chapter 24

Introduction to the Urinary System

  • Components of the urinary system

    • Kidneys:

    • Function: Filter blood to remove waste products.

    • Converts the filtered material (filtrate) into urine.

    • Ureters:

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

    • Bladder:

    • Structure: Expandable muscular sac capable of storing up to 1 liter of urine.

    • Urethra:

    • Function: Eliminate urine from the body.

Urinary System Anatomy

Urinary System (Figure 24.1a)

  • Components illustrated:

    • Diaphragm

    • Adrenal gland

    • Kidneys

    • Renal artery

    • Hilum (where vessels, nerves, ureter enter the kidney)

    • Renal vein

    • Inferior vena cava

    • Descending abdominal aorta

    • Ureters

    • Parietal peritoneum

    • Urinary bladder

    • Urethra

Urinary System (Figure 24.1b)

  • Components illustrated:

    • Adrenal gland

    • T12 vertebra

    • Left kidney

    • Right kidney

    • L3 vertebra

    • 12th rib

    • Ureters

    • Urinary tract

    • Urinary bladder

    • Urethra

Functions of the Kidneys

  • Removal of wastes from blood:

    • Wastes are excreted in urine.

Other Functions of the Kidneys

  • Regulation of blood ion concentrations:

    • Major ions: Sodium (Na+), Potassium (K+), and Calcium (Ca2+).

  • Maintaining acid-base balance:

    • Alters levels of Hydrogen ions (H+) and bicarbonate ions (HCO3⁻).

Kidney Function and Regulation

  • Regulation of blood pressure:

    • Alters the amount of fluid lost in urine.

    • Helps to regulate blood volume.

    • Releases renin enzyme, required for production of angiotensin II (hormone that increases blood pressure).

Gross Anatomy of the Kidney

Kidney Characteristics

  • Shape and Structure:

    • Two symmetrical, bean-shaped organs.

    • Concave medial border known as hilum

    • Where vessels, nerves, and ureter connect to the kidney.

Kidney Location and Support

  • Kidney positioning:

    • Located on posterior abdominal wall.

    • Lateral to the vertebral column.

    • Left kidney: Between levels of T12 and L3 vertebrae.

    • Right kidney: 2 cm inferior to left kidney to accommodate the liver.

    • Partially protected by rib cage and vulnerable to forceful blows.

    • Positioned in retroperitoneal space with only the anterior surface covered by parietal peritoneum.

Kidney Anatomy

  • Fibrous capsule (Renal Capsule):

    • Structure: Connective tissue adhered to the external surface of the kidney.

    • Functions:

    • Maintains shape.

    • Protects from trauma.

    • Prevents pathogen penetration.

Sectional Anatomy of the Kidney

  • Cortex:

    • Outer section of the kidney.

  • Medulla:

    • Inner section of the kidney.

  • Renal pyramids:

    • 8-15 per kidney.

    • Portions of medulla divided by renal columns.

  • Renal columns:

    • Extensions of the cortex, forming a wide base at the external edge of the medulla, meeting the cortex.

    • Apex of renal pyramids termed renal papilla.

  • Renal sinus:

    • Medially located urine drainage area organized into minor calyces, major calyces, and renal pelvis.

  • Minor calyces:

    • Funnel-shaped structures associated with renal pyramids that merge to form major calyx.

  • Renal pelvis:

    • Formed from merged major calyces.

    • Merges at the medial edge of the kidney with the ureter.

    • Houses renal arteries, veins, lymph vessels, nerves, and fat around the pelvis.

Functional Anatomy of the Kidney: Nephron

Nephron Overview

  • Nephron:

    • Microscopic functional filtration unit of the kidney.

    • Consists of:

    • Renal corpuscle

    • Renal tubules

    • All of the renal corpuscle and most of the renal tubules reside in the cortex.

Renal Corpuscle

  • Structure:

    • Enlarged region of nephron within the renal cortex.

    • Composed of:

    • Glomerulus:

      • A ball of capillaries.

      • Blood entering by the afferent arteriole, exiting through the efferent arteriole.

    • Glomerular capsule:

      • Encloses the glomerulus.

Renal Tubules

  • Components:

    • Proximal convoluted tubule (PCT)

    • Nephron loop (Loop of Henle):

    • Contains descending limb and ascending limb.

