What are the organs of the urinary system?
- 2 kidneys
- urinary bladder
- 2 ureters
- urethra
What are the functions of the kidneys?
- FILTER blood
- REABSORB water
- REABSORB needed substances into blood
(remaining water and waste removed in urine)
REGULATES - blood volume and composition
- blood pH
- blood pressure
- contribute to metabolism
(calcitriol, erythropoietin, gluconeogenesis)
Where are your kidneys? In relation to the peritoneum?
- between T12 and L3
- partially protected by 11th/12th rib
- right kidney lower than left
- retroperitoneal (kidneys and ureters)
What are the three layers that surround and protect the kidneys?
- renal capsule
- adipose capsule
- renal fascia
Identify the hilum, cortex, medulla, minor and major calyces and the renal pelvis.
- renal hilum => ureter, blood, lymphatic vessels
- renal cortex => outer layer of kidney
- renal medulla => area in center (made of renal pyramids, each kidney has 8-18)
- major calyx => collect urine from minor calyces, empty into renal pelvis
- minor calyx => drain pyramids as papillae
- renal pelvis => funnel-shaped tube continuous with ureter
Trace urine flow.
renal pyramid => minor calyx => major calyx => renal pelvis => ureter
Does urine change after it leaves the collecting duct?
Nope
What is the functional unit of the kidney?
NEPHRON => number CONSTANT from birth, can function on 2/3 of one kidney, FORMS the urine (2 classes = cortical, juxtamedullary)
What are the parts of a nephron?
- RENAL CORPUSCLE \+ glomerulus \+ glumerular (Bowman’s) capsule - RENAL TUBULE \+ proximal convoluted tubule \+ nephron loop (loop of Henle) \+ distal convoluted tubule \+ collecting duct
What three processes are used to make urine?
1 - glomerular filtration
2 - tubular reabsorption
3 - tubular secretion
What process(es) take place in each section?
FILTRATION => filtered bad stuff out, good stuff taken back (ONLY in renal corpuscle)
REABSORPTION => picking good stuff back out
SECRETION => tossing bad stuff into waste basket
What are the two types of nephrons and how do they differ?
CORTICAL => 85% of nephrons; almost entirely in CORTEX
JUXTAMEDULLARY => long nephron LOOPS deeply invade medulla, ascending limbs have THICK and THIN segments, important in production of concentrated urine
Why is the structure of the efferent and afferent arterioles so critical to the functioning of the kidney?
- AFFERENT arteriole divides into ball shaped capillary network called the GLOMERULUS (larger than efferent)
- capillaries rejoin to form EFFERENT arterioles, then peritubular capillaries
How much kidney do you need to survive?
2/3rds of one kidney to survive
Describe the vascular system surrounding the nephron.
- blood supplied to the kidneys by renal artery
- VASA RECTA => a series of straight capillaries in the medulla
What three structures make up the filtration membrane?
- fenestrated endothelium
- basement membrane of glomerulus
- slit membrane (between pedicels)
What does each layer hold back from the filtrate?
- fenestrated endothelium (prevents the passage of cells)
- basement membrane of glomerulus (prevents passage of large proteins)
- slit membrane (prevents passage of medium sized proteins)
What type of epithelium is found in the proximal convoluted tubules? What functions occur here?
- CUBOIDAL and they have MICROVILLI
- about 65% of water and 100% of some solutes are reabsorbed here
What is the juxtaglomerular apparatus? How does it function to control blood pressure locally and systemically?
- specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole
- helps regulate arterial blood pressure and rate of filtration
- secretes RENIN when blood pressure DECREASES to convert angiotensinogen to angiotensin 1
How is angiotensin II formed, and how does it increase blood pressure?
- angiotensin 1 is converted to angiotensin 2 in the lungs
- angiotensin 2 is a potent VASOCONSTRICTOR = INCREASES blood pressure
What are mesangial cells?
hold together afferent arteriole and distal convoluted tube
What two types of cells make up the distal convoluted tubules and collecting ducts, and what are the functions of these cells?
PRINCIPAL CELLS => respond to antidiuretic hormone (ADH) and aldosterone
INTERCALATED CELLS => secrete H+ (rids body of excess acid)
What force drives fluid out of the glomerular capillaries?
glomerular hydrostatic pressure
What two forces oppose glomerular filtration?
- plasma colloid osmotic pressure
- capsular hydrostatic pressure (in Bowman’s capsule)
What is tubular reabsorption?
