Urinary System: Structure, Function, and Regulation

Urinary System Overview

Functions of the Kidneys

The kidneys are vital organs responsible for filtering blood, removing waste, regulating fluid and electrolyte balance, and maintaining acid-base homeostasis.

• Excretion of metabolic wastes: Removes urea, creatinine, and other waste products.

• Regulation of blood volume and pressure: Adjusts water and salt balance.

• Regulation of electrolyte balance: Controls levels of sodium, potassium, calcium, etc.

• Acid-base balance: Maintains pH by excreting hydrogen ions and reabsorbing bicarbonate.

• Hormone production: Secretes erythropoietin (stimulates RBC production) and renin (regulates blood pressure).

External & Internal Anatomy of the Kidney

The kidneys are bean-shaped organs located retroperitoneally. The right kidney sits slightly lower than the left due to the liver's position.

• External anatomy: Renal capsule (fibrous covering), renal hilum (entry/exit for vessels and ureter).

• Internal anatomy: Cortex (outer region), medulla (inner region with renal pyramids), renal pelvis (collects urine).

Pathway of Urine Flow

Urine is produced in the nephrons and flows through the following structures:

• Nephron → Collecting duct → Minor calyx → Major calyx → Renal pelvis → Ureter → Bladder → Urethra

Hematuria

Hematuria is the presence of blood in urine, indicating possible injury, infection, or disease in the urinary tract.

Nephron Structure and Function

What are Nephrons?

Nephrons are the functional units of the kidney, responsible for filtering blood and forming urine. Each kidney contains about 1 million nephrons.

Renal Capsule and Renal Tubule

• Renal capsule: Consists of the glomerulus (capillary network) and Bowman's capsule (surrounds glomerulus).

• Renal tubule: Three major parts:

  • Proximal convoluted tubule (PCT): Reabsorbs most filtered substances.

  • Nephron loop (Loop of Henle): Descending and ascending limbs; concentrates urine.

  • Distal convoluted tubule (DCT): Regulates reabsorption and secretion, influenced by hormones.

Collecting Duct

The collecting duct receives urine from multiple nephrons and is involved in final water and solute reabsorption.

• Cell types: Principal cells (regulate water and sodium balance), Intercalated cells (regulate acid-base balance).

Glomerulus

The glomerulus is a tuft of capillaries where blood filtration occurs. Blood enters via the afferent arteriole and exits via the efferent arteriole.

• Filtrate: Fluid filtered from blood, containing water, ions, glucose, amino acids, and wastes.

Urine Production

• Urine is produced from filtrate after reabsorption and secretion.

• Consists of water, urea, creatinine, ions, and other wastes.

• Average adult urine volume: 1-2 liters per day.

Processes of Urine Formation

Three Main Processes

Urine formation involves three processes:

• Glomerular filtration: Passive process; blood pressure forces water and solutes through the filtration membrane.

• Tubular reabsorption: Movement of substances from filtrate back into blood; can be active or passive.

• Tubular secretion: Addition of substances from blood into filtrate; important for removing excess ions and wastes.

Filtration Membrane Functions

• Allows passage of water and small solutes.

• Prevents passage of blood cells and large proteins.

Forces Determining Net Filtration Pressure (NFP)

NFP is the sum of forces favoring and opposing filtration:

• Glomerular hydrostatic pressure (favors filtration)

• Capsular hydrostatic pressure (opposes filtration)

• Blood colloid osmotic pressure (opposes filtration)

Equation:

$\text{NFP} = \text{Glomerular hydrostatic pressure} - (\text{Capsular hydrostatic pressure} + \text{Blood colloid osmotic pressure})$

Glomerular Filtration Rate (GFR)

GFR is the volume of filtrate formed per minute by all nephrons.

• GFR is directly proportional to NFP, surface area, and membrane permeability.

Equation:

$\text{GFR} = \text{NFP} \times \text{Filtration coefficient}$

Intrinsic and Extrinsic Controls

• Intrinsic controls (Renal autoregulation): Maintains GFR despite changes in blood pressure.

• Extrinsic controls (Neural & hormonal): Adjusts GFR to maintain systemic blood pressure.

Renin release pathways: Granular cells release renin via three mechanisms: direct stimulation by low BP, sympathetic stimulation, and reduced NaCl in distal tubule.

Aldosterone mechanism: Increases sodium reabsorption, leading to increased water retention and higher blood pressure.

Tubular Reabsorption and Secretion

Tubular Reabsorption

Reabsorption is the process of moving substances from filtrate back into blood.

• Most abundant cation: Sodium (Na+); transport is primarily active.

• Proximal convoluted tubule: Site of most reabsorption (glucose, amino acids, water, ions).

• Nephron loop: Concentrates urine by reabsorbing water and salts.

• Distal convoluted tubule & collecting duct: Reabsorption is hormonally regulated (aldosterone, antidiuretic hormone).

Hormonal Regulation

• Aldosterone: Increases Na+ reabsorption.

• Antidiuretic hormone (ADH): Increases water reabsorption.

• Parathyroid hormone: Increases Ca2+ reabsorption.

Tubular Secretion

Secretion is the process of moving substances from blood into filtrate.

• Removes excess ions (K+, H+), drugs, and wastes.

• Helps regulate acid-base balance.

Diuretics

Diuretics are substances that increase urine output by inhibiting reabsorption of water and/or solutes.

• Overhydrated: Produces dilute urine.

• Dehydrated: Produces concentrated urine.

Renal Clearance and Urine Characteristics

Renal Clearance

Renal clearance measures the efficiency of the kidneys in removing a substance from blood.

• Equation:

$\text{C} = \frac{\text{U} \times \text{V}}{\text{P}}$

• C = clearance rate

• U = concentration in urine

• V = urine flow rate

• P = concentration in plasma

Normal GFR: 90-120 mL/min

Urine Composition and Properties

• Chemical composition: Water, urea, creatinine, ions, uric acid.

• Color: Yellow due to urochrome; abnormal colors may indicate disease.

• Odor: Slightly aromatic; ammonia smell indicates bacterial action; fruity odor may indicate diabetes.

• pH: Normally 4.5-8.0; average is 6.0.

• Specific gravity: 1.001-1.035; water is 1.000.

Urinary Tract Structures and Micturition

Ureters, Bladder, and Urethra

• Ureters: Transport urine from kidneys to bladder.

• Bladder: Stores urine; capacity ~500 mL.

• Urethra: Conducts urine out of the body.

Micturition

Micturition is the process of urination (voiding). Also called urination or voiding.

• Controlled by both voluntary and involuntary mechanisms.

• Incontinence: inability to control urination.

• Retention: inability to empty bladder.

Developmental Aspects and Clinical Relevance

Urinary System Development

• Females have more urinary tract infections due to shorter urethra and proximity to anus.

Summary Table: Hormonal Regulation of Urine Formation (Inferred from "Table 25.2")

Hormone	Site of Action	Effect
Aldosterone	DCT, Collecting Duct	Increases Na+ reabsorption, increases water retention, raises BP
Antidiuretic Hormone (ADH)	Collecting Duct	Increases water reabsorption, concentrates urine
Parathyroid Hormone	DCT	Increases Ca2+ reabsorption
Renin-Angiotensin System	Systemic	Increases BP via vasoconstriction and aldosterone release

Example: Renal Clearance Calculation

If a substance has U = 50 mg/mL, V = 2 mL/min, and P = 1 mg/mL:

$\text{C} = \frac{50 \times 2}{1} = 100 \text{ mL/min}$

Additional info: Some details, such as the exact content of Table 25.2 and the "Control of Micturition chart," were inferred based on standard textbook content.