Ch 25 - The Urinary System

Urinary System

contains the kidneys, ureters, bladder, and urethra

Homeostatic functions of the Kidneys

maintains homeostasis by managing the volume and composition of fluid reservoirs, primarily blood; it filters blood to become urine

• Regulation of blood ionic compounds (Na+, K+, Cl-)

• Regulation of blood pH (H+, HCO-3)

• Regulation of blood volume (H20) - can bring water back to blood

• Regulation of blood pressure - increased urine output = increased BP = increased BV

• Maintenance of blood osmolarity - measures conc. of body fluids

• Production of hormones: calcitrol and erythropoietin

• Excretion of metabolic wastes and foreign substances

• Regulation of blood glucose level

calcitrol and erythropoietin

calcitrol actives vitamin D and stimulates reabsorption of calcium (increases blood calcium level)

erythropoietin is secreted by the kidneys

Where are the kidneys located

attached to the back muscles in the retroperitoneal space (space behind the peritoneal cavity)

• kidneys are partially protected by the lower ribs

Renal Fascia (external renal anatomy)

thin, outer covering of the kidney

• it anchors to other substances

Adipose capsule (external renal anatomy)

middle layer of the external layers

• it protects and anchors

Renal capsule (external renal anatomy)

innermost layer of the external layers of the kidney

• it is continuous with the ureter

Renal hilum

entrance where renal artery, renal vin, ureter, nerves, and lymphatics enter the kidneys

Renal cortex (internal renal anatomy)

outer pink

• located between the renal capsule and the renal medulla

Renal medulla

inner region of the kidney (darker pink)

• is organized into cone-shaped structures called renal pymraids

Renal pyramids

secreting apparatus and tubules

• they are composed of tiny tubules and forms the renal medulla

Renal columns

they anchor the cortex

• medullary extensions of the renal cortex that project inward between the renal pyramids

Tubules

bring urine to the collecting ducts

Renal pelvis

urine collecting area inside the kidney

Ureter

transports urine to urinary bladder

Pathway of urine drainage

Collecting duct → papillary duct → minor calyx → major calyx → renal pelvis → ureter → urinary bladder

Urethra

conveys urine to the outside

Urinary bladder

assists in micturition

Blood supply of the kidney

renal artery → segmental artery → interlobar artery → arcutae arteries → cortical radiate arteries → afferent arterioles → glomerular capillaries → efferent arterioles → pertubular capillaries → pertibular venules → cortical radiate veins → arcuate veins → interlobar veins → renal vein

Nephron

functional unit of the kidney

• contains a distal convoluted tubule, nephron loop (ascending and descending limb), proximal convoluted tubule, and renal corpuscle

nephron loop = formed by ascending and descending loop

Renal corpusle

contains two parts

1. glomerulus

2. Bowman’s capsule

is the space where the nephron starts

Glomerulus

mass of capillaries that is fed by afferent arterioes and drain into the efferent arteriole

Bowman’s capsule

has a visceral layer of podocytes, which wrap around the capillaries

• filtrate is collected between the visceral and parietal layers

• Glomerular endothelial cells have large pores (fenestrations) and are leaky

• Basal Lamina lies between endothelium and podocytes

Podocytes

for pedicles (“feet”), which are filtration slits

• they are extensions from the cytoplasm

large cell that forms the viscera layer

pedicles = specialized, finger-like cytoplasmic extensions of podocyte cells that wrap around glomerular capillaries in the kidney, forming a crucial component of the blood filtration barrier

filtration slit

allows small molecules to pass

• podocyte cells in the glomerulus

Filtration Membrane

1. Fenestration (pore) of glomerular endothelial cells → prevents filtration of blood cells but allows all components of blood plasma to pass through; innermost

2. Basement membrane of glomerulus → prevents filtration of larger proteins;

3. Slit membrane between pedicles → prevents filtration of medium-sized proteins

(why a healthy person should not have proteins or RBCs in their urine)

Cortical Nephrons

most common (80-85% of nephrons); most parts are in the renal cortex

• renal corpuscle in outer portion of cortex

• short loops of Henle extend only into outer region of medulla

• create urine with osmolarity similar to blood

Juxtamedullary Nephrons

15% of nephrons; closer to medullary, deeper in kidneys

• renal corpsucle deep in cortex with long nephron loops

• receive blood from peritubular capillaries and vasa recta

• ascending limb has thick and thin regions

• enable kidney to secrete very concentrated urine (during times of dehydration)

