The Urinary System: Kidney Function and Filtration

Chapter 19: The Urinary System

19.1 Functions of the Kidneys

The kidneys are essential organs that maintain homeostasis by regulating the composition and volume of body fluids. They perform several critical functions necessary for survival and health.

• Regulation of Extracellular Fluid Volume and Blood Pressure: The kidneys adjust the volume of blood and interstitial fluid, influencing blood pressure through mechanisms such as water reabsorption and the renin-angiotensin-aldosterone system.

• Regulation of Osmolarity: By controlling the amount of water and solutes reabsorbed or excreted, the kidneys maintain the osmolarity of body fluids near 300 mOsm.

• Maintenance of Ion Balance: The kidneys regulate the concentrations of key ions (e.g., Na+, K+, Ca2+, Cl-) in the blood.

• Homeostatic Regulation of pH: By excreting hydrogen ions (H+) and reabsorbing bicarbonate (HCO3-), the kidneys help maintain acid-base balance.

• Excretion of Wastes: Metabolic wastes (e.g., urea, creatinine), toxins, and drugs are removed from the body via urine.

• Production of Hormones: The kidneys produce hormones such as erythropoietin (stimulates red blood cell production), renin (regulates blood pressure), and activate vitamin D.

19.2 Anatomy of the Urinary System

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

• Major Organs: Kidneys, ureters, urinary bladder, and urethra.

• Kidney Structure: Each kidney contains an outer cortex and an inner medulla. The functional unit is the nephron, which spans both regions.

• Vascular Supply: Blood enters the kidney via the renal artery, branches into smaller arteries and arterioles, and is filtered in the glomerulus. Blood exits via the renal vein.

• Nephron Anatomy: Each nephron consists of a renal corpuscle (glomerulus + Bowman's capsule), proximal tubule, loop of Henle (descending and ascending limbs), distal tubule, and collecting duct.

• Juxtaglomerular Apparatus: Specialized structure where the distal tubule contacts the afferent and efferent arterioles, involved in regulating blood pressure and filtration rate.

Example: The juxtaglomerular apparatus contains macula densa cells (detect NaCl) and granular cells (secrete renin).

19.3 Overview of Kidney Function

The kidneys maintain homeostasis through three main processes: filtration, reabsorption, and secretion. These processes occur along the nephron and modify the composition of the filtrate to form urine.

• Filtration: Movement of fluid and small solutes from blood into the nephron at the renal corpuscle. The filtered fluid is called filtrate.

• Reabsorption: Return of useful substances (water, ions, nutrients) from the filtrate back into the blood, mainly in the proximal tubule and loop of Henle.

• Secretion: Active transport of additional substances (e.g., H+, K+, drugs) from blood into the nephron tubule for excretion.

Key Equation:

$\text{Amount excreted} = \text{Amount filtered} - \text{Amount reabsorbed} + \text{Amount secreted}$

Example: About 180 L of plasma is filtered daily, but only 1.5 L of urine is excreted, indicating extensive reabsorption.

Nephron Segments and Functions

Segment of Nephron	Processes
Renal corpuscle (glomerulus + Bowman's capsule)	Filtration of mostly protein-free plasma from the capillaries into the capsule
Proximal tubule	Isosmotic reabsorption of organic nutrients, ions, and water; secretion of metabolites and xenobiotics
Loop of Henle	Reabsorption of ions in excess of water to create dilute fluid in the lumen; contributes to concentrated interstitial fluid in the renal medulla
Distal nephron (distal tubule + collecting duct)	Regulated reabsorption of ions and water for salt and water balance and pH homeostasis

19.4 Filtration

Filtration is the process by which water and solutes are forced from the blood into the nephron at the renal corpuscle. This process is selective and depends on several anatomical and physiological barriers.

• Filtration Fraction: The percentage of renal plasma flow that becomes filtrate (about 20%).

• Filtration Barriers:

  • Glomerular Capillary Endothelium: Fenestrated capillaries allow passage of water and small solutes but block blood cells.

  • Glycocalyx: Negatively charged layer that repels proteins.

  • Basement Membrane: Acellular layer that acts as a physical and charge barrier.

  • Epithelium of Bowman's Capsule: Podocytes with foot processes and filtration slits regulate passage of molecules.

  • Mesangial Cells: Support glomerular capillaries and can contract to regulate filtration surface area.

Pressures Affecting Filtration

Filtration is driven and opposed by several pressures:

• Capillary Blood Pressure (PH): Hydrostatic pressure (~55 mmHg) that favors filtration.

• Capillary Colloid Osmotic Pressure (π): Due to plasma proteins (~30 mmHg), opposes filtration by pulling fluid back into capillaries.

• Capsule Fluid Pressure (Pfluid): Hydrostatic pressure in Bowman's capsule (~15 mmHg), opposes filtration.

Net Filtration Pressure Equation:

$\text{Net Pressure} = P_H - \pi - P_{fluid} = 10\ \text{mmHg into Bowman's capsule}$

Glomerular Filtration Rate (GFR)

GFR is the volume of fluid filtered from the glomerular capillaries into Bowman's capsule per unit time (typically ~125 mL/min or 180 L/day).

• Influenced by:

  • Net filtration pressure (renal blood flow and blood pressure)

  • Filtration coefficient (surface area and permeability of filtration slits)

• Regulation: GFR is kept relatively constant by autoregulation mechanisms, including changes in arteriole resistance and feedback from the juxtaglomerular apparatus.

Autoregulation of GFR

• Myogenic Response: Intrinsic ability of vascular smooth muscle to respond to pressure changes, similar to other arterioles.

• Tubuloglomerular Feedback: Paracrine signaling (ATP, adenosine) from the macula densa in response to NaCl concentration in the filtrate; granular cells secrete renin when blood pressure or Na+ is low.

• Hormonal and Autonomic Regulation: Changes in arteriole resistance and filtration coefficient via hormones and sympathetic nervous system.

Summary Table: Factors Affecting GFR

Factor	Effect on GFR
Increased resistance in afferent arteriole	Decreases GFR
Increased resistance in efferent arteriole	Increases GFR
Decreased resistance in afferent arteriole	Increases GFR
Decreased resistance in efferent arteriole	Decreases GFR

Key Terms

Important terms to know include: kidneys, ureter, urinary bladder, urethra, nephron, glomerulus, Bowman's capsule, proximal tubule, loop of Henle, distal tubule, collecting duct, juxtaglomerular apparatus, filtration, reabsorption, secretion, glomerular filtration rate (GFR), podocytes, mesangial cells, myogenic response, tubuloglomerular feedback, macula densa, granular cells.

Learning Outcomes

• List and describe the six functions of the kidneys.

• Diagram the anatomic relationship between the vascular and tubular elements of the nephron.

• Describe the three processes of the nephron.

• Diagram the volume and osmolarity changes of filtrate as it passes through each section of the nephron.

• Describe the filtration barriers between the blood and the lumen of the nephron and explain how they can be modified to control filtration.

• Describe the pressures that promote and oppose glomerular filtration.

• Define glomerular filtration rate.

• Explain how GFR can be influenced by local and reflex control mechanisms.

Additional info: The nephron is the functional unit of the kidney, and its structure and function are central to understanding renal physiology and the regulation of body fluid balance.