solid food (30%) Metabolic water or water of oxidation (10%) 3

4 Water Balance and ECF Osmolality Water output Urine (60%) and feces (4%) Insensible losses (28%), sweat (8%) Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH) 4

5 Water Intake and Output 5

6 Regulation of Water Intake The hypothalamic thirst center is stimulated: By a decline in plasma volume of 10% 15% By increases in plasma osmolality of 1 2% Via baroreceptor input, angiotensin II, and other stimuli 6

7 Regulation of Water Intake Thirst is quenched as soon as we begin to drink water Feedback signals that inhibit the thirst centers include: Moistening of the mucosa of the mouth and throat Activation of stomach and intestinal stretch receptors 7

8 Regulation of Water Intake: Thirst Mechanism 8

9 Regulation of Water Output Obligatory water losses include: Insensible water losses from lungs and skin Water that accompanies undigested food residues in feces Sensible water loss of 500ml in urine Kidneys excrete mosm of solutes to maintain blood homeostasis Urine solutes must be flushed out of the body in water 9

10 Influence and Regulation of ADH Water reabsorption in collecting ducts is proportional to ADH release Low ADH levels produce dilute urine and reduced volume of body fluids High ADH levels produce concentrated urine Hypothalamic osmoreceptors trigger or inhibit ADH release Factors that specifically trigger ADH release include prolonged fever; excessive sweating,vomiting, or diarrhea; severe blood loss; and traumatic burns 10

11 Electrolyte Balance Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance Salts are important for: Neuromuscular excitability Secretory activity Membrane permeability Controlling fluid movements Salts enter the body by ingestion and are lost via perspiration, feces, and urine 11

12 Sodium in Fluid and Electrolyte Balance Changes in plasma sodium levels affect: Plasma volume, blood pressure ICF and interstitial fluid volumes Renal acid-base control mechanisms are coupled to sodium ion transport 12

13 Regulation of Sodium Balance: Aldosterone Sodium reabsorption 65% of sodium in filtrate is reabsorbed in the proximal tubules 25% is reclaimed in the DCT When aldosterone levels are high, all remaining Na+ is actively reabsorbed Water follows sodium if tubule permeability has been increased with ADH 13

14 Regulation of Sodium Balance: Aldosterone The renin-angiotensin mechanism triggers the release of aldosterone This is mediated by the juxtaglomerular apparatus, which releases renin in response to: Sympathetic nervous system stimulation Decreased filtrate osmolality Decreased stretch (due to decreased blood pressure) Renin catalyzes the production of angiotensin II, which prompts aldosterone release 14

15 Regulation of Sodium Balance: Aldosterone Low aldosterone cause Na excretion and water will follow it High aldosterone levels will cause Na absorption. For the water to be absorbed ADH must also be present Adrenal cortical cells are also directly stimulated to release aldosterone by elevated K+ levels in the ECF Aldosterone brings about its effects (diminished urine output and increased blood volume) slowly 15

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17 What is the acid base balance Acid-base balance is defined by the concentration of hydrogen ions. In order to achieve homeostasis, there must be a balance between the intake or production of hydrogen ions and the net removal of hydrogen ions from the body. 17

18 An Acid Molecules containing hydrogen atoms that can release hydrogen ions in solutions are referred to as an acid. An example of an acid is hydrochloric acid (HCL) 18

19 A Base A base is an ion that can accept a hydrogen ion. An example of a base is is the bicarbonate ion.( HCO3 - ) 19

20 How is Acid-Base balance measured Hydrogen ion concentration is expressed on a logarithm scale using ph units (part/percentage hydrogen). 7.0 being neutral Body systems carefully control ph of the body within the range of

21 ph A low ph corresponds to a high hydrogen ion concentration The term Acidosis refers to the addition of excess hydrogen ions and the body has a ph that falls below

22 ph A high ph corresponds to a low hydrogen concentration The term Alkalosis refers to excess removal of hydrogen ions from the body and has a ph that rises above

23 How the Body defends against fluctuations in ph Three Systems in the body: 1.Buffers in the blood 2.Respiration through the lungs 3.Excretion by the kidneys 23

24 Buffers in the Blood Buffers are substances that neutralize acids or bases Bicarbonate which is a base and carbonic acid in the body fluids protect the body against changes in acidity These buffer systems serve as a first line of defense against changes in the acidbase balance 24

25 Respiration through the lungs Carbon Dioxide which is formed during cellular metabolism forms carbonic acid in the blood decreasing the ph When the ph drops respiration rate increases this hyperventilation increases the amount of CO2 exhaled thereby lowering the carbonic acid concentration and restoring homeostasis 25

