Chapter 49 - The Urinary System

-rids body of waste products

learning outcomes:

• name and locate the organs of urinary system
• List several functions of the kidneys in addition to urine formation
• Name the major nitrogenous wastes and identify their sources
• Define excretion and identify the systems that excrete wastes

consists of six organs: two kidneys, two ureters, urinary bladder, and urethra

Functions of the kidneys

-filters blood plasma, separates waste form useful chemicals, returns useful substances to blood, eliminates waste

-regulate blood volume and pressure by eliminating or conserving water

-regulate the osmolarity of the body fluids by controlling the relative amounts of water and solutes eliminated

-secretes enzyme, renin, which activates hormonal mechanism that control blood pressure and electrocyte

-secretes erythropoietin, which stimulates the production of red blood cells

-collaborate the with lungs to regulate PCO2 (percent of carbon dioxide) and acid base balance of body fluids

-final step in synthesizing hormone calcitriol, which contributes to calcium homeostasis

-gluconeogenesis from amino acids in extreme starvation

waste- any substance that is useless to the body

metabolic waste - waste substances produced by the body

Uric acid - product of nucleic acid catabolism

Creatinine - product of creatine phosphate catabolism

Blood urea nitrogen (BUN) - expression of the level of nitrogenous waste in the blood

Excretion

Four body systems carry out excretion

-respiratory system

• CO2, small amounts of other gases and water

-Integumentary system

• water, inorganic salts, lactic acid, urea in sweat

-Digestive system

• Water, salts, CO2, lipids, bile pigments, cholesterol, other metabolic waste and food residue

-Urinary system

• many metabolic wastes, toxins, drugs, hormones, salts and water

Anatomy of the Kidney

learning outcomes:

-describe the location and general appearance of the kidney

-identify the external and internal features of the kidney

-trace the flow of fluid through the renal tubules

-describe the nerve supply to the kidney

Gross anatomy

-lateral surface is convex and medial is concave with a slity called the hiilum

three protective connective tissues coverings

• renal fascia - nids it to abdominal wall
• perirenal fat capsule - cushions kidney and holds it in place
• Fibrous capsule - encloses kidney protecting it from trauma and infection

Renal parenchyma - glandular tissue that forms urine

• renal sinus: contains blood and lymphatic vessels, nerves, and urine-collecting structures

Two zones of the parenchyma

-Outer renal cortex

-Inner renal medulla

• renal columns - extensions of the cortex that project inward toward sinus
• renal pyramids - 6-10 with broad base facing cortex and renal papilla facing sinus

Minor Calyx: cup that nestles the papilla of each pyramid; collects its urine

Major Calyces: formed by convergence of two or three major calyces

Rena Calyces: formed by convergence of two or three major calyces

Ureter: a tubular continuations of the pelvis that drains the urine down to the bladder

Renal artery divides into segmental arteries in the kidney

• In the cortes, peritubular capallaries branch off of the efferent arterioles supplying the tissue near the glomerulus, the proximal and distal convoluted tubules

The Nephron

• each kidney has about 1.2 million nephrons

• each composed of two principle parts

  -Renal corpuscle filters the blood plasma

  - Renal tubule: long coiled tube that converts the filtrate into urine

• Renal corpuscle consists of the glomerulus and a two layered glomerular (Bowman) capsule that encloses glomerulus

-parietal (outer) layer of Bowman capsule

-viscera; (inner) layer of Bowman capsule

• capsular space separates the two layers of Bowman capsule

The Renal Tubules

• Renal tubule - a duct that leads away form the glomerular capsule and ends at the tip of the medullary pyramid

  • Divided into 4 regions
  • Proximal convoluted tubule, nephron loop, distal convoluted tubule: parts of one nephron
  • Collecting duct receives fluid from many nephrons

• Proximal convoluted tubule (PCT) arises from glomerular capsule

• Nephron loop - long U shaped portion of renal tubule

  • descending limb and ascending limb

• Distal convoluted tubules (DCT) - begins shortly after the ascending limb reenters the cortex

