Urinary System

Urinary System Functions

• removal of wastes from blood

  • and toxins/medications

• formation/concentration of urine

• storage of urine

• excretion of urine

• regulation of homeostasis

  • electrolytes, blood pressure, erythrocyte levels, H₂O

4 Homeostatic Functions of Urinary System

1. regulates blood volume and blood pressure

2. regulates plasma ion concentrations

3. helps stabilize blood pH

4. conserves valuable nutrients

Regulates Blood Volume and Blood Pressure

• by adjusting volume of water lost in urine

• releases renin

• releases erythropoietin (increases O2 in blood)

Regulates Plasma Ion Concentrations

• sodium, potassium. and chloride ions (by controlling quantities lost in urine)

• calcium ion levels (through synthesis of calcitriol (vit D))

Helps Stabilize Blood pH

• by controlling loss of hydrogen ions and bicarbonate ions in urine

Conserves Valuable Nutrients

• by preventing excretion while excreting organic waste products

Parts of Urinary System

• kidneys (2)

• ureters (2)*

• urinary bladder (1)*

• urethra (1)*

*part of urinary tract

Location of the kidneys? Peritoneum?

• kidneys are retroperitoneal (behind the peritoneum)

• peritoneum = double layer membrane that surrounds most abdominal organs

Kidneys are held in place by:

• the overlying peritoneum

• contact with adjacent organs

• supporting connective tissues

  • fibers capsule, perinephric fat capsule, renal fascia

  • all three layers connected by collagen fibers and anchored to peritoneum and deep fascia posteriorly

Parts of Kidneys

• hilum- medial depression, entry/exit point for renal artery/vein.nerve and ureters

• renal sinus- space/cavity within medial kidney, contains renal pelvis

• cortex- area closest to renal capsule laterally

• medulla- inner layer (medial to cortex)

Blood Flow Through Kidneys

• kidneys receive 20% to 35% to total cardiac output

• 1200mL of blood flows through kidneys each minute

• kidney receives blood through renal artery

Artery blood flow in kidneys

1. renal artery enters kidney at hilum and branches as it flows toward the cortex

2. segmental arteries- in renal sinus

3. interlobar arteries- travel through renal columns

4. arcuate arteries

5. cortical radiate arteries

6. afferent arterioles

Afferent arterioles

• enter glomerular capsule and form a capillary bed called a glomerulus

• blood leaves glomerulus in efferent arteriole vessels then forms peritubular capillaries and (sometimes) vasa recta

Vein blood flow through kidneys

• particular capillaries converge to form:

  • cortical radiate veins, arcuate veins, interlobar veins, renal vein

  • NO segmental veins

  • arcuate arteries and veins form boundary b/w cortex and medulla in kidneys

Renal Corpuscle

The Nephron? Made up of?

• basic functional unit of kidneys

• where blood is filtered and the fluid removed is concentrated into urine to almost urine

• Made up of:

  • renal corpuscle- glomerulus and glomerular capsule

  • PCT- proximal convoluted tubule

  • nephron loop

  • DCT- distal convoluted tubule

The Nephron

2 types of nephrons

• cortical nephron (85%)

• juxtamedullary nephron

What are Juxtamedullary Nephrons?

• have long nephron loops

• reach far into renal medulla where solute concentration is high in peritubular fluid

• these nephrons are important in concentrating our urine

Urine formation (7 steps)

1. blood enters “leaky” glomerulus

2. fluid and some solutes leave blood

3. filtrate is caught by glomerular capsule and enters PCT

4. as filtrate flows through PCT, Tehran loop and DCT, water and some solutes are reabsorbed back into blood. some substances are secreted into nephron

5. filtrate flows into collected duct - some final changes occur

6. by the time filtrate leaves collecting duct, it can be called urine

7. urine passes through papilla into minor calyx to major calyx to renal pelvis to ureter to urinary bladder

What is being filtered out?

• metabolic waste products/nitrogenous wastes

  • urea- AA breakdown

  • creatinine- muscle contraction/creatine phosphate breakdown

  • uric acid- DNA/RNA breakdown

  • ammonia- AA breakdown

Renal Corpuscle Structure

• glomerular capillaries

• glomerular capsule

• podocytes

• mesangial cells

Glomerular capillaries

• single layer of endothelial cells plus basement membrane (dense layer)

Glomerular Capsule

• made up of simple squamous epithelium (capsular epithelium)

• capsular space separates capillaries from glomerular capsule

Podocytes

• cells that wrap around glomerular capillaries

Mesangial Cells

• cells that lie between capillaries and help regulate capillary blood flow

Renal Corpuscle - The Filtration Membrane

• glomerular capillaries

• dense layer

• filtration slits in podocytes

• Only small solutes get through to enter capsular space and PCT

What 3 forces are at work in the renal corpuscle?

