Urinary System: Structure, Function, and Physiology

Urinary System

Overview

The urinary system, also known as the renal system, is responsible for removing waste products from the blood, regulating fluid and electrolyte balance, and maintaining homeostasis. It consists of several organs that work together to produce, store, and eliminate urine.

• Kidneys: Produce urine by filtering blood.

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

• Urinary bladder: Stores urine until excretion.

• Urethra: Transports urine from the bladder to the outside of the body.

Functions of the Urinary System

• Adjusts blood volume and blood pressure.

• Regulates blood plasma concentrations of sodium, potassium, chloride, and other ions.

• Stabilizes blood pH.

• Conserves valuable nutrients by preventing their loss in urine.

• Removes drugs and toxins from the bloodstream.

Kidneys

Protection and Structure

The kidneys are protected by visceral organs anteriorly and muscles posteriorly. Each kidney is surrounded by three concentric layers of connective tissue:

• Fibrous capsule: Collagen fibers covering the entire surface of the organ.

• Perinephric fat: Thick layer of adipose tissue surrounding the fibrous capsule.

• Renal fascia: Dense fibrous outer layer that fuses with the fascia of abdominal wall muscles.

Kidney Anatomy

• Renal medulla: Inner portion, appears striated, contains cone-shaped renal pyramids. The tip of each pyramid is the renal papilla.

• Cortex: Outer layer, appears granular, dips down between pyramids to form renal columns.

Nephron: The Functional Unit

Structure of the Nephron

Nephrons are the functional units of the kidney where filtration occurs. There are two types:

• Cortical nephrons: Located mostly in the cortex.

• Juxtamedullary nephrons: Extend deep into the medulla, important for concentrating urine.

Each nephron consists of:

• Renal corpuscle: Contains the glomerulus (capillary network) and Bowman's capsule. Filtrate is forced out of the blood into the capsule.

• Renal tubule:

  • Proximal convoluted tubule (PCT): Initial segment, most reabsorption occurs here.

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

  • Distal convoluted tubule (DCT): Final adjustment via reabsorption and secretion.

Blood Supply in the Kidney

Renal Circulation

At rest, 15-30% of cardiac output is directed to the kidneys, allowing for efficient filtration and homeostasis. The blood supply follows a complex branching pattern:

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

• Blood that is not filtered leaves via efferent arteriole (not a vein), then enters peritubular capillaries or vasa recta (important for reabsorption).

• Veins follow a similar path: Cortical radiate veins → Arcuate veins → Interlobar veins → Renal vein → Inferior vena cava (IVC).

Urine Formation

Overview

The main function of the nephron is to control the composition of body fluids and remove waste from the blood. Urine formation involves three key processes:

• Glomerular filtration

• Tubular reabsorption

• Tubular secretion

Urine carries wastes such as urea, creatinine, and uric acid, along with excess water and electrolytes.

Filtration

• Occurs exclusively at the glomerulus, where capillaries are wrapped in podocytes (specialized cells for filtration).

• Afferent arteriole is larger than efferent arteriole, increasing pressure and driving filtration.

• Filtration rate is controlled by net filtration pressure (NFP):

  • Glomerular hydrostatic pressure

  • Capsular hydrostatic pressure

  • Blood colloid osmotic pressure

• About 180 L/day is filtered, but only 0.5-1.5 L is excreted as urine.

Equation for Net Filtration Pressure (NFP):

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

Glomerular Filtration Rate (GFR)

• GFR is directly proportional to NFP.

• Most important factor is glomerular hydrostatic pressure, influenced by arteriole diameter.

• Constriction or dilation of afferent/efferent arterioles can increase or decrease GFR.

Regulation of GFR

• Autoregulation keeps GFR relatively constant:

  • Myogenic mechanism: Smooth muscle in arterioles responds to changes in blood pressure.

  • Tubuloglomerular feedback: Macula densa cells detect changes in filtrate and signal adjustments.

• Neural control: Sympathetic nervous system can constrict arterioles during stress or shock, reducing GFR.

• Hormonal control: Renin-angiotensin-aldosterone system (RAAS) maintains systemic blood pressure and GFR.

Renin-Angiotensin-Aldosterone System (RAAS)

• Renin is secreted in response to low blood pressure, low NaCl, or sympathetic stimulation.

• Renin converts angiotensinogen (from liver) to angiotensin I, which is converted to angiotensin II by ACE (from lungs).

• Angiotensin II effects:

  • Stimulates aldosterone secretion (increases sodium and water reabsorption).

  • Stimulates ADH secretion (increases water reabsorption).

  • Stimulates thirst and vasoconstriction.

Other Hormones

• Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are secreted when blood volume/pressure increases, promoting sodium and water excretion.

• Hypertension may be treated with ACE inhibitors to block the RAAS pathway.

Tubular Reabsorption and Secretion

Tubular Reabsorption

• Filtration is not selective; reabsorption is needed to reclaim useful substances.

• Reabsorption occurs via passive (osmosis, diffusion) or active transport mechanisms.

• Key substances reabsorbed:

  • Amino acids

  • Organic acids (lactic, citric, uric, ascorbic)

  • Phosphate, sulfate, calcium, potassium, sodium (about 70%)

• Transport maximum: Each carrier protein has a limit to how much it can transport.

Sodium Reabsorption

• Key to reabsorption of water and other solutes; establishes osmotic and electrical gradients.

• Occurs via Na+/K+ ATPase pumps and co-transport with other molecules.

• Electrical gradient: Movement of Na+ creates a negative charge, attracting anions.

• Osmotic gradient: Water follows sodium by osmosis.

• About 99% of filtered water and sodium are reabsorbed.

Tubular Secretion

• Reverse of reabsorption; moves substances from blood into the tubule for excretion.

• Regulates acid-base balance by secreting hydrogen and bicarbonate ions.

• Removes wastes (urea, uric acid, creatinine, bile acids, ammonia) and drugs from the blood.

Regulation of Urine Concentration and Volume

• Depends on water content and the osmolarity of interstitial fluid around the distal convoluted tubule (DCT) and collecting duct (CD).

• Antidiuretic hormone (ADH) increases water reabsorption by inserting aquaporins into the tubule wall.

• Concentrated urine is produced during dehydration; dilute urine when hydrated.

Urea and Uric Acid Excretion

• Urea: Byproduct of amino acid metabolism; filtered and partially reabsorbed.

• Uric acid: Byproduct of nucleic acid metabolism (purines); almost all filtered uric acid is reabsorbed, but about 10% is secreted back into the filtrate.

• Excess uric acid can precipitate and form crystals (gout).

Urine Composition

• Varies depending on hydration and other factors.

• Usually 95% water.

• Contains urea, uric acid, creatinine.

• Glucose, proteins, ketones, and blood cells are not usually present (may indicate disease if found).

• Normal urine volume: 0.6–2.5 L/day (50–60 mL/hour is typical).

• Oliguria (<30 mL/hour) may indicate kidney failure.