26 Excretion by the Kidneys The kidneys play the primary role in maintaining long term control of Acid-Base balance The kidney does this by selecting which ions to retain and which to excrete The kidneys adjust the body s Acid-Base balance 26

27 The Importance of the Body s Buffering Systems Can be quickly realized if one considers the low concentration of hydrogen ions in the body fluids and the relatively large amounts of acids produced by the body each day Example: 80 milliequvilalants of hydrogen is either ingested or produced each day by metabolism. Whereas the hydrogen ion concentration of the body fluids normally is only about.0004meq/l 27

28 Acid-Base Balance Normal ph Alkalosis or alkalemia arterial blood ph rises above 7.45 Acidosis or acidemia arterial ph drops below 7.35 (physiological acidosis) 28

29 Sources of Hydrogen Ions Most hydrogen ions originate from cellular metabolism Breakdown of phosphorus-containing proteins releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic acid Fat metabolism yields organic acids and ketone bodies Transporting carbon dioxide as bicarbonate releases hydrogen ions 29

30 Hydrogen Ion Regulation Concentration of hydrogen ions is regulated sequentially by: Chemical buffer systems act within seconds The respiratory center in the brain stem acts within 1-3 minutes Renal mechanisms require hours to days to effect ph changes 30

31 Chemical Buffer Systems Strong acids all their H+ is dissociated completely in water Weak acids dissociate partially in water and are efficient at preventing ph changes Strong bases dissociate easily in water and quickly tie up H+ Weak bases accept H+ more slowly (e.g.,hco3 and NH3) 31

32 Strong and Weak Acids 32

33 Chemical Buffer Systems One or two molecules that act to resist ph changes when strong acid or base is added Three major chemical buffer systems Bicarbonate buffer system Phosphate buffer system Protein buffer system Any drifts in ph are resisted by the entire chemical buffering system 33

34 Bicarbonate Buffer System A mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well) If strong acid is added: Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid) The ph of the solution decreases only slightly HCl + NaHCO3 = H2CO3 + NaCl 34

35 Bicarbonate Buffer System If strong base is added: It reacts with the carbonic acid to form sodium bicarbonate (a weak base) The ph of the solution rises only slightly NaOH + H2CO3 = NaHCO3 + H2O This system is the only important ECF buffer 35

36 Phosphate Buffer System Nearly identical to the bicarbonate system Its components are: Sodium salts of dihydrogen phosphate (H2PO4 ), a weak acid Monohydrogen phosphate (HPO42 ), a weak base HCl + Na2HPO4 = NaH2PO4 + NaCl NaOH + NaH2PO4 = Na2HPO4 + H2O This system is an effective buffer in urine and intracellular fluid 36

37 Protein Buffer System Plasma and intracellular proteins are the body s most plentiful and powerful buffers Some amino acids of proteins have: Organic acid groups (weak acids) COOH (carboxyl) R-COOH RCOO- + H+ Groups that act as weak bases NH2 (amino) R-NH2 R-NH3 Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base 37

38 Physiological Buffer Systems The respiratory system regulation of acid-base balance is a physiological buffering system There is a reversible equilibrium between: Dissolved carbon dioxide and water Carbonic acid and the hydrogen and bicarbonate ions CO2 + H2O H2CO3 H + + HCO3 38

39 Physiological Buffer Systems During carbon dioxide unloading, hydrogen ions are incorporated into water When hypercapnia or rising plasma H+ occurs: Deeper and more rapid breathing expels more carbon dioxide Hydrogen ion concentration is reduced Alkalosis causes slower, more shallow breathing, causing H+ to increase Respiratory system impairment causes acid-base imbalance (respiratory acidosis or respiratory alkalosis) 39

40 Renal Mechanisms of Acid- Base Balance Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body The lungs can eliminate carbonic acid (volatile acid) by eliminating carbon dioxide Only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids and ketones) and prevent metabolic acidosis The ultimate acid-base regulatory organs are the kidneys 40

41 Renal Mechanisms of Acid- Base Balance The most important renal mechanisms for regulating acid-base balance are: Conserving (reabsorbing) or generating new bicarbonate ions Excreting bicarbonate ions Losing a bicarbonate ion is the same as gaining a hydrogen ion (the blood becomes acidic); reabsorbing a bicarbonate ion is the same as losing a hydrogen ion (the blood becomes alkaline) 41

42 Renal Mechanisms of Acid- Base Balance Hydrogen ion secretion occurs in the PCT and in the collecting ducts Hydrogen ions come from the dissociation of carbonic acid 42

