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Summary - FLG 221 (Flg221)
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THEME 1: RENAL PHYSIOLOGY AND ACID-BASE EQUILIBRIUM
1.1: OVERVIEW OF THE FUNCTIONS OF THE RENAL SYSTEM
1. kidney’s role in regulating the composition of body fluids
Urinary functions
- AIM: homeostasis in body fluids
➢ Regulate ECF volume, osmolarity and electrolyte balance by handling water and
electrolytes
• Osmolarity is caused by different substances (ions: Na+, K+, Cl-) in the fluids.
➢ Regulate acid-base balance by controlling H+ and HCO3- levels
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• Acid-base balance: pH of ECF.
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• pH is determined by concentration of hydrogens (high H+ = low pH= acidic).
➢ Excrete metabolic end-products and foreign substances like urea, uric acid and
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creatinine
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• Urea: metabolic end product of amino acids. Amino acids have nitrogen atoms
which cannot be kept in the body and need to be excreted via urea.
• Uric acid: metabolic end product of nucleic acids.
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• Creatinine: From creatine phosphate found in muscle cells
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➢ Retain adequate amounts of essential substances.
• Filtrate initially contains all substances that are still essential, and the kidneys
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will reabsorb these substances and not excrete them in the urine.
Multiple functions of the kidneys
- Rid the body of waste materials (ingested or produced by metabolism)
- Control volume and electrolyte composition of the body fluids.
- Maintain stable internal environment for cells to perform activities.
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- Regulate body fluid osmolality and electrolyte concentrations
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- Regulate water and electrolyte balances.
➢ To maintain homeostasis: water excretion should match electrolyte intake.
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➢ Intake of water and electrolytes is governed by a person’s eating and drinking habits,
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requiring kidneys to adjust excretion rate to match intakes of electrolytes.
- Regulate arterial pressure
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- Regulate acid-base balance
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, - Regulate erythrocyte production
- Secrete, metabolize and excrete hormones
- Gluconeogenesis/glucose synthesis
➢ Kidneys produce glucose from amino acids and other precursors during
gluconeogenesis (prolonged fasting)
➢ During complete renal failure, potassium, acids, fluid and other substances
accumulate in the body and cause death unless hemodialysis is initiated to restore
body fluid and electrolyte balances.
2. kidney’s role in maintaining pH balance
Acid-base: pH
- pH (potential of hydrogen/power of hydrogen): Scale used to specify the acidity or
basicity
➢ scale runs from 1 to 14
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• >7: Acidic
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• =7: neutral
• <7: Basic
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➢ ECF pH is between 7.35 & 7.45.
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• Water = pH 7
• ECF pH <7.35: Acidosis Pathologic conditions but the body has various mechanisms
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• ECF pH> 7.45: Alkalosis to regulate the pH and keep it between the mentioned range
- Lower pH values→ Acidic solutions
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- Higher pH values → Basic or alkaline solutions
- The pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in
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the solution (a lower pH indicates a higher concentration of hydrogen ions and vice
versa)
➢ An abnormal increase in hydrogen ions/drop in pH is dangerous because
protons/hydrogen ions have a positive charge and will bind to proteins (which carry
any negative charges), change the proteins’ conformation → Change their activity.
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Origin of hydrogen ions in the body
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- Origin of H+-ions in the body:
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➢ Mainly produced from metabolism of carbohydrates, fats and proteins.
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➢ Some from diet → intake of Acidic foods
- Either volatile or non-volatile acids:
➢ Volatile acids from CO2 produced in metabolism of carbohydrates and fats:
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, • Upon hydration, volatile acids produce hydrogen ions
➢ Non-volatile acids from anything other than Co2
• Eg) amino acids with sulfur atoms → produce Sulphuric acid and hydrochloric
acid, and in conditions of low insulin and oxygen/O2 → Lactic acid
The role of the kidneys and lungs in acid-base balance
- HA: non-volatile acids (net endogenous acid production)
- NaA: sodium salt of non-volatile acid RNAE
Volatile acids
- renal net acid excretion
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non-volatile acids
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- Fats and carbohydrates are broken down to produce ATP (krebs cycle and oxidative
phosphorylation)
➢ Oxygen and insulin are required
➢ Water and carbon dioxide (volatile acid) are produced from the ATP making
processes
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➢ When there are low oxygen levels (during exercise for example), non-volatile acids
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can be produced (one of them being lactic acid)
• Non-volatile acids are produced from protein breakdown
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- The lungs get rid of the carbondioxide/volatile acids
➢ More CO2 o be removed → Fast breathing
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➢ Less CO2 to be removed→ Slow breaths
- The kidneys can get rid of nonvolatile acids
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➢ They can excrete hydrogen ions (Renal net acid excretion)
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, - Buffers (eg: NaHCO3) can be used to buffer the acid but they need to be
replaced/refilled
➢ Organic anions like Citrate can be used to form bicarbonate ions (HCO3)
- Every day, bicarbonate ions are lost in the feces, and this is the same as increasing the
amount of volatile acids
➢ Because if there is less bicarbonate to buffer, the nonvolatile acids will increase.
Defence mechanisms against fluctuations in pH
pH of ECF is maintained within narrow range 7.35-7.45
- 1. Extracellular and intracellular buffering
➢ Operates very quickly/immediately
• When the hydrogen ions increase, they can get bound to a base (eg: Bicarbonate
anion) and this neutralizes the hydrogen ions
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- 2. adjustments in pCO2 (partial pressure of CO2) by alterations in the ventilatory rate of
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the lungs
➢ Increase in breathing/ventilatory rate causes more CO2 to be exhaled (and vice
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versa).
➢ Also operates fairly quickly (not as fast as buffer system)
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- 3. adjustments in RNAE (Renal net acid excretion)
➢ This system takes a bit longer to operate.
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3. kidney’s role in regulating arterial blood pressure
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- They regulate arterial blood pressure by excreting variable amounts of sodium and
water.
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- They contribute to short term arterial pressure regulation by secreting hormones and
vasoactive factors or substances (ie: renin) which form vasoactive products (ie:
angiotensin II)
- Regulation of 1,25-Dihydroxyvitamin D3 production
➢ They produce calcitriol (1,25-dihydroxyvitamin D3)/active form of vitamin D, by
hydroxylating vitamin D at number 1 position.
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➢ Calcitriol is needed for normal calcium deposition in bone and calcium reabsorption
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by the GI tract.
➢ Calcitriol plays a role in calcium and phosphate regulation.
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4. kidney’s role in ridding the body of waste products and foreign chemicals
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