1. Functions of the Urinary System
Elimination of N wastes, toxins, drugs
Regulate aspects of homeostasis
Water, electrolytes, pH
Blood pressure
Red blood cell production
Activation of vitamin D
Kidneys filter liters of fluid daily. Cleanup of wastes from metabolism, digestion, toxins. Lungs and skin also excrete some substances, but kidney is major player.
Nitrogen from amino acids made into urea as waste
Others: sodium, chloride, potassium, calcium, hydrogen ions, creatinine
Kidneys are the “poster organ” for homeostasis:
Output varies from 2-1/2 liter/day to 1 liter/hour
Kidneys produce renin (hormone) which helps regulate blood pressure
Erythropoietin hormone stimulates RBC production in bone marrow.
Vit D activation: sterol converted by UV on skin to precursor, then liver, then to kidneys for conversion to active Vit D, stimulated by PTH from parathyroid gland. Vit D essential for absorption of calcium, phosphorus from digestive tract.
Remainder of urinary system provides transport and temporary storage for waste collected by kidneys.

2. Organs of the Urinary system
Kidneys
Ureters
Urinary bladder
Urethra
Kidneys on the dorsal side of the abdominal cavity, beneath the parietal peritoneum. Th4, separate compartment from digestive system. Tucked up underneath the 12th rib, which gives it some protection.
Adrenal gland sits on top, but is a separate organ.
Curved shape, ie, kidney bean, indentation is the hilus, where point of entry and exit of arteries, veins, and nerves. Well vascularized, since its function is to clean blood. Renal artery branches off from main thoracic artery descending from heart. This ensures good blood pressure, essential to kidney function.
Kidney is surrounded by a fibrous renal capsule. Adipose capsule - Surrounds the kidney, Provides protection to the kidney - mechanical padding and holds kidney in icorrect location. If kidney falls lower, ureters may kink and not drain urine correctly, causing hydronephrosis, damages kidney.
Kidneys: principle organ, cortex, medulla. Ureters: transport urine to bladder
Urinary bladder: stores urine ( ml.) Urethra: carries urine from body, two sphincters male vs female bladder infections.
Figure 15.1a

3. Nephron Basic unit that forms urine
Filters fluid and reabsorbs needed substances
Bowman’s capsule
Tubule: proximal, loop of Henle, distal, collecting
How is blood cleaned? Due to nephrons. The structural and functional units of the kidneys
Main structures of the nephrons
Bowman’s capsule
Renal tubule
Renal tubule. About 3 cm long. Twists and turns = convoluted. Extends down to loop of Henle, reverses direction and ascends as distal tubule. Then joins collecting duct, which eventually joins up to form calyces and funnels urine to renal pelvis.
Inner wall of PT is covered with microvilli, suited for absorption. Note: opposite of filtration.

4. Tubular and Vascular Nephron Components
Blood vessels associated with tubules:
Arterioles: afferent, efferent
Capillaries: glomerular, peritubular, vasa recta
Blood vessels associated with tubules
Arterioles: afferent, efferent
Capillaries: glomerular, peritubular, vasa recta
Glomerulus = A specialized capillary bed,within a glomerular (also called Bowman’s) capsule. Attached to arterioles on both sides (maintains high pressure)
Large afferent arteriole, narrow efferent arteriole
Capillaries are covered with podocytes from the inner layer of the glomerular capsule. Podocytes are cells with many extensions, pods/feet, holes between these foot processes are filtration slits. This forms the porous membrane essential to filtration.
Filtration: starts with blood under high arterial pressure forced through this capillary bed. Smaller diameter efferent arteriole ads to high pressure, which forces fluid and solutes out of capillaries and into capsule of renal tubule. This fluid then goes into proximal tubule.
Also note second capillary bed: peritubular capillaries that surround renal tubule. These arise from efferent arteriole of the glomerulus. These are normal, low pressure capillaries, designed for absorption. Reclaim most of fluid pushed out of blood in first bed. These cap’ys attached to a venule, Cling close to the renal tubule and reabsorb (reclaim) some substances from collecting tubes. Vasa recta extends into medulla region, uses countercurrent exchange.
Figure 15.5

5. Formation of Urine 1. Filtration - passive, nonselective
2. Reabsorption - water, glucose, amino acids
3. Secretion - active; H+, NH4+, K+, drugs
Filtration - filters fluid from capillaries into glomerular capsule
Nonselective passive process. Works as long as BP is adequate. Many molcs pushed out: through capillary wall: urea, glucose, ions, amino acids. Blood cells cannot pass out to the capillaries
Rate of filtration: 1/2 c per minute = 180 L per day. Resting rate under local chemical controls; stress causes sympathetic nervous system to reduce flow.
Filtrate is collected in the glomerular capsule and leaves via the renal tubule
2. Reabsorption: 180 L reduced to 1.5 L per day. Gradients created between urine, tubule cells, interstitial space and blood in capillaries. Tubule interior surface has cells with microvilli, which selectively absorb molcs from urine. These molcs pass into interstitial space behind these cells, taken up by peritubular capillaries. Water flows passively, by osmosis. Detail on next slide.
3. Secretion - active process. Compounds which need to be eliminated are transferred into urine. Some may not have been filtered out initially (drugs), some may still remain and need to be pushed out (K+, creatinine).
Figure 15.6