- the process by which the nephron removes water and solutes from the tubular fluid (pre-urine) and returns them to the circulating blood
- reabsorption takes place mainly in the PROXIMAL convoluted tubule of the nephron
- reabsorbs water, glucose, amino acids, urea, and ions (esp. Na+, Cl+, K+)
What is reabsorbed in the PCT?
- 65% of water
- 100% of glucose
- 100% amino acids
- 50% urea
- 65% of potassium
- 65% chloride
- 67% of sodium
How is sodium moved from the filtrate to the peritubular capillaries?
sodium ions are reabsorbed by ACTIVE TRANSPORT
What do we mean by “where sodium goes, water follows”?
“the body has a bunch of semi-permeable membranes that only allow some particles to pass through”
- water follows sodium => OSMOSIS
- sodium follows water => DIFFUSION
What do we mean by a transport maximum, and what happens when it is exceeded?
- maximum amount of a substance that can be reabsorbed per unit time (mg/min.)
- when EXCEEDED => the plasma concentration for a substance at which the transport maximum is exceed and begins to spill into the urine => RENAL THRESHOLD
How does ADH affect the DCT and collecting ducts? What does aldosterone do here?
ADH => stimulates cells to put water channels (aquaporins) in the membrane (responsible for facultative water reabsorption)
ALDOSTERONE => causes principal cells to synthesize sodium pumps / potassium to be secreted in urine
What is tubular secretion, and what are the two main functions of tubular secretion?
“the transfer of materials from peritubular capillaries to the renal tubular lumen”
- secretion of substances to eliminate them from body (ions, nitrogen containing waste (urea, creatinine), and certain drugs)
- secretion of H+ to maintain blood pH
What is meant by “facultative water reabsorption”?
- water withdrawn from the plasma filtrate as a regulated negative feedback control of hydration status under the influence of ADH
- in the absence of ADH the collecting tubule is impermeable to water and therefore no water is reabsorbed
How is a dilute urine produced? A concentrated urine?
DILUTE => absence of ADH (tubules absorb more solutes than water) [4x more dilute than plasma]
CONCENTRATED => created by long loops of Henle of juxtamedullary nephrons with a countercurrent mechanism [4x more conc. than plasma]
What is meant by the countercurrent mechanism?
establishes a concentration gradient in the interstitial fluid of the medulla of the kidney
What is diabetes insipidus?
- disorder of salt and water metabolism marked by intense thirst and heavy urination
- occurs when the body can’t regulate how it handles fluids
- the condition is caused by a HORMONAL abnormality and NOT related to diabetes
Describe the ureters. How do they conduct urine to the bladder? What do we find at the junction of the ureters and the bladder?
- muscosa, muscularis, fibrous coating
- mucosa is TRANSITIONAL EPITHELIUM
- mucus protects ureters from urine pH and concentration
- PERISTALSIS aids in movement of urine (and hyrdostatic pressure, gravity)
- outer coat (adventitia) contains blood vessels and sensory nerves
- renal calculi => shock wave lithotripsy
Describe the urinary bladder (location, layers, type of epithelium, trigone, internal and external urethral sphincters).
- posterior to pubic symphysis
- mucosa, submucosa, muscularis (detrusor muscle), serous coat
- when the bladder is empty, the mucosa has numerous folds called RUGAE
- rugae and transitional epithelium allow the bladder to expand as it fills
- TRIGOME => no rugae, between openings of ureters and internal urethral orifice, where mucosa is tightly bound to muscularis
- internal sphincter = involuntary control
- external sphincter = voluntary contol
How does micturition occur?
- when the bladder is full of urine, stretch receptors in the bladder wall trigger the micturition reflex
- the internal urethral sphincter relaxes, allowing for urine to pass out of the bladder into the urethra
- both of these reactions are involuntary
How does the urethra differ in the male and the female? How are they the same?
MALE => ~20 cm long, seminal vesicle on back of bladder, prostate gland surround urethra, has 3 division: prostatic urethra, membranous urethra, spongy urethra, discharges semen
FEMALE => ~4 cm long, none of that ^^^
BOTH => tube leading from bladder to exterior, has muscularis and mucosa layer of circular smooth muscle, discharge urine
How do we evaluate kidney function?
Blood Urea Nitrogen (BUN) => urea is a breakdown product of proteins
- influenced by diet (not the best measure of kidney function)
Creatinine => end product of muscle metabolism
- normally constant
- can determine creatinine clearance
Of BUN and serum creatinine, which one is influenced by diet?
BUN
What does inulin clearance give us? Clearance of PAH?
