The Urinary System: Structure and Function (Chapter 25 Study Notes)

The Urinary System

Overview and Major Organs

The urinary system is essential for maintaining the body's internal environment by regulating water, solute concentrations, and removing metabolic wastes. It consists of the kidneys, ureters, urinary bladder, and urethra.

• Kidneys: Filter blood, regulate fluid and electrolyte balance, and produce urine.

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

• Urinary bladder: Temporarily stores urine.

• Urethra: Conducts urine out of the body.

Functions of the Kidneys

The kidneys perform several vital functions to maintain homeostasis:

• Regulate total body water volume and solute concentration

• Regulate ion concentrations in extracellular fluid (ECF)

• Maintain long-term acid-base balance

• Excrete metabolic wastes, toxins, and drugs

• Produce erythropoietin (stimulates RBC production) and renin (regulates blood pressure)

• Activate vitamin D (calcitriol) for calcium regulation

• Carry out gluconeogenesis during prolonged fasting

Gross Anatomy of the Kidneys

Location and External Structure

The kidneys are retroperitoneal, bean-shaped organs located in the superior lumbar region (T12–L3). The right kidney is slightly lower due to the liver. Each kidney is protected by the rib cage and is capped by an adrenal gland.

External Anatomy

• Renal hilum: Medial indentation where ureters, blood vessels, lymphatics, and nerves enter/exit.

• Supportive tissue layers:

  • Renal fascia: Anchoring outer layer of dense connective tissue

  • Perirenal fat capsule: Fatty cushion

  • Fibrous capsule: Transparent capsule preventing infection spread

Internal Gross Anatomy

The kidney has three main regions:

• Renal cortex: Light-colored, superficial, granular appearance

• Renal medulla: Darker, reddish-brown, contains cone-shaped pyramids

• Renal pelvis: Funnel-shaped tube continuous with the ureter

Renal Pyramids, Columns, and Calyces

• Renal pyramids: Cone-shaped structures in the medulla; papilla points internally

• Renal columns: Extensions of cortical tissue separating pyramids

• Lobe: Each pyramid plus surrounding cortex (about 8 per kidney)

• Major and minor calyces: Collect urine from papillae and funnel it into the renal pelvis

Urine Flow Pathway

Urine flows from the papilla of the pyramid → minor calyces → major calyces → renal pelvis → ureter → bladder.

Blood Supply of the Kidneys

The kidneys receive about 25% of cardiac output per minute via the renal arteries. Blood flows through a series of arteries and capillaries, allowing for filtration and reabsorption.

Nephrons: The Functional Units of the Kidney

Structure of a Nephron

Each kidney contains about one million nephrons, which filter blood and form urine. Each nephron consists of a renal corpuscle and a renal tubule, and empties into collecting ducts.

Renal Corpuscle

• Glomerulus: Tuft of fenestrated capillaries; site of filtration

• Glomerular (Bowman's) capsule: Surrounds the glomerulus; has parietal (simple squamous) and visceral (podocyte) layers

Renal Tubule and Collecting Duct

• Proximal convoluted tubule (PCT): Cuboidal cells with microvilli for reabsorption and secretion

• Nephron loop (Loop of Henle): Descending and ascending limbs with different epithelial types

• Distal convoluted tubule (DCT): Cuboidal cells, fewer microvilli, mainly for secretion

• Collecting duct: Principal cells (water/Na+ balance) and intercalated cells (acid-base balance)

Types of Nephrons

• Cortical nephrons: 85% of nephrons, mostly in cortex, short loops

• Juxtamedullary nephrons: Long loops extend deep into medulla, essential for concentrating urine

Nephron Capillary Beds

• Glomerulus: Specialized for filtration; fed and drained by arterioles

• Peritubular capillaries: Surround tubules in cortex; reabsorb water and solutes

• Vasa recta: Long, straight vessels in medulla; maintain concentration gradient

Juxtaglomerular Complex (JGC)

The JGC is a specialized structure important for regulating filtrate formation and blood pressure. It includes:

• Macula densa: Chemoreceptors in the distal nephron loop, sense NaCl content

• Granular (JG) cells: Mechanoreceptors in arteriole walls, secrete renin

• Extraglomerular mesangial cells: Pass signals between macula densa and granular cells

Urine Formation

Three Major Processes

1. Glomerular filtration: Passive process; blood pressure forces water and solutes through the filtration membrane into the glomerular capsule

2. Tubular reabsorption: Selective movement of substances from filtrate back into blood

3. Tubular secretion: Selective movement of substances from blood into filtrate

Glomerular Filtration

Filtration occurs across a three-layered membrane:

• Fenestrated endothelium: Excludes blood cells

• Basement membrane: Repels large anions and proteins

• Foot processes of podocytes: Form filtration slits, restrict large proteins

Filtration Pressures

Filtration is driven by hydrostatic pressure in the glomerular capillaries (HPGC), opposed by hydrostatic pressure in the capsule (HPCS) and colloid osmotic pressure (OPGC):

• HPGC: 55 mm Hg (outward)

• HPCS: 15 mm Hg (inward)

• OPGC: 30 mm Hg (inward)

Net Filtration Pressure (NFP):

$NFP = HP_{GC} - (HP_{CS} + OP_{GC})$

$NFP = 55 - (15 + 30) = 10 \ \text{mm Hg}$

Glomerular Filtration Rate (GFR)

GFR is the volume of filtrate formed per minute by both kidneys. It is directly proportional to:

• Net filtration pressure (NFP)

• Total surface area available for filtration

• Filtration membrane permeability

Regulation of GFR

• Intrinsic controls (renal autoregulation):

  • Myogenic mechanism: Smooth muscle response to pressure changes

  • Tubuloglomerular feedback: Macula densa senses NaCl, adjusts arteriole diameter

• Extrinsic controls: Neural (sympathetic) and hormonal (renin-angiotensin-aldosterone) mechanisms regulate GFR to maintain blood pressure

Tubular Reabsorption and Secretion

Reabsorption

Most filtrate is reabsorbed into the blood via active (ATP-dependent) and passive (diffusion, osmosis) processes. Reabsorption can occur through transcellular or paracellular routes.

• Proximal convoluted tubule: Major site for reabsorption of water, ions, and nutrients

• Nephron loop: Descending limb reabsorbs water; ascending limb reabsorbs Na+, Cl−

• Distal convoluted tubule and collecting duct: Fine-tune reabsorption under hormonal control (e.g., aldosterone, ADH)

Secretion

Tubular secretion removes additional wastes and regulates blood pH by moving substances from blood into the filtrate.

Summary Table: Nephron Types

Feature	Cortical Nephrons	Juxtamedullary Nephrons
Location	Mostly in cortex	Close to cortex-medulla junction
Nephron loop length	Short	Long, deep into medulla
Function	General filtration	Concentrated urine formation

Key Terms and Concepts

• Filtrate: Plasma-derived fluid filtered by the glomerulus

• Renin: Enzyme that initiates the renin-angiotensin-aldosterone system (RAAS) to regulate blood pressure

• Podocyte: Specialized cell in the visceral layer of the glomerular capsule

• Macula densa: Senses NaCl concentration in the distal nephron

• Principal cell: Regulates water and sodium balance in the collecting duct

• Intercalated cell: Regulates acid-base balance in the collecting duct