• Glomerular Hydrostatic Pressure (GHP)

  • blood pressure in glomerular capillaries

• Capsular Hydrostatic Pressure (CsHP)

  • pressure of filtrate in capsular space

• Blood Colloid Osmotic Pressure (BCOP)

  • force of solutes in blood that draws water back into glomerular capillaries

Where is GHP high? Why? CsHP?

• GHP is higher in glomerular capillaries than other capillary beds - 50mm HG vs 35 mm Hg

  • bc the efferent arteriole is smaller in diameter than afferent arteriole

• CsHP is about 15mm HG

  • this pressure of filtrate being pushed into PCT

  • Opposes GHP

How do you calculate Net Hydrostatic Pressure (NHP)?

• CsHP - GHP = NHP

How to calculate Net Filtration Pressure (NFP_

• NHP - BCOP = NFP

How many liters of filtrate do kidneys produce a day? What % is reabsorbed? How much urine is produced in 24 hours?

• 180 Liters of filtrate per day

• 99% is reabsorbed by nephron and collecting ducts

• 1.8 L of urine per 24 hours

What is Glomerular Filtration Rate (GFR)?

• the amount of filtrate the kidneys produce in one minute (approx. 125 mL/min)

What does rate of filtration depend on? What does an increase and decrease in pressure do?

• depends on BP entering glomerulus

• Increase= increased filtrate

• decrease= decreased filtrate

What would happen to filtration if there was a small drop in pressure? Why is this bad?

• small drop in pressure would eliminate filtration

• with our filtration we lose ability to eliminate wastes and maintain pH and blood volumes

Regulation of GFR

• auto regulation

• accomplished by myogenic mechanisms

• a drop in systemic BP= contraction of efferent arteriole

• a rise in systemic BP = constriction of afferent arteriole

What hormones regulate GFR?

• renin-angiotensin system

• natriuretic peptides

Renin-angiotensin system

• renin secretion from juxtaglomerular complex

• drop in BP

• nervous stimulation

• decline in osmotic concentration of filtrate in DCT

Natriuretic peptides

• secreted by heart in response to high blood volume

• causes vasodilation of afferent arterioles

• causes vasoconstriction of efferent arterioles

• causes a decrease in Na+ (and water) reabsorption at nephron

What is Juxtaglomerular Complex made up of?

• made up of cells of the macula dense and juxtaglomerular cells

What does the macula dense do? What do the juxtaglomerular cells do?

• macula densa= stimulate juxtaglomerular cells in response to a drop in Na+ levels in filtrate

• juxtaglomerular cells= secrete renin in response to a decline in BP in afferent arteriole, nervous stimulation, or when stimulated by cells of macula dense

What do renin release do?

• renin release concerts angiotensinogen to angiotensin I

Angiotensin I

• is cleaved into angiotensin II in the lungs by the enzyme angiotensin converting enzyme (ACE)

Angiotensin II

• causes constriction of efferent arteriole, aldosterone release, thirst, ADH release, increase heart rate and vasoconstriction in periphery

Erythropoietin

• released by the juxtaglomerular complex

• released in response to low blood O2 levels

• stimulates red bone marrow to increase RBC production

Autonomic Regulation

• regulates GFR

• sympathetic nervous system stimulation causes vasoconstriction in afferent arteriole

• this decreases GFR and filtrate production during sympathetic activation

  • during fight or flight response

  • during strenuous exercise

Reabsorption

• certain substance are reabsorbed (remove) from the filtrate into the interstitial fluid of kidneys

Secretion

• certain substances are added to the filtrate from the interstitial fluid of the kidneys

Where is filtrate? What is peritubular fluid?

• filtrate is contained in the nephron

• peritubular fluid is the extracellular fluid outside the nephorn

• peritubular fluid contents are taken up by peritubular capillaries and vasa recta

Peritubular Fluid Concentration

• has vastly differing solute concentrations depending on where in the renal medulla you look

• near cortex= 300mOsm/L, deep in medulla= 1200mOsm/L

Proximal Convoluted Tubule (PCT), What is reabsorbed? What is filtered?

