Urinary System: Structure, Function, and Regulation

Urinary System Overview

Primary Functions of the Urinary System

The urinary system is essential for maintaining the body's internal environment by regulating water, solute concentrations, and removing metabolic wastes. The kidneys are the major excretory organs, performing several critical functions:

• Regulation of water volume and solute concentration: Ensures homeostasis of body fluids.

• Regulation of ion concentrations in extracellular fluid (ECF): Maintains electrolyte balance.

• Long-term acid-base balance: Controls pH of blood and body fluids.

• Excretion of metabolic wastes, toxins, and drugs: Removes harmful substances from the body.

• Production of erythropoietin: Hormone that stimulates red blood cell production.

• Production of renin: Hormone that regulates blood pressure.

• Activation of vitamin D: Essential for calcium absorption.

• Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources during fasting.

Main Structures of the Urinary System

Supportive Tissue Layers of the Kidney

Three layers of supportive tissue surround each kidney, providing protection and anchorage:

• Renal fascia: Anchoring outer layer of dense fibrous connective tissue.

• Perirenal fat capsule: Fatty cushion that protects the kidney from trauma.

• Fibrous capsule: Transparent capsule that prevents the spread of infection to the kidney.

Structure and Function of the Kidneys

Internal Anatomy of the Kidney

The kidney has three distinct internal regions:

• Renal cortex: The outer, granular-appearing region.

• Renal medulla: The inner region, composed of cone-shaped medullary (renal) pyramids.

• Renal pyramids: Separated by renal columns, which are inward extensions of cortical tissue.

• Lobe: Consists of a medullary pyramid and its surrounding cortical tissue.

Renal Pelvis and Urine Flow

The renal pelvis is a funnel-shaped tube continuous with the ureter. Urine flows through the following structures:

• Minor calyces: Cup-shaped areas that collect urine draining from pyramidal papillae.

• Major calyces: Collect urine from minor calyces and empty into the renal pelvis.

• Renal pelvis: Collects urine and channels it into the ureter.

Urine flow: Renal pyramid → minor calyx → major calyx → renal pelvis → ureter

Nephrons: Structure and Function

Renal Corpuscle and Glomerulus

Nephrons are the structural and functional units of the kidney, responsible for filtering blood and forming urine. Each kidney contains over one million nephrons, each consisting of a renal corpuscle and a renal tubule.

• Glomerulus: A tuft of highly porous capillaries that efficiently forms filtrate.

• Glomerular capsule (Bowman's capsule): Cup-shaped structure surrounding the glomerulus.

• Filtrate: Plasma fluid processed by renal tubules to form urine.

Renal Tubule

The renal tubule is about 3 cm long and drains into the collecting duct. It consists of three major parts:

1. Proximal convoluted tubule (PCT): Confined to the cortex, lined with cuboidal cells with microvilli for increased reabsorption and secretion.

2. Nephron loop (Loop of Henle): U-shaped structure with descending and ascending limbs.

3. Distal convoluted tubule (DCT): Functions more in secretion than reabsorption and drains into the collecting duct.

Collecting Ducts

Collecting ducts receive filtrate from many nephrons, run through medullary pyramids, and deliver urine through papillae into minor calyces.

Types of Nephrons

• Cortical nephrons: Make up 85% of nephrons, almost entirely in the cortex.

• Juxtamedullary nephrons: Originate near the cortex-medulla junction, have long nephron loops extending into the medulla, essential for producing concentrated urine.

Renal tubules are associated with two capillary beds: glomerulus and peritubular capillaries/vasa recta.

Glomerulus: Capillary Bed

The glomerulus is specialized for filtration and is fed and drained by arterioles. The afferent arteriole enters the glomerulus, and the efferent arteriole leaves, feeding into peritubular capillaries or vasa recta. Blood pressure in the glomerulus is high due to the larger diameter of afferent arterioles and high resistance of arterioles.

Peritubular Capillaries and Vasa Recta

Peritubular capillaries are low-pressure, porous vessels adapted for absorption of water and solutes. Vasa recta are long, thin-walled vessels parallel to nephron loops of juxtamedullary nephrons, essential for forming concentrated urine.

Juxtaglomerular Complex (JGC)

Each nephron has a juxtaglomerular complex, important for regulating filtrate formation and blood pressure. It consists of:

• Macula densa: Chemoreceptors monitoring NaCl content.

• Granular cells (JG cells): Mechanoreceptors monitoring blood pressure and secreting renin.

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

Formation of Urine

Three Major Renal Processes

Urine formation involves three processes:

1. Glomerular filtration: Produces cell- and protein-free filtrate.

2. Tubular reabsorption: Selectively returns 99% of substances from filtrate to blood.

3. Tubular secretion: Selectively moves substances from blood to filtrate.

Glomerular Filtration

Glomerular filtration is a passive process driven by hydrostatic pressure. It allows molecules smaller than 3 nm (water, glucose, amino acids, nitrogenous wastes) to pass, while plasma proteins remain in the blood to maintain colloid osmotic pressure.

