MLAB 2401: Clinical Chemistry Keri Brophy-Martinez Renal Unit :Overview
Dec 14, 2015
Renal Physiology
• Three renal processes– Glomerular filtration– Tubular reabsorption– Tubular secretion
Glomerular Filtration
• Glomerulus filters incoming blood– How?
• High pressure in the glomerular capillaries sets up a pressure difference
• Semipermeable glomerular basement membrane allows small dissolved solutes, which have a positive charge to pass
• Glomerular filtrate, which is cell-free, protein free is made and can be measured by the GFR( glomerular filtration rate)
Tubular Reabsorption
• Substance movement from tubular lumen to peritubular capillary plasma for regulation of concentration
– Assists in conservation of nutrients– Prevents electrolyte loss– Assists in acid-base imbalance
• Occurs by Active and Passive transport– Active Transport
• Tubular epithelial cells use energy to bind and transport substances across plasma membrane to blood
• Transports Glucose, Amino acids, Proteins, Electrolytes• Maintains the sodium pump to prevent the cells from swelling up
– Passive Transport• No energy requirement• Substances, such as water and urea, move from higher concentrations to lower
concentrations
Tubular Reabsorption
• Occurs in proximal convoluted tubule– Receives the glomerular filtrate– Return the bulk of substances back to blood circulation
• Proteins, glucose completely reabsorbed• Sodium, chloride partially reabsorbed• Creatinine no reabsorption
– Secretes products of kidney tubular metabolism, such as hydrogen ions and drug
Tubular Secretion
• Occurs in the proximal convoluted tubule• How is this used?
– Movement of substances from peritubular capillary plasma to tubular lumen that were not previously eliminated
– Secretion of products from tubule cell cellular metabolism into filtrate, such as hydrogen
Distal Convoluted Tubule
• Most of reabsorption complete• Function
– Make small adjustments to benefit electrolyte and acid-base balance
– Adjustments occur under control of ADH (antidiuretic hormone) and aldosterone
Functions of the Kidney
• Urine formation• Fluid and electrolyte balance• Regulation of acid-base balance• Excretion of waste products of
protein metabolism• Excretion of drugs and toxins• Secretion of hormones
Water Balance
• Kidney assists with water balance through water loss or water conservation
• Water balance regulated by ADH/AVP– Secretion of ADH
• Increased plasma osmolality or decreased intravascular volume
– Action of ADH• ADH increases the permeability of the distal
convoluted tubules and collecting ducts to water• Results in water reabsorption and concentration
of urine• Activates thirst mechanism
Aldosterone
• Origin– Adrenal cortex– Influenced by renin-angiotension system
• Secretion– Decreased blood flow or blood pressure– Decreased plasma sodium
• Action– Stimulates sodium reabsorption in distal tubules and
potassium and hydrogen ion secretion– Increases water retention– Raises blood pressure
Acid-Base Balance
• Renal system assists with constant control of overall pH– Conserve bicarbonate ions– Secretes H+ ions attached to
phosphate– Secretes H+ ions attached to
ammonia
Elimination of Nonprotein Nitrogen Compounds
• Waste products formed by degradative metabolism of proteins, amino acids and nucleic acids
• Main players are urea, creatinine and uric acid– Urea
• 75% of the nonprotein nitrogens excreted daily• By product of ammonia breakdown• 40-60% reabsorbed
– Creatinine• Formed from creatine, which is found in muscle• Not reabsorbed by the tubules
– Uric acid• Waste product of purine metabolism (nucleic acid)
– Ammonia• Product of amino acid and protein catabolism
Endocrine Functions
• Produces hormones– Renin– Erythropoietin
– 1,25- dihydroxy vitamin D 3
– Prostaglandins
• Target for certain hormones produced by other endocrine glands
Hormones: Renin
• Released by juxtaglomerular cells• Stimulated by decreases in fluid volume
or blood pressure • Effect of:
– Promote sodium reabsorption and water conservation
– Promotes secretion of aldosterone• Bottom line: Increase blood pressure
Hormones: Erythropoietin
• Made by kidney• Production regulated by blood
oxygen levels (hypoxia)
• Effect of:– Stimulate RBC production
Hormones: 1,25-Dihydroxy Vitamin D3
• Made in the kidneys• Active form of vitamin D• Determines phosphate and
calcium balance and bone calcification in the human body
Hormones: Prostaglandins
• Fatty acids formed from dietary fatty acids
• Produced in the kidney• Function
– Increase renal blood flow (vasodilator)– Sodium and water excretion– Antagonistic to renin release