URINARY SYSTEM

Lecture Notes

I. Major Functions-The basic idea of the Urinary system is to alter blood conditions.

A. Regulates blood volume and blood pressure by altering water reabsorption levels)

B. Regulate blood pH by regulating H+, NH4+ (ammonia) and HCO3– (bicarbonate) concentrations

C. Controls ionic composition of blood ( Na, Cl, Ca, K)

D. Eliminate urea and ammonia (which were produced by liver)

E. Produce Hormones (EPO and Calcitriol)

F. Deaminate amino acids to produce glucose for gluconeogenesis

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List functions of the system and relate to other systems.

II. Organs of urinary system

A. Kidney

1. Gross anatomy
a. capsule= connective tissue protective layer

b. cortex (with nephron tubules) filled with interstitial fluid with high osmotic pressure

c. medulla (with urine-conducting ducts) filled with interstitial fluid with higher osmotic pressure

2. Blood flow in kidney
a. renal arteries enter hilus followed by series of arteries delivering blood to afferent arteriole which regulates blood flow to glomerular capillary.

b. After glomerular capillary, blood flows through efferent arterioles then to peritubular and vasa recta capillary networks which surround nephron tubules.
(The glomerular capillaries fiter blood to form the filtrate. The peritubular and vasa recta capillary networks exchange (reabsorb or secrete solutes and water) with the fitrate already in nephron tubules)

c. Blood then drains from capillaries to various venis and finally to the renal vein.

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List the urinary vessels and structures that blood flows through, starting with abdominal artery and ending with inferior vena cava.

3. Filtrate ( and urine) flow in system
a. Nephron ( glomerular capsule and tubules) =site of filtrate production and modification to form unine

b. Papillary ducts =drain urine from nephron collecting duct tubule

c. Minor/major calyces =drain urine from papillary ducts

d. Renal pelvis==drain urine from calyces into ureter

B. Ureters (continuing urine flow into urinary bladder)

C. Urinary bladder
folded mucosal layer (as in stomach) allows storage of urine
muscular layer (as in stomach, too) promotes movement of urine out to urethra

D. Urethra (urine exits the body)

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Trace the flow of filtrate/urine from glomerular capsule to out of the body.

III. Histology

A. Nephron

1. Glomerular corpuscle
a. Glomerular capillary forms filtration membrane which allows small substances to be filtered out of blood.
Filtration membrane consists of fenestratrated capillary walls, non-cellular layer and podocyte cells that act as filters to prevent larger particles (cells and proteins) from exiting blood and entering capusular space

b. Glomerular (Bowman's) capsule surrounds glomerular capillary
and captures the fluid passed through filter membrane (filtrate).

2. tubules
a. Proximal convoluted tubule (PCT) made up of cuboidal cells with microvilli, designed for maximum reabsorption (like those in intestine). Membranes contain Na/H, Na/NH4, Na/Glucose and Na/Amino acid symporter pumps. All pumps actively transporting Na (with solute) into interstitial fluid surrounding tubules. Pumping Na into interstitial fluid increases cortex osmotic pressure. Membrane permeable to water.

b. thin and descending part of Loop of nephron (Henle) made up of simple squamous epithelium. Has Cl and Na pumps. Permeable to water.

c. thick ascending part of loop and early Distal convoluted tubule (DCT) made up of simple cuboidal to columnar cells. Contains Na and Cl pumps. DCT has juxtaglomerular apparatus (JGA) next to afferent arteriole that produces renin when filtrate flow is low (same as low blood pressure). DCT also has hormonally (PTH) activated Ca pumps. Impermeable to water.

d. collecting ducts are made up of cuboidal cells with cells that have receptors for Aldosterone (activating Na/K pumps) and ADH (increasing water pores and permeability)..

B. Ureters (transport urine)


1. Mucosa of transitional epithelium (stretchable epithelium )
2. Muscularis (long and circular smooth muscle layer)
3 Fibrous coat- serosaC. Urinary bladder (store and eject urine)
1. Mucosa of transitional epithelium is folded into rugae both allow for increased storage of urine.
2. Submucosa
3. Muscularis of smooth muscle (also called the detrusor muscle) provides force for urine ejection). Muscularis will
expel urine via micturition reflex (spinal stretch reflex)
4. Skeletal muscle sphincter at base of uriaray bladdert can be controlled by conscious action
5. Fibrous coat- Serosa or adventitia
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IV. Physiology of nephron

A. Three processes
1. Glomerular filtratration forms the filtrate.Filtrate components determined by filtering membrane function.

