Acid-base Balance, pH Homeostasis PDF

PA PAGE 1 G E 1 https://edu.umch.de Acid-base ba...

PA PAGE 1 G E 1 https://edu.umch.de Acid-base balance. www.umfst.ro 2024 Lecturer Dr. Florina Gliga Assist. Dr. Horațiu Sabău Assist. Dr. Andreea Tinca Introduction PAGE 2 Key words Acidemia: an increase in blood [H+] Alkalemia: a decrease in blood [H+] Acidosis: a pathophysiologic process that tends to acidify body fluids Alkalosis: a pathophysiologic process that tends to alkalinize body fluids Hypoxia: reduced level of tissue oxygenation Hypoxemia: a decrease in the partial pressure of oxygen (pO2) in the blood Normocapnia: normal arterial pCO2 (40 mmHg) Hypocapnia: a decrease in arterial pCO2 Hypercapnia: an increase in arterial pCO2 Acid base balance PAGE 3 Acid–base physiology deals with the maintenance of normal hydrogen ion concentration (abbreviated as [H+]) in body fluids. The normal [H+] in the extracellular fluid is about 40 nmol/L or 40 nEq/L (range 38–42 nmol/L), which is precisely regulated. Maintaining balance: 1. buffer systems 2. lungs 3. kidneys Acid base balance PAGE 4 Netter's Essential Physiology, Mulroney, Susan E., PhD; Myers, Adam K., PhD.Pages 185-199. © 2016. Acid base balance PAGE 5 Buffer systems A buffer is a solution of a weak acid and its salt (or a weak base and its salt) that can bind H+ and therefore resist changes in [H+]. Buffering does not remove H + from the body – it is a short-term solution to the problem of excess H+, mopping it up temporarily-> are the first line of defense against wide fluctuations in pH. Principal buffers in the human body Intracellular space Extracellular space 1. Phosphate 1. Bicarbonate 2. Hemoglobin 2. Plasma proteins Acid base balance PAGE 6 Buffer systems The most important is the bicarbonate buffer system. The roles of this system are, as followed: reversibly binds hydrogen ions and therefore is going to maintain the pH in a normal range can shift carbon dioxide through carbonic acid to hydrogen ions and bicarbonate and also vice-versa allows quick buffering The acid-base imbalances that overcome this buffer system can be compensated by: – changing the rate of ventilation – excretion of excess acid or base; the kidneys are slower to compensate. The proteins have the highest buffering capacity of all buffer solutions of the human body but this is a slow system. Acid base balance PAGE 7 Buffer systems ↑[H+]: the bicarbonate component of the buffer accepts (H+)-> carbonic acid (H2CO3)-> CO2 + H2O H++HCO3− ⇌H2CO3⇌CO2+H2O At the first stage, the bicarbonate concentration decreases and pCO2 increases. the CO2 is eliminated through the lungs-> the bicarbonate/pCO2 ratio is subsequently brought back toward normal ↓[H+]: the carbonic acid component of the buffer will dissociate to supply H+ H2CO3→ H++HCO3− The ventilation rate will decrease, retaining CO2 to increase the pCO2 , thus normalizing the bicarbonate/pCO2 ratio: CO2+H2O→H2CO3 Acid base balance PAGE 8 Lung After buffers, the lungs are the second line of defense against pH disturbance (takes several hours to complete). Alveolar ventilation is controlled by chemoreceptors located: centrally in the medulla peripherally in the carotid body and aortic arch. Blood [H+] and pCO2-> the chemoreceptors-> alter alveolar ventilatory rate. ↑[H+]= ↓pH= acidosis-> stimulates ventilatory rate-> decreases pCO2->↑pH ↓[H+]= ↑pH= alkalosis-> depresses ventilatory rate-> retention of pCO2-> ↓pH Acid base balance PAGE 9 Kidney 1 mmol/kg/day of fixed acid is produced from the diet. If not removed, this acid is retained and plasma [HCO3 −] decreases. The maintenance of [HCO3 −] is achieved by three renal mechanisms: 1. Reabsorption of filtered HCO3− (the Na/H antiporter) 2. Generation of new HCO3− by titratable acid (TA) excretion 3. Formation of HCO3− from generation of