6. Tubular Reabsorption
Overall: returns water and solutes to blood capillaries
1. Na+ moved by active transport from tubule cells to interstitial fluid and diffuses to capillaries
2. Cl- passively follows Na+ (balanced charge)
3. Water reabsorbed with salts
4. glucose and amino acids are cotransported with Na+ into tubule cells; then they diffuse into the interstitial fluid
ONLY energy input is to move Na+ out of tubule cell (step 1)
Orient students to Xsec of tubule. Peritubular cap’y
KEY POINT - ATP used to move Na + out of tubule cell. All other movement is by diffusion or facilitated diffusion, or cotransport (glu, am ac).
Process occurs mainly in proximal tubule and some into distal tubule.
Over a million nephrons per kidney. Each kidney filters about liters of blood per 24/hr day. Produces liters of urine. Concentration ratio of about 100:1 or more.

7. Tubular Secretion
Tubular secretion: removes other substances from blood
Purposes: regulate chemical levels in body, excrete harmful chemicals
Substances secreted: penicillin, cocaine, marijuana, pesticides, preservatives, H+, NH4+, potassium
Secretion - active process. Compounds which need to be eliminated are transferred into urine. Some may not have been filtered out initially (drugs), some may still remain and need to be pushed out (K+, creatinine).

8. Formation of Dilute Urine
3 phases. Dependent on different permeability of tubule along its length. Follow legend. Note passive and active transport. Allows more dilute urine than is blood plasma.
Inside kidney, process occurs in this orientation, cortex to medulla. Countercurrent exchange in vasa recta allows solute gradient to remain, cortex vs medulla. Analogy with heat transfer in whale tail.
Most reabsorption occurs in PT, but some occurs in DT and in collecting duct.
Same setup for concentrated urine, but Increased ADH causes permeability of collecting duct to change, allows water to be reabsorbed from collecting ducts.
Figure 15.10

9. Regulation of Water and Electrolyte Reabsorption
Regulation is primarily by hormones
Antidiuretic hormone (ADH) prevents excessive water loss in urine
Aldosterone regulates Na+ content of extracellular fluid
Triggered by the renin-angiotensin mechanism
Cells in the kidneys (juxtaglomerular apparatus) and hypothalamus are active monitors
ADH acts on kidneys to cause more reabsorption of water by collecting duct cells. Usly constant low level of ADH in blood, so rate of water retention can be adjusted in both directions. Levels of ADH under control of hypothalamus, as dictated to pituitary.
Aldosterone affects Na+ content, and th4 alters blood volume and pressure. Trigger mechanism is via the kidney itself:
kidney cells called juxtaglomerular apparatus detect low blood pressure or volume in arterioles. This triggers adjacent cells to secrete renin which is a hormone/enzyme. Renin activates angiotensin (made by liver). Angiotensin triggers adrenal cortex to secrete aldosterone, which increases rate of Na+ reabsorption by distal tubules. Every Na+ reabsorbed means that Cl- is reabsorbed and K+ is excreted. Important to maintain correct balance of electrolytes.
Aldosterone stim by rising K+ or falling Na+, but mostly by renin-angiotensin.

10. Regulation of Blood Solutes, Volume
Interaction btw regulation of blood solute concentration and blood volume, th4 blood pressure.
LEFT DIAG: Suppose water content drops, such that solutes are more concentrated.
Osmoreceptors in the hypothalamus detect these increased solutes, send nervous impulse to posterior pituitary, which releases ADH, anti diuretic hormone. ADH acts on kidneys to cause more reabsorption of water by duct cells. Usly constant low level of ADH in blood, so rate of water retention can be adjusted in both directions.
RIGHT DIAG: Suppose that blood volume drops, kidney cells called juxtaglomerular apparatus detect low blood pressure or volume in arterioles. Secrete renin which is a hormone/enzyme. Renin activates angiotensin (made by liver). Angiotensin triggers adrenal cortex to secrete aldosterone, which increases rate of Na+ reabsorption by distal tubules. Every Na+ reabsorbed means that Cl- is reabsorbed and K+ is excreted. Important to maintain correct balance of electrolytes.
Aldosterone - major factor regulating Na+ content
Aldosterone stim by rising K+ or falling Na+, but mostly by renin-angiotensin.

11. Maintaining Water Balance
Water intake must equal water output
Sources for water intake
Ingested foods and fluids
Water produced from metabolic processes
Sources for water output
Vaporization out of the lungs
Lost in perspiration
Leaves the body in the feces
Urine production
Ingested fluid = 90%, only 10% from metabolism of cells (300 mL per day)
Dilute urine is produced if water intake is excessive
Less urine (concentrated) is produced if large amounts of water are lost
Proper concentrations of various electrolytes must be present
Urine is major route for loss of electrolytes. 1/4 of blood supply goes thru kidneys each minute.