Tubular Reabsorption

Tubular reabsorption quickly reclaims most of the tubular contents and returns them to the blood. The proximal convoluted tubule is the site of most reabsorption, including all nutrients, 65% of Na+ and water, many ions, almost all uric acid, and about half of urea.

• Nephron loop: Descending limb allows water to leave; ascending limb allows solutes to leave.

• Distal convoluted tubule and collecting duct: Reabsorption is hormonally regulated by ADH, aldosterone, atrial natriuretic peptide, and parathyroid hormone.

Tubular Secretion

Tubular secretion is the process of moving substances from peritubular capillaries into the filtrate. It is important for disposing of drugs, eliminating undesirable substances, ridding the body of excess K+, and controlling blood pH.

Pressures Involved in Filtration

Filtration is governed by outward and inward pressures:

• Outward pressure: Hydrostatic pressure in glomerular capillaries (HPgc), about 55 mm Hg.

• Inward pressures: Hydrostatic pressure in capsular space (HPcs, 15 mm Hg) and colloid osmotic pressure in capillaries (OPgc, 30 mm Hg).

• Net filtration pressure (NFP): $\text{NFP} = \text{HP}_{gc} - (\text{HP}_{cs} + \text{OP}_{gc}) = 55 - (15 + 30) = 10 \text{ mm Hg}$

Glomerular Filtration Rate (GFR)

GFR is the volume of filtrate formed per minute by both kidneys (approx. 120-125 mL/min). It is directly proportional to net filtration pressure, surface area available for filtration, and membrane permeability.

Regulation of Urine Volume and Concentration

Intrinsic and Extrinsic Controls

Kidneys regulate GFR to maintain homeostasis. Intrinsic controls (renal autoregulation) maintain GFR within a mean arterial pressure (MAP) range of 80–180 mm Hg. Extrinsic controls (neural and hormonal) maintain systemic blood pressure.

Purpose	Intrinsic Control (Renal Autoregulation)	Extrinsic Control
Mechanisms	Myogenic, Tubuloglomerular feedback	Hormonal (renin-angiotensin-aldosterone), Neural (SNS)
Operating Conditions	MAP 80–180 mm Hg	MAP outside 80–180 mm Hg

Normal and Abnormal Characteristics of Urine

Urinalysis and Chemical Composition

Urinalysis examines urine for signs of disease and illegal substances. Normal urine is 95% water and 5% solutes, including urea, uric acid, creatinine, and various ions. Abnormal concentrations or components may indicate pathology.

Substance	Name of Condition	Possible Causes
Glucose	Glycosuria	Diabetes mellitus
Proteins	Proteinuria, albuminuria	Nephropathology, excessive exertion, pregnancy
Ketone bodies	Ketonuria	Starvation, untreated diabetes
Hemoglobin	Hemoglobinuria	Hemolytic anemia, burns
Bile pigments	Bilirubinuria	Liver disease, obstruction
Erythrocytes	Hematuria	Bleeding urinary tract
Leukocytes	Pyuria	Urinary tract infection

Physical Characteristics of Urine

• Color and transparency: Clear, pale to deep yellow; abnormal colors may indicate disease.

• Odor: Slightly aromatic when fresh; ammonia odor develops upon standing.

• pH: Slightly acidic (~pH 6); varies with diet and health.

• Specific gravity: Ranges from 1.001 to 1.035.

Ureters, Urinary Bladder, and Urethra

Structure and Function

The urinary system includes the kidneys, ureters, urinary bladder, and urethra. Ureters transport urine from kidneys to bladder; the bladder stores urine; the urethra expels urine from the body.

Ureters

Slender tubes that convey urine from kidneys to bladder. They begin at L2 as a continuation of the renal pelvis and enter the base of the bladder through the posterior wall. Increased bladder pressure closes the distal ends of the ureters, preventing backflow.

Urinary Bladder

The bladder is a muscular sac for temporary storage of urine, located retroperitoneally on the pelvic floor. It has openings for ureters and urethra, and the trigone is a smooth triangular area outlined by these openings. The bladder expands during filling without a significant rise in internal pressure.

Urethra

The urethra is a muscular tube that drains the urinary bladder. It has internal and external sphincters, with the internal sphincter being involuntary and the external sphincter voluntary. The female urethra is short and tightly bound to the anterior vaginal wall, while the male urethra is longer and carries both semen and urine.

Micturition (Urination)

Process of Micturition

Micturition is the act of emptying the urinary bladder. Three simultaneous events must occur:

1. Contraction of detrusor muscle by the autonomic nervous system (ANS).

2. Opening of internal urethral sphincter by the ANS.

3. Opening of external urethral sphincter by the somatic nervous system.

Urinary incontinence: In adults, usually caused by weakened pelvic muscles. Urinary retention: Bladder unable to expel urine, often treated by catheterization.