PH

Essentials of Human Anatomy & Physiology - The Urinary System

Functions of the Urinary System

  • Disposal of Waste Products:

  • Kidneys excrete waste products in the urine, including:

    • Nitrogenous wastes

    • Toxins

    • Drugs

    • Excess ions

  • Kidneys produce erythropoietin which stimulates red blood cell production and helps regulate blood pressure by controlling renal blood flow

  • Regulatory Functions:

  • Functional Unit of the Urinary System- Nephrons are the basic structural and functional units of the kidneys, responsible for filtering blood, reabsorbing essential substances, and excreting waste through urine.

  • Renin Production:

    • Helps maintain blood pressure.

  • Erythropoietin Production:

    • Stimulates red blood cell production.

  • Vitamin D Conversion:

    • Converts vitamin D to its active form.

Organs of the Urinary System

  • Kidneys

  • Ureters

  • Urinary Bladder

  • Urethra (consists of smooth muscle and skeletal muscle)

Kidneys

Anatomy & Location

  • Kidneys form the same week as ears, therefore they could affect each other

  • Positioned retroperitoneally (behind the peritoneum).

  • The right kidney is slightly lower than the left due to the liver's position.

Structure

  • Average Size:

  • Approximately 12 cm long and 6 cm wide.

  • Renal Hilum:

  • Entrance for structures including ureters, renal vessels, and nerves.

  • Adrenal Glands:

  • Sit atop each kidney.

  • Protective Layers:

  • Fibrous capsule- encloses kidney

  • Perirenal fat capsule- surrounds the kidney (cushion)

  • Renal fascia- anchors kidney to surrounding structures

Regions of the Kidney

  • Renal Cortex:

  • Outer region.

  • Renal Medulla:

  • Deeper region consisting of renal pyramids and renal columns.

  • Renal Pelvis:

  • Funnel-shaped tube that collects urine from calyces.

Blood Supply

  • Renal Artery:

  • Supplies blood to each kidney (about ¼ of cardiac output).

  • Renal Vein:

  • Drains blood from the kidney to the inferior vena cava.

Blood flow through the kidney:

  • aorta → renal artery → segmental artery → interlobar artery → arcuate artery → cortical radiate artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → cortical radiate vein → arcuate vein → interlobar vein → renal vein → inferior vena cava.

Nephrons

Overview

  • Functional Units:

  • Each kidney has over a million nephrons.

  • Two main structures of a nephron:

  • Renal Corpuscle:

    • Composed of glomerulus and Bowman’s capsule.

  • Renal Tubule:

    • Subdivided into proximal convoluted tubule (PCT), nephron loop, and distal convoluted tubule (DCT).

Types of Nephrons

  • Cortical Nephrons:

  • Located in the cortex.

  • Juxtamedullary Nephrons:

  • Located at the cortex-medulla junction; nephron loop extends into the medulla.

Capillary Structures

  • Glomerulus:

  • High-pressure capillary bed specialized for filtration.

  • Peritubular Capillaries:

  • Low-pressure capillaries for reabsorption of solutes and water.

Pathway of Renal Tubule:

  1. Proximal Convoluted Tubule (PCT):

    • The first segment of the renal tubule where most reabsorption of water, glucose, amino acids, and ions occurs.

  2. Nephron Loop (Loop of Henle):

    • Divided into descending and ascending limbs.

      • Descending Limb: Permeable to water, allowing for its reabsorption.

      • Ascending Limb: Impermeable to water but actively transports ions out into the interstitial fluid, contributing to the concentration of urine.

  3. Distal Convoluted Tubule (DCT):

    • The final segment of the renal tubule where further reabsorption of ions and water occurs under hormonal control (e.g., aldosterone affecting sodium).

After the DCT, the filtrate then moves into collecting ducts for final processing before becoming urine.Urine Formation

Processes Involved

  1. Glomerular Filtration:

  • Passive process where water and small solutes move from blood to Bowman’s capsule.

  • Filtrate forms as blood pressure forces fluid through capillary walls.

  1. Tubular Reabsorption:

  • Peritubular capillaries reabsorb useful substances like water, glucose, amino acids, and ions from renal tubules.

  • Most reabsorption occurs in the PCT.

  1. Tubular Secretion:

  • Materials like hydrogen and potassium ions move from peritubular capillaries back into the renal tubules for elimination.

Characteristics of Urine

  • Production:

  • Daily production ranges from 1.0 to 1.8 liters.

  • Differences Between Urine and Filtrate:

  • Filtrate contains all blood plasma components except proteins; urine is what remains after reabsorption of water and nutrients.

  • Typical Composition:

  • Clear yellow color due to urochrome, slightly acidic pH (around 6), contains urea, uric acid, creatinine, sodium, and potassium ions.

  • Gout- increase of uric acid

  • Urea- end product of protein breakdown

  • Specific gravity (concentration of urine)- between 1.001 and 1.035

Ureters

  • Function:

  • Transport urine from kidneys to the urinary bladder via peristalsis.

  • Length:

  • Approximately 25–30 cm (10–12 inches).

Urinary Bladder

Structure and Function

  • Shape and Composition:

  • Smooth muscular sac that temporarily stores urine.

  • Trigone:

  • Triangular region at the base with openings for ureters and urethra.

Muscle Structure

  • Detrusor Muscle:

  • Composed of three layers of smooth muscle.

Capacity

  • Can hold approximately 500 mL of urine comfortably, expandable up to twice that amount.

Urethra

Differences in Males and Females

  • Function:

  • Females: carries urine only.

  • Males: carries urine and sperm.

  • Control Mechanism:

  • Internal sphincter (involuntary) and external sphincter (voluntary).

Length

  • Female: 3-4 cm (1.5 inches)

  • Male: 20 cm (8 inches)

Micturition

Process of Urination

  • Micturition involves contraction of the bladder and relaxation of the sphincters when sufficient urine is collected (200 mL).

  • Osmotic and Oncotic pressure

  • Lumbar Plexus gives sensation when you need to urinate

  • The autonomic nervous system also plays a role in regulating bladder control, facilitating both voluntary and involuntary responses to the urge to void.

Fluid, Electrolyte, and Acid-Base Balance

Blood composition depends on 3 factors:

  • diet

  • cellular metabolism

  • urine output

Water occupies 3 main fluid compartments:

  • Intracellular fluid (ICF)

    • fluid inside cells

  • Extracellular Fluid (ECF)

    • fluid outside cells

  • Plasma

    • links external and internal enviroments

Functions of Kidneys

  • Regulate blood composition, maintain pH, and balance electrolytes and water.

Water Intake and Regulation

  • Total body water varies by age and sex, requiring balance of intake through food, drink, and metabolic processes.

  • Thirst Mechanism:

  • Controlled by osmoreceptors in the hypothalamus.

Electrolyte Balance

  • Hormonal regulation (e.g., aldosterone) influences sodium and water balance.

  • Water follows salt

Acid-Base Balance

  • Maintained primarily by kidneys and buffering systems:

  • Bicarbonate buffer system, phosphate buffer system, protein buffer system

Developmental Aspects

  • Infants:

  • Kidneys develop early in gestation, with urination by the third month.

  • Aging Effects:

  • Decreased filtration rate, increased urgency/frequency, urinary retention common in older males.