MEDS2001 Key Concepts of Physiology Module 5: Fluid Balance Notes

Lecture: The Kidney as a filter Functions of the Kidneys

- Regulation of extracellular fluid volume and blood pressure - Regulation of plasma osmolality - Maintenance of plasma ionic composition - Regulation of plasma pH • Excretion of wastes - Production of hormones

- Erythropoietin • 1,25-(OH)2-Vitamin D3 - Renin – leading to increased Angiotensin II and Aldosterone

- Gluconeogenesis

Fuctional unti of Kidneys: Nephrons

- Blood supply from afferent arteriole and out through the efferent arteriole → GLomerula capillary

Fundamentals

- Filtration → Fluid and solutes move from the blood to the Bowman’s capule - Reabsorption → back into the blood - Secretion → From blood to lumen - Excretion

Renal Clearance is the equivalent volume of plasma from which a substance is completely removed by kidney per unit time.

- Determines whether the kidney is functioning or not.

Inulin clearance as a measure of GFR

- Glomerular FIltration rate is measured by inulin which is not produced within the body.

- It is used as it is freely filtered and not reabsorbed. Therefore, whatever enters is whatever exits via excretion

Creatinine is easier to use than insulin

- Creatine is produced in muscle from creatine and is also freely filtered and gets fully excreted.

- Production rate depends on: - Body weight - fatter, lower mass due to muscle loss - Age - less muscle mass when younger - Gender - females have less - Ethnicity - Diet

- Formulas are available to take these factors into account - As GFR decreases, plasma creatine increases as less is getting filtered from the body →

therefore, GFR is not efficient Filtration Fraction

- Renal blood flow (RBF) = ~1.2 L/min - Renal plasma flow (RPF) = ~600 ml/min - GFR = ~125 ml/min - Filtration Fraction = GFR/RPF = 20% - The kidneys produce 180 L/day of

filtrate, and reabsorb 179 L of it - 1L excreted - depends on H2O intake