Ch 14 Urinary System

The Juxtaglomerular Apparatus

• The distal convoluted tubule cells contacting JG cells are called macula densa (chemo or osmoreceptors) that respond to changes in the solute concentration of the filtrate in the tubule.

Vasa Recta

• Vasa recta are capillaries of the juxtamedullary nephrons that loop and have a hairpin configuration, forming a bundle of long straight vessels.

Glomerular Filtration

• Urine formation begins when waste and water and dissolved materials are filtered out of the glomerular capillary.
• Urinary excretion = glomerular filtration + tubular secretion – tubular reabsorption.
• The glomerular capillaries are more permeable than the capillaries in other tissues.
• Filtration pressure = forces favoring filtration (glomerular capillary hydrostatic pressure & capsular osmotic pressure) – forces opposing filtration (capsular hydrostatic pressure & glomerular capillary osmotic pressure).

Regulation of GFR

• Neural regulation involves sympathetic nerves that can cause the constriction or relaxation of the afferent arteriole, resulting in a change of GFR, upon activation of chloride ion levels.
• Renal autoregulation involves the juxtaglomerular apparatus (JGA) secreting vasoconstriction substances to the afferent arteriole, in response to GFR changes and NaCl levels.

Functions of the Urinary System

• Excretion – removal of waste material from the blood plasma and disposal of this waste in the urine.
• Elimination – removal of waste from other organ systems, including the digestive system, respiratory system, and skin.
• Water balance – kidney tubules regulate water reabsorption and urine concentration.
• Regulation of pH – volume, and composition of body fluids.
• Production of erythropoietin – for hematopoieses, and renin for blood pressure regulation.

Anatomy of the Urinary System

• Kidneys – a pair of bean-shaped organs located retroperitoneally, responsible for blood filtering and urine formation.
• Renal capsule – a layer of fibrous connective tissue covering the kidneys.
• Renal cortex – outer region of the kidneys where most nephrons are located.
• Renal medulla – inner region of the kidneys where some nephrons are located, also where urine is collected to be excreted outward.

Internal Anatomy of the Kidney

• Renal pyramid – connective tissues in the renal medulla binding various structures together.
• Renal pelvis – central urine collecting area of renal medulla.
• Hilum – concave notch of kidneys where renal artery, renal vein, ureter, nerves, and lymphatic vessels converge.
• Ureter – a tubule that transports urine (mainly by peristalsis) from the kidney to the urinary bladder.
• Urinary bladder – a spherical storage organ that contains up to 400 ml of urine.
• Urethra – a tubule that excretes urine out of the urinary bladder to the outside, through the urethral orifice.

Microscopic Anatomy

• Each kidney consists of about 1 million basic functional units called nephrons where blood filtering and urine formation occur.
• Each nephron is composed of the following parts: afferent arteriole → glomerulus → bowman's capsule → efferent arteriole → proximal convoluted tubule (PCT) → descending limb of loop of henle → loop of henle → ascending limb of loop of henle → distal convoluted tubule (DCT) → collecting duct (not part of the nephron).

Urine Formation

• Urine formation involves 4 processes: filtration, reabsorption, concentration, and secretion.
• Filtration – small molecules are filtered from glomerulus to bowman's capsule.
• Reabsorption – nutrient molecules are transported from PCT and DCT to peritubular capillaries.
• Concentration – water is reabsorbed from descending limb of loop of henle and from collecting duct into peritubular capillaries.
• Secretion – waste or harmful substances are transported from peritubular capillaries to PCT and DCT.

Glomerular Filtration

• Glomerular filtration begins when waste, water, and dissolved materials are filtered out of the glomerular capillary.
• Urinary excretion = glomerular filtration + Tubular secretion - Tubular reabsorption
• Glomerular capillaries are more permeable than capillaries in other tissues.
• Filtration pressure = forces favoring filtration (Glomerular capillary hydrostatic pressure & capsular osmotic pressure) – forces opposing filtration (capsular hydrostatic pressure & Glomerular capillary osmotic pressure).

Regulation of GFR

• Neural regulation involves sympathetic nerves, which can cause constriction or relaxation of the afferent arteriole, resulting in a change of GFR.
• Renal autoregulation involves the juxtaglomerular apparatus (JGA) secreting vasoconstriction substances to either afferent arteriole, in response to GFR changes and NaCl levels.