NH4 + The kidneys reabsorb filtered bicarbonate in the proximal tubules and generate new bicarbonate in the distal tubules, where there is a net secretion of hydrogen ions. Acid base balance PAGE 10 Parameters that are investigated in order to assess acid base balance disturbances: 1. pH 2. pO2 (partial pressure of oxygen) 3. pCO2 (partial pressure of carbon dioxide) 4. Standard bicarbonate 5. Buffer base 6. Reserve alkalinity 7. Anion gap 8. Base excess Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. Acid base balance PAGE 11 1. pH The pH= the negative logarithm (base 10) of the molar concentration of dissolved hydrogen ions. Arterial blood the normal range: 7.35-7.45. Measures of the acidity or alkalinity of the blood: above 7.45 = alkalinity below 7.35= acidity Values compatible with life in mammals are Arterial Blood Gases Made Easy, Hennessey, Iain AM, MBChB (Hons) BSc (Hons) MMIS FRCS; Japp, Alan G, limited to a pH range between 6.8 and 7.8 MBChB (Hons) BSc (Hons) MRCP PhD. © 2016. Acid base balance PAGE 12 2. PO2 PO2=Partial pressure of oxygen PaO2=Partial pressure of oxygen in arterial blood PvO2=Partial pressure of oxygen in venous blood Normal values of PaO2= 75-100 mmHg while in venous blood is 40 mmHg. Modified mostly by pulmonary conditions which are not allowing a proper oxygenation. Arterial Blood Gases Made Easy, Hennessey, Iain AM, MBChB (Hons) BSc (Hons) MMIS FRCS; Japp, Alan G, MBChB (Hons) BSc (Hons) MRCP PhD. © 2016. Acid base balance PAGE 13 3. PCO2 The values are dependent on breathing. The values are controlled by the brain. Increasing of pCO2 will lead to decrease of pH. Decreasing of pCO2 will lead to increase of pH. Medical Biochemistry, Dominiczak, Marek H.; Szczepańska-Konkel, Mirosława. © 2019. Acid base balance PAGE 14 4. Standard bicarbonate Concentration of bicarbonate when the body has the following: pCO2 of 40 mmHg, oxygenation 100% and t=37°c. Normal value in arterial blood is 24 mEq/l. Decrease will lead to decrease of pH. Increase will lead to increase of pH. Acid base balance PAGE 15 5. Buffer base Represents the sum of all buffering agents in the blood. Normal buffer base (NBB)= at a pH value of 7.4 , pCO2 40 mmHg, temperature of 37°C. Normal value 46 mEq/l. 6. Reserve alkalinity Concentration of bases in plasma Normal range is 24-29 mmol/l. Acid base balance PAGE 16 7. Anion gap: In plasma (serum), the number of cations must equal the number of anions to maintain electroneutrality. Usually measured: Na+ Cl− HCO3− Acid base balance PAGE 17 8. Base excesses Excess or deficit in the amount of base present in the blood The reference range is from is -2 to +2 mEq/l Defined under a standardized pressure of carbon dioxide and a standardized blood pH of 7.40. Disorders of Acid–Base Balance PAGE 18 Classification: 1. Acidosis: respiratory metabolic 2. Alkalosis: respiratory metabolic Medical Biochemistry, Fifth Edition, Baynes, John W., PhD; Dominiczak, Marek H., MD, Dr Hab Med, FRCPath, FRCP (Glas), 2019, Elsevier Disorders of Acid–Base Balance PAGE 19 1. Respiratory acidosis: Changes in ventilations which lead to the increase of pCO2 Leads to a decreased pH The bicarbonate level is normal (primary change-lung) 2. Respiratory alkalosis: Changes in ventilations which lead to the decrease of pCO2 Leads to an increased pH The bicarbonate level is normal (primary change-lung) Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. Disorders of Acid–Base Balance PAGE 20 3. Metabolic acidosis: Caused by the decrease of bicarbonate in the blood with increasing H+ Leads to a decreased pH The pCO2 level is normal (primary change-extracellular fluid) 4. Metabolic alkalosis: Caused by increasing bicarbonate levels with decreasing H+ Leads to an increased pH The pCO2 level is normal (primary change-extracellular fluid) Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. Compensatory mechanisms PAGE 21 Looking at the Henderson–Hasselbalch equation: pCO2 is controlled by the lungs= the respiratory component of the acid–base balance plasma bicarbonate concentration is controlled by the kidneys= the metabolic component of the acid–base balance Medical Biochemistry, Dominiczak, Marek H.; Szczepańska-Konkel, Mirosława. 