VT 111 Lec 13 Urinary System

Waste Excretion

Metabolic Waste Products 

• potentially harmful substances to the body

  • must be eliminated

    • of no further use

    • can be harmful if allowed to accumulate

  • examples

    • carbon dioxide (CO2) and water (H2O)

    • nitrogenous wastes, primarily urea

    • bile salts and pigments

    • various salts

Routes for Waste Product Elimination

• respiratory system

  • carbon dioxide, water vapor

• sweat glands

  • water, salts, urea

• digestive system

  • bile salts, pigments

• urinary system

  • urea, salts, water, other soluble waste products

The Urinary System

• single most important route for removal of waste products 

  • removes nearly all soluble waste from blood

  • transports soluble waste out of the body

• major route for elimination of excess water

Parts of the Urinary System

• kidneys (2)

• ureters (2)

• urinary bladder (1)

• urethra (1)

Kidney Function

• production of urine to facilitate elimination of metabolic waste materials

• maintenance of homeostasis through:

  • blood filtration, reabsorption, secretion

  • fluid balance regulation

    • antidiuretic hormone (ADH), aldosterone

  • acid-base balance regulation

  • production of hormones

    • Erythropoietin (EPO), prostaglandins

  • blood pressure regulation

The Kidneys 

• located in dorsal abdominal area

  • ventral to first few lumbar vertebrae

  • on either side of first few lumbar vertebrae

• retroperitoneal to the abdominal cavity

• surrounded by layer of perirenal fat

• right kidney more cranial than left (except pigs)

Gross Anatomy of the Kidneys 

• fibrous connective tissue capsule

• hilus: indented area on medial side

  • ureters, nerves, blood and lymph vessels enter and leave

• renal pelvis: funnel-shaped area inside hilus/hilum

• renal cortex – surrounds medulla

• renal medulla – shape can vary (multilobar vs. unilobar)

• calyx – cup-like extension of pelvis; directs urine to pelvis

Gross Anatomy of Kidneys Continued…

1. Multilobar

2. Unilobar

Porcine Kidneys

Interior Structure of the Kidney

• Cortex

• Medulla

• Medullary pyramid

• Minor calyces

• Major calyces

• Pelvis

• Ureter

• Fat

Microscopic Anatomy of Kidneys 

• nephron = basic functional unit

  • number varies with size of the animal

    • cat ~200,000/kidney

    • human ~ 1 million/kidney

• composed of:

  • renal corpuscle

  • proximal convoluted tubule (PCT)

  • loop of Henle

  • distal convoluted tubule (DCT)

Renal Corpuscle 

• located in renal cortex

• glomerulus (capillaries) surrounded by Bowman’s capsule (2 layers)

• filters blood in first stage of urine production: Bowmaglomerular filtrate

Bowman’s Capsule

• Double-walled capsule

  • Inner (visceral) layer

    • Visceral layer

      • Adheres closely to capillaries

      • Composed of podocytes that create a permeable layer

  • Outer (parietal) layer

    • Simple squamous epi

  • Space between layers

    • Capsular space

    • Where filtration from glomerulus enters

      • Collects into convoluted tubule

Glomerulus 

Proximal Convoluted Tubule (PCT)

• continuation of capsular space of Bowman’s capsule

  • Lined with cuboidal epithelial cells with brush border

• twists through cortex

• reabsorption and secretion functions

• glomerular filtrate now called tubular filtrate

Loop of Henle

• continues from PCT, descends into medulla, makes a U-turn, and heads back into cortex

  • At U-turn, narrows and wall thins

    • Simple squamous epithelial cells

      • No brush border

• ascending wall becomes thicker again

Ascending Limb of Loop of Henle 

• Ascending limb has active transport pumps

• No water channels, no water reabsorption here

• Na+ is actively transported from loop

• Amount of Na+ reabsorbed is dependent on concentration of filtrate

• Volume does not change here

• Concentration decreases due to loss of Na+

Distal Convoluted Tubule (DCT)

• continuation of ascending loop of Henle

  • twists through cortex

• DCTs from all nephrons  in the kidney empty into collecting ducts

  • empty into calyces à renal pelvis

  • primary site of antidiuretic hormone (ADH) action

  • regulation of potassium and acid-base balance

Movement of Nutrients & Wastes

Nerve Supply to the Kidney

• primarily from sympathetic portion of the autonomic nervous system

  • not essential for kidney function

  • transplants

• sympathetic stimulation causes vasoconstriction of renal vessels

  • temporarily decreases urine function

Blood Supply to the Kidney 

• LARGE supply

  • ~25% of CO

  • All circulating blood passes through every 4-5 min

• renal artery (branch off aorta) enters at hilus

• subdivides to become series of afferent glomerular arterioles

• afferent glomerular arterioles carry blood to renal corpuscle

• glomerular capillaries filter some plasma out of the blood: glomerular filtrate

