Anatomy and Physiology Chapter 26: Urinary System

Urinary System Functions

• The urinary system is the major excretory system of the body, responsible for filtering blood to remove wastes and conserving water.
• The kidneys filter a large volume of blood to remove waste products, which form urine consisting of excess water, ions, metabolic waste, and toxic substances.
• The kidneys also regulate blood volume and pressure, electrolyte levels, pH, and red blood cell synthesis.

Functions of the Kidneys

• Excrete waste products from the blood, with 21% of cardiac output filtered per minute.
• Regulate blood volume and pressure by controlling extracellular fluid volume.
• Regulate electrolyte levels, including Na+, Cl-, K+, Ca2+, HCO3-, and HPO42-.
• Regulate extracellular fluid pH by secreting H+.
• Regulate red blood cell synthesis by secreting erythropoietin.
• Regulate vitamin D synthesis to help regulate blood Ca2+ levels.

Anatomy of the Urinary System

• The kidney has two main regions: the cortex and the medulla.
• The renal columns are part of the cortical tissue that extends into the medulla.
• The renal pyramids are cone-shaped structures in the medulla, with the base projecting into the cortex and the apex pointing towards the sinus.

Nephron Structure

• A nephron is the functional and histological unit of the kidney.
• The nephron consists of the renal corpuscle, proximal convoluted tubule, nephron loop, and distal convoluted tubule.
• The nephron loop is divided into the descending limb and the ascending limb.
• The proximal convoluted tubule and distal convoluted tubule are both lined with simple cuboidal epithelium.

The Renal Corpuscle

• The renal corpuscle is the filtration unit of the nephron.
• It consists of the glomerular (Bowman's) capsule and the glomerulus.
• The glomerular capsule is a double-walled chamber that surrounds the glomerulus.
• The glomerulus is a network of capillaries that receives blood from the afferent arteriole.

Glomerular Capsule and Filtration

• The glomerular capsule is lined with a parietal layer (simple squamous epithelium) and a visceral layer (specialized podocytes).
• The visceral layer wraps around the glomerular capillaries.
• Fenestrae (window-like openings) in the endothelial cells of the glomerular capillaries and filtration slits (gaps between podocyte processes) allow for filtration.

Filtration Membrane

• The filtration membrane consists of the capillary endothelium, basement membrane, and podocytes.
• The first stage of urine formation occurs here, as fluid from the blood in the capillaries moves across the filtration membrane into the lumen of the Bowman's capsule.

Juxtaglomerular Apparatus

• The juxtaglomerular apparatus is a specialized structure near the glomerulus, responsible for renin production.
• It consists of juxtaglomerular cells (smooth muscle in the afferent arteriole) and the macula densa (specialized tubule cells in the distal convoluted tubule).

Urine Production

• Urine production involves three major steps: filtration, tubular reabsorption, and tubular secretion.
• Filtration occurs in the glomerular capillaries, where fluid and small molecules are forced out of the blood into the lumen of the Bowman's capsule.
• Tubular reabsorption occurs in the renal tubules, where cells contain transport proteins that move water and filtered molecules back into the blood.
• Tubular secretion occurs in certain tubule cells, where additional solutes are transported from the blood into the filtrate.

Filtration Pressure

• Filtration pressure is the pressure gradient responsible for filtration, forcing fluid from the glomerular capillary across the membrane into the lumen of the Bowman's capsule.
• The filtration pressure is the difference between the glomerular capillary pressure (GCP) and the sum of the capsular hydrostatic pressure (CHP) and the blood colloid osmotic pressure (BCOP).

Regulation of Glomerular Filtration Rate

• Intrinsic mechanisms, such as autoregulation, involve changes in the degree of constriction in afferent arterioles.
• Extrinsic mechanisms, such as the sympathetic nervous system and hormones, occur during severe conditions like hemorrhage or dehydration.### Reabsorption in the Nephron Loop
• The descending limb of the nephron loop is highly permeable to water and moderately permeable to ions (Na+, Cl-) and molecules (urea).
• Water moves out of the descending limb by osmosis, while solutes move in by diffusion.
• By the end of the thin segment, the filtrate volume has decreased by 15% and its concentration has increased to 1200 mOsm/L.

Reabsorption in the Ascending Limb

• The ascending limb has a simple squamous epithelium that is impermeable to water but permeable to solutes.
• Solutes exit the ascending limb, reducing the filtrate's concentration.
• The thick segment of the ascending limb has simple cuboidal epithelium that is impermeable to both water and solutes.
• The thick segment has ATP-powered pumps and symporters that remove solutes from the filtrate, contributing to the kidneys' ability to conserve water.