2. Tubular reabsorption allows body to reclaim substances potentially lost in filtrate.
a) driving force for most reabsorption is active transport of sodium ions into interstitial fluid of cortex. This drives osmotic pressure up in the cortex. Reabsorption of ions in loop increase osmotic pressure of interstitial fluid of medulla. Some solutes are actively reabsorbed with Na. Water and some other solutes passively move down their electrochemical gradient determined by Na levels.
b. urea is reabsorbed into medullary space and poorly recovered into loop-this keeps urea levels high outside the tubules (second important factoring increasing medulary osmotic gradient after Na reabsorption)

3. Tubule secretion allows body to dispose of substances that are potentially harmful in high concentrations.
driving force is always active pumping.
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V. Glomerular filtration
A. Filtered substances
1. Fluid is forced through glomerular capillary membrane by blood pressure (about 10 mmHg)
about 180 liters/day are filtered but only about 2 liters/day are urinated (1-2%) therefore most of filtrate is reclaimed

2. Water, glucose, amino acids, urea, small proteins and ions are filtered through filter membrane into capsule to form filtrate..

3. Large proteins, viruses and cells are not filtered. filtrate = blood without plasma proteins (some hormones, most antibodies, fibrinogen and albumin)  and cells

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Compare and contrast which blood components are filtered through the filter membrane into the capsule

B. Regulation of Glomerular filtration rate ( and therefore Blood volume and blood pressure.)
rate regulated to maintain proper flow for appropriate levels of reabsorption and secretion.

1. GFR is directly dependent on blood pressure.
a. If GFR (BP) is too high, filtrate flows too fast and substances are NOT reabsorbed (-urine flow increases-water is lost-blood volume drops-blood pressure drops.)

b. If GFR (BP) is too low, filtrate flows too slow and substancesare retained too much (-urine flow decreases-water is preserved-blood volume increases-blood pressure increases.)

2. Mechanisms of GFR regulation

a.. Autoregulation
1) Myogenic mechanism-very rapid changes
stretching of smooth muscle in afferent arteriole, due to rising blood pressure, results in vasoconstriction. Vasoconstriction reduces flow through tubules so less water is lost.
relaxing of smooth muscle in afferent arteriole, due to lowering blood pressure, results in vasodilation. Vasodilation returns flow to normal so reabsorption is normalized.


2) Tubuloglomerular feedback
Macula densa cells of Juxtaglomerular apparatus (at junction of capsule and DCT tubule) sense water and ion leves and provide feedback to afferent arterioles near glomerulus via production of vasodilator (NO)

b. ANS regulation of vasoconstriction of afferent arteriole ( usually only during extreme sympathetic stimulation)

Decreased BP would cause increased Sympathetic stimulation which causes increased vasoconstriction of afferent arteriole. This results in decreased GFR (with associated increased time for water reabsorption and decrease in urine production) hence less water loss which builds blood volume and blood pressure.

c.. Endocrine system
Increased ANP (stimulated by high blood pressure) increases filtration (by relaxing glomerular capillaries)

Increased Angiotensin II (stimulated by increased Renin, which promotes Ang II) decreases filtration rate by constricting afferent arterioles.

 

VI. Tubular reabsorption (movement from tubule into interstitial fluid and then blood)

A) Proximal convoluted tubule -most reabsorption of substances occurs here
Sodium ions reabsorbed via active symporter pumps.
all glucose and amino acids move with Na in symporter pumps.
most Cl– and almost all HCO3- ions passively diffuse down their electrical gradient, following Na+
positive ions (K+ and Ca++) diffuse down their electrical gradient, following negatively charged HC03 and Cl-
most water moves to accumulation of ions in interstitial fluid (movement of Na+, Cl– and other dissolved substance) by osmosis (osmotic pressure being higher in interstitial fluid than filtrate)

B). Descending loop of Henle (always permeable to H2O)
passive H2O reabsorption following osmotic gradient set up by Na and CL reabsorption by ascending loop in medulla (and accumulation of urea also in medulla)

C. Ascending loop of Henle (always impermeable to H2O)
does not allow water back into filtrate but pumps out Na and Cl
(Medullary osmotic gradient magnified by this countercurrent multiplier)

D. Distal convoluted tubules
1. Ca+ reabsorption promoted by PTH.
2. more Cl and Na+ ions reabsorbed promoting little more water reabsorption

From PCT to DCT all reabsorption is obligatory-it will always occur. In the collecting ducts, hormones determine reabsorption.

