Renal lecture 3
Acid-Base balance
Regulation of plasma calcium
Control of micturition
Regulation of [H+]
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Acid-Base Balance
• Arterial pH less than 6.8 or greater than
8.0 is not compatible with life
• Acidosis
– Exists when blood pH falls below 7.35
• Alkalosis
– Occurs when blood pH is above 7.45
Comparison of pH Values of
Common Substances
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Acid-Base Balance
• Consequences of fluctuations in pH
– Changes in excitability of nerve and muscle
cells
– Marked influence on enzyme activity
– Changes influence K+ levels in body
Lines of Defense Against pH Changes
• Chemical buffer systems
• Respiratory system
• Kidneys
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• The kidneys help remove excess chemicals from the blood.
It is the kidneys that ultimately remove (from the body) H+ ions and other
components of the pH buffers that build up in excess. Acidosis that results from
failure of the kidneys to perform this excretory function is known as metabolic
acidosis. However, excretion by the kidneys is a relatively slow process, and
may take too long to prevent acute acidosis resulting from a sudden decrease in
pH (e.g., during exercise).
• The lungs provide a faster way to help control the pH of the blood.
The increased-breathing response to exercise helps to counteract the pHlowering effects of exercise by removing CO2, a component of the principal pH
buffer in the blood. Acidosis that results from failure of the lungs to eliminate CO2
as fast as it is produced is known as respiratory acidosis.
Chemical Buffer Systems
• Minimize changes in pH by binding with or yielding free
H+
• First line of defense
• Body has four buffer systems
– HCO3- buffer system
• Primary ECF buffer
- Protein buffer system
• Primary ICF buffer; also buffers ECF
– Hemoglobin buffer system
– Phosphate buffer system
• Important urinary buffer; also buffers ICF
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Respiratory System
• Second line of defense again changes in
pH
• Acts at a moderate speed
• Regulates pH by controlling rate of CO2
removal
• Carbon dioxide in the blood is converted to
bicarbonate ion and transported in the
plasma
• Increases in hydrogen ion concentration
produces more carbonic acid
• Excess hydrogen ion can be blown off with
the release of carbon dioxide from the lungs
• Respiratory rate can rise and fall depending
on changing blood pH
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Kidneys
• Third line of defense against change in hydrogen
ion concentration
• Kidneys require hours to days to compensate for
changes in body-fluid pH
• Control pH of body fluids by adjusting
– H+ elimination in urine (secretion)
– HCO3- reabsorption and synthesis
– Ammonium secretion
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CO2
HCO3- + H+
HCO3-
H+ : Phosphoric
acid, NH4+
Acid-Base Imbalances
• Can arise from either respiratory
dysfunction or metabolic disturbances
• Deviations divided into four general
categories
– Respiratory acidosis
– Respiratory alkalosis
– Metabolic acidosis
– Metabolic alkalosis
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Respiratory Acidosis
• Result of abnormal CO2 retention arising from
hypoventilation
• Possible causes
–
–
–
–
Lung disease
Depression of respiratory center by drugs or disease
Nerve or muscle disorders that reduce respiratory muscle activity
Holding breath
• Compensations
– Chemical buffers immediately take up additional H+
– Kidneys are most important in compensating for respiratory
acidosis
Respiratory Alkalosis
• Primarily due to excessive loss of CO2 from body as result of
hyperventilation
• Possible causes
–
–
–
–
Fever
Anxiety
Aspirin poisoning
Physiologic mechanisms at high altitude
• Compensations
– Chemical buffer systems liberate H+
– If situation continues a few days, kidneys compensate by conserving H+
and excreting more HCO3-
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Metabolic Acidosis
• Includes all types of acidosis other than those caused by excess
CO2 in body fluids
• Causes
–
–
–
–
Severe diarrhea
Diabetes mellitus
Strenuous exercise
Uremic acidosis
• Compensations
– Buffers take up extra H+
– Lungs blow off additional H+ generating CO2
– Kidneys excrete more H+ and conserve more HCO3-
Metabolic Alkalosis
• Reduction in plasma pH caused by relative deficiency of
noncarbonic acids
• Causes
– Vomiting
– Ingestion of alkaline drugs
• Compensations
– Chemical buffer systems immediately liberate H+
– Ventilation is reduced
– If condition persists for several days, kidneys conserve H+ and
excrete excess HCO3- in the urine
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Patient evaluation The presence of an acid-base disturbance may be
suspected on the basis of clinical presentation or by results of laboratory data
(eg, a low HCO3-). Evaluation of any acid-base disorder can then be
approached in a stepwise manner
ACID-BASE Parameters
Vit D and the kidney
-conversion of Vit D to calcitriol
-Enhances Ca2+ absorption
-Increases Ca2+ in plasma
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Kidney, bone and calcium balance
99% of Ca2+ in bone
PTH - parathyroid hormone
Calcitonin - thyroid gland
Urine Transport,
Storage, and Elimination
Takes place in the urinary tract:
– ureters
– urinary bladder
– Urethra
Normal Urine
- Is a clear, sterile solution
- Yellow color (pigment urobilin) generated in kidneys
from urobilinogens
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Micturition (Urination)
bladder fills
detrusor muscles
stretch
stretch receptors
(involuntary)
micturition reflex
(involuntary)
• spinal reflex
• can be overridden voluntarily
(voluntary)
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3 Micturition Reflex Problems
1.
–
2.
3.
Sphincter muscles lose tone:
leading to incontinence
Control of micturition can be lost due to:
–
a stroke
–
Alzheimer’s disease
–
CNS problems affecting cerebral cortex or
hypothalamus
In males, urinary retention may develop if enlarged
prostate gland compresses the urethra and restricts urine
flow
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Renal system and ageing
• kidneys appear scarred and grainy
• kidney cells die
• by age 80, kidneys have lost a third of their mass
• kidney shrinkage due to loss of glomeruli
• proteinuria may develop
• renal tubules thicken
• harder for kidneys to clear certain substances
• bladder, ureters, and urethra lose elasticity
• bladder holds less urine
Dialysis
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Renal transplantation
• At the end of this lecture you should
Understand the importance of maintaining a physiological pH
Be familiar with the chemical, respiratory and renal mechanisms
that maintain pH
Be familiar with the role of the kidney in the regulation of plasma
calcium concentration.
Understand the micturition reflex
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