Secretion of Hydrogen and Potassium Into the Renal Tubule

• Hydrogen ions are secreted into the filtrate by an antiport mechanism in the proximal convoluted tubule, exchanging H+ for Na+.
• Hydrogen ions are derived from the peritubular capillaries and the reaction between CO2 and water in the tubule cells.
• Sodium ions and HCO3- are symported across the basal membrane into the interstitial fluid and then diffuse into the peritubular capillaries.
• Hydrogen ions and K+ are secreted into the filtrate by antiport mechanisms in the distal convoluted tubule.

Urine Concentration Mechanism

• The kidneys can produce urine with concentrations ranging from 65 mOsm/kg to 1200 mOsm/kg.
• The ability to control urine volume and concentration depends on countercurrent mechanisms, medullary concentration gradient, and hormonal mechanisms.

Countercurrent Mechanisms

• The countercurrent mechanism involves fluid in separate structures flowing in opposite directions, allowing for material exchange.
• The countercurrent multiplier in the nephron loop is responsible for the high solute concentration in the interstitial fluid of the medulla.
• The countercurrent exchanger in the vasa recta maintains the high solute concentration in the interstitial fluid.

Summary of Urine Formation

• About 180 L of filtrate enter the proximal convoluted tubules daily.
• Glucose, amino acids, Na+, Ca2+, K+, Cl-, water, and other substances are reabsorbed into the interstitial fluid in the proximal convoluted tubules.
• The filtrate then enters the descending limbs of the nephron loops, where water diffuses out and solutes diffuse in.
• The filtrate's volume is reduced by 80% and its osmolality increases to 1200 mOsm/kg by the time it reaches the end of the nephron loops.
• The filtrate then enters the ascending limbs, where solutes are removed and water is reabsorbed, decreasing the filtrate's osmolality to 100 mOsm/kg.
• The filtrate enters the distal convoluted tubules and collecting ducts, where it is further regulated by hormonal mechanisms.

Regulation of Urine Concentration and Volume

• Filtrate reabsorption in the proximal convoluted tubules and descending limbs is obligatory and relatively constant.
• Filtrate reabsorption in the distal convoluted tubules and collecting ducts is tightly regulated and can change dramatically depending on body conditions.
• Regulation of urine concentration and volume involves hormonal mechanisms, including the renin-angiotensin-aldosterone mechanism and the antidiuretic hormone (ADH) mechanism.

Renin-Angiotensin-Aldosterone Hormone Mechanism

• The mechanism is initiated under low blood pressure conditions and counteracts dropping blood pressure.
• Renin is released by juxtaglomerular cells, converting angiotensinogen to angiotensin I.
• Angiotensin-converting enzyme (ACE) in the lungs converts angiotensin I to angiotensin II, a potent vasoconstrictor that stimulates aldosterone secretion, thirst, and ADH secretion.
• Aldosterone acts on the distal convoluted tubules and collecting ducts to increase sodium reabsorption and water reabsorption.

Effect of Aldosterone on the DCT

• Aldosterone increases the synthesis of the Na+-K+ pump and other Na+ transport proteins in the distal convoluted tubules and collecting ducts.
• The Na+-K+ pump increases sodium reabsorption and potassium secretion across the basal membrane of tubule cells.
• Other Na+ transport proteins increase sodium transport across the apical membrane of tubule cells.

Antidiuretic Hormone Mechanism

• Antidiuretic hormone (ADH) is produced by hypothalamic neurons and stored in the posterior pituitary.
• Osmoreceptors in the hypothalamus detect increased osmolality of the interstitial fluid, stimulating ADH release.
• Baroreceptors in the atria of the heart and some vessels can also stimulate ADH release when blood pressure drops.
• ADH acts on the distal convoluted tubules and collecting ducts to increase water reabsorption, countering any decrease in blood pressure and/or increase in solute concentration.

