Carriage of Blood Gases 2020-2021 PDF

Carriage of Blood gases Kath Taylor slides adapted from Darrell Brooks UM1010 ISCM 1 CVS-R physiology  Lecture Outline How oxygen is carried in the blood Factors that affect the carriage of oxygen, and how this changes in different areas of the body How carbon...

Carriage of Blood gases Kath Taylor slides adapted from Darrell Brooks UM1010 ISCM 1 CVS-R physiology  Lecture Outline How oxygen is carried in the blood Factors that affect the carriage of oxygen, and how this changes in different areas of the body How carbon dioxide is carried in the blood Importance of the bicarbonate buffer system Schematic diagram showing the basic structure of a single haemoglobin A molecule, including two α-globin chains (green), two β- globin chains (yellow), each containing a haem–iron complex (blue).  Deoxygenated and Oxygenated Haemoglobin The transition from ‘tense’ to ‘relaxed’ haemoglobin. In its deoxygenated ‘tense’ form, the crevice containing the haem molecule is narrow, restricting the access of oxygen to its binding site. As each oxygen molecule binds, the position of the haem molecule changes which affects the interaction between adjacent globin chains, relaxing the molecule and so allowing easier access of subsequent oxygen molecules to their binding site. Contin Educ Anaesth Crit Care Pain, Volume 12, Issue 5, October 2012, Pages 251–256, Hb saturation with 02 and partial pressure  The relationship between percentage saturation of Hb with oxygen, and partial pressure of oxygen is a sigmoid curve. In the alveoli, the partial pressure of oxygen is about 104mmHg, which means that Hb is almost 100% saturated; it has a high AFFINITY for oxygen Deoxygenated blood (contracting skeletal muscle)  In systemic veins, the partial Oxygenated blood in systemic arteries pressure of oxygen is about 40mmHg, and Hb is around 77% saturated  This means that some of the oxygen has been released for use in aerobic respiration Deoxygenated  In systemic veins Hb has a LOWER blood in systemic veins affinity for oxygen. (average at rest) Effect of pH on affinity of haemoglobin for oxygen  When pH is reduced High blood pH (high concentration of (7.6) hydrogen ion), the affinity of Hb for oxygen reduces Normal blood pH  This means that at any (7.4) PO the SaO2 is lower – Low blood pH 2 more oxygen has been (7.2) released  The curve shifts to the right (Note curve down) Why is this an advantage? When is this a disadvantage? Effect of P 2 on affinity of haemoglobin for oxygen CO  Blood also carries carbon Low blood PCO 2 dioxide  When blood PCO2 is high, Normal blood PCO the affinity of Hb for 2 oxygen falls High blood PCO  The curve shifts to the 2 right, and more oxygen is released (Note curve down) Effect of Temperature on haemoglobin O2 affinity  Increasing temperature Low temperature (20°C, 68°F) reduces affinity and shifts the curve to the right  Decreasing temperature shifts the curve to the left Normal blood temperature (37°C, 98.6°F) High temperature (43°C, 110°F) Effect of 2,3 DPG on affinity of haemoglobin  2,3-Bisphosphoglyceric acid (2,3-BPG), is a three-carbon Decreased 2,3 BPG isomer of a glycolytic intermediate  Present in human red blood cells at @ 5 mmol/L  binds with greater affinity to deoxygenated haemoglobin  promotes the release of the Increased 2,3 BPG remaining oxygen  2,3-BPG increases x5 within 1-2 hrs in patients with chronic anaemia  Decreases with dialysis and transfusion (RBC stress) Foetal Hb affinity for oxygen  Foetal Hb (α2γ2) predominates during most of gestation HbF. Foetal Hb has a higher affinity for oxygen than maternal HbA.  This is essential because it means that under conditions where maternal Hb is releasing some oxygen, foetal Hb can take it up.  This allows effective transfer of oxygen from maternal to foetal blood Foetal Maternal Transfer of oxygen to tissues  Myoglobin also has greater affinity for O2 than haemoglobin (curve shifted left)  Accepts O2 from haemoglobin when PO2 in blood is low and release O2 in muscle (e.g. during exercise).  The myoglobin binding curve lacks the sigmoidal shape of the haemoglobin binding curve because of the single O2 binding site in each molecule. CO2 transport in blood  CO2 produced as a waste product of metabolism.  Transported to lungs for excretion.  Different ‘pools’ of CO2 in blood.  Bicarbonate contributes the overwhelming majority of CO2.  carbonic anhydrase catalyses the reaction to produce carbonic acid which then readily dissociates to form hydrogen ions and bicarbonate CO2 transport in blood  A small amount of CO2 dissolves in plasma  Most carbon dioxide diffuses into red blood cells  Some carbon dioxide attaches to Hb, displacing oxygen (note O2 affinity)  Most carbon dioxide reacts with water catalysed by carbonic anhydrase to produce bicarbonate and hydrogen ion  Bicarbonate diffuses out of the red blood cell and is replaced by chloride ions (chloride shift)  In the lungs the process is reversed The importance of the bicarbonate ion  One of the most important buffer systems in the body  A reversible chemical reaction that can absorb or release hydrogen ions (H+).  Hydrogen ion concentration must be kept constant. Changes in pH affect:  The affinity of Hb for oxygen  The rate of enzyme reactions (optimal within a very narrow range)  The ionisation states of many substances, disrupting their structure e.g. DNA The importance of bicarbonate ion  Mechanisms involved in the bicarbonate balance  Carbon dioxide production from metabolism (Krebs cycle )  Exhalation of carbon dioxide by the lungs  Hydrogen ion excretion by the kidney  Bicarbonate reabsorption The importance of bicarbonate ion  Other tissues  Stomach – parietal cells: acid secretion  Pancreas – duct cell: pancreatic juice Metabolic and respiratory disorders  Metabolic processes affect bicarbonate concentration, and respiratory processes affect PCO. 2  Changes in ventilation compensate for metabolic disorders, and renal excretion of acid compensates for respiratory disorders.  In primary acid–base disorders, the underlying disorder will be evident from examination of the pH, PCO , and serum bicarbonate. 2 Metabolic and respiratory disorders Metabolic disorders  Respiratory compensation for primary metabolic disorders begins within minutes as chemoreceptors sense the change in extracellular pH and signal the respiratory centre to change minute ventilation.  In a primary metabolic acidosis, the acidaemia stimulates an increase in minute ventilation and subsequent decrease in PCO ; conversely, a metabolic 2 alkalosis results in hypoventilation and increased PCO. 2 Respiratory disorders  Renal compensation for primary respiratory disorders is a much slower process, taking 6–12 hours to respond to sustained changes in pH.  In a primary respiratory acidosis, the kidneys increase bicarbonate synthesis, excrete more organic acids, and reclaim more bicarbonate from the proximal tubule.  Learning outcomes Demonstrate the physiological process of respir ation and its nervous control Demonstrate the significance of maintaining normal acid-base balance within the body and the role of the respiratory system in it.