Kidney Functions and Nephron Roles

Overview

Lecture on the urinary system (Chapter 26) focusing on kidney functions and nephron processes.
Emphasis on filtration, reabsorption, minor secretion, and systemic regulation roles.

Nephrons are the functional units where urine formation occurs.
Main processes:
- Filtration: pressure-driven movement of fluid and small solutes out of blood; size-dependent.
- Reabsorption: recovery of needed “good” substances filtered out.
- Secretion: smaller role; adds additional solutes to filtrate.
Filtration removes both desirable and waste solutes; reabsorption adjusts to conserve needed amounts.

Excretion of wastes and foreign substances via filtration and selective reabsorption.
Regulation of ionic composition:
- Adjusts reabsorption of electrolytes (e.g., sodium, chloride, calcium) based on needs.
- Can increase or decrease conservation depending on dietary intake.
Acid-base balance:
- Maintains blood pH through renal compensation.
- Specialized cells conserve or excrete hydrogen ions based on pH status.
Osmolarity and water balance:
- Water follows solutes; kidneys maintain body fluid osmolarity.
- Urine can be four times more concentrated or more dilute than plasma.
- Regulates water conservation or expulsion to control blood volume.
Blood pressure regulation:
- Directly via blood volume control.
- Indirectly via renin secretion initiating the RAAS pathway.

Function	Mechanism/Process	Outcome
Excretion of wastes	Filtration of blood; limited secretion	Removal of metabolic wastes and foreign substances
Ion composition regulation	Adjusted reabsorption of electrolytes	Maintains proper sodium, chloride, calcium levels
Acid-base balance	Renal compensation via H+ handling	Stabilizes blood pH
Osmolarity control	Coupled solute-water handling	Proper body fluid osmolarity
Blood volume control	Variable water reabsorption	Increases or decreases blood volume
Blood pressure regulation	Volume changes; renin release (RAAS)	BP adjusted up or down
Erythropoiesis support	Erythropoietin secretion	Increases RBC production
Calcium homeostasis	Calcitriol activation	Enhances dietary calcium absorption

Nephron: functional unit of the kidney where urine forms.
Filtration: pressure-driven movement of fluid and small solutes from blood; size-dependent.
Reabsorption: return of needed substances from filtrate to blood.
Secretion: transport of substances from blood into filtrate.
Renal compensation: kidney-adjusted H+ handling to maintain pH.
Osmolarity: concentration of solute particles in body fluids.
Renin: enzyme initiating RAAS to regulate blood pressure.
Erythropoietin: hormone stimulating red blood cell production.
Calcitriol: active vitamin D; promotes calcium absorption.