      • Ascending limb further divided into thin segments and thick segments.

    • Distal convoluted tubule (DCT)

    • Collecting tubule

    • Collecting duct

Functional Anatomy of the Kidney: Types of Nephrons

Nephron Types

  • Classification:

    • Based on the relative position in the cortex and length of the loop.

  • Cortical nephrons:

    • Most numerous in cortex.

    • Short nephron loop, just penetrating the medulla.

  • Juxtamedullary nephrons:

    • Long nephron loops extending deep into the medulla.

    • Help establish a salt concentration gradient in interstitial space to allow for regulation of urine concentration.

Functional Anatomy of the Kidney: Collecting Tubules and Ducts

  • Nephron drainage:

    • Several collecting tubules drain into a collecting duct.

Blood Flow and Filtered Fluid Flow

Blood Flow Through the Kidney

  • Volume:

    • 20% to 25% of resting cardiac output (~1 liter per minute).

  • Filtrate formation:

    • Occurs when blood flows through the glomerulus, allowing some plasma components to enter the capsular space.

Arteries

  • Blood delivery:

    • Each kidney receives blood via the renal artery, which arises from the abdominal aorta.

    • Blood travels through several arteries to enter the afferent arteriole.

Arterioles and Capillaries

  • Afferent arteriole:

    • Enters the renal corpuscle forming the glomerulus.

  • Efferent arteriole:

    • Remaining blood exits the glomerulus and branches into either peritubular capillaries or vasa recta.

    • Peritubular capillaries:

    • Intertwined around proximal and distal convoluted tubules.

    • Vasa recta:

    • Associated with the nephron loop.

    • All blood moves through both capillary beds (glomerular capillaries and either peritubular capillaries or vasa recta) and drains into a network of veins.

Veins

  • Drainage:

    • Drain peritubular and vasa recta capillary beds, travel through several veins to the renal vein, and then drains into the inferior vena cava.

Blood Supply to the Kidneys

Blood Supply Diagram (Figure 24.8)

  • Key components illustrated:

    • Peritubular capillaries (associated with convoluted tubules)

    • Medulla

    • Cortex

    • Nephrons and their segments (DCT, nephron loop, PCT, etc.)

    • Various arteries and veins associated with kidney anatomy.

Blood Flow and Filtered Fluid Flow: Filtrate, Tubular Fluid, and Urine Flow

Definitions

  • Filtrate:

    • Blood that flows out of the glomerulus, moving across the wall of glomerular capillaries into the capsular space.

  • Tubular fluid:

    • Filtrate is renamed as it exits the glomerular capsule.

    • Items Filtered:

      • Water, ions, glucose, wastes.

    • Items Not Filtered:

      • Blood cells, proteins.

  • Urine:

    • Tubular fluid is renamed as it leaves the collecting ducts.

    • Flows from the papillary duct, located within the renal papilla, into the renal sinus, then from minor calyx to major calyx, and into the renal pelvis.

    • Finally, the renal pelvis connects to the ureter which leads to the urinary bladder (where urine is stored) and is excreted from the body through the urethra.

Filtration Membrane in the Renal Corpuscle

Characteristics of the Filtration Membrane

  • Structure:

    • Porous, thin, negatively charged structure formed by the glomerulus and the visceral layer of the glomerular capsule.

    • Composed of three sandwiched layers:

    • Endothelium of glomerulus:

      • Innermost layer closest to the lumen, fenestrated which allows plasma and dissolved substances to filter while restricting large structures (e.g., erythrocytes).

    • Basement membrane of glomerulus:

      • Porous middle layer that restricts large plasma proteins.

    • Visceral layer of glomerular capsule:

      • Outermost layer composed of specialized cells (podocytes) with long processes (pedicels).

      • These wrap around the glomerular capillaries, support the capillary wall, and do not completely ensheathe it.

      • Filtration slits separate podocytes and restrict passage of most small proteins.

Pressures Associated with Glomerular Filtration

  • Pressures that influence filtration:

    • Hydrostatic pressure of blood in the glomerulus promotes material moving out of blood.

    • Colloidal osmotic blood pressure and fluid pressure in capsular spaces promote material moving into blood.

    • Net Filtration Pressure (NFP):

    • Calculated as difference between pressures:
      ext{NFP} = ext{Hydrostatic Pressure} - ( ext{Colloidal Osmotic Pressure} + ext{Fluid Pressure})

  • Glomerular filtration rate (GFR):

    • Rate of filtration volume formed.

