BPK 205 Lecture 30 & 31: Fluid & Electrolyte/Calcium Balance - Fall 2023

BPK 205 Intro to Human Physiology Lecture 30 Fluid & Electrolyte Balance Lecture 31 Calcium Homeostasis 1  Lecture Objectives – FE Balance Describe Fluid & Electrolyte homeostasis in the body – Water balance – Vasopressin regulates water reabs...

BPK 205 Intro to Human Physiology Lecture 30 Fluid & Electrolyte Balance Lecture 31 Calcium Homeostasis 1  Lecture Objectives – FE Balance Describe Fluid & Electrolyte homeostasis in the body – Water balance – Vasopressin regulates water reabsorption in the collecting duct Aldosterone regulates Na+ reabsorption in the distal tubule and collecting duct Describe how the Renin-Angiotensin-Aldosterone system(RAAS) helps maintain blood pressure – Integration of renal and cardiovascular systems Explain how acid-base balance is maintained 2  Lecture Objectives – Calcium balance Structure and growth of bone – Roles of osteoblasts and osteoclasts Calcium homeostasis – Hormones, role of bone, the kidneys and GI system 3 Readings Chapter 20: Fluid and Electrolyte Balance Chapter 23: “Tissue and Bone Growth” and “Calcium Balance” 4  Fluid & Electrolyte Homeostasis Why is it important? Na+ and water ECF volume and osmolarity (also, blood volume) K+ Cardiac and muscle function / excitable cells Ca2+ Exocytosis, muscle contractions, bone formation, many other functions – H+ and HCO3 pH balance The body must maintain mass balance: Input + cellular production = cellular consumption + excretion Excretion route is mainly via the kidney What comes into the body in excess of needs must be excreted Fluid and electrolyte balance involves INTEGRATION of respiratory, renal and cardiovascular systems 5  Water is the most abundant Maintaining Water Balance compound in the body! ~30 (F) -42 (M) Litres The kidneys can conserve or remove body fluid. They do so by regulating the amount of H2O reabsorbed Figs. 20.2 & 20.3 6 Filtrate osmolarity changes through the nephron Fig. 20.4 Final osmolarity (concentration) of urine depends on hormonal regulation of H2O reabsorption in the distal nephron 7 Vasopressin (ADH) influences H2O reabsorption Hormone released by the posterior pituitary gland – Also known as Anti-Diuretic Hormone (ADH) Stimuli for release: Fig. 20.6 – High plasma osmolarity – Low blood volume – Low blood pressure Controls the addition of water pores (aquaporin-2; AQP2) into the apical membrane of collecting duct cells – Result: Increased water reabsorption & more concentrated urine – Response may be graded and depends on amount of hormone released 8 Stimuli for Vasopressin Release This is a good example of INTEGRATION 9 Fig 20.6 Aldosterone regulates Na+ reabsorption Aldosterone: – Steroid hormone – Synthesized in & released from the adrenal cortex – Acts on principal (P) cells of the distal tubule & collecting duct Results in increased Na+ reabsorption Main stimuli for Aldosterone release: – Angiotensin II (via low blood pressure and the Renin- Angiotensin-System; RAS) − Hyperkalemia (High [K+] in plasma) Fig. 20.9 10 Low blood pressure stimulates renin secretion via 3 routes 11 Fig. 20.10 The Renin-Angiotensin-Aldosterone System Low BP OR Low BV Angiotensinogen The liverTis continually producing and ANG I secreting angiotensinogen into the blood ACE ACE = Angiotensin-converting ANG II enzyme - located on all vascular Aldosterone release from adrenal cortex endothelial cells Increased Na+ reabsorption ANG II has many other targets! - Cardiovascular control center: increase CO - Arterioles: ANG II is a powerful vasoconstrictor - Hypothalamus: increase Vasopressin release - Hypothalamus: induce thirst 12 - Hypothalamus: salt appetite See Fig. 20.10 Low BP & the Renin-Angiotensin System Fig 20.10 13 Acid-Base Balance Fig. 20.14 14 Maintaining pH Homeostasis The body uses 3 mechanisms to maintain normal pH 1. Buffers cellular proteins, hemoglobin, phosphates, HCO3- 2. Regulation of Ventilation (rate & depth) Regulates blood PCO2 (and therefore H+) 3. Kidneys Regulation of H+ and HCO3- secretion/reabsorption CA CO2 + H2O H2CO3 H+ + HCO3- 15 Maintaining pH Homeostasis àRespiratory Compensation (Review) áPlasma PCO2 or H+ âPO2 2 stimuli: – Increased H+ – Increased PCO2 Respiratory compensation for changes in pH is immediate CA Fig. 20.15 CO2 + H2O H2CO3 H+ + HCO3- 16 Maintaining pH Homeostasis à Renal Compensation H+ secreted and HCO3- reabsorbed at the proximal tubule Fine regulation of acid-base balance occurs at the collecting duct – Intercalated (I) cells (interspersed between P cells) Contain high levels of carbonic anhydrase Type A I cells secrete H+ and reabsorb HCO3- – Response to acidosis Type B I cells secrete HCO3- and reabsorb H+ – Response to alkalosis 17 Calcium in the Body 18 Structure of Bone Composition of Bone Bone is composed largely of calcified extracellular matrix. Epiphysis is the end of a long bone. Compact bone is dense and used for support. Epiphyseal plate Blood vessels is the site of bone growth. Spongy bone or trabecular bone forms a calcified Diaphysis is lattice. the shaft of a long bone. Bone marrow Bone is remodeled throughout life Calcified extracellular matrix called hydroxyapatite: Ca10(PO4)6(OH)2 Osteoblasts synthesize bone Osteoclasts resorb bone Both are important for calcium homeostasis 19 Calcium Homeostasis Three main mechanisms regulate calcium homeostasis in the body : 1. Parathyroid hormone (PTH): increases calcium resorption from bone, increases calcium reabsorption (kidney), increases calcium absorption (GI) 2. Calcitriol (Vit D3): increases calcium absorption (GI) 3. Calcitonin: increases deposition of calcium in bone (bone formation) 20 Parathyroid Hormone 21 Calcitriol (Vitamin D3) 22 Calcitonin Inhibits osteoclasts Decreases renal reabsorption Minor role in adults 23 Calcium Homeostasis Summary: PTH increases reabsorption of Ca2+ that is filtered by the kidney, increases bone resorption and increases calcitriol levels Calcitriol increases absorption of Ca2+ from the GI and increases resorption from bone Calcitonin increases bone formation and decreases reabsorption of Ca2+ that is filtered by the kidney 24 Lecture Review Questions Chapter 20, Level 1 Questions 1-6, 11-16 Chapter 20, Level 2 Questions 20-22 Chapter 23 Level 1 Question 6 Chapter 23 Level 2 Question 12 25