Acid-base and body fluid physiology

Body Fluids 

Regulation of ECF composition 

Defence of Tonicity 

• Vasopressin secretion and thirst mechanism: 
  • Increased osmolality ECF ->  thirst + vasopressin release ->  increased water intake and retention in kidneys ->  dilution (negative feedback loop) 
  • Normal osmolality = 280-295 mosm/kg  

Defence of Volume  

• Determined by amount of osmotically active solutes = Na+  
• Mechanisms: 
  • Na+  
    • ECF volume decreased ->  BP falls ->  GFR falls ->  reduced Na+ filtered 
    • Aldosterone ->  increased tubular reabsorption of Na+  
  • Water  
    • Hypovolaemia ->  angiotensin II ->  secretion of ADH + aldosterone  
    • Secretion of BNP/ANP ->  natriuresis and diuresis  

Specific Ionic Composition 

• Ca2+ feedback to parathyroids and calcitonin- secreting cells  
• Mg+ feedback incompletely understood  
• Na+ / K+ also dependent on H+ and pH  

Defence of H+  

• Buffer systems: Plasma proteins, Hb and carbonic acid- bicarbonate  
  • Blood 
    • H2CO3 -> H+ + HCO3- 
    • HProt -> H+ + Prot – 
    • HHb -> H+ + Hb- 
• Interstitial fluid 
  • H2CO3 -> H+ + HCO3- 
• Intracellular fluid  
  • HProt -> H+ + Prot – 
  • H2PO4- -> H+ +HPO42- 
• Henderson – Hasselbalch Equation: pH = pK + log [A-]/[HA] 
• Most effective buffers are those with pK close to pH in which they operate 

Interpreting ABG

1. Determine adequacy of oxygenation (normal PO2 = 80- 100 mmHg) 
2. Determine pH status  
   • <7.35 defines acidaemia
   • >7.45 defines alkalaemia  
3. Determine respiratory component – normal PCO2 35- 45 mmHg  
4. Determine metabolic component – normal HCO3- 22-26 mmol/L
5. Assess level of compensation

Renin-Angiotensin System 

• Angiotensinogen = made by liver 
  • Increased level by glucocorticoid, thyroid hormone, oestrogen, cytokines and angiotensin II 
• Renin = acid protease secreted by juxtaglomerular cells of kidney (t ½ 80 mins) 
  • Regulation  
    • Stimulatory – increased sympathetic activity in renal nerves, increased circulating catecholamines, prostaglandins  
    • Inhibitory – Increased Na+/Cl- reabsorption across macular densa, increased afferent arteriolar pressure, angiotensin II and ADH  
  • Conditions which increase renin secretion – Na+ depletion, diuretics, hypotension/ hypovolaemia, cardiac failure, cirrhosis, constriction of renal artery or aorta, psychologic stimuli  
• Angiotensin converting enzyme (ACE) = in lungs, also inactivates bradykinin  
• Angiotensin II = t ½ 1-2 mins 
  • Actions: 
    • Potent vasoconstrictor  
    • Increase NE release from post ganglionic sympathetic neurons + potentiates pressor effect via area prostrema  
    • Stimulates aldosterone secretion from adrenal cortex ->  Na+/H2O retention 
    • Stimulates ADH secretion from pituitary ->  H2O retention 
    • Contracts mesangial cells to reduced eGFR  
    • Decreases sensitivity to baroreflex 
    • Dipsogenic effect = Increases thirst via subfornical organ (SFO) and organum vasculosum of lamina terminalis (OVLT) 
  • Two receptors:  
    • AT1 (G protein coupled) – activity increases cytosolic free Ca2+ ->  tyrosine kinase activation  
      • Vascular smooth muscle – R downregulated by A II activity  
      • Adrenal cortex – R upregulated (more sensitive)  
    • AT2 – coded on X chromosome – (G protein coupled) – activity opens K+ channels/ increase NO and cGMP (brain) 
  • Reduced activity in cirrhosis/ hyponatraemia  
  • Does not penetrate BBB 
• Tissue RAAS systems = eyes, exocrine pancreas, ovary/testis, heart, adrenal cortex, intermediate lobes of pituitary and brain (contribute little to circulating renin pool) 
• Pharmacologic manipulation 
  • PG inhibitors/ BB (indomethacin, propranolol) reduce renin secretion 
  • Renin inhibitor (enalkiren) stop production angiotensin I 
  • ACE inhibitors and ARBs  
• Goldblatt hypertension = constriction of renal artery causing increased renin secretion  

