Key Concepts for the Urinary System

Chapter 25 (pg. 997-1029)

 

Urinary System

Purpose

Regulate volume and chemistry of blood.

Remove waste and fluids

Regulate red blood cell counts-EPO

Gluconeogenesis- makes glucose out of non-carbohydrates. This is normally a

Liver function, this is caused from prolonged fasting.

 

Organs

Kidneys

Ureters

Urinary bladder

Urethra

Location and External Anatomy

Retroperitoneal-T12-L3, surrounded by three membranes:

Renal capsule-fibrous/tough, protects kidneys from infection in surrounding

tissue

Adipose capsule-fatty mass cushions and protects. Also attaches the kidney

to the posterior wall.

Renal fascia-outer membrane attaches the kidney to adrenal gland to surrounding

structures.

Internal Anatomy

Regions

Cortex-lighter

Medulla-darker, pyramids

Pelvis-flat, funnel shaped. Joins with ureter at hilus

Calyces-extensions of pelvis

Calyces major and Calyces minor surround the papillae

Blood and Nerve Supply (Illustration pg. 999)

Renal arteries deliver of the total cardiac output to kidneys each minute

Perpendicular branches of abdominal aorta, the right artery is longer than left

Each artery branches into 5 segmental arteries just outside hilus of kidney

They then branch into lobar arteries inside kidney

They then divide to form several interlobular arteries

At the base of the medulary pyramid they then branch into the arcuate arteries

Renal Veins

Interlobular

Arcuate

Interlobar

Renal veins

There are no lobar or segmental veins

 

 

Renal Plexus (ANS)

Nephron

The structural and functional units of the kidneys

Each nephron consists of:

Glomerulus

Bowmans capsule

Renal tubule

Glomerulus-fenestrated capillaries

Forms filtrate

Solution is a protein free, solute rich fluid, which passes from blood

Bowmans capsule

Particles of (CHON) filtered out

Bowmans capsule

Parietal Layer-simple squamous, no filtration function

Visceral layer- by glomerulous, has branching epithelial cells called

Podocytes (foot cells). They terminate in foot processes, which

intertwine as they cling to the basement membrane of the

glomerulus.

The clefts or openings between the foot processes called filtration

slits, allow filtrate to enter the capsular space.

Renal tubule

3 parts

Proximal convoluted tubule, loop on Henle and distal

convoluted tubule

Collecting ducts- receives filtrate from many nephrons

Run through the medullary pyramids and give them a striped

appearance.

They come together at the papilla and form papillary ducts

which deliver urine to the minor calyces.

Principle cells-no microvilli

Intercalated cells-microvilli

Proximal Convoluted Tubule

Simple cuboidal epithelial cells with brush border

absorb water from filtrate and secrete substances into it

Loop of Henle

Descending-proximal part-same as proximal convoluted tubule

Distal part-thin segment-simple squamous-permeable to water

Ascending-cuboidal/columnar-thick segments

Distal Convoluted Tubule

Cuboidal, but thinner than proximal convoluted tubule, reduced

microvilli/brush border

Cortical Nephrons represent 85% of the nephrons in the kidneys and are

found mostly in the cortex

Juxtamedullary nephrons represent 15% in the kidneys, these are

important in urine concentration, they have more extensive thin

segments, and their loops of Henle invade deeply the medulla

Capillary Beds of Nephrons

Glomerulus

Fed and drained by arterioles, the afferent arteriole and the

efferent arteriole

2 reasons why glomerular pressure is high

1)Arterioles are high-resistance vessels

2) Afferent arteriole has a larger diameter than efferent

Peritubular capillaries

Arise from efferent arterioles, tend not to break up into

peritubular capillaries, instead they form venules

They absorb water and solute removed from nephron

99% is reclaimed

Vasa Recta (Straight-vessel)

Thin walls

Extend deep into the medulla paralleling the longest loops of

Henle

Vascular resistance in microcirculation

High pressure in the efferent arterioles reinforces the high

glomerular pressure

Juxtaglomerular Apparatus

Controls filtration rate

Mechanoreceptors=pressure

Chemo receptors=solute content

Juxtaglomerular cells-Afferent arteriole wall pressure

Macula Densa-Distal convoluted tubule wall chemistry

Filtration Membrane

Porous, between blood and capsule has 3 layers

1)fenestrated epithelium (tunica intima)

2) gel like basement membrane (basal lamina of other layers)

negatively charged glycoproteins repel many

macromolecules, others stopped at filtration slits

3)visceral membrane of glomerular capsule (podocytes)Net Filtration Pressure (NFP)

NFP=HP (hydrostatic P=glomerulous) (OP osmotic pressure +HP

capsular) (pg. 1007 left column, upper paragraph for better looking

formula)

Glomerular Filtration Rate (GFR)

FR is directly proportional to NFP

3 factors governing filtration rate at capillary beds

1)surface area-not easily changed

2)permeability-not easily changed

3)NFP

Try to keep these constant, why is this important?

