Regulation of water and Electrolyte
Balance (III)
Calcium & acid-base balance
Lecture Outline
I. 
II. 
Calcium balance
• 
Renal handling of calcium ions
• 
Hormonal control of plasma concentrations
Acid-base balance
• 
• 
• 
Essentials
Disturbances
Defense mechanisms
Fluid Head and Metabolism. Calcium & acid-base balance.
1
I. Calcium Balance
•  Hypercalcemia = high plasma calcium
•  Hypocalcemia = low plasma calcium
•  Calcium balance critical because
•  Triggers exocytosis
•  Triggers secretion
•  Triggers muscle contraction
•  Increases contractility of cardiac and smooth muscle
Fluid Head and Metabolism. Calcium & acid-base balance.
Routes of Calcium Exchange
• 
• 
Organs involved: Kidneys, Digestive tract, Bone, Skin
Hormones involved: Parathyroid hormone, Calcitriol
(Vitamin D3), Calcitonin
G & S: Figure 19.20
Fluid Head and Metabolism. Calcium & acid-base balance.
2
Renal Handling of Calcium
•  Blood calcium
- Bound to carrier proteins, Ca2+ + Protein  Ca-Protein
- Free calcium in plasma—freely filtered at glomerulus
•  70% reabsorbed in proximal tubules
•  19–20% reabsorbed in thick ascending limbs
of the loops of Henle
•  9–10% reabsorbed in distal tubules
•  Reabsorption in loops of Henle and distal tubules is regulated
Fluid Head and Metabolism. Calcium & acid-base balance.
Parathyroid Hormone Increases Plasma Calcium
•  PTH released from parathyroid glands
•  Stimulus = decrease Ca2+ in plasma
•  Actions
•  Increase Ca2+ reabsorption by kidneys
•  Stimulates activation of calcitriol in kidneys which:
•  Stimulates Ca2+ absorption in digestive track
•  Ca2+ reabsorption in kidneys
•  Stimulates resorption of bone
•  Stimulates small increase in calcium absorption
•  Overall effect: increase blood calcium
Fluid Head and Metabolism. Calcium & acid-base balance.
3
PTH and Calcium Balance
[Ca2+] in plasma
Parathyroid glands
PTH secretion
Negative
feedback
[PTH] in plasma
Kidneys
Phosphate reabsorption
Ca2+ reabsorption
Calcium excretion
in urine
Bone
1,25-(OH2)D3 activation
Ca2+ resorption
[1,25-(OH2)D3] in plasma
Negative
feedback
Kidneys
Gastrointestinal tract
Ca2+ absorption
[Ca2+] in plasma
G & S: Figure 19.21
Fluid Head and Metabolism. Calcium & acid-base balance.
II. Acid-Base Balance
•  Normal pH of arterial blood = 7.35–7.45
•  pH < 7.35 = acidosis
•  pH > 7.45 = alkalosis
•  Complications with acid-base disturbance
•  Conformation change in protein structure
•  Changes in excitability of neurons
•  Changes in potassium balance
•  Cardiac arrhythmias
•  Vasodilation
Fluid Head and Metabolism. Calcium & acid-base balance.
4
Inputs and Outputs of Acid
G & S: Figure 19.23
Fluid Head and Metabolism. Calcium & acid-base balance.
Respiratory Disturbances
Carbon dioxide is a source of acid
CA
CO2 + H2O  H2CO3  HCO3- + H+
•  Normal PCO2 arterial blood = 40 mm Hg
•  Sources of CO2: metabolism
•  Output of CO2: through respiratory system
•  Increases in plasma [CO2]  respiratory acidosis
•  Decreases in plasma [CO2]  respiratory alkalosis
Fluid Head and Metabolism. Calcium & acid-base balance.
5
Metabolic Acidosis /
Decrease pH through
something other than
carbon dioxide
•  High protein diet
•  High fat diet
•  Heavy exercise
•  Severe diarrhea (loss
of bicarbonate)
Metabolic Alkalosis
Increase pH through
something other than
carbon dioxide
• Excessive vomiting (loss
of hydrogen ions)
• Consumption of alkaline
products (baking soda)
• Renal dysfunction
•  Renal dysfunction
Fluid Head and Metabolism. Calcium & acid-base balance.
Defense Mechanisms Against
Acid-Base Disturbances
Three lines of defense
•  Buffering of hydrogen ions
•  Respiratory compensation
•  Renal compensation
Fluid Head and Metabolism. Calcium & acid-base balance.
6
Buffering
•  Quickest defense against changes in pH
•  Most important ECF buffer = bicarbonate
•  HCO3- + H+  H2CO3
•  ICF Buffers
•  Proteins: Protein- + H+  H•Protein
•  Phosphates: HPO42- + H+  H2PO4-
Fluid Head and Metabolism. Calcium & acid-base balance.
Respiratory Compensation
•  Second line of defense
•  Takes minutes to
have effect
•  Regulates pH by varying
ventilation
•  Increase
ventilation 
decreases CO2
•  Decrease
ventilation 
increases CO2
G & S: Figure 19.24
Fluid Head and Metabolism. Calcium & acid-base balance.
7
Renal Compensation
•  Third line of defense
•  Takes hours to days
•  Regulate excretion of hydrogen ions and
bicarbonate in urine
•  Regulate synthesis of new bicarbonate in renal
tubules
Fluid Head and Metabolism. Calcium & acid-base balance.
Renal Handling of Hydrogen and Bicarbonate Ions:
a) Proximal tubule
• Proximal tubule: Bicarbonate reabsorption coupled to
hydrogen ion secretion
G & S: Figure 19.25
Fluid Head and Metabolism. Calcium & acid-base balance.
8
b) Distal tubule and collecting duct
• Distal tubule and collecting duct: Secretion of hydrogen ions
coupled to synthesis of new bicarbonate ions
G & S: Figure 19.26
Fluid Head and Metabolism. Calcium & acid-base balance.
Severe acidosis: Glutamine Metabolism in Proximal
Tubules
Glutamine metabolism to produce new bicarbonate and secrete
hydrogen in form of ammonium
G & S: Figure 19.27
Fluid Head and Metabolism. Calcium & acid-base balance.
9