Urinary System: Structure, Function, and Filtration

Introduction to the Urinary System

Overview

The urinary system is essential for removing waste products from the body and maintaining homeostasis. It is closely associated with the reproductive system, sharing embryonic development and anatomical relationships, and is collectively referred to as the urogenital (UG) system.

• Primary function: Elimination of metabolic wastes

• Additional roles: Regulation of blood volume, pressure, and composition

Functions of the Urinary System

Expected Learning Outcomes

• Name and locate the organs of the urinary system

• List several functions of the kidneys in addition to urine formation

• Name the major nitrogenous wastes and identify their sources

• Define excretion and identify the systems that excrete wastes

Organs of the Urinary System

• Kidneys

• Ureters

• Urinary bladder

• Urethra

Location: Kidneys are retroperitoneal, positioned against the posterior abdominal wall.

Functions of the Kidneys

• Filter blood and excrete toxic metabolic wastes

• Regulate blood volume, pressure, and osmolarity

• Regulate electrolytes and acid-base balance

• Secrete erythropoietin to stimulate red blood cell production

• Help regulate calcium levels via calcitriol synthesis

• Clear hormones from blood

• Detoxify free radicals

• In starvation, synthesize glucose from amino acids (e.g., glutamine to alpha-ketoglutarate)

Nitrogenous Wastes

Definition and Types

• Waste: Any substance useless to the body or present in excess

• Metabolic waste: Waste substance produced by the body

Major Nitrogenous Wastes

Name	Source	Chemical Structure
Ammonia	Protein catabolism (amino acid deamination)	NH3
Urea	Liver converts ammonia to urea	CO(NH2)2
Uric acid	Nucleic acid catabolism	C5H4N4O3
Creatinine	Creatine phosphate catabolism	C4H7N3O

• Blood urea nitrogen (BUN): Normal: 10–20 mg/dL

• Azotemia: Elevated BUN, may indicate renal insufficiency

• Uremia: Toxic effects of nitrogenous waste accumulation (symptoms: diarrhea, vomiting, dyspnea, arrhythmia)

Excretion

Definition

Excretion is the process of separating wastes from body fluids and eliminating them from the body.

Excretory Systems

• Respiratory system: Excretes CO2, small amounts of other gases, and water

• Integumentary system: Excretes water, inorganic salts, lactic acid, urea in sweat

• Digestive system: Excretes water, salts, lipids, bile pigments, cholesterol, and other metabolic waste

• Urinary system: Excretes many metabolic wastes, toxins, drugs, hormones, salts, and water

Anatomy of the Kidney

Expected Learning Outcomes

• Describe the location and general appearance of the kidneys

• Identify the external and internal features of the kidney

• Trace the flow of blood through the kidney

• Trace the flow of fluid through the renal tubules

• Describe the nerve supply to the kidney

Kidney Position and Associated Structures

• Located against the posterior abdominal wall (T12–L3)

• Right kidney is slightly lower due to the liver

• Rib 12 crosses the middle of the left kidney

• Retroperitoneal: kidneys, ureters, bladder, renal artery/vein, adrenal glands

Gross Anatomy of the Kidney

• Shape and size: About the size of a bar of soap; lateral surface convex, medial concave (hilum)

• Protective coverings:

  • Renal fascia: binds to abdominal wall

  • Perirenal fat capsule: cushions and holds kidney

  • Fibrous capsule: encloses and protects from trauma/infection

Internal Structure

• Renal parenchyma: Glandular tissue forming urine, encircles renal sinus

• Renal sinus: Contains vessels, nerves, urine-collecting structures, and adipose tissue

• Zones:

  • Outer cortex

  • Inner medulla (renal pyramids and columns)

• Lobe: One pyramid and its overlying cortex

• Urine flow: Minor calyx → Major calyx → Renal pelvis → Ureter → Bladder

Renal Circulation

Blood Supply

• Kidneys receive about 21% of cardiac output

• Renal artery → Segmental arteries → Interlobar arteries → Arcuate arteries → Cortical radiate arteries → Afferent arterioles → Glomerulus