2019. Compensatory mechanisms PAGE 22 These changes can be compensated by the opposite system (which is not affected). Compensatory mechanisms: 1. Respiratory acidosis: kidney (will increase the reabsorption of bicarbonate) 2. Respiratory alkalosis: kidney (will decrease the reabsorption of bicarbonate) Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. Compensatory mechanisms PAGE 23 3. Metabolic acidosis: lung (hyperventilation to decrease pCO2) 4. Metabolic alkalosis: lung (will decrease ventilation) Therefore, the pH can be restored to normal values, but the pCO2 and the bicarbonate will be altered. Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. Compensatory mechanisms PAGE 24 Normal acid–base Uncompensated respiratory Partially compensated Fully compensated balance. acidosis. metabolic acidosis. metabolic acidosis. Arterial Blood Gases Made Easy Hennessey, Iain AM, MBChB (Hons) BSc (Hons) MMIS FRCS; Japp, Alan G, MBChB (Hons) BSc (Hons) MRCP PhD. © 2016. Compensatory mechanisms PAGE 25 Physiology, Sixth Edition, Costanzo, Linda S., PhD, 2018 by Elsevier Arterial Blood Gas analysis PAGE 26 Investigation of: acid-base status partial pressure of oxygen partial pressure of carbon dioxide can be performed from arterial and venous blood. These parameters are particularly beneficial in the critical care settings in patients with a variety of respiratory and critical illnesses. 1. Arterial blood gas (ABG) analysis: mostly used. 2. Venous blood gas (VBG) analysis: obtaining a venous blood sample is quicker, simpler and less painful than obtaining an arterial blood sample and avoids the potential complications of arterial puncture. Arterial Blood Gas PAGE 27 Differences between arterial and venous gas blood analysis. Arterial blood Venous blood pH 7.40 7.36 Pco2 40 mm Hg 45 mm Hg Po2 80-100 mm Hg 40 mm Hg HCO3 24 mEq/L 26 mEq/L Arterial Blood Gas PAGE 28 An arterial blood sample can be obtained from many sites, including: radial artery femoral artery brachial artery dorsalis pedis artery axillary artery Arterial Blood Gases Made Easy, Hennessey, Iain AM, MBChB (Hons) BSc (Hons) MMIS FRCS; Japp, Alan G, MBChB (Hons) BSc (Hons) MRCP PhD. © 2016. Arterial Blood Gas PAGE 29 + H 35–45 nmol/L pH 7.35–7.45 Common ABG Values PCO2 35–45 mmHg PO2 >80 mmHg Bicarb 22–28 mmol/l BE −2 to +2 mmol/l SO2 >96% Lactate 0.4–1.5 mmol/l K 3.5–5 mmol/l Na 135–145 mmol/l Cl 95–105 mmol/l + iCa 1–1.25 mmol/l Hb 13–18 g/dl Glucose 3.5–5.5 mmol/l ABG interpretation PAGE 30 Always consider the clinical context when interpreting acid–base status. Metabolic compensation takes days to occur, respiratory compensation REMEMBER takes minutes. Overcompensation does not occur. An apparent compensatory response could represent an opposing primary process.  References PAGE 31 1. Arterial Blood Gases Made Easy, Hennessey, Iain AM, MBChB (Hons) BSc (Hons) MMIS FRCS; Japp, Alan G, MBChB (Hons) BSc (Hons) MRCP PhD. © 2016. 2. Netter's Essential Physiology, Mulroney, Susan E., PhD; Myers, Adam K., PhD.Pages 185-199. © 2016. 3. Netter's Integrated Review of Medicine, Kunnirickal, Steffne. © 2021. 4. Medical Biochemistry, Dominiczak, Marek H.; Szczepańska-Konkel, Mirosława. © 2019. 5. Clinical Biochemistry: An Illustrated Colour Text, Murphy, Michael, MA MD FRCP FRCPath; Srivastava, Rajeev, MS FRCS FRCPath; Deans, Kevin, PhD FRCP FRCPath. © 2019. 6. Physiology, Sixth Edition, Costanzo, Linda S., PhD, 2018 by Elsevier