Chapter 19: The Kidneys – Structure and Function in Human Physiology

The Urinary System: Overview

Functions of the Urinary System

The urinary system plays a critical role in maintaining homeostasis by regulating the composition and volume of body fluids.

• Regulate plasma ionic composition: Controls levels of ions such as Na+, K+, Ca2+, Cl-.

• Regulate plasma volume: Adjusts water excretion to maintain blood pressure.

• Regulate plasma osmolarity: Maintains solute concentration in plasma.

• Regulate plasma pH: Excretes H+ and conserves HCO3- to balance acid-base status.

• Remove metabolic waste products and foreign substances: Eliminates creatinine, uric acid, urobilinogen, and xenobiotics.

• Other functions:

  • Secrete erythropoietin (stimulates red blood cell production) and renin (regulates blood pressure).

  • Activate vitamin D3 to calcitriol (important for calcium homeostasis).

  • Perform gluconeogenesis (glucose synthesis from non-carbohydrate sources).

Structures of the Urinary System

The urinary system consists of organs that produce, transport, store, and excrete urine.

• Kidneys: Form urine; located retroperitoneally.

• Ureters: Transport urine from kidneys to bladder.

• Bladder: Stores urine until excretion.

• Urethra: Excretes urine from bladder to outside the body.

Microscopic Anatomy of the Kidney

Nephron Structure

The nephron is the functional unit of the kidney, responsible for urine formation.

• Renal corpuscle:

  • Glomerulus: Capillary network for filtration.

  • Bowman's capsule: Receives filtrate; directs flow into renal tubules.

• Renal tubules:

  • Proximal convoluted tubule

  • Loop of Henle: Descending limb, thin ascending limb, thick ascending limb

  • Distal convoluted tubule

  • Collecting duct

Cortical vs. Juxtamedullary Nephrons

Nephrons are classified based on their location and structure.

• Cortical nephrons: Short loop of Henle; most numerous (80–85%).

• Juxtamedullary nephrons: Long loop of Henle extends into medulla; responsible for medullary osmotic gradient, crucial for concentrating urine.

• Both types produce urine.

Basic Renal Exchange Processes

Overview of Exchange Processes

Renal function involves four main processes:

• Filtration: Movement of fluid from blood into nephron (Bowman's capsule).

• Reabsorption: Movement from tubules back into blood (peritubular capillaries).

• Secretion: Movement from blood into tubules (opposite of reabsorption).

• Excretion: Removal of substances from the body via urine.

Glomerular Filtration

Filtration Mechanism

Glomerular filtration is the process by which protein-free plasma moves from the glomerulus to Bowman's capsule.

• Glomerular Filtration Rate (GFR): $GFR = 125\ \text{mL/min} = 180\ \text{L/day}$

• Filtrate must cross three barriers:

  • Capillary endothelial layer

  • Basement membrane (negatively charged, repels proteins)

  • Surrounding epithelial layer (podocytes)

Forces Affecting Filtration

Filtration is determined by hydrostatic and osmotic pressures.

• Forces favoring filtration:

  • Glomerular hydrostatic pressure (GHP): 55 mm Hg

  • Bowman's capsule colloid osmotic pressure: 0 mm Hg (no proteins in filtrate)

• Forces opposing filtration:

  • Capsule hydrostatic pressure (CHP): 15 mm Hg

  • Glomerular osmotic pressure (GOP): 30 mm Hg

Net Filtration Pressure Equation:

$\text{Net Filtration Pressure} = P_{\text{H}} - \pi - P_{\text{fluid}} = 55 - 30 - 15 = 10\ \text{mm Hg}$

Glomerular Filtration Rate (GFR)

GFR is relatively constant and influenced by:

• Net filtration pressure

• Filtration coefficient:

  • Surface area of glomerular capillaries

  • Permeability of interface between capillary and Bowman's capsule

Regulation of GFR

Intrinsic Regulation

GFR is autoregulated by mechanisms within the kidney.

• Myogenic regulation: Smooth muscle in afferent arteriole contracts in response to stretch.

• Tubuloglomerular feedback: Macula densa cells secrete paracrine factors in response to increased fluid flow, affecting afferent arteriole diameter.

Extrinsic Regulation

GFR is also regulated by hormones and autonomic neurons.

• Sympathetic neurons: Constrict afferent and efferent arterioles, decreasing GFR.

• Angiotensin II: Vasoconstrictor, reduces GFR.

