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Anatomy and Physiology of the Kidney:A micro-engineering marvel that maintains osmotic homeostasis in our body
Larry M. Frolich, Ph.D.Yavapai College, Dept. of Biology
Kidney Brainstorm
Take 30 seconds to describe what the kidney does
Adjective/phrase 1:
Adjective/phrase 2:
Adjective/phrase 3:
Baseline physiology understanding is often common sense: What do you already know about the kidney and what it does?
The Challenge: Appreciate the kidney’s elegant fluid flow engineering solution to minimizing fluid loss while maintaining osmotic balance and secreting metabolic waste products
Where kidney is located, flow of urine out of body Internal organization of kidneyUrine production in kidney—a 3-step process1. Filtration2. Reabsorption and Secretion3. ConcentrationDiabetes and Kangaroo Rats—do you really understand the kidney?
To get there, we need to know:
Paired, mid-abdomenRetroperitoneal
“kidney punch”“dolor de los riñones”
Urine constantly flows to bladder through urethraUrine collects in bladder and passes through urethra at urination
Where are kidneys?
Inside the kidney
One lobe of kidney is medullary pyramid plus cortex around it
Cortex has concentration of glomeruli where filtration happens
Medulla or core of pyramid appears striated due to oriented collecting ducts that unite into minor calyx at apex of pyramid
Minor calices are open spaces where urine collects at apex of pyramid
Minor calices empty into larger major calices which merge to form renal pelvis
Urine passes from renal pelvis into ureter
Ureter--from kidney to bladderLAYERS OF URETER
External connective tissue--adventitia
Middle muscular layer--muscularis
Smooth Muscle
Inner Longitudinal
Outer Circular
External longitudinal (on distal third)
Peristaltic action moves urine to bladder (and stones!!)
Inner lining of transitional (stretchy) epithelium--Mucosa
Bladder
Muscular (what kind?) sac that fills with urine from uretersAnterior against pubis in pelvisFilled with urine expands into abdomenBlood supply from internal iliac arteriesInnervation is autonomic from hypogastric plexus
Layers of bladder wallOuter connective tissue--adventitiaMiddle muscular layer (“detrusal” or expulsor)--inner and outer longitudinal fibers around middle circular fibersInner transitional (stretchy) epitheliumBladder can expand 15 times its empty volume to hold 500 ml of urineTrigone is triangle between ureters/urethra--persistent sight of infection
UrethraDrains urine from bladder to outside
Female = short tube
Males = long tubeProstatic, Membranous, Spongy (penile) portions
Also carries sperm
Internal Urethral SphincterBetween bladder + urethra
Thickening of detrusor (smooth muscle)
External Urethral SphincterWithin urogenital diaphragm
Skeletal muscle = voluntary control urination
External Urethral OrificeMales = end of penile urethra
Females = anterior to vaginal opening, posterior to clitoris
Micturition = UrinationEmptying bladder
Stretch receptors in bladder respond when bladder fullParasympathetic signals detrusor muscle to contract and internal urinary sphincter to open (also inhibits sympathetic pathways that would prevent urination)Other brain receptors can inhibit urination by relaxing detrusor, and keep external urinary sphincter closed Voluntary contraction of abdominal wall muscles increases abdominal pressureVoluntary relaxation of external urethral sphincter
Ascent of the kidney in developmentKidneys from intermediate mesoderm
Pronephric kidney in fetus shows segmental body plan
Fish with dorsal renal tissue lateral to vertebral column for most of length
In human, metanephric kidney migrates from inferior to superior
Variation in kidney shape not uncommon (horseshoe kidney
Ureter also from intermediate mesoderm
Nephron: the functional unit of the kidney(understand how one nephron works and you understand how the kidney works)
One million nephrons per kidney (140 miles of tubing within each kidney!)Always oriented with glomerulus towards cortex, collecting duct heading towards calyx at apex of pyramid in center of kidney
How does the kidney remove waste products from the blood and maintain osmotic balance in the body? (THE STORY OF BOWMAN AND HENLE)
1. Filtration: Bowman’s Capsule
2. Reabsorption and Secretion: Proximal Convoluted Tubules
3. Concentration of Urine: Collecting Duct—Loop of Henle establishes concentration gradient
Friedrich Gustav Jakob Henle (1809-1885)
Sir William Bowman(1816-1892)
3 Steps—the quick story:1. Plasma filters out of blood and into
tubules at Bowman’s Capsule2. In tubules, water and non-waste
solutes are reabsorbed back into blood. Wastes from blood are secreted into tubules
3. In collecting duct, water osmoses out and urine is concentrated to final form
Collectingduct
How does a nephron work?
Kidneys are 0.5% of total body weight but receive over 20% of blood pumped by the heart
Another view of the same
Step 1. Filtration—Bowman’s Capsule/Glomerulus
Constant Glomerular Filtration Rate (GFR)—about 100 mL/min = 150 L/daySpecial epithelial cells (podocytes) surround capillaryFenestrated endothelial cells of capillaries, and slits between podocytes allow plasma with dissolved solutes to leave blood at rapid rate and filter into capsular spaceMolecules smaller than 3nm filter through including water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes. Proteins and cells are too large.
PODOCYTES
Step 2. Reabsorption and Secretion—PCT
Six percent of resting ATP use, or caloric energy is expended hereWater, ions, and glucose are reclaimed from glomerular filtrate back into capillaries that surround tubule by reabsorptionSodium is actively pumped across epithelial cells of tubule (Na-K pump using ATP)Wastes including uric acid are secreted from blood in surrounding capillaries into interior of tubule
Step 3. Concentration—Collecting Duct. (Loop of Henle creates concentration gradient)
Active transport of NaCl on Ascending Loop of Henle sets up counter-current exchangeConcentration gradient into center of medulla allows concentration of urine in collecting ductHormonal control of permeability of collecting duct membrane determines ultimate concentration of urine
Review of nephron function
RESORPTION: DIABETES AND THE PCT
Constant thirst and bed-wetting are symptoms of diabetes. Why?High sugar levels in blood leads to high sugar levels in filtrate at glomerulusThis sugar-rich filtrate cannot be fully reabsorbed across proximal convoluted tubule resulting in higher volume of fluid, with dissolved sugar left in tubule.This leads a need for more frequent urination (or bed-wetting in children) and frequent thirst to replace lost fluidEarly 20th century physicians diagnosed diabetes by tasting the patient’s urine!
Diabetes and the PCT
Kangaroo Rats and