Urinary

System

Learn this and you could be a real “whiz!”

Functions of the Urinary System

• Excretion

• Regulation of blood pressure and

volume

• Regulation of blood solute

concentrations

• Regulation of extracellular fluid pH

• Regulation of RBC synthesis

• Vitamin D synthesis

Organs of the Urinary System • Kidney – External

– Bean shaped, fist sized

– Perirenal fat encases each kidney

– Renal capsule surrounds each kidney

– Renal fascia surrounds perirenal fat

– Hilum – renal artery and nerves, renal vein and ureter

– Renal sinus

Organs of the Urinary System • Kidney – Internal

– Cortex

– Medulla

– Renal columns

– Renal pyramids

• Bases of pyramids separate medulla from cortex

• Renal papillae – points of pyramids – Minor calyces

– Major calyces – minor calyces merge to form these which merge to form renal pelvis

Organs of the Urinary System • Kidney – Internal

– Nephron

• Functional unit of kidney

• Juxtamedullary – long loops of Henle

• Cortical – shorter loops of Henle

– Renal Corpuscle

• Bowman’s capsule

• glomerulus

Terms to Know • Bowman’s capsule (glomerular capsule) • Glomerulus • Proximal convoluted tubule • Podocyte cells • Fenestrae and filtration slits = filtration membrane • Afferent arteriole • Efferent arteriole • Juxtaglomerular cells and macula densa = juxtaglomerular

apparatus • Proximal convoluted tubule • Loop of Henle • Distal convoluted tubule • Collecting ducts

Pathway

Through the

Kidney

Pathway Through the Kidney

• Afferent arteriole glomerulus (Efferent arteriole) filtration membrane proximal convoluted tubule loop of Henle (descending limb) loop of Henle (ascending limb) distil convoluted tubule collecting ducts

Pathway

Through the

Kidney

Renal Arteries and Veins • Renal artery segmental arteries

interlobar arteries arcuate arteries interlobar arteries afferent arteriole efferent arteriole peritubular capillaries vasa recta interlobular veins arcuate veins interlobular veins renal vein

Renal Arteries and Veins

Urine Production • 3 processes essential for urine production

– Filtration – production of filtrate

– Tubular reabsorption

– Tubular secretion

Urine Production • Filtration

– Movement of water and solutes across the

filtration membrane

– Less than 1% of the 180L of filtrate produced

daily becomes urine

– Filtrate contains small amounts of proteins that

are reabsorbed by endocytosis in proximal

convoluted tubule

Urine Production • Filtration Pressure

– Glomerular capillary pressure (GCP) – moves fluid out of the capillary into Bowman’s capsule

• Afferent arteriole dilation, increase GCP

• Efferent arteriole constriction, increase GCP

– Capsular pressure (CP) – pressure of filtrate inside Bowman’s capsule

– Blood colloid osmotic pressure (BCOP) – proteins do not move out of the blood and form an osmotic force

Regulation of Glomerular Filtration Rate (GFR)

• Autoregulation

– Kidneys maintain GFR

• Increase blood pressure = afferent arteriole constricts to

prevent increase in renal blood flow and filtration

pressure

• Filtrate rate of flow past macula densa will send signals to

constrict/dilate afferent arteriole

• Sympathetic Stimulation

– Norepinephrine stimulates constriction of afferent

arteriole

Urine Production Tubular Reabsorption

• Proximal Convoluted Tubule

– Apical surface = inside tubule

– Basal surface = outer wall of tubule

1. Sodium is in high concentration in the lumen of the tubule

• Many substances are symported with sodium into the cells lining the proximal tubule

2. Sodium concentration increases in the cells and must be actively transported out of the cells

3. Symported substances cross the basal membrane by facilitated diffusion

• Water will follow by osmosis

Urine Production

Urine Production

Urine Production Tubular Reabsorption

• Loop of Henle

– Descending

• Highly permeable to water and moderately to urea, sodium and ions

– Water moves out; solutes move into the filtrate

• Volume of filtrate reduced another 15%

– Ascending

• Permeable to solutes, not to water

• Solutes diffuse out of the filtrate in the thin segment

• sodium, potassium, chlorine are transported out of the filtrate in the thick segment

Urine Production

Urine Production Tubular Reabsorption

• Distil Convoluted Tubule and Collecting Ducts

– Chlorine is symported with sodium through the apical surface

– Interstitial fluid solute concentration is high, so osmosis occurs

– Permeability of the collecting duct and distil convoluted tubule to water is hormonally controlled

– Renal tubules are not as permeable to urea, urate ions, creatine, sulfates, phosphates, and nitrates

Urine Production Tubular Secretion

• Movement of materials that become toxic in high concentrations into the nephron for elimination

• Hydrogen produced by carbon dioxide combining with water is antiported with sodium

Medulla Concentration Gradient • Cortex = 300 mOsm/kg

• Medulla increases in concentration to 1200 mOsm/kg

• Concentration of Medulla is maintained by

– Active transport of sodium and the symport of potassium, chlorine, and other ions out of the thick portion of the ascending limb into the medulla

– Impermeability of ascending limb to water

– Reabsorbed water and solutes enter the blood supply

– Active transport of ions into the medulla

– Urea diffuses into the medulla

Urine Concentration and Volume Hormonal Mechanisms • Renin – Angiotensin – Aldosterone

• Renin is secreted by juxtaglomerular apparatus if – Blood pressure in afferent arteriole decreases – Sodium concentration of filtrate decreases as it passes the macula densa

• Renin angiotensinogen angiotensin I angiotensin II vasoconstriction increased blood pressure

• Angiotensin II increases aldosterone secretion and ADH secretion

• Aldosterone increased sodium transport back into blood – Increased blood potassium stimulates aldosterone secretion;

decreases in blood potassium decreases aldosterone secretion

Urine Concentration and Volume Hormonal Mechanisms

• Antidiuretic Hormone (ADH)

– Secreted by posterior pituitary

– Hypothalamus osmoreceptor cells sense small osmolarity changes

• Osmolarity too high ADH released

• Osmolarity too low ADH shut off

– Blood pressure also influences ADH secretion

• BP too high ADH shut off

• BP too low ADH released

Urine Concentration and Volume Hormonal Mechanisms

• Atrial Naturetic Hormone (ANH)

– Secreted by cardiac cells (stretch)

– Inhibits sodium reabsorption in kidney

– Inhibits ADH to increase urine volume to lower blood pressure and blood volume

– Dilates arteries and veins

Urine Concentration and Volume

CVP – central venous pressure AVP – vasopressin or ADH ANH – atrial naturetic hormone (peptide)

Plasma Clearance

• Represents the volume of plasma that is cleared of a specific substance each minute

– Can be use to determine GFR (Inulin used)

– Clearance value for urea and creatinine

• Increase plasma concentration and lower than normal clearance = kidney failure and reduced GFR

– Can also be used to determine the volume of plasma flowing through the kidney each minute

Tubular Load and Tubular Maximum • Tubular Load – total amount of a substance that

passes into the nephron each minute

• Tubular Maximum – maximum rate at which a substance can be actively reabsorbed

Urine Movement • Micturition Reflex – activated when the bladder

is stretched and results in micturition, the elimination of urine

• Urine, you’re out!