• Filtration coefficient alteration: Podocyte filtration slits and mesangial cell contraction can change permeability.

Reabsorption

Mechanisms of Reabsorption

Reabsorption returns filtered substances to the blood, primarily in the proximal tubule.

• Transepithelial (transcellular) transport: Movement through epithelial cells.

• Paracellular pathway: Movement between cells via tight junctions.

Principles Governing Tubular Reabsorption

• Active transport: Sodium reabsorption via Na+/K+-ATPase.

• Secondary active transport: Symport with sodium (e.g., glucose, amino acids).

• Passive reabsorption: Epithelial junctions, facilitated diffusion.

• Receptor-mediated endocytosis: For plasma proteins.

Transport Maximum

Carrier proteins have a maximum rate of transport, known as the transport maximum.

• Renal threshold: Plasma concentration at which a solute (e.g., glucose) begins to appear in urine.

• Glucose reabsorption:

  • Apical membrane: secondary active transport (SGLT)

  • Basolateral membrane: facilitated diffusion (GLUT)

Graphical Relationship: Filtration, reabsorption, and excretion rates of glucose show a plateau at transport maximum.

Regional Specialization of Renal Tubules

• Proximal tubule: Mass reabsorber (70% sodium and water, 100% glucose); large surface area due to brush border; leaky tight junctions allow paracellular transport.

• Distal tubule and collecting duct: Regulated reabsorption and secretion; tight junctions limit paracellular transport; hormone receptors regulate water and solute transport.

• Loop of Henle: Water conservation; establishes medullary osmotic gradient.

Secretion

Mechanism and Substances Secreted

Secretion moves substances from peritubular capillaries into the tubules, using similar transport mechanisms as reabsorption but in the opposite direction.

• Secreted substances: Potassium, hydrogen ions, choline, creatinine, penicillin, benzoate, salicylate, saccharine.

Excretion and Clearance

Excretion

The amount of a substance excreted is determined by filtration, reabsorption, and secretion.

• Excretion equation: $\text{Excreted} = \text{Filtered} + \text{Secreted} - \text{Reabsorbed}$

• Depends on filtered load (GFR × plasma concentration), secretion rate, and reabsorption rate.

Clearance

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

• Clearance equation: $\text{Clearance} = \frac{\text{Excretion rate}}{\text{Plasma concentration}}$

• Inulin and creatinine are used to measure GFR.

• If clearance of a substance (Cx) > GFR, it is secreted; if Cx < GFR, it is reabsorbed.

Clearance Table

Comparison of clearance rates for common substances:

Substance	Clearance Rate (mL/min)	Renal Processing
Creatinine	140	Secretion
Inulin	125	No reabsorption or secretion
Potassium	120	Reabsorption
Chloride	13	Reabsorption
Sodium	0.9	Reabsorption
Glucose	0	Reabsorption

Summary Table: Filtration and Reabsorption Rates

Substance	Filtration Rate (mmol/day)	Reabsorption Rate (mmol/day)	% Filtered Reabsorbed
Water	180	178.5	99.2%
Glucose	800	800	100%
Urea	54	27	50%
Na+	25.50	24.65	96.7%
K+	7.40	6.90	93.2%
Ca2+	0.99	0.98	99.0%
Cl-	18.00	17.70	98.3%
HCO3-	4.32	4.18	96.7%

Sites of Reabsorption and Secretion in Renal Tubules

Tubule Segment	Substances Reabsorbed	Substances Secreted
Proximal tubule	Na+, Glucose, Amino acids, Vitamins, Urea, Choline, Water	H+
Loop of Henle (descending limb)	Water
Loop of Henle (ascending limb)	Na+, K+, Cl-, Mg2+, Ca2+
Distal tubule	Na+, Ca2+, Cl-, Water	K+, H+
Collecting duct	Water, HCO3-, Urea	H+, K+

Key Terms

• Nephron: Functional unit of the kidney.

• Glomerulus: Capillary network for filtration.

• Bowman's capsule: Surrounds the glomerulus, collects filtrate.

• GFR (Glomerular Filtration Rate): Rate at which plasma is filtered through the glomerulus.

• Reabsorption: Return of filtered substances to the blood.

• Secretion: Addition of substances from blood to tubule.

• Excretion: Removal of substances from the body via urine.

• Clearance: Volume of plasma cleared of a substance per unit time.

Additional info: Some explanations and table entries have been expanded for clarity and completeness based on standard physiology textbooks.