Chapter 26: The Urinary System – Structure, Function, and Physiology

Chapter 26: The Urinary System

Learning Objectives and Key Terms

• Describe the organization and gross anatomy of the urinary system.

• Explain the structure and function of the nephron.

• Describe the blood supply to the kidney and nephron.

• Understand the mechanisms of urine formation and regulation.

• Discuss the micturition reflex and urinary incontinence.

Key Terms: Excretion, Elimination, Renal cortex, Renal medulla, Renal pelvis, Renal lobe, Renal papilla, Renal capsule, Bowman's capsule, Nephron, Glomerulus, Loop of Henle, DCT (Distal Convoluted Tubule), PCT (Proximal Convoluted Tubule), Collecting duct, Peritubular space, Ureter, Cortical nephron, Juxtamedullary nephron, Interlobar artery/vein, Interlobular artery/vein, Afferent arteriole, Efferent arteriole, Vasa recta, Juxtaglomerular Apparatus, Macula densa, Juxtaglomerular cells, Renin, Urea, Creatinine, Glomerular Filtration, GFR, Countercurrent mechanism, Aldosterone, Antidiuretic Hormone, Parathyroid Hormone, Micturition reflex, Incontinence, Osmolarity.

Overview of the Urinary System

Organization

The urinary system consists of four main organs that work together to remove waste products from the blood and regulate fluid and electrolyte balance.

• Kidneys: Filter blood and produce urine.

• Ureters: Transport urine from the kidneys to the bladder.

• Bladder: Stores urine until excretion.

• Urethra: Conducts urine from the bladder to the exterior of the body.

Functions

1. Excretion and Elimination: Removal of metabolic wastes (such as urea, creatinine, and uric acid) from the body.

2. Homeostatic Regulation: Maintains blood volume, pressure, plasma ion concentrations, and pH balance.

The Anatomy of a Kidney

Location and Structure

The kidneys are retroperitoneal organs located on either side of the vertebral column. Each kidney has an outer cortex and an inner medulla. The medulla contains pyramids ending in papillae, which drain into calyces and then the renal pelvis. The kidney is surrounded by a capsule for protection.

• Renal lobe: Consists of a pyramid, overlying cortex, and adjacent columns.

• Columns: Extensions of cortex between pyramids.

The Functional Unit of the Kidney: The Nephron

Nephron Structure

The nephron is the microscopic structural and functional unit of the kidney responsible for urine formation.

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

• Proximal convoluted tubule (PCT): Site of most reabsorption.

• Loop of Henle: Descending and ascending limbs; involved in concentration of urine.

• Distal convoluted tubule (DCT): Further reabsorption and secretion.

• Collecting duct: Receives urine from multiple nephrons and delivers it to the renal pelvis.

Types of Nephrons

• Cortical nephrons: Located mostly in the cortex; short loops of Henle.

• Juxtamedullary nephrons: Located near the cortex-medulla junction; long loops of Henle extend deep into the medulla, crucial for concentrating urine.

The Blood Supply to the Kidney and Nephron

Renal Circulation

Blood enters the kidney via the renal artery, which branches into segmental, interlobar, arcuate, and interlobular arteries. Blood is filtered in the glomerulus, then exits via the efferent arteriole to the peritubular capillaries (cortical nephrons) or vasa recta (juxtamedullary nephrons), and finally drains into the renal vein.

Juxtaglomerular Complex (JGA)

Structure and Function

• Macula densa: Specialized cells in the distal tubule that sense sodium concentration.

• Juxtaglomerular (JG) cells: Located in the afferent arteriole; secrete renin in response to low blood pressure or low sodium.

The JGA is an endocrine structure that regulates blood pressure and glomerular filtration rate (GFR) via the renin-angiotensin-aldosterone system (RAAS).

Renal Physiology: Forming Urine

Purpose of Urine Formation

The kidneys remove metabolic wastes and maintain homeostasis by forming urine.

• Urea: From amino acid breakdown.

• Creatinine: From muscle metabolism (creatine phosphate).

• Uric acid: From nucleic acid metabolism.

Basic Processes of Urine Formation

1. Filtration: Hydrostatic pressure forces water and solutes across the wall of the glomerular capillaries into Bowman's capsule.