• Blood leaves glomerulus and enters efferent glomerular arteries

Blood Supply to the Kidney Continued…

• Efferent glomerular arterioles divide into capillaries that surround rest of nephron

  • peritubular capillaries

• Site of oxygen transfer to cells of nephron

• Tubular reabsorption and tubular secretion also occur at this levelPeritubular capillaries converge to form venules à larger veins à renal vein

• Renal vein leaves kidney at hilus

• Joins caudal vena cava to return to heart

• “clean blood”

Mechanisms of Renal Action 

• filtration of the blood

• reabsorption of useful substances

  • back into the bloodstream

• secretion of waste products

  • from the blood

Filtration of Blood 

• occurs in renal corpuscle

  • afferent →  efferent arterioles

    • blood pressure much higher than in other capillaries

      • Only ~30% lower than aorta

    • due to pressure from difference in size between afferent and efferent glomerular arterioles

• high blood pressure in glomerular capillaries forces some plasma into capsular space of Bowman’s capsule

  • large fenestrations in capillary endothelium

  • glomerular filtrate formed

    • no proteins or blood cells → too large to fit through fenestrations

      • may see in urine if damage to glomerulus

• glomerular filtration rate (GFR)

  • depends on rate of blood flow to kidney

  • ml/min

  • On average, ~25% of plasma is removed from circulation each minute!

Glomerular FIltrate 

• Flows through fenestrations in capillaries à into capsular space à into tubules (now tubular filtrate)

• Fluid with small molecules

  • Some is waste

  • Some the body still needs!

    • K, Na, Ca, Mg, a.a., Cl, H2O

  • The body needs to reabsorb the needed elements out of the glomerular filtrate before it leaves the body.

    • → reabsorption

      • Takes these elements out of tubules and BACK into the blood stream

        • Passive: osmosis, diffusion

        • Active: transport across cell membranes

Reabsorption Part 1

• useful substances exit into tubules of nephron (as well as wastes): secretion

  • sodium, potassium, calcium, magnesium, glucose, amino acids, chloride, bicarbonate,     and water

  • these are needed by the body!

• Reabsorption à process to move from nephron tubules back to peritubular capillaries

  • Passive or active

Sodium Reabsorption

• sodium in tubular filtrate attaches to carrier protein

  • carried into cytoplasm of PCT epithelial cell

    • Requires energy!

  • Glucose and amino acids hitch a ride on the same carrier protein

    • Passive transport

      • Sodium cotransport

• sodium actively pumped out of PCT cell into interstitial fluid, where it moves into peritubular capillaries

• sodium ions also reabsorbed in ascending loop of Henle and DCT

  • usually exchanged for hydrogen, ammonium, or potassium ions

  • under influence of aldosterone (hormone from adrenal gland)

Sodium Reabsorption Continued

• Sodium movement (Na+) creates an electrical imbalance

  • Actively pumped from tubule into tubular epithelial cell and then into interstitial space

• Chloride (Cl-) diffuses into same space to restore neutrality

• Water also follows due to osmosis

  • Remember, Na has brought along glucose and amino acids

  • Allows for passive reabsorption

    • Some wastes get reabsorbed, too (BUN)

Reabsorption Part 2

• chloride ions diffuse from tubular filtrate into epithelial cells and interstitial space

  • in response to electrical imbalance created by sodium removal

• some of the water in the filtrate moves by osmosis into interstitial space and peritubular capillaries

  • after sodium, glucose, amino acids, and chloride have left tubular filtrate (change in concentration gradient)

• urea also passively absorbed

  • Waste product, but some hangs around

    • BUN

Reabsorption Part 3 

• potassium

  • diffuses into interstitial fluid in the PCT, ascending loop of Henle and DCT

• calcium

  • reabsorbed and moves through epithelial cells in the PCT, ascending loop of Henle, and DCT

    • under influence of Vitamin D, parathyroid hormone (PTH), and calcitonin (thyroid)

• magnesium

  • reabsorbed from PCT, ascending loop of Henle, and the collecting duct

    • PTH increases its reabsorption

Reabsorption Part 4

• ~65% of tubular reabsorption happens in PCT

• ~80% of water, sodium, chloride and bicarbonate and 100% of glucose are reabsorbed in PCT

• Remainder of absorption

  • DCT

  • Loop of Henle

  • Collecting ducts

Secretion

• not all wastes are filtered by glomerulus

• excess wastes are secreted primarily in DCT by secretion

  • hydrogen, potassium, ammonia

• some medications also eliminated by secretion

  • E.g. penicillin, sulfonamides

  • where would these drugs concentrate?