  -shorter and less coiled than PCT

  -DCT is the end of the nephron

• Collecting duct - receives fluid from the DCTs of several nephrons as it passes back into the medu;;a

  -numerous collecting ducts converge toward the tip of the medullary pyramid

  -Papillary duct: formed by merger of several collecting ducts

  • 30 papillary ducts end in the tip of each papila
  • collecting and papillary ducts lined with simple cuboidal epithelium

Cortical Nephrons

• 85% of all nephrons
• Short nephron loops

Juxtamedullary nephrons

• 15% of all nephrons
• Very long nephron loops, maintain salinity gradient in the medulla and helps conserve water

Urine Formation 1: Glomerular filtration

learning outcomes:

-describe the process by which the kidney filters the blood plasma, including the relevant cellular structure of the glomerulus

-Explain the forces the promote oppose filtration, and calculate the filtration pressure if given the magnitude of these forces

-describe how the nervous system, hormones, and the nephron itself regulate filtration

1. glomerular filtration creates a plasma like filtrate of the blood
2. Tubular reabsorption removes useful solutes from the filtrate, returns them to the blood
3. Tubular secretion removes additional wastes from the blood, adds them to the filtrate
4. water conservation removes water from the urine and returns it to blood; concentrates wastes

-kidneys convert blood plasma to urine in three stages

1. glomerular filtration - a special case of the capillary fluid exchange process in which water and soome solutes in the blood plasma pass from the capillaries of the glomerulus into the capsular space of the nephron
2. tubular reabsorption and secretion
3. water conservation

Glomerular filtrate: the fluid in the capsular space - similar to blood plasma except that it has almost no protein

Tubular fluid: fluid from the proximal convoluted tubule though the distal convoluted tubule - substances have been removed or added by tubular cells

Urine: fluid that enters the collecting duct - undergoes little alteration beyond this point except for changes in water content

The filtration Membrane

filtration membrane - three barriers through which fluid passes

• Fenestrated endothelium of glomerular capillaries
  • highly permeable
• Basement membrane
  • blood plasma is 7% protein, the filtrate is only 0.03%
• Filtration slits
  • Podocyte cell extensions wrap around the capillaries to form a barrier layer with 30 nm filtration slits
  • negatively charged which is an obstacle for large anions

-almost any molecule smaller than 3 nm can pass freely through the filtration membrane

• ex. water, electrolytes, glucose, fatty acids, amino acids

-some substances of low molecular weight are bound to the plasma proteins and cannot get through the membrane

-most calcium, iron, and thyroid hormone

• unbound fraction passes freelin into the filtrate

-Proteinurie (albuminurie); presence of protein in urine

-Hematuria: presence of blood in the urine

Filtration pressure

• blood hydrostatic pressure

  much higher in glomerular capillaries

• because afferent arteriole is large than efferent arteriole

• larger inlet and smaller outlet

Hydrostatic pressure in capsular space

• glomerulur filtration rate

  -total amountn of filtrate produced equals 50 to 60 times the amount of blood in the body

  • 99% of filtrate is reabsorbed since only 1 to 2 L urine expected per day

Urine Formation 2: Tubular reabsorption and Secretion

• PCT reabsorbs about 65% of glomerular filtrate, removes some substances from the blood, and secretes them into the tubular fluid for disposal in urine

Tubular reabsorption - process of reclaiming water and solutes from the tubular fluid and returning them to the blood

-sodium reabsorption is the key to everything else

• creates an osmotic and electrical gradient that drives the reabsorption of water and other solutes
• most abundant cation in filtrate
• creates sleep concentration gradient that favors its diffusion into the epithelial cells

Two types of transport proteins in the apical cell surface are responsible for sodium uptake

-sodium is prevented from accumulating in the epithelial cells by NA-K pumps in the basal surface of the epithelium