Effect of ADH on Renal Tubule Water Movement

• ADH binds to its receptor in the plasma membranes of the distal convoluted tubule cells and collecting duct cells.
• The binding of ADH activates a G protein mechanism, which increases the rate of cAMP synthesis.
• cAMP promotes the insertion of aquaporin-2 containing cytoplasmic vesicles into the apical membranes of the distal convoluted tubules and collecting ducts.
• Water then moves by osmosis out of the distal convoluted tubules and collecting ducts into the tubule cells through the aquaporin-2 water channels.### Atrial Natriuretic Hormone (ANH)
• Produced by cells in the right atrium of the heart when they are stretched more than normal
• Increases blood volume, which causes a stretch in the atrium, leading to the production of ANH
• Functions to decrease blood volume by:
  • Inhibiting Na+ reabsorption, resulting in less water reabsorption
  • Inhibiting ADH production
  • Increasing urine production
  • Decreasing venous return, which lowers the volume in the right atrium

Regulation of Blood Pressure

• Blood volume is maintained within a normal range through a series of control centers and responses
• When blood volume increases outside the normal range, the control centers respond to:
  • Decrease ADH and aldosterone secretion, reducing water reabsorption
  • Dilate renal arteries, increasing urine production
  • Stimulate the heart to secrete ANH, increasing urine production
• These responses cause blood volume to decrease, returning to the normal range

Urine Movement and Micturition Reflex

• Ureters bring urine from the renal pelvis to the urinary bladder
• The urinary bladder is a hollow muscular container lined with transitional epithelium
• The detrusor muscle is the smooth muscle part of the bladder wall
• The trigone is the triangular area between the entry of the two ureters and the exit of the urethra
• Hydrostatic pressure forces urine through the nephron and peristalsis moves urine through the ureters
• Parasympathetic stimulation increases the frequency of peristalsis, while sympathetic stimulation decreases it
• The micturition reflex is activated when the urinary bladder is stretched, leading to:
  • Contraction of the detrusor muscle
  • Relaxation of the external urethral sphincter
  • Urine flow through the urethra

Effects of Aging on the Kidneys

• Gradual decrease in kidney size, but only one-third of one kidney is necessary for homeostasis
• Decrease in blood flow through the kidneys
• Decrease in the number of glomeruli and the ability to secrete and reabsorb substances
• Decreased ability to concentrate urine and respond to ADH and aldosterone
• Reduced ability to participate in vitamin D synthesis, contributing to calcium deficiency and osteoporosis

Fluid, Electrolyte, and Acid-Base Balance

• Maintaining a homeostatic balance of electrolytes and fluids is important for proper body function
• Concentration of major solutes:
  • Intracellular cation: K+
  • Interstitial fluid cation: Na+
  • Plasma cation: Na+
  • Intracellular anion: Protein and phosphate
  • Interstitial fluid anion: Cl-
  • Plasma anion: Cl-
• Exchange between compartments:
  • Hydrostatic pressure and osmotic pressure regulate fluid movement
  • Osmotic pressure has the greatest influence on maintaining fluid homeostasis
• Regulation of intracellular fluid:
  • Selectively permeable plasma membrane
  • Large molecules synthesized by the cell cannot leave
  • Transport proteins in the plasma membrane, some of which use ATP for pumps

Regulation of Fluid Balance

• Thirst is the sensation that induces an urge to drink liquids
• Mechanisms that control thirst:
  • Hypothalamic osmoreceptors
  • Arterial and juxtaglomerular apparatus baroreceptors
  • Dryness of the mouth
  • Distension of the stomach
• Regulation of fluid output:
  • Influencing urine output and reducing thirst
  • Mechanisms that monitor blood pressure, which is directly related to blood volume

Regulation of Electrolyte Balance

• Electrolytes are formed when molecules dissociate into ions in water
• Regulation of electrolytes involves the coordinated participation of several organ systems
• Regulation of sodium:
  • Dominant extracellular cation that exerts substantial osmotic pressure
  • Kidneys are the major route for sodium excretion
  • Aldosterone increases sodium reabsorption in the kidney
  • Sodium levels affect blood pressure because water follows sodium
  • ANH increases sodium and water excretion
• Regulation of potassium:
  • Important for exocytosis, including neurotransmitters
  • Needed for muscle contraction
  • Regulates action potential in cardiac muscle
  • Regulated by the kidneys, digestive tract, and bones
  • Balance between deposition and reabsorption from bone
  • Balance between absorption from the intestines and excretion by the kidneys
• Regulation of calcium, magnesium, and phosphate ions:
  • Important functions in the body
  • Regulated by the kidneys, digestive tract, and bones
  • Imbalances lead to various symptoms and disorders

Explore the complex processes of the kidneys in filtering blood and removing toxins. This quiz covers the Urinary System chapter in Seeley's Anatomy and Physiology, 13th Edition.