Natriuretic Factors 

• Myocardial cells secrete ANP/BNP when stretched  
• NaCl intake increases/ ECF expanded ->  secretory granules increase ->  release substances causing natriuresis ->  lower blood pressure  
• Actions =  via NPR A and NPR B receptors  
  • Dilate afferent arterioles and relax mesangial cells = increase eGFR = Na+ excretion  
  • Inhibit Na+ reabs in tubules  
  • Increase capillary permeability to extravasate fluid  
  • Relax vascular smooth muscle  
  • Inhibit renin and counteract pressors/ Angiotensin II  
• Types: 
  • Atrial natriuretic peptide (ANP) 
  • Brain natriuretic peptide (BNP) 
    • Ventricles, brain  
  • C-type natriuretic peptide (CNP) 
    • Brain, pituitary, kidneys and vascular endothelium  
    • Primarily paracrine mediator

Body Fluids 

Marrow: 

• Haematopoiesis = formation of RBC/WBC/PLT 
  • Occurs in bone marrow for adults (red marrow – active, yellow – inactive) 
  • Extramedullary (liver and spleen) in foetus or disease bone marrow states  
• Haemopoietic stem cells (HSC) = progenitor to blood cells  
• Proliferation depends on stem cell factor (SCF) = EPO, colony stimulating factors (GM-CSF – granulocyte- macrophage) + IL-1,6,3 

WCC

Granulocytes (polymorphonucleocytes = PMN)

• Contain cytoplasmic granules with active substances: 
• Neutrophils = neutrophilic granules 
  • t ½ 6 hours  
  • Produce 100 billion neutrophils daily  
  • Involved inflammatory response = presence of bacteria ->  chemotaxis via chemokines (leukotrienes, C5a, polypeptides) ->  opsonization of bacteria (tasty opsonins – IgG + complement proteins) ->  bind to neutrophil ->  phagocytosis ->  exocytosis of bacteria + granules containing antimicrobial proteins (defensins a/b) + NADPH oxidase for respiratory burst ->  bactericidal  
• Eosinophils = granules dye acidic stain  
  • Similar diapedesis and chemotaxis  
  • Abundant in mucosa of GI/ urinary and respiratory tracts  
  • Increased in asthma 
• Basophils  = basophilic granules 
  • Contain histamine and heparin – released when activated by histamine releasing factor from T lymphocytes  
  • Immediate hypersensitivity reactions (urticaria ->  anaphylaxis)

Mast cells

• Heavily granulated, wandering cells  
• Found in areas of rich connective tissue/ beneath epithelium 
• Granules contact heparin, histamine, proteases  
• IgE R ->  involved in responses initiated by IgE and IgG, such as parasitic infections  
• Release TNF- a ->  Ab independent mechanism = natural immunity  
• Marked mast cell degranulation ->  anaphylaxis  

Lymphocytes

• Large round nuclei + scanty cytoplasm  
• Most formed in lymph nodes/ thymus/ spleen  
• 2% body lymphocytes in blood stream, rest in lymphoid organs  
• T cells: Mature in thymus  
  • Cytotoxic (CD8) 
  • Helper (CD4) = TH1 – cellular immunity via IL-2 and IFN y, TH2 – humoral immunity via IL-4/5 
  • Memory – can produce accelerated response to second exposure  
• B cells: Mature in liver/ marrow  
  • Plasma  = produce Ab 
  • Memory 
• Natural killer cells (NK)

Monocytes

• Abundant agranular cytoplasm + kidney shaped nuclei 
• In blood = life span 72 hours  
• Enter tissues = macrophages, life span ~ 3months  
  • Kupffer cells (liver) 
  • Pulmonary alveolar macrophage 
  • Microglia of brain  
• “Reticular endothelial system” 
• Activated by lymphokines from T lymphocytes ->  chemotaxis ->  engulf bacteria  

RBC

• Biconcave discs, carry haemoglobin  
• Life span 120 days 
• 3 x 10¹³ RBC (900g Hb) in circulation  
• Regulated by EPO from kidney  
• Morphology:  
  • Spectrin + ankyrin protein make up membrane skeleton  
  • Mean corpuscular volume (MCV) normal ~ 90 
    • >95fL = macrocytes and <80fL = microcytes  
  • Mean corpuscular haemoglobin (MCH) normal ~30 
    • <25g/dL = hypochromic  
• Osmotic fragility: shrink in solutions with greater osmotic pressure than plasma  
  • Low osmotic pressure ->  swell  ->  spherical shape ->  lose Hb (haemolysis) 
  • Hereditary spherocytosis = spherocytic cells, haemolyse more readily in hypotonic solutions, removed by spleen ~ hereditary haemolytic anaemia 
  • Glucose 6 phosphate dehydrogenase (G6PD) deficiency ->  increased haemolysis  
    •  G6PD enzyme catalyses initial step oxidation of glucose to form NADPH, required for maintaining cell fragility  
• Contain haemoglobin molecule = 4 subunits, haeme conjugated to polypeptide globin  
  • HbA (adult) = 2 polypeptide chains each with haeme, (a2b2) 
  • HbA₂ = 2.5%, contain a2d2 
  • HbA1C = glycated Hb, glucose attached to terminal valine in each b chain 
  • HbF = foetal, (a2g2) ->  oxygen content at certain PO2 is greater than HbA, as it binds 2,3-BPG less avidly ->  allows maternal-foetal circulation 
• Reactions of haemoglobin  
  • Oxygen (affinity shifted by pH, temperature, 2,3-BPG) 
  • Drugs/ oxidizing agents ->  methaemoglobin  
  • CO 
• Catabolism of Hb ->  haeme + globin 
  • Haem – biliverdin – bilirubin – excreted in bile 
  • Iron is reused  
  • Exposure to UV light converts BR to lumirubin (shorter t ½) thus phototherapy can be used to treat infants with jaundice due to haemolysis