    • Increased NFP results in increased GFR, leading to more solutes and water remaining in tubular fluid and decreased filtrate reabsorption.

Regulation of Glomerular Filtration Rate

  • Importance:

    • Helps kidneys control urine production based on physiological conditions (e.g., hydration status).

    • Influenced by changing luminal diameter of afferent arteriole and altering surface area of filtration membrane.

  • Neural and hormonal control:

    • Sympathetic stimulation reduces glomerular pressure and GFR through vasoconstriction of afferent arteriole, thus decreasing surface area of glomerulus and blood flow into glomerulus.

Reabsorption and Secretion in Tubules and Collecting Ducts

Complete Reabsorption Processes

  • Glucose reabsorption:

    • Occurs at the glomerulus and at the Proximal Convoluted Tubule (PCT) via glucose transport proteins; usually 100% reabsorbed in healthy individuals.

Amino Acid and Protein Reabsorption

  • Small and medium-sized proteins can be in filtrate; all proteins and amino acids are transported back to the blood via protein transporters in the PCT.

Regulated Reabsorption

Sodium Reabsorption

  • Sodium is reabsorbed in the glomerulus, PCT, ascending loop, DCT, and cortical region of the collecting duct.

  • Regulation by hormones occurs near the end of the tubule, including aldosterone (which increases Na+ reabsorption and K+ secretion) and dietary alterations.

Water Reabsorption

  • Primarily by osmosis, occurring:

    • Between cells.

    • Through aquaporins.

    • An average of 180 L filtered daily, with all but 1.5 L reabsorbed.

    • Most water is reabsorbed in the PCT.

Nephron Loop and Collecting Duct

  • Osmolarity:

    • Increases deeper into the kidney, ranging from ~300 mOsm near cortex to approximately 1400 mOsm in the deepest part of the medulla, facilitating reabsorption due to osmotic gradients.

Hormonal Control of Water Reabsorption

  • Antidiuretic hormone (ADH):

    • Increases water reabsorption, decreases urine volume, increases urine concentration, and can cause urine coloration changes based on concentration.

Secretion in Tubules and Collecting Ducts

Substance Management

  • Substances eliminated as waste:

    • Prevents accumulation of cellular wastes, various hormones, and metabolites.

    • Some substances are filtered at the glomerulus, while others are secreted along the tubule pathway.

Nitrogenous Waste Elimination

  • Key products include:

    • Urea: Produced from protein breakdown, both reabsorbed and secreted.

    • Uric Acid: Produced from nucleic acid breakdown, both reabsorbed and secreted.

    • Creatinine: Produced from creatine metabolism, primarily secreted.

Drug and Bioactive Substance Secretion

  • Occurs mainly in the PCT:

    • Involves drugs (like penicillin), metabolic wastes (like bilirubin), and hormone metabolites.

Urine Characteristics, Transport, Storage, and Elimination

Characteristics of Urine

  • Composition:

    • Urine is a product of nephron function, typically sterile unless contaminated.

    • Composed of 95% water and 5% solutes (e.g., Na+, Cl-, K+).

Abnormal Components

  • May include glucose, ketones, proteins (in excess), bile, erythrocytes, hemoglobin, leukocytes, and nitrites.

  • Daily volume: Average between 1-2 L, varying due to several factors like fluid intake, blood pressure, and temperature.

  • Minimum urine volume necessary to eliminate wastes: 0.5 L.

Factors Influencing Urine Volume

Table Summary of Influences

  • Decrease Urine Volume:

    • Increase in ADH, aldosterone; decrease in atrial natriuretic peptide; decrease in fluid intake; decrease in blood pressure.

  • Increase Urine Volume:

    • Decrease in ADH, aldosterone; increase in atrial natriuretic peptide; increase in fluid intake; increase in blood pressure; diabetes; diuretics.

Urine pH and Specific Gravity

  • pH: Usually between 4.5 and 8.0, influenced by diet and metabolism.

  • Specific Gravity: Slightly higher than that of water due to solutes, with hydration increasing urine output and lowering specific gravity.

Urine Color

  • Ranges from almost clear to dark yellow depending on urobilin concentration; may appear clouded due to protein, bacteria, or other causes.