2. Reabsorption: Removal of water and solutes from filtrate across tubular epithelium into peritubular space.

3. Secretion: Transport of solutes from peritubular space across tubular epithelium into tubular fluid.

Glomerular Filtration

Filtration Barrier and Filtrate Composition

• The filtration membrane consists of capillary endothelium, basement membrane, and podocyte filtration slits.

• Filtrate contains water, electrolytes, glucose, amino acids, and metabolic wastes, but no large proteins or blood cells.

Glomerular Filtration Rate (GFR)

GFR is the amount of filtrate produced per minute by both kidneys (normal: ~125 mL/min in adults).

Regulation of GFR

1. Autoregulation: Local adjustment of afferent and efferent arteriole diameter to maintain stable GFR.

2. Hormonal Regulation: Involves the renin-angiotensin-aldosterone system (RAAS), atrial natriuretic peptide (ANP), and antidiuretic hormone (ADH).

3. Autonomic Regulation: Sympathetic nervous system can decrease GFR during stress or blood loss.

The Countercurrent Mechanism

Mechanism and Function

The countercurrent mechanism in the loop of Henle allows for efficient reabsorption of water and solutes by having fluid flow in opposite directions in adjacent limbs.

• Descending limb: Permeable to water, not solutes; water exits into the medulla.

• Ascending limb: Impermeable to water; actively transports Na+ and Cl- out into the medulla.

This mechanism creates a concentration gradient in the medulla, essential for producing concentrated urine.

Producing a Concentrated Urine

Role of Antidiuretic Hormone (ADH)

• ADH increases the permeability of the collecting duct to water, allowing more water to be reabsorbed and producing concentrated urine.

• In the absence of ADH, the collecting duct is impermeable to water, resulting in dilute urine.

Secretion at the PCT, DCT, and Collecting Duct

• PCT: Secretes drugs, toxins, and some ions.

• DCT: Secretes ions (K+, H+), acids, drugs, and toxins.

• Collecting Duct: Secretes K+ and H+ as needed for acid-base balance.

Summary of Renal Function

1. Glomerulus: Filtrate produced at the renal corpuscle has the same osmotic concentration as plasma (minus plasma proteins).

2. PCT: Active removal of ions and organic substrates; reduces filtrate volume and maintains osmotic balance.

3. Loop of Henle: Water moves out of the tubule in the descending limb; Na+ and Cl- are actively transported out in the ascending limb, creating the medullary osmotic gradient.

4. DCT and Collecting Duct: Final adjustments to urine composition and volume occur here, regulated by hormones (ADH, aldosterone).

5. Vasa Recta: Absorbs solutes and water reabsorbed in the loop of Henle and collecting ducts, maintaining the concentration gradient of the medulla.

Micturition Reflex and Urination

Neural Control of Urination

• Stretch receptors in the bladder wall send signals to the spinal cord and brain when the bladder fills.

• Parasympathetic stimulation contracts the bladder and relaxes the internal urethral sphincter.

• Voluntary relaxation of the external urethral sphincter allows urination.

Incontinence: Loss of voluntary control over urination.

Table: Comparison of Cortical and Juxtamedullary Nephrons

Feature	Cortical Nephron	Juxtamedullary Nephron
Location	Mostly in cortex	Near cortex-medulla junction
Loop of Henle	Short	Long, extends deep into medulla
Function	Majority of filtration	Concentration of urine
Associated Capillaries	Peritubular capillaries	Vasa recta

Key Equations

• Glomerular Filtration Rate (GFR):

$GFR = K_f \times (P_{GC} - P_{BS} - \pi_{GC})$

Where $K_f$ is the filtration coefficient, $P_{GC}$ is glomerular capillary hydrostatic pressure, $P_{BS}$ is Bowman's space hydrostatic pressure, and $\pi_{GC}$ is glomerular capillary oncotic pressure.

• Renal Clearance:

$C = \frac{U_x \times V}{P_x}$

Where $C$ is clearance, $U_x$ is urine concentration of substance x, $V$ is urine flow rate, and $P_x$ is plasma concentration of substance x.

Example: Regulation of Blood Pressure by the Kidney

When blood pressure drops, the juxtaglomerular cells release renin, activating the RAAS pathway. This leads to increased sodium and water reabsorption, raising blood volume and pressure.

Additional info: The notes have been expanded with academic context, including definitions, physiological mechanisms, and equations for GFR and renal clearance. The table comparing nephron types is inferred from standard textbook content.