Urine Volume Concentration

• urine volume is determined by amount of water contained in tubular filtrate when it reaches the renal pelvis

• controlled by actions of 2 hormones

  • antidiuretic hormone (ADH)

    • from posterior pituitary

    • acts on DCT and collecting ducts promote water reabsorption

    • if absent, polyuria results

  • aldosterone

    • from adrenal cortex

    • Increases reabsorption of sodium into bloodstream in DCT and collecting ducts → osmotic imbalance → water follows

ADH & Aldosterone

Regulation of Blood Pressure (BP)

• kidneys help maintain homeostasis by their role in regulating blood pressure

  • renin-angiotensin-aldosterone system responds when BP falls

• Monitoring cells

  • Afferent glomerular arterioles

    • Juxtaglomerular cells

      • Monitor blood pressure

• Ascending limb of loop of Henle

  • Macula densa

    • Monitors NaCl concentration

Regulation of Blood Pressure (BP) Continued 

• If BP falls or NaCl decreases

• renin is released

• Enzyme that splits angiotensin

  • angiotensin I converted to angiotensin II by ACE

  • angiotensin II causes arterial constriction and stimulates release of aldosterone

• increased amounts of sodium and water reabsorbed back into bloodstream, causing an increase in blood volume

• as blood volume increases, so does blood pressure

Ureters

• tubes that exit the kidney at the hilus and then connect to the urinary bladder at the neck

• trigone: arrangement of openings of ureters into bladder and opening from bladder into urethra

• a continuation of the renal pelvis

• each ureter leaves its kidney at the hilus

• composed of 3 layers:

  • outer fibrous layer

  • middle muscular layer

    • smooth muscle propels urine by peristalsis

      • Doesn’t require gravity

  • inner epithelial layer

    • Lines with transitional epithelium

    • allows ureters to stretch when urine passes through

Ureters Continued… 

• enter bladder at an oblique angle

• openings collapse when bladder is full

  • prevent backup of urine into ureters

  • peristalsis still allows urine to enter bladder

Urinary Bladder

• stores urine as it is produced

• lined with transitional epithelium

• releases urine periodically from the body

• 2 parts: muscular sac neck

Anatomy of the Urinary Bladder

• size and position vary depending on amount of urine it contains

  • transitional epithelium stretches as bladder fills with urine

  • detrusor muscle (smooth) contracts to expel urine

• circular sphincter muscles (skeletal) around neck of bladder

  • provides voluntary control over urination process

Function of the Urinary Bladder

• collect urine

  • kidneys constantly produce urine

• store urine

• release urine

Urination = Micturition = Uresis

• expulsion of urine from the urinary bladder into the urethra for elimination from the body

• 2-3 steps in process:

  • urine accumulation

  • muscle contraction

  • sphincter muscle control

Control of Urination

• urine accumulation

  • bladder constantly accumulates urine

  • pressure of filling bladder reaches certain trigger point

  • stretch receptors in bladder wall are activated

• muscle contraction

  • spinal reflex is activated

  • motor impulse sent to detrusor muscle

    • smooth muscle in bladder wall contracts

      • This is the sensation of the need to urinate

  • bladder emptied if animal is not housebroken

Control of Urination Continued… 

• sphincter muscle control

  • voluntary control of sphincter around neck of bladder offers temporary control of urination

  • the fuller the bladder, the more pressure on the sphincter muscle

  • eventually the sphincter muscle relaxes

    • urine is released

    • “accident”

  • full bladders also have thinner walls

    • More susceptible to damage (trauma)

Urethra

• continuation of the neck of the bladder; lined with transitional epithelium

• carries urine from bladder to the external environment

• runs through pelvic canal

• much longer in male; runs along ventral aspect of penis

Anatomy of the Urethre

Female Urethra:

• shorter and straighter

• opens on floor (ventral portion) of vestibule of the vulva

• lined with transitional epithelium which allows it to expand

Male Urethra:

• longer and curved

• runs along the ventral aspect of the penis

• lined with transitional epithelium which allows it to expand

Function of the Urethra 

Female Urethra:

• strictly a urinary function

• carries only urine

Male Urethra:

• has both urinary and reproductive functions

• carries urine or semen

• semen (sperm + seminal fluid) enters the urethra as it passes through pelvic canal

• At ejaculation, sphincter of bladder neck closes

Accessory Sex Glands