-secondary active transport: NA transporting symports in apical cell membrane do not consume ATP, are considered an example of secondary active transport for their dependance on the NA-K pumps

• negative chloride ions follow the positive sodium ions by electrical attraction

The proximal convoluted tubule

Two routes of reabsorption

• transcellular route- substances pass through the cytoplasm of the PCT epithelial cells and out their base

• Paracellular route- substances pass between PCT cells, junctions between cells all significant amounts of water to pass, solvent drag - water carries with it a variety of dissolved solutes

• Potassium, magnesium, and phosphate ions diffuse through the paracellular route with water

• Phosphate is also cotransported into the epithelial cells with NA

• some calcium is reabsorbed through the paracellular route in the PCT

• Glucose is cotransported with NA+ by sodium-glucose transport (SGLT) proteins

• Urea diffuses through the tubule epithelium with water reabsorbs 40% to 60% in tubular fluid

  • kidneys remove about half of the urea from the blood; creatinine is not reabsorbed at all

The Transport Maximum

• there is a limit to the amount of solute that the renal tubules can reabsorb
• limited by the number of transport proteins in the plasma membrane
• if all transporters are occupied as solute molecules pass
  • excess solutes appear in urine
• Transport maximum is reached when transporters are saturated
• each solute has its own transport maximum

Tubule secretion

• tubular secretion - process in which renal tubule extracts chemicals from capillary blood and secretes them into tubular fluid

• two purposes in proximal convoluted tubule and nephron loop

  -waste removal

  -acid-base balance / homeostasis

The Nephron Loop

-Primary function of nephron loop is to generate salinity gradient that enables collecting duct to concentrate the urine and conserve water

The Distal Convoluted Tubule and Collecting Duct

-fluid arriving in the DCT still contains about 20% of the water and 7% of the salts from glomerular filtrate

-Two kinds of cells

• Principal cells
  • most numerous
  • have receptors for hormones
  • involved int salt and water balance
• Intercalated cells
  • involved in acid-base balance by secreting H+ into tubule lumen and reabsorbing K+

-Aldosterone - the “salt retaining hormone”

• steroid secreted by the adrenal cortex
• created when sodium concentration falls or when potassium concentration rises or if there is a drop in blood pressure

-functions of aldosterone

• acts on thick segment of nephron loop, DCT, and cortical portion of collecting duct

• stimulates the reabsorption of more sodium and secretion of potassium

• water and CL follow the sodium

• Net effect is that the body retains NaCl water

  -helps maintain blood volume and pressure

• urine volume is reduced

• urine has an elevated potassium concentration

Composition and Properties of Urine

• urinalysis - the examination of the physical and chemical properties of urine
• Appearance - clear, almost colorless to deep amber-yellow color due to urochrome pigment from breakdown oh hemoglobin
  • cloudiness or blood could suggest urinary tract infection, trauma, or stones
  • Pyuria: puss in the urine
  • Hematuria: blood in urine due to UTI, trauma, or kidney stones
• Odor - bacteria degrade urea to ammonia, some foods impart aroma
• Specific gravity - compare to distilled water
• PH range: 4.5 to 8.2; usually 6.0
• Chemical composition 95% water 5% solutes
  • normal to find: urea, NaCl, KCl, creatinine, uric acid, phosphates, sulfates, traces of calcium, magnesium, and sometimes bicarbonate, urochrome, and a trace of bilirubin
  • abnormal to find: glucose, free hemoglobin, albumin, ketones, bile pigments

Urine Volume

• normal volume for average adult - 1 to 2L a day
• Polyuria - output in excess of 2L/day
• Oliguria - output of less tan 500mL/day
• Anuria - 0 to 100 mL/day

-low output form kidney disease, dehydration, circulatory shock, prostate enlargement

-low urine output of less that 400mL.day, the body cannot maintain a safe, low concentration of waste in the plasma