Flow of fluid through a cortical nephron

Bowman’s capsule → proximal convoluted tubule (PCT) → descending limb of the nephron loop → ascending limb of the nephron loop → distal convoluted tubule (drains into collecting duct)

Proximal convoluted tubule

the primary site of reabsorption in the kidney's nephron

• reabsorbs 100% of glucose

Distal convoluted tubule

located between the loop of Henle and collecting duct

• selectively reabsorbs Ca2+, Na+, and K+

attaches to the collecting duct

Flow of fluid through a juxtamedullary nephron

Bowman’s capsule → PCT → descending limb → thin ascending limb → thick ascending limb → DCT

Juxtaglomerular Apparatus

has two types of cells

the ascending loop contacts the afferent arteriole at the macula dense → in wall of ascending loop of afferent arteriole

the wall of the arteriole contains smooth muscle cells called juxtaglomerular cells

• the apparatus regulates blood pressure in the kidneys

When blood pressure is low, what do juxtaglomeular cells do?

They secrete renin in response to low BP and low BV (activates the renin-angiotensin-aldosterone (RAAS) mechanism to increase BP body-wide

Steps:

1. Detects low BP and blow BV, JG cells secrete renin

2. Renin stimulates conversion of angiotensinogen into angiotensin I

3. Converting enzyme in the lungs then coverts angiotensin I into angiotensin II

4. The active form of angiotensin II stimulates the adrenal cortex to secrete aldosterone and the arterioles to begin vasoconstriction of the systemic arterioles

5. The release of aldosterone stimulates sodium reabsorption by the kidneys, increasing vascular volume and arterial BP

6. Vasoconstriction of the systemic arterioles raises arterial BP and BV

Pedicles

long cytoplasmic extensions off of podocytes

Glomerular Filtration

First step of urine formation

• driven by BP

• opposed by capsular hydrostatic pressure and blood colloid osmotic pressure

• water and small molecules move OUT of the glomerulus

in one day, 150-180 liters of water pass out into the glomerular capsule

Capsular Hydrostatic Pressure

the pressure exerted by the fluid inside the Bowman's capsule (approx. 15–18 mmHg) that pushes fluid back into the glomerular capillaries

Blood Colloid osmotic Pressure

the inward-pulling force (approx. 25–30 mmHg) exerted by plasma proteins, mainly albumin, that draws water from the interstitial fluid back into capillaries

Glomerular filtration rate

amount of filtrate form by the kidneys each minute

• too high GFR = substances pass too quickly and are not reabsorbed (too much filtration)

• too little GFR = nearly all substances are reabsorbed and some waste products are not adequately excreted (too much reabsorption)

Renal autoregulation

controls GFR

two components:

1. myogenic mechanism: increased BP stretches the afferent arteriole. The stretched smooth muscle in the wall contracts, causing vasoconstriction of afferent arteriole

• less blood enters the glomerulus, decreasing GFR

2. tubuloglomerular feedback: rapid delivery of electrolytes due to high blood pressure. Macula dense detects high NaCl and signals the afferent arteriole to constrict. Blood flow into glomerulus decreases and GFR decreases back to normal

Angiostensin II

decrease BV or BP stimulates production of angiotensin II

causes constriction of the afferent and efferent arterioles

• decreased GFR

Tubular Reabsorption and Secretion

steps 2 and 3 of urine formation

• much of the filtrate is reabsorbed, especially water, glucose, amino acids, and ions (glucose is full reabsorbed and none is secreted)

• secretion helps manage pH and rid the body of toxic and foreign substances

in a healthy person, their urine should contain no proteins or RBCs

Water and variable water reabsoprtion

water reabsorption occurs primarily along the PCT and descending limb

variable water reabsorption occurs primarily along the DCT and collecting duct

Solute reabsorption and variable solute reabsorption

solute reabsorption occurs primarily along the PCT, ascending limb, DCT, and collecting duct

variable solute reabsorption primarily occurs at the PCT, DCT, and collecting duct

Obligatory reabosprtion

always occurring

• 90% of water follows the solutes that are reabsorbed

PCT (reabsorption and secretion)

major site of reabsorption

• glucose and amino acids are 100% reabsorbed via active transport

• Cl-, K+, and urea are reabsorbed by passive transport (diffusion)