E . collecting ducts (reabsorption/secretion due to levels of hormone, ADH or Aldosterone)

ADH
1. if high levels of ADH then more water pores made in cells so more water reabsorbed (forming little concentrated urine)
2. ) if low levels of ADH then less water pores made in cells so water not reabsorbed (forming copius dilute urine)

Aldosterone
 c) if high levels of aldosterone then more Na/K+ pumps made in cells so more Na reabsorption/K secretion
d) if low levels of aldosterone less Na/K+ pumps made in cells so less Na reabsorption/K secretion.

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Identify which filtrate components are reabsorbed. Describe the importance of generating an osmotic gradient with Na pumping and urea recycling to reabsorption of other ions and water.

 VII. Tubular secretion-always actively move ions from interstitial fluid into filtrate A PCT
H+ ions secreted in exchange for Na+ , NH4 can also be secreted with this route (both of these processes alters body fluid pH

B. Collecting ducts
K+ secretion under the influence of increased Aldosterone -
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Identify which ions are primarily secreted at each tubule.

VII. Formation of urine (95% H2O, 5% solutes)
Medullary osmotic gradient is produced by Na pumping and urea recycling. Gradient has more concentrated medullary extracellular fluid than cortical fluid. As such filtrate loses water at bottom of loop but cannot reclaim because ascending loop limb is not permeable to water.
A.. Dilute urine -- produced when body is over hydrated
no water is lost by collecting tubules so urine is hypotonic and large volume

B. Concentrated urine -produced whern body is dehydrated (ADH produced)
water is reabsorded so urine is hypertonic and small volume

Describe how the medullary osmotic gradient coupled with varying ADH production affect urine concentration and volume.

IX. Hormonal regulation of blood pressure

1 ANP (Atrial Natriuretic Peptide)-produced by atrial cardiac muscle cells
a. Action-
Decreases retention of Na by nephron cells
Decreases production of Aldosterone by zona glomerulosa cells of adrenal cortex
Decreases production of ADH by neurosecretory cells of hypothalamus/posterior pituitary

Overall response is to lower blood volume and pressure

b. production stimulated by high blood volume and pressure in atria of heart.

2. Renin-Angiotensin-Aldosterone produced by juxtaglomerular appratus cells in nephron, ACE makes Angiotensin from Renin, Angiotensin promotes Aldosterone production.
a. Action-
Increase production of Aldosterone by nephron cells (Increased Na retention, followed by water)
Increase vasoconstriction by smooth muscle cells in arteriole (so less filtration)

b. production stimulated by low blood volume and blood pressure
Overall response is to increase blood volume and pressure.

3. ADH (antidiuretic hormone) -produced by neurosecretory cells in hypothalamus that extend to posterior pituitary
a. Actions
Increases retention of water by tubule cells, then reduces volume of urine but increasing water and blood volume
Increases vasoconstriction of smooth muscle cells in arterioles increasing blood pressure under extreme conditions

Overall response is to increase blood volume and pressure
b. stimulated by high osmotic pressure (dehydration, loss of blood) as measured by hypothalamic osmoreceptors

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Compare and contrast neural vs hormonal feedback of filtrate production.

Construct the hormonal feedback loops that lower and raise blood volume and pressure. CLICK TO VIEW AND PRINT THE COMPLETE HORMONE CYCLE TEMPLATE.

X Reguation of pH by urinary system
during low blood pH (acidosis) ......increased Hydrogen secretion (and increased bicarbonate reabsorption) reduces free H ions in blood (thereby increasing blood pH)

during high blood pH (alkalosis) ......decreased Hydrogen secretion (and decreased bicarbonate reabsorption) increases free H ions in blood (thereby decreasing blood pH)

Professor Thomas M. Lancraft

Human Anatomy and Physiology Courses 
at St. Petersburg College
St. Petersburg/Gibbs Campus

5/2008