Overview
This lecture covers the structure and function of the kidney, emphasizing key processes and hormonal regulation relevant for MCAT testing.
Major Functions of the Kidney
- Filters blood to remove waste and excess substances.
- Regulates blood pressure and blood volume for fluid homeostasis.
- Maintains electrolyte (salt and water) balance.
- Controls acid-base (pH) balance by excreting hydrogen ions and using the phosphate buffer system.
- Produces erythropoietin, which stimulates red blood cell production in bone marrow.
- Activates vitamin D, essential for calcium absorption.
Kidney Structure and Nephron Anatomy
- The kidney is located in the lower abdomen with a cortex (outer) for filtration and medulla (inner) for urine concentration.
- The nephron is the kidney's functional unit, with thousands per kidney.
Nephron Processes
- Filtration: Occurs in the glomerulus, where blood is filtered by Starling forces (hydrostatic and oncotic pressure).
- Reabsorption: At the proximal and distal convoluted tubules, nutrients like glucose and amino acids are reabsorbed into the body.
- Secretion: Waste products and ions (e.g., potassium, urea) are secreted into the filtrate.
- Excretion: Final urine is excreted via the ureter after passing through the collecting duct.
Sodium-Potassium Pump and Hormonal Control
- Sodium-potassium ATPase pumps reabsorb sodium and secrete potassium, crucial for neuron function and blood pressure.
- Aldosterone (steroid hormone from adrenal glands) increases sodium reabsorption and potassium secretion, raising blood pressure over time.
- High sodium diets can contribute to hypertension due to limited mechanisms for sodium excretion.
Loop of Henle and Collecting Duct
- The Loop of Henle creates a concentration gradient (countercurrent multiplier system) in the medulla, enabling passive water reabsorption.
- Ascending limb: Pumps out sodium, impermeable to water.
- Descending limb: Permeable to water, impermeable to salts.
- Collecting duct reabsorbs water through aquaporins, concentrating urine.
Hormonal Regulation of Water Reabsorption
- Antidiuretic hormone (ADH/vasopressin, a peptide hormone) opens aquaporins in the collecting duct, allowing rapid water reabsorption.
- Aldosterone and ADH both increase blood pressure but use different mechanisms (steroid vs peptide, chronic vs acute response).
Key Terms & Definitions
- Nephron — Functional unit of the kidney, responsible for filtration, reabsorption, secretion, and excretion.
- Filtrate — Fluid filtered from blood entering the nephron, destined for excretion unless reabsorbed.
- Glomerulus — Capillary network in the nephron where filtration occurs.
- Bowman’s Capsule — Surrounds the glomerulus; collects filtrate.
- Aldosterone — Steroid hormone that increases sodium reabsorption and potassium secretion in the nephron.
- Antidiuretic Hormone (ADH)/Vasopressin — Peptide hormone that increases water reabsorption by opening aquaporins.
- Aquaporins — Water channels in the collecting duct membrane.
- Countercurrent Multiplier System — Process in the Loop of Henle that concentrates solutes in the medulla to favor water reabsorption.
- Erythropoietin — Hormone from the kidney that stimulates red blood cell production.
Action Items / Next Steps
- Memorize the six major kidney functions for the MCAT.
- Review the structure and processes of the nephron, especially locations of filtration, reabsorption, secretion, and excretion.
- Study hormonal regulation (aldosterone vs ADH), their mechanisms, and effects.
- Understand the role of the Loop of Henle in urine concentration.