43 Reabsorption of Bicarbonate Carbon dioxide combines with water in tubule cells, forming carbonic acid Carbonic acid splits into hydrogen ions and bicarbonate ions For each hydrogen ion secreted, a sodium ion and a bicarbonate ion are reabsorbed by the PCT cells Secreted hydrogen ions form carbonic acid; thus, bicarbonate disappears from filtrate at the same rate that it enters the peritubular capillary blood 43

44 Reabsorption of Bicarbonate Carbonic acid formed in filtrate dissociates to release carbon dioxide and water Carbon dioxide then diffuses into tubule cells, where it acts to trigger further hydrogen ionsecretion 44

45 Generating New Bicarbonate Ions Two mechanisms carried out collecting ducts cells generate new bicarbonate ions Both involve renal excretion of acid via secretion and excretion of hydrogen ions or ammonium ions (NH4+) 45

46 Hydrogen Ion Excretion Dietary hydrogen ions must be counteracted by generating new bicarbonate The excreted hydrogen ions must bind to buffers in the urine (phosphate buffer system) Collecting duct cells actively secrete hydrogen ions into urine, which is buffered and excreted Bicarbonate generated is: Moved into the interstitial space via a cotransport system Passively moved into the peritubular capillary blood 46

47 Hydrogen Ion Excretion In response to acidosis: Kidneys generate bicarbonate ions and add them to the blood An equal amount of hydrogen ions are added to the urine 47

48 Ammonium Ion Excretion This method uses ammonium ions produced by the metabolism of glutamine in PCT cells Each glutamine metabolized produces two ammonium ions and two bicarbonate ions Bicarbonate moves to the blood and ammonium ions are excreted in urine 48

49 Ammonium ion Excretion 49

50 Bicarbonate Ion Secretion When the body is in alkalosis, type B intercalated cells of collecting ducts : Exhibit bicarbonate ion secretion Reclaim hydrogen ions and acidify the blood The mechanism is the opposite of type A intercalated cells and the bicarbonate ion reabsorption process Even during alkalosis, the nephrons and collecting ducts excrete fewer bicarbonate ions than they conserve 50

51 Respiratory Acidosis and Alkalosis Result from failure of the respiratory system to balance ph PCO2 is the single most important indicator of respiratory inadequacy PCO2 levels Normal PCO2 fluctuates between 35 and 45mm Hg Values above 45 mm Hg signal respiratory acidosis Values below 35 mm Hg indicate respiratory alkalosis 51

52 Respiratory Acidosis and Alkalosis Respiratory acidosis is the most common cause of acid-base imbalance Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic fibrosis, or emphysema Respiratory alkalosis is a common result of hyperventilation 52

53 Metabolic Acidosis All ph imbalances except those caused by abnormal blood carbon dioxide levels Metabolic acid-base imbalance bicarbonate ion levels above or below normal (22-26 meq/l) Metabolic acidosis is the second most common cause of acid-base imbalance Typical causes are ingestion of too much alcohol and excessive loss of bicarbonateions Other causes include accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, and kidney failure 53

54 Metabolic Alkalosis Rising blood ph and bicarbonate levels indicate metabolic alkalosis Typical causes are: Vomiting of the acid contents of the stomach Intake of excess base (e.g., from antacids) Constipation, in which excessive bicarbonate is reabsorbed 54

55 Respiratory and Renal Compensations Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the other system The respiratory system will attempt to correct metabolic acid-base imbalances The kidneys will work to correct imbalances caused by respiratory disease 55

56 Respiratory Compensation Metabolic acidosis has low ph: Bicarbonate level is low Pco2 is falling below normal to correct the imbalance The rate and depth of breathing are elevated 56

57 Respiratory Compensation Metabolic alkalosis has high ph: High levels of bicarbonate Correction is revealed by: Rising PCO2 Compensation exhibits slow, shallow breathing, allowing carbon dioxide to accumulate in the blood 57

58 Renal Compensation To correct respiratory acid-base imbalance, renal mechanisms are stepped up Respiratory Acidosis has low ph Has high PCO2 (the cause of acidosis) In respiratory acidosis, the respiratory rate is often depressed and is the immediate cause of the acidosis High bicarbonate levels indicate the kidneys are retaining bicarbonate to offset the acidosis 58

59 Renal Compensation Respiratory Alkalosis has high ph Low PCO2 (the cause of the alkalosis) Low bicarbonate levels The kidneys eliminate bicarbonate from the body by failing to reclaim it or by actively secreting it 59

60 Developmental Aspects Water content of the body is greatest at birth (70-80%) and declines until adulthood, when it is about 58% At puberty, sexual differences in body water content arise as males develop greater muscle mass Homeostatic mechanisms slow down with age Elders may be unresponsive to thirst clues and are at risk of dehydration The very young and the very old are the most frequent victims of fluid, acid-base, and electrolyte imbalances 60