• water is passively reabsorbed by osmosis

Reabsorption in the Loop of Henle

descending limb - only water reabsorption

• the wall is highly permeable to water, water leaves by osmosis

ascending limb - only solute reabsorption (K+, Na+, Cl- symporters)

• only has thick walls, so no water reabsorption can occur

• active transport reabsorption in thick ascending limb

Reabsorption in DCT and collecting ducts

• they are sites of regulated and variable reabsorption

PTH stimulates reabsorption of Ca2+ and inhibits phosphate reabsorption in the PCT, enhancing its extertion

Aldosterone stimulates Na+-Cl- symporters to reabsorb ions

• also stimulates K+ secretion (takes potassium from blood and brings to urine)

Facultative water reabsorption - 10%

• regulated by ADH (secreted by posterior pituitary, increases NaCl reabsorption)

Reabsorption and secretion of HCO3-, secretion of H+

Glomeular blood hydrostatic pressure

the chief force pushing water and solutes out of blood across the filtration membrane

Urine concentration

varies with ADH and his highly variable

• high intake of fluids results in dilute urine of high volume (more pee but less color)

• low intake of fluids results in concentrated urine of low volume (less pee and more color)

Formation of Dilute Urine

glomerular filtrate and blood have the same osmolarity, but tubular osmolarity changes due to concentration gradient in medulla

When diluted urine is formed, the osmolarity in the tubule

• increases in descending limb

• decreases in ascending limb

• decreases MORE in the collecting duct

Thick ascending limb (formation of diluted urine)

• has low water permeability

• Symporters actively resorb (Na+, K+, Cl-)

• Solutes LEAVE, water stays in tubule

Collecting duct (formation of diluted urine)

• low water permeability in absence of ADH

• water stays in tubule, large volumes of diluted urine produced

Formation of concentrated urine

juxtamedullary nephrons w/ long loops form concentrated urine

• osmotic gradient is formed by countercurrent multiplier

  • solutes pumped out of ascending limb, but water stays in the tubule

  • medullary osmolarity is increased in vertical direction

ADH

changes the permeability of collecting ducts

• makes the collecting ducts more permeable to water

water leaves by osmosis (water is reabsorbed back into the body), urine becomes concentrated

Formation of concentrated urine steps

1. Symporters in ascending limb cause buildup of Na+ and Cl- in renal medulla

2. Countercurrent flow through nephron loop establishes osmotic gradient

3. Principal cells in collecting duct reabsorb more water when ADH is present

4. Urea recycling causes buildup of urea in renal medulla

Summary of Renal Function

Step 1: Glomerulus

• filtrate produced at renal corpuscle has the same composition as blood plasma (minus plasma proteins)

Step 2: PCT

• produces osmotic water flow out of tubular fluid

• reduces volume of filtrate

Step 3: PCT and descending limb

• Water moves into peritubular fluids, leaving highly concentrated tubular fluid

  • reduction in volume occurs by obligatory water reabsorption

Step 4: thick ascending limb

• tubular cells actively transport Na+ and Cl- out of tubule (osmolarity is decreased)

Step 5: DCT and collecting ducts

• final adjustments in composition of tubular fluid

• exposure to ADH determine final urine concentration

Step 6: Urine production

• ends when fluid enters the renal pelvis

ADH

secreted by the posterior pituitary

• increase water content in the blood and decreases water content in the urine

• as a result, water being secreted causes BV to increase

Normal urine

95% water

Nitrogen Wastes: urea (most abundant), uric acid, creatinine

Electrolytes - H+ ions, sodium, potassium, calcium

Toxins - from bacteria, drugs

Pigments - urochromes from RBC breakdown

Hormones

Abnormalities in urine

• albumin

• glucose

• RBCs

• ketone bodies

• microbes

Ureters

transport urine from renal pelvis by peristaltic waves (transport urine from kidneys to urinary bladder)

• no anatomical valve at the opening of the ureter into bladder

Bladder

when the bladder fills, it compresses the opening and prevents backflow of urine into renal pelvis

the bladder is distensible (capable of being stretched), muscular organ with a capacity averaging 700-800 mL

• responsible for storage and elimination of urine

What happens during urination?

• Detrusor muscle contracts

• Internal urethral sphincter relaxes (involuntary)

• External urethral sphincter relaxes (voluntary control)