MLAB 2401: Clinical Chemistry Keri Brophy-Martinez

Renal Unit :Overview

Kidney physiology

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

How does it work?

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

References

• Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins.

• Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .

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