Platelets

• Anuclear granulated bodies  
• t ½ 4 days 
• Formed from megakaryocytes (giant cells in marrow) cytoplasm 
• 60-75% in circulation, remaining in spleen 
• Splenectomy = increase platelet count  
• Membrane have R for collagen, ADP, vWF and fibrinogen  
• Cytoplasm contains two granule types: 
  • Dense = serotonin, ADP, adenine nucleotides ->  for platelet activation 
    • Three kinds ADP R – P2Y1, P2Y2, P2X1  
  • a= clotting factors, PDGF  
• Vessel wall injury ->  PLT adhere to exposed collagen + vWF ->  granule release + PAF from N/monocytes = platelet aggregation  
• Regulated via colony stimulating factors for megakaryocytes + thrombopoietin

Immunity: 

• Innate = R bind to common bacteria to activate defense mechanisms 
  • Neutrophils, macrophages and natural killer cells  
  • Toll- like receptors – eg. TLR 4 binds bacterial lipopolysaccharide + protein CD14 initiates transcription for proteins needed for innate response   
• Acquired = Lymphocytes activated by Ag, form clones that produce Ab. 2 components: 
  • Humoral – circulating Ab ->  bacterial infection  
  • Cellular – mediated by T lymphocytes ->  find cells which display certain antigen, kill via perforins ->  virus/fungi/few bacteria, delayed allergic reaction, transplant rejection
• Cytokines = hormone like molecules which regulate immune responses in paracrine fashion  
• Complement system = various plasma enzymes, include >30 proteins synthesised in liver  
  • Function in both innate and adaptive immunity  
  • Three pathways of enzymes to activate: 
    • Classic via immune complexes  
    • Mannose binding lectin pathway 
    • Alternative/ properdin via contact with intruder  
  • Functions:  
    • Opsonisation, chemotaxis 
    • Cell lysis via insertion of MAC (membrane attack complex) -> makes cell permeable to water/ ions and leads to cell lysis  
    • Activate B cells  
    • Help dispose of waste products after apoptosis 
  • Specific proteins: 
    • C5a, C3a, C4a mediate chemotaxis, facilitate release of histamines from mast cells  
    • C3b is an opsonin, forms ‘MAC’ 
• Autoimmunity = failed processes which eliminate Ab to self Ag  
  • B cell mediated/ T cell mediated 
  • Organ specific/ systemic  
    • Type 1 DM (Ab against pancreatic islet cells) 
    • Myasthenia gravis (Ab against nicotinic cholinergic R) 
    • MS (Ab against myelin protein) 
    • Graves (Ab against TSH R) 
  • Molecular mimicry = production of Ab against invader Ag which cross react with normal body constituents  
    • Rheumatic fever (portion of cardiac myosin resembles portion of streptococcal M protein ->  Ab damage heart) 
  • Bystander effect = inflammation sensitizes neighbourhood T cells, which should otherwise be inactivated  
• Tissue transplantation = rejection occurs via T cells  
  • Azathioprine – kills rapidly dividing cells ~ kills T cells 
  • Glucocorticoids – inhibit IL-2 thus T cell proliferation 
  • Cyclosporin/ tacrolimus – prevent dephosphorylation of NF-AT  
    • Activation of T cell ->  increased intracellular calcium ->  activates calcineurin via calmodulin ->  dephosphorylates transcription factor NF-AT ->  increases gene activity for IL-2 ->  stimulatory cytokine 
• Acquired Immunodeficiency Syndrome (AIDS) caused by Human Immunodeficiency virus (HIV) = virus destroys CD4 cells ->  failure to proliferate CD8 and B cells ->  loss of immune function  

Plasma  

• Contain proteins (albumin, globulin and fibrinogen fractions) 
• Functions: 
  • Osmotic pressure  
  • Buffering capacity of blood 
• Hypoproteinaemia = prolonged starvation, malnutrition, liver disease and nephrotic syndrome ->  oedema  

Lymph 

• Tissue fluid enters lymphatic vessels  
• Drains into venous blood via thoracic and right lymphatic ducts  

Last Updated on September 24, 2021 by Andrew Crofton