Functions of Nephron Components

• Renal capsule: filtration of H2O and dissolved substances from the plasma.
• Glomerulus: filtration of H2O and dissolved substances from the plasma.
• Glomerular capsule: receives the glomerular filtrate.
• Proximal convoluted tubule:
  • Reabsorption of glucose, amino acids, creatine, lactic acid, citric, uric, and ascorbic acids; phosphate, sulfate, calcium, K, and Na by active transport.
  • Reabsorption of proteins by pinocytosis.
  • Reabsorption of H2O by osmosis.
  • Reabsorption of Cl- and other negatively charged ions by electrochemical attraction.
  • Active secretion of substances such as penicillin and hydrogen ions.

Descending and Ascending Limbs of Nephron Loop

• Descending limb of nephron loop: reabsorption of H2O by osmosis.
• Ascending limb of nephron loop: reabsorption of Na, K, and Cl- by active transport.

Distal Convoluted Tubule

• Reabsorption of Na by active transport.
• Reabsorption of H2O by osmosis.
• Active secretion of hydrogen ions.
• Secretion of K both actively and by electrochemical attraction (passive).

Collecting Duct

• Reabsorption of H2O by osmosis.

Juxtaglomerular Apparatus

• Located at the point of contact between the distal convoluted tubule and the afferent and efferent arterioles.
• JG cells are mechanoreceptors that detect changes in blood pressure in the afferent arterioles and secrete renin.
• Hormonal regulation involves the JGA secreting a hormone called renin, which activates an inactive hormone from the liver, resulting in an active hormone (angiotensin II) that causes constriction of afferent arteriole and release of aldosterone from adrenal cortex, leading to salt and water retention.

GFR

• The rate of filtration varies with filtration pressure.
• Filtration pressure changes with the diameters of the afferent and efferent arterioles.
• Constriction of afferent arterioles due to sympathetic stimulation decreases glomerular filtration rate.
• As the osmotic pressure in the glomerulus increases, filtration decreases.
• As the hydrostatic pressure in a glomerular capsule increases, filtration increases.
• The kidney produces 125 ml of glomerular fluid per minute, most of which is reabsorbed.
• The volume of filtrate varies with the surface area of the glomerular capillary.

Regulation of Filtration Rate

• Glomerular filtration rate remains relatively constant but may increase or decrease when needed.
• Increased sympathetic activity decreases GFR.
• When tubular fluid NaCl decreases, the macula densa causes the JG cells to release renin, which leads to vasoconstriction, affecting GFR, and secretion of aldosterone, which stimulates tubular Na+ reabsorption.

Role of ADH (Antidiuretic Hormone)

• Concentration of H2O in the blood decreases.
• Increase in the osmotic pressure of body fluids stimulates osmoreceptors in the hypothalamus.
• Hypothalamus signals the post.pituitary gland to release ADH.
• Blood carries ADH to the kidneys.
• ADH causes the distal convoluted tubules and collecting ducts to become more permeable and increase H2O reabsorption by osmosis.
• Urine becomes more concentrated, and urine volume decreases.

Urine Formation

• Urine formation involves 4 processes: filtration, reabsorption, concentration, and secretion.
• Small molecules in blood plasma are filtered from the glomerulus to Bowman's capsule.
• Nutrient molecules are transported from PCT and DCT to peritubular capillaries.
• Water is reabsorbed from descending limb of loop of Henle and from collecting duct into peritubular capillaries.
• Waste or harmful substances are transported from peritubular capillaries to PCT and DCT.

Mechanism of Forming Dilute and Concentrated Urine

• Urine formation and adjustment of blood composition involve three major processes: glomerular filtration, tubular reabsorption, and secretion.
• The preritubular capillary is adapted for reabsorption. It carries low-pressure blood and is very permeable.
• Most reabsorption (70%) occurs in the proximal tubule.
• Different modes of transport reabsorb various substances in particular segments of the renal tubule.
• Glucose and amino acids are reabsorbed by active transport. H2O is reabsorbed by osmosis. Proteins are reabsorbed by pinocytosis.