Too fast and too much=poor absorption

Too slow=everything reabsorbed, even waste we should eliminate

 

Renal Auto regulation

Maintain constant GFR, 2 methods

1)Myogenic mechanism-tunica media vasoconstrict increasing blood

pressure and stretches wall. Tunica media vasoconstricst blood pressure

goes down causes vasodilatation

2)Tubuloglomerular feedback mechanism- when pressure and osmolality

goes down, this caused vasodilatation of afferent arterioles. When flow,

pressure and olsmolality goes up vasoconstriction of afferent arterioles

Sympathetic Nervous System

Stress or emergency overcomes renal auto regulation

Blood to vital organs goes away from kidneys

Renin-aniotensin mechanism

Release of renin (JG cells) causes a chemical cascade

Angiotensin I = II

Angiotensin II

1)     Smooth muscle vasoconstrictor, blood pressure goes up

2)     Decrease GFR

3)     Stimulates aldosterone released by adrenal cortex, kidneys reabsorb more Na+ and H2O, blood volume and blood pressure goes up

When to start renin-angiotensin

When systemic blood pressure is below 80mm Hg

Low filtrate flow rate/osmolarity

Tubular Reabsorption

Our total blood volume filters into the renal tubules about every 45 mins. 99% is

reabsorbed

Reabsorption rate may be passive (no ATP required) or active (at least on of its

steps is driven by ATP directly or indirectly)

Active reabsorption-Na+ is very common in filtrate, 80% of energy from active

reabsorption

2 sodium and 3 potassium, pumps ATP, keeps tubule cells low in salt this

creates electrochemical gradient

Passive tubular reabsorption-obligatory water absorption, and obliged to follow

sodium

Secondary active transport-glucose, amino acids, vitamins are substances

reabsorbed by a carrier molecule. Lets sodium follow gradient, and carries

something else along

Non-reabsorbed substances

Substances include-urea, uric acid and creatinine

No carriers, not lipid soluble, too big for pores, tight junctions and nitrogenous

Absorptive capabilities of different regions

Proximal convoluted tubule-most active area of reabsorption, reabsorbs anything

Loop on Henle-selective reabsorption

Descending- water volume

Ascending- Na, K,Cl (active),Ca+2 and Mg+2

 

Key Concepts for Fluid and Electrolyte Balance

Chapter 26 (pg. 1034-1059)

 

Body water content

Water content for a health man is 60% and 50% for a health woman, about 40 L

Fluid compartments

Intracellular fluid compartment (ICF)-in a healthy male of average size ICF

Accounts for 25 L of the 40 L of body water. This consists of trillions of

compartments: the cells

Extracellular fluid (ECF)- internal environment of each cell, there are 2

subcompartments- 1) plasma and 2)interstitial fluid, the fluid in a microscopic

spaces between tissue cells.

Composition of fluids

Electrolyte- osmolarity concentration, ionic bonds; ionize or dissolve in water; dissolve easier and more important for osmolarity

Electrolyte concentrations are expressed in milliequivalents per liter (mEq/L)

mEq/L= ionic concentration (mg/L) no. of electrical charges

atomic weight of ion on one ion

Intra and extracellular fluid (pg. 1036 illustration)

Na+ / K+- sodium potassium pumps

Movement of fluid from compartments

Anything that changes the solute concentration in any compartment leads to

net water flows or movement

Plasma circulates throughout the body and links the external and internal

environments (lungs, GI, kidneys) exchanges occur continuously

Water balance and extracellular fluid osmolality

Input=ingest=food, drink and metabolic water

Output-urine (60%), perspiration (8%), insensible (expired air, 28%), feces

(4%)

Thirst-hypothalamus-salivary glands obtain the water they require from the blood,

So when blood volume is down, increase in thirst-about 10%

Osmoreceptors lose water by osmosis to the hypertonic ECF, or excited by

baroreceptors inputs, these events cause thirst-2-3%

 

Regulation of water output

Obligatory water losses=500ml of urine a day

Kidneys regulate water and sodium solutes

Disorders of water balance

Dehydration-output is greater than input; fever, confusion, kidney pain and

hypovolemic shock

Hypotonic hydration-overhydration-nausia, cramping, edema, coma, death

can be corrected by manitol IV which makes osmolaity go up

Edema-swelling-accumulation of fluid in interstitial space. Increased blood

pressure and congestive heart failure are factors that accelerate fluid loss from

blood

Electrolyte balance

Salt balance in body-90% of solutes in ECF

Major osmotic pressure, water follows salt, we reabsorb and reuse

GI disorders can lead to large salt losses in feces or vomit

Sodium regulation

No sodium receptors!

60-65% reabsorbed at PCT

25% at loop of Henle

Influence of aldosterone

Causes sodium and water reabsorption at distal convoluted tubules and collecting

ducts

When aldosterone release in inhibited, no sodium reabsorption occurs beyond

the distal tubule.

Juxtaglomerular apparatus- responds to sympathetic stimulation, decreased

osmolality or decreased stretch, the JG cells release renin. Renin catalyzes the

series of reactions that produce angiotensin II that prompts aldosterone release.

Adrenal cortical cells also directly stimulated to release aldosterone by elevated

K+ levels. Aldosterone brings effects slowly, over hours or even days