Clearance of inulin => not absorbed or secreted, how fast it is cleared = GFR
Clearance of PAH (para amino hippuric acid) => not absorbed, actively secreted = good measure of renal plasma flow
What does urine normally look like?
transparent yellow
lots of water intake = clear
What would you expect to find in urine?
IONS => Na+, Cl-, K+ & nitrogenous wastes: ammonia, creatinine, urea and uric acid & foreign chemicals: drugs, pesticides, food additives, etc.
SUSPENDED SOLIDS (trace amounts) => bacteria, blood cells, casts
What would you NOT find in a normal urine sample?
- albumin
- glucose
- acetone (diabetes mellitus)
- bile (liver disease, obstruction of bile ducts, hemolytic disease)
What is a cast?
impressions of kidney tubules
What is the normal pH of urine? Why?
pH = 4.6 - 8
- it is the job of the kidneys to excrete acid into the urine
What is the normal specific gravity of urine?
1.001 - 1.035
What factors would increase the specific gravity of urine? Decrease it?
INCREASE
- damage to the tubules of kidney (a fixed specific gravity) = 1.010
- diabetes mellitus (“sugar diabetes”) = 1.030
- emesis or fever = 1.040
DECREASE
- diabetes insipidus (lack of ADH) = 1.003
What is the macula densa? What does its cells monitor?
- end of ascending loop of Henle
- Na+, Cl-, and water
How does renal autoregulation (myogenic mechanism and tubuloglomerular mechanism) affect the kidneys?
myogenic mechanism => vascular smooth muscle contracts when stretched
- when BP increases, afferent arteriole CONTRACTS
- when BP decreases, afferent arteriole DILATES
tubuloglomerular mechanism => macula densa cells monitor Na+, Cl-, and water
- when these substances decrease (low BP), JGA dilates afferent arteriole and INCREASES blood pressure in glomerulus
- opposite occurs with high blood pressure
How does hormonal regulation affect the kidneys?
- vessels in the kidney have sympathetic innervation
- at rest, efferent/afferent are maximally dilated
- MODERATE stimulation constricts both EQUALLY
- STRONG stimulation constricts afferent > efferent, reducing glomerular filtration rate (GFR)
- ^^^ this triggers release of RENIN
How does neural regulation affect the kidneys?
- secretes RENIN when blood pressure DECREASES to convert angiotensinogen to angiotensin 1
- angiotensin 1 is converted to angiotensin 2 in the LUNGS
- angiotensin 2 is a potent VASOCONSTRICTOR = INCREASES blood pressure
- angiotensin 2 stimulates release of ALDOSTERONE and acts on kidney to INCREASE Na+ reabsorption
- angiotensin 2 stimulates THIRST center of hypothalamus and release of ADH from the posterior pituitary gland
What do angiotensin-converting enzyme (ACE) inhibitors do?
prevents conversion of angiotensin 1 to angiotensin 2
What effect does RENIN have on peritubular capillary pressure?
DECREASES by constricting afferent arteriole and increases fluid reabsorption
What effect does RENIN have on glomerular mesangial cells?
causes them to CONTRACT => reduces total surface area of glomerular capillaries available for filtration, causing more fluid to be retained
Making urine is like…
…making chicken soup.
The only place where filtration takes place is…
…the renal corpuscle.
The inner layer of the filtration membrane holds back _____ _____ from the blood.
formed elements
When we think about the proximal convoluted tubule we are thinking about…
…tubular reabsorption.
When we think about the distal convoluted tubule and collecting duct we are thinking about…
…tubular secretion.
Which is larger: the afferent arteriole or efferent arteriole?
afferent arteriole
Urine is changed after it leaves the…
…renal papilla.
Order the parts of the nephron:
1 renal corpuscle 2 proximal convoluted tubule 3 nephron loop 4 distal convoluted tubule 5 collecting duct
The filtration membrane holds back _____ _____ and _____.
blood cells
proteins
The inner layer of Bowman’s capsule is made up of cells called:
podocytes (with fingerlike projections called pedicels)
What force promotes filtration?
blood hydrostatic pressure (pressure in blood in glomerulus)
What forces opposes filtration?
blood colloid osmotic pressure
hydrostatic pressure of fluid in Bowman’s space
Osmotic pressure is going to be determined by…
…the number solute particles.
The part of the nephron that is responsible for producing concentrated urine is…
…nephron loop.
Tubular secretion:
rids the body of waste, drugs, H+
In order for water to be reabsorbed in the collecting duct…
…ADH needs to be present.
A concentrated urine is produced due to the concentration of…
…sodium in the medulla.
