Urinary System: Structure, Function, and Regulation

Urinary System Overview

Functions of the Kidney

• Excretion of metabolic wastes: The kidneys remove urea, creatinine, uric acid, and drugs from the blood.

• Regulation of blood volume and pressure: By adjusting water excretion and releasing renin, kidneys help control blood pressure.

• Electrolyte balance: Kidneys regulate sodium, potassium, calcium, and other ions.

• Acid-base balance: Kidneys excrete hydrogen ions and reabsorb bicarbonate to maintain pH.

• Hormone production: Kidneys produce erythropoietin (stimulates RBC production) and activate vitamin D.

Nephron Structure and Capillary Networks

Nephron Capillary Networks

• Afferent arteriole: Brings blood to the glomerulus.

• Efferent arteriole: Carries blood away from the glomerulus.

• Difference: The afferent arteriole has a larger diameter than the efferent arteriole, creating higher pressure in the glomerulus to drive filtration.

• Renal corpuscle: Consists of the glomerulus and Bowman's capsule; site of blood filtration.

• Tubule segments: Proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

Filtration Membrane Structure

Layers and Cell Types

• Filtration membrane: Composed of three layers:

  • Fenestrated endothelium of glomerular capillaries

  • Basement membrane

  • Podocyte foot processes (visceral layer of Bowman's capsule)

• What is filtered: Water, glucose, amino acids, ions, and small molecules pass through; blood cells and most proteins do not.

Urine Formation

Normal and Abnormal Components

• Normal urine: Contains water, urea, creatinine, uric acid, ions (Na+, K+, Cl-), and small amounts of other substances.

• Abnormal urine: Presence of glucose, proteins, blood, or ketones may indicate disease.

Steps in Urine Formation

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

Filtration at Bowman's Capsule

• Substances filtered: Water, electrolytes, glucose, amino acids, and small molecules.

Glomerular Filtration Rate (GFR) and Regulation

Factors Influencing GFR

• Hydrostatic pressure: Higher pressure in glomerular capillaries increases filtration.

• Osmotic pressure: Opposes filtration; due to plasma proteins in capillaries.

• Diameter of arterioles: Constriction or dilation of afferent/efferent arterioles alters GFR.

Net Filtration Pressure (NFP)

• Definition: The pressure driving fluid out of the glomerulus into Bowman's capsule.

• Equation:

$\text{NFP} = (\text{Glomerular hydrostatic pressure}) - (\text{Capsular hydrostatic pressure} + \text{Blood colloid osmotic pressure})$

• Increase in GFR: Caused by increased glomerular hydrostatic pressure or decreased plasma protein concentration.

• Decrease in GFR: Caused by decreased blood pressure, increased capsular pressure, or increased plasma proteins.

Regulation of GFR

• Autoregulation: Myogenic mechanism and tubuloglomerular feedback maintain stable GFR.

• Hormonal regulation: Renin-angiotensin-aldosterone system (RAAS) increases GFR during low blood pressure; atrial natriuretic peptide (ANP) increases GFR during high blood volume.

• Neural regulation: Sympathetic stimulation constricts afferent arterioles, reducing GFR during stress.

Tubular Reabsorption and Secretion

Reabsorption in the Proximal Convoluted Tubule (PCT)

• Major site of reabsorption: About 65% of filtrate is reabsorbed here, including glucose, amino acids, and ions.

• Mechanisms: Active transport, facilitated diffusion, and osmosis.

Countercurrent Multiplication and Exchange

• Countercurrent multiplication: Occurs in the loop of Henle; creates an osmotic gradient in the medulla for water reabsorption.

• Countercurrent exchange: Occurs in the vasa recta; maintains the medullary osmotic gradient.

Hormonal Regulation

• Antidiuretic hormone (ADH): Increases water reabsorption in the collecting ducts.

• Aldosterone: Increases sodium reabsorption in the distal tubule and collecting duct.

• Atrial natriuretic peptide (ANP): Inhibits sodium reabsorption, increasing urine output.

Effects of Low/High ADH

• Low ADH: Produces dilute urine; less water reabsorbed.

• High ADH: Produces concentrated urine; more water reabsorbed.

Urine Concentration and Dilution

• Concentration: Occurs in the presence of ADH; water is reabsorbed from the collecting duct due to the medullary osmotic gradient.

• Dilution: Occurs in the absence of ADH; water remains in the filtrate.

Bladder Function and Voiding

Bladder Muscle and Sphincters

• Detrusor muscle: Contracts during urination (voiding).

• Internal urethral sphincter: Involuntary; relaxes during voiding.

• External urethral sphincter: Voluntary; under conscious control.

Effects of Autonomic Stimulation

• Sympathetic stimulation: Relaxes detrusor muscle, contracts internal sphincter (urine storage).

• Parasympathetic stimulation: Contracts detrusor muscle, relaxes internal sphincter (urination).

Stimulus for Voiding

• Stretch receptors in the bladder wall send signals to the spinal cord when the bladder is full, initiating the micturition reflex.

Summary Table: Hormonal Regulation of Kidney Function

Hormone	Source	Main Effect
ADH (Antidiuretic Hormone)	Posterior pituitary	Increases water reabsorption in collecting ducts
Aldosterone	Adrenal cortex	Increases Na+ reabsorption, K+ secretion
ANP (Atrial Natriuretic Peptide)	Atria of heart	Decreases Na+ reabsorption, increases urine output

Example:

During dehydration, ADH secretion increases, leading to more water reabsorption and concentrated urine. In contrast, after drinking excess water, ADH secretion decreases, resulting in dilute urine.

Additional info: This guide expands on the question prompts by providing definitions, mechanisms, and examples for each topic, ensuring a comprehensive review for exam preparation.