12. Blood Buffers - Review
Blood pH must remain between 7.35 and 7.45 to maintain homeostasis
Buffer molecules react to prevent changes in H+ concentration
Bind to H+ when pH drops
Release H+ when pH rises
Three major chemical buffer systems
bicarbonate, phosphate, protein
Respiratory buffer system
Blood pH must remain between 7.35 and 7.45 to maintain homeostasis
Alkalosis – pH above 7.45
Acidosis – pH below 7.35
Most ions originate as byproducts of cellular metabolism
Respiratory buffer system
Carbon dioxide in the blood is converted to bicarbonate ion and transported in the plasma
Increases in hydrogen ion concentration produces more carbonic acid
Excess hydrogen ion can be blown off with the release of carbon dioxide from the lungs
Respiratory rate can rise and fall depending on changing blood pH

13. Renal Mechanisms of Acid-Base Balance
Slower than chemical regulation, but effective at maintaining blood pH
1. Reabsorption of bicarbonate ions
2. Excretion of H+ (as NH4+)
Urine pH varies from 4.5 to 8.0
HCO3- is nearly all reabsorbed from filtrate into blood by combining it with H+ from kidney tubule cells, such that CO2 is formed and diffuses back into blood. Any excess H+ secreted will be excreted in urine.
H+ excreted (as NH4+) if pH goes too low. This results in extra HCO3-, originally from CO2 + H2O-> H2CO3. Same as creating new HCO3-.
Urine pH shows ability of renal tubules to excrete acidic or basic ions.

14. Urinary Bladder Smooth, collapsible, muscular sac
Temporarily stores urine
Ureters - Slender tubes attaching the kidney to the bladder
Peristalsis aids gravity in urine transport.
Folds of mucosa in bladder act as blocks to prevent backflow from bladder into ureters.
Kidney stones - chunks of uric acid salts that block ureter. Very painful. Lithotripsy (treatment w/ ultrasound waves while submersed) works, but is tricky procedure. Pulverizes crystals, so need to aim carefully (3D).
Bladder - Trigone – smooth triangular shape of bladder defined by three openings: Two from the ureters, One to the urethra
Three layers of smooth muscle (detrusor muscle)
Mucosa made of transitional epithelium. Non absorptive cells.
Walls are thick and folded in an empty bladder. Bladder can expand significantly without increasing internal pressure. Can hold 500 ml to 1 L when fully expanded.
Figure 15.6

15. Urethra
Thin-walled tube that carries urine from the bladder to the outside of the body by peristalsis
Release of urine is controlled by two sphincters
Internal urethral sphincter (involuntary)
External urethral sphincter (voluntary)
250 mls of urine accumulate in bladder, contracts and pushes some into urethra. Presence there will notify person that nature is calling. If not voided, contractions stop. For a while. Urine production is continuous; reflex will reoccur.
Incontinence can be due to several factors. Toddlers do not have muscle control yet. Usly after months. Later at night, while sleeping.
Bladder infection causing irritation of mucosa.
Old age - decreases control over external sphincter.
Often, muscle control is difficult for women after several rounds of childbirth. Treatment can involve medications, exercises, and surgery.

16. Characteristics of Urine Used for Medical Diagnosis
Colored yellow due to the pigment urochrome and solutes
Sterile
Slightly aromatic
Normal pH of around 6
Specific gravity of to 1.035
Colored somewhat yellow due to the pigment urochrome (from the destruction of hemoglobin) and solutes
Sterile - can be drunk to avoid dehydration. Has allowed earthquake victims to survive for days under rubble.
Slightly aromatic - urea. Bact break it down readily to ammonia, stronger odor.
Normal pH of around 6.. Can vary, depending on diet. High protein - acidic. More veggies - more alkaline.
Specific gravity of to reflects the concentration of solutes, and th4 functioning of kidneys.
Solutes: Na, K+, urea, uric acid, creatinine, ammonia, bicarbonate ions, other ions.
Abnormal : glucose, blood proteins, RBC hemoglobin, WBC, bile.

17. Disorders of the Urinary System
Kidney stones
Acute and chronic renal failure
Therapies:
Dialysis
Kidney transplant
Problems occur quickly if kidneys do not function correctly. Uremia - buildup of nitrogenous wastes, pH drops. Leads to diarrhea, vomiting, convulsions, coma, death.
Hemodialysis - uses artificial kidney. Tubing which is selectively permeable. Blood is pumped through, 4-6 hrs X 3/wk. Single filtration process that uses diffusion to transfer chems OUT of blood. Puts some buffers for H+ into blood.
CAPD - continuous ambulatory peritoneal dialysis. Uses fluid put into peritoneal cavity as exchange reservoir. Drained later, along with excess salts, toxins. Partially effective.
Issues w/ kidney transplants. Any living donor must be friends with recipient. Why? To prevent organ sales. Read Issues with Organ Transplants, pp in Johnson..