Chapter 19 – The Kidneys: Structure, Function, and Physiology

Chapter 19 – The Kidneys

Overview of Kidney Functions

The kidneys are essential organs responsible for maintaining homeostasis by regulating fluid balance, electrolyte concentrations, and waste excretion. Their complex structure and function are central to the body's internal environment.

• Six Functions of the Kidneys:

  • Regulation of extracellular fluid volume and blood pressure

  • Maintenance of osmolarity

  • Regulation of ion balance

  • Excretion of metabolic wastes and foreign substances

  • Regulation of pH

  • Production of hormones (e.g., erythropoietin, renin)

• Normal Osmolarity: Blood osmolarity is typically about 300 mOsm/L. Kidney filtrate osmolarity varies throughout the nephron, starting similar to plasma and becoming more concentrated or diluted depending on segment and physiological state.

• Two Layers of the Kidneys: The kidney consists of the cortex (outer layer) and medulla (inner layer).

Nephron Structure and Function

The nephron is the functional unit of the kidney, responsible for filtration, reabsorption, secretion, and excretion.

• Filtration: Occurs in the glomerulus and Bowman's capsule.

• Reabsorption: Primarily in the proximal tubule, but also in the Loop of Henle, distal tubule, and collecting duct.

• Secretion: Occurs in the proximal tubule, distal tubule, and collecting duct.

• Excretion: Final urine leaves via the collecting duct to the ureter.

Primary Functions of Nephron Segments

• Proximal Tubule: Reabsorbs most filtered water, ions, and nutrients; secretes some substances.

• Loop of Henle: Establishes osmotic gradient; reabsorbs water (descending limb) and ions (ascending limb).

• Distal Tubule: Fine-tunes reabsorption and secretion of ions.

• Collecting Duct: Regulates final urine concentration and volume.

Filtration and Excretion Quantities

• Fluid Filtered: About 180 liters/day filtered through the glomerulus; only about 1.5 liters/day excreted as urine.

• Volume of Excretion Calculation: $\text{Excretion} = \text{Filtration} - \text{Reabsorption} + \text{Secretion}$

• Plasma Filtration: Approximately 20% of plasma entering the glomerulus is filtered into Bowman's capsule; less than 1% of filtered plasma is excreted as urine.

Filtration Barrier and Pressures

• Structures Keeping Plasma Proteins in Plasma:

  • Glycocalyx (endothelial cell surface layer)

  • Glomerular basement membrane

  • Podocytes (with filtration slits)

• Glomerular Filtration Pressure:

  • Hydrostatic pressure (glomerular capillaries)

  • Colloid osmotic pressure (plasma proteins)

  • Capsular hydrostatic pressure (Bowman's capsule)

• Glomerular Filtration Rate (GFR) Factors: GFR is affected by net filtration pressure and filtration coefficient.

• Filtration Coefficient Factors: Surface area of glomerular capillaries and permeability of filtration barrier.

Autoregulation of GFR

The kidneys maintain a relatively constant GFR across a range of blood pressures through autoregulatory mechanisms.

• Myogenic Response: Smooth muscle in afferent arteriole contracts when stretched, reducing GFR.

• Tubuloglomerular Feedback: Macula densa cells sense flow and signal afferent arteriole to constrict or dilate.

Transport Mechanisms in the Nephron

• Active Transport: Direct use of ATP to move substances (e.g., Na+ reabsorption).

• Secondary Active Transport: Uses energy from another gradient (e.g., glucose reabsorption via Na+-glucose cotransporter).

• Passive Reabsorption: Movement down concentration gradients (e.g., water via osmosis).

• Endocytosis: Uptake of large molecules (e.g., proteins).

• Saturation: Occurs when all transporters are occupied; example: glucose appears in urine (glucosuria) when plasma glucose exceeds transport maximum.

• Peritubular Capillary Pressure: Low hydrostatic pressure and high oncotic pressure favor reabsorption from interstitial fluid into capillaries.

• Transport Types Comparison:

  • Direct Active Transport: Uses ATP directly (e.g., Na+-K+ pump)

  • Secondary Indirect Active Transport: Uses gradient created by primary active transport (e.g., Na+-glucose cotransport)

  • Tertiary Indirect Active Transport: Uses gradient from secondary transport (e.g., organic anion transporters)

Secretion, Excretion, and Clearance

• Secretion: Movement of substances from blood into nephron tubule for excretion.

• Excretion Calculation: $\text{Excretion} = \text{Amount filtered} - \text{Amount reabsorbed} + \text{Amount secreted}$

• Excretion vs. Clearance: Excretion is the amount of substance removed; clearance is the volume of plasma cleared of a substance per unit time.

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

• Inulin: Used to measure GFR because it is freely filtered and neither reabsorbed nor secreted.

Micturition (Urination)

• Urine Storage: The urinary bladder stores urine; internal and external sphincters keep urine in the bladder at rest.

• Urine Excretion: When the bladder is full, stretch receptors trigger the micturition reflex, relaxing sphincters and allowing urine to exit via the urethra.

• Urine Retention Limit: Voluntary control is limited; eventually, the reflex overrides conscious control.

Key Terminology

• Apical membrane/surface: The side of epithelial cells facing the nephron lumen.

• Basolateral membrane/surface: The side facing interstitial fluid and blood.

• Ureter: Tube carrying urine from kidney to bladder.

• Urinary bladder: Organ for urine storage.

• Urethra: Tube for urine excretion.

• Retroperitoneum: Anatomical space behind the peritoneum where kidneys are located.

• Nephron: Functional unit of the kidney.

• Glomerulus: Capillary network for filtration.

• Bowman's capsule: Surrounds glomerulus; collects filtrate.

• Proximal tubule, Loop of Henle, Distal tubule, Collecting duct: Segments of nephron with specialized functions.

• Distal nephron: Includes distal tubule and collecting duct.

• Juxtaglomerular apparatus: Structure regulating GFR and blood pressure.

• Filtration, Reabsorption, Secretion, Excretion: Key nephron processes.

• Filtrate: Fluid filtered from blood into nephron.

• Glycocalyx, Glomerular basement membrane, Podocytes: Components of filtration barrier.

• Mesangial cells: Support glomerular capillaries.

• Macula densa: Senses flow in distal tubule.

• Granular/juxtaglomerular cells: Release renin.

• Transcellular transport: Through cells.

• Paracellular transport: Between cells.

• Saturation, Specificity, Competition: Properties of transport proteins.

• Glucosuria/glycosuria: Glucose in urine.

• Peritubular capillary pressure: Favors reabsorption.

• Clearance: Volume of plasma cleared per unit time.

• Inulin: Marker for GFR.

• Micturition: Urination process.

Summary Table: Nephron Segment Functions

Segment	Main Function	Key Transport
Proximal Tubule	Bulk reabsorption of water, ions, nutrients	Active and passive transport
Loop of Henle	Osmotic gradient creation	Passive (water), active (ions)
Distal Tubule	Fine-tuning ion balance	Active transport
Collecting Duct	Final urine concentration	Regulated water/ion transport

Summary Table: Filtration Barrier Components

Component	Function
Glycocalyx	Repels proteins, maintains selectivity
Glomerular Basement Membrane	Physical and charge barrier
Podocytes	Filtration slits, structural support

Example: Glucose Reabsorption and Saturation

Glucose is reabsorbed in the proximal tubule via secondary active transport. When plasma glucose exceeds the transport maximum, glucose appears in urine (glucosuria).

Additional info: The kidneys' ability to filter and reabsorb large volumes is essential for maintaining homeostasis, and their regulatory mechanisms ensure precise control of blood composition and volume.