• diabetes - any metablolic disorder resulting in polyuria

• at least four forms of diabetes

  • diabetes mellitus type 1, type 2, and gestational diabetes ( from pregnancy)
  • high concentration of glucose in renal tubule
  • glucose opposes the osmotic reabsorption of water
  • More water passes in the urine
  • Glycosuria - glucose in the urine

• Diabetes insipidus

  -ADH hypersecretion causes not enough water to be reabsorbed in the collecting duct

  -more water passes in the urine

• Diuretics - any chemical that increases urine volume

  -some increase GFR

  -reduce tubular reabsorption of water

• commonly used to treat hypertension and congestive heart failure by reducing the body’s fluid volume and blood pressure

Urine Storage and Elimination

• urine is produced continually
• Does not drain continually from the body
• Urination is episodic - occuring when we allow it
• Made possible by storage apparatus and neural controls of this timely release

The Ureters

• ureters - retroperitoneal, muscular tube that extends the kidney to the urinary bladder
  • about 25 cm long
  • passes posterior to bladder and enters it from below
  • flap of mucosa acts as a valve into bladder
  • keeps urine from backing up in the ureter when bladder contracts

-Three layers of ureter

• Adventitia - connective tissue layer that connects ureter to surrounding structures

• Muscularis - two layers of smooth muscle with third layer in lower ureter

        -urine enters, it stretches and contracts in peristaltic wave 
  

• Mucosa - transitional epithelium

  • begins at minor calyces and extends through the bladder

  -lumen very narrow, easily obstructed kidney stones

The Urinary Bladder

• urinary bladder - muscular sac located on floor of pelvic cavity

-three layers

• parietal peritoneum, superiorly, fibrous adventitia other areas

• Muscularis: detrusor muscle: three layers of smooth muscle

• Mucosa: transitional epithelium

  • rugae - conspicuous wrinkles in relaxed bladder

• Trigone - smooth-surfaced triangular area marked with opening of ureters and urethra

• capacity - moderate fullness is 500mL, maximum fullness is 700 to 800 mL

  - highly distensible

  -As it fills, it expands a lot

  -rugae flatten

Kidney stones

• renal calculus (kidney stone) - hard granule of calcium phosphate, calcium oxalate, uric acid or a magnesium salt called struvite
• Form in the renal pelvis
• usually small enough to pass unnoticed in the urine flow

-large stones might block renal pelvis or ureter and can cause pressure buildup in kidney which destroys nephrons

• passage of large jagged stones is excruciatingly painful and may damage ureter causing hematuria

-causes include hypercalcemia, dehydration, pH imbalances, frequent UTIs, or enlarged prostate gland causing urine retention

-treatment includes stone-dissolving drugs, ofen surgery, or lithotripsy (nonsurgical technique that pulverizes stones with ultrasound)

The Urethra

• female urethra: 3 to 4 cm long
• bound to anterior wall of vagina
• internal urethral sphincter
  • detrusor muscle thickening
  • smooth muscle under involuntary control
• External urethral sphincter
  • where the urethra passes through the pelvic floor
  • skeletal muscle under voluntary control

Urinary Tract Infection (UTI)

• Cystitis - infection of the urinary bladder

  -especially common in females due to short urethra

  -frequently triggered by sexual intercourse

  -can spread up the ureter causing pyelitis

• Pyelitis - infection of the renal pelvis

• Pyelonephritis - infection that reaches the cortex and the nephrons

Voiding Urine

-between acts of urination, the bladder is filling

• detrusor muscle relaxes

urethral sphincters are tightly closed

• accomplished by sympathetic pathway from upper lumbar spinal cord
• postganglionic fibers travel through the hypogastric nerve to the detrusor muscle (relax) and internal urethral sphincter (excite)

Micturition - the act of urinating

Renal Insufficiency and hemodialysis

• renal insufficiency - a state which the kidneys cannot maintain homeostasis du to extensive destruction of their nephrons
• causes of nephron destruction