• Efferent arterioles → Peritubular capillaries (cortex) or Vasa recta (medulla)

• Veins: Cortical radiate veins → Arcuate veins → Interlobar veins → Renal vein → Inferior vena cava

The Nephron

Structure and Function

• Each kidney contains about 1.2 million nephrons

• Two main parts:

  • Renal corpuscle: Filters blood plasma (glomerulus + glomerular capsule)

  • Renal tubule: Converts filtrate to urine (PCT, nephron loop, DCT, collecting duct)

Renal Corpuscle

• Parietal layer: Simple squamous epithelium

• Visceral layer: Podocytes wrap around capillaries

• Capsular space: Separates the two layers

• Vascular pole: Entry/exit of arterioles

• Urinary pole: Beginning of renal tubule

Renal Tubule

• PCT: Longest, most coiled, with microvilli for absorption

• Nephron loop: U-shaped, descending and ascending limbs, thick (cuboidal) and thin (squamous) segments

• DCT: Shorter, less coiled, end of nephron

• Collecting duct: Receives fluid from multiple nephrons, merges into papillary duct

Flow of filtrate: Glomerular capsule → PCT → Nephron loop → DCT → Collecting duct → Papillary duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Bladder → Urethra

Types of Nephrons

Type	Proportion	Features
Juxtamedullary	15%	Long loops, maintain medullary salinity gradient, vasa recta
Cortical	85%	Short loops, peritubular capillaries

Urine Formation I: Glomerular Filtration

Stages of Urine Formation

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

4. Water conservation

Glomerular filtrate: Fluid in the capsular space, similar to plasma but nearly protein-free.

Tubular fluid: Fluid in the tubule, modified by reabsorption and secretion.

Urine: Fluid in the collecting duct, undergoes little alteration except water content.

Glomerular Filtration Membrane

• Three barriers:

  • Fenestrated endothelium (70–90 nm pores, excludes blood cells)

  • Basement membrane (proteoglycan gel, negative charge, excludes large molecules)

  • Filtration slits (podocyte extensions, 30 nm slits, negative charge)

• Allows passage of molecules < 3 nm (water, electrolytes, glucose, amino acids, nitrogenous wastes, vitamins)

• Excludes most proteins, blood cells, and large anions

Clinical relevance: Damage to the membrane can cause proteinuria (protein in urine) or hematuria (blood in urine).

Filtration Pressure

• Depends on hydrostatic and osmotic pressures

• Blood hydrostatic pressure (BHP): ~60 mm Hg (high due to afferent arteriole size)

• Capsular hydrostatic pressure: ~18 mm Hg

• Colloid osmotic pressure (COP): ~32 mm Hg

• Net filtration pressure (NFP): $NFP = BHP - (COP + Capsular Pressure)$

High glomerular pressure makes kidneys vulnerable to hypertension, which can lead to nephrosclerosis and renal failure.

Glomerular Filtration Rate (GFR)

• GFR: Amount of filtrate formed per minute by both kidneys

• Male: ~180 L/day; Female: ~150 L/day

• 99% of filtrate is reabsorbed; only 1–2 L urine excreted per day

Clinical importance: Too high GFR leads to dehydration; too low leads to azotemia (waste retention).

Regulation of Glomerular Filtration

• Controlled by adjusting glomerular blood pressure

• Three mechanisms:

  • Renal autoregulation (myogenic mechanism, tubuloglomerular feedback)

  • Sympathetic control

  • Hormonal control (renin-angiotensin-aldosterone system)

Renal autoregulation: Nephrons adjust their own blood flow and GFR. Myogenic mechanism responds to stretch; tubuloglomerular feedback involves the juxtaglomerular apparatus (macula densa, granular cells, mesangial cells).

Sympathetic control: Reduces GFR during exercise or shock, redirecting blood to vital organs.

Renin-angiotensin-aldosterone system: Activated by low blood pressure, increases GFR and blood pressure via vasoconstriction and sodium/water retention.

Equation for GFR: $GFR = NFP \times K_f$ where $K_f$ is the filtration coefficient (permeability and surface area).

Additional info: The notes above are expanded with academic context for clarity and completeness.