• cells forming wall of PCT have microvilli to increase surface area

• filtrate here is similar to plasma without proteins

• the PCT reabsorbs 60-70% of filtrate produced

• reabsorption of glucose, AAs, ions

• reabsorption of water by osmosis

• secretion of H+

• all reabsorption into peritubular fluid

Nephron Loop (Loop of Henle)

• the “loop” reabsorbs 50% of water that is left in filtrate and 66% of Na+ and Cl-

• Descending limb and ascending limb have differing permeabilities

  • (thin) descending limb is permeable to water, but not silutes

  • (thick) ascending limb is not permeable to water or solutes, but has active transport mechanisms for removal of Na+ and Cl- from filtrate

Ascending Limb (thick)

• cells of thick ascending limb actively transport Na+, K+ and Cl- out of filtrate into cell at apical surface

• cells then actively transport Cl- and K+ out basal surface into peritubular fluid

• cells then also use Na+/K+ ATPase pump to return K+ into cells and pump Na+ out into peritubular fluid

• K+ then diffuses back into filtrate through passive channels

Different permeabilities of descending vs. ascending limbs

• helps create osmotic gradient in medulla

• facilitates reabsorption of water and solutes

Countercurrent Multiplication

• between ascending and descending limbs of loop

• permits passive reabsorption of water from tubular fluid

• picture drawn in notebook

Countercurrent Multiplication

Distal Convoluted Tubule

• 15-20% of original filtrate reaches DCT

• Reabsorption of Na+ and Cl- through active transport

• reabsorption of water

• secretes K+ and H+ from peritubular fluid into filtrate

What control is DCT reabsorption under?

• under hormonal control

• aldosterone increases Na+ reabsorption

• ADH increases water reabsorption

• effects of aldosterone and ADH are opposed by natriuretic peptides

Collecting Duct (what is reabsorbed?)

• reabsorption also regulated by ADH and aldosterone

  • aldosterone- more Na+ reabsoprtion

  • ADH- more water reabosprtion

  • both opposed by natriuretic peptides

• Reaborbs HCO3- and H+

• urea is also reabsorbed

Anti-diuretic Hormone

• The DCT and collecting duct are impermeable to water except in the presence of ADH

  • stimulates water channels to be produced and embedded in the apical surface of the DCT and collecting duct cells

  • water is then reabsorbed into peritubular fluid

  • facultative water reabsorption

  • this mechanism reabsorbs about 26L per day

  • diabetes insipidus

Peritubular capillaries and Vasa recta

• take up peritubular fluid and carry away both water and solutes and returns them to the circulation

Normal Urine

• clear/light yellow

• sterile (no bacteria)

• volume: 700mL - 2000mL per day

• pH= 6.0

• protein= none (unless after intense workout for short period)

• blood= none (unless after intense workout for short period)

• glucose= none (unless w/ diabetes mellitus)

Ureters

• muscular tubes that extend from the kidneys to the urinary bladder

• peristaltic contractions occur about every 30 seconds to move urine along toward bladder

3 tissue layers of Ureters

• inner transitional epithelium

• middle smooth muscle layer

• superficial connective tissue

Urinary Bladder

• located w/in pelvic cavity - posterior to pubic symphysis

• “boat-shaped”

Where do ureters enter bladder? Urethra?

• ureters enter bladder on posterior/inferior portion

• urethra exits bladder inferior to ureter openings

• urinary trigone

4 tissue layers of urinary bladder

• inner transitional epithelium (mucosa)

• submucosa

• muscular layer (detrusor muscle)

• superficial connective tissue layer

Urethra (What is it? what does it do? how long?)

• tubular structure that extends from urinary bladder to exterior

• transports urge from bladder to exterior

• tissue lining urethra varies from transitional epithelium near bladder to stratified squamous epithelium near external environment

• 18-20cm long in males and 3-5cm long in females

What is where the urethra exits the bladder?

• a circular band of of smooth muscle called

• where urethra passes through pelvic floor musculature, there is a circular band of skeletal muscle called: external urethral sphincter

• provide voluntary control over discharge of urine

Male Bladder and Urethra

Female Bladder and Urethra

Voiding/Urination

1. stretch receptors in the wall of the urinary bladder are bladder are stimulated

2. neuron signals are sent through pelvic nerves into spinal cord where motor neurons controlling the detrusor muscle are stimulated

3. urination involves conscious relaxation of external urethral sphincter, which causes relaxation of internal urethral sphincter

Incontinence

• inability to control urination

• damage to urethral sphincters

• damage to pelvic nerves

• damage to central nervous system

UTI

• bacterial infection of urinary bladder (lower UTI) OR bladder and ureters and renal pelvis (upper UTI)

• prevention through hydration and proper hygiene (front to back)

• more common in females

• symptoms

  • frequency (feel like you have to go to the bathroom all the time), pain with urination, flank pain, fever

Kidney Stones

• mineral deposits that form in urine in the kidneys

• painful as they pass through ureters

• calcium oxalate (80%)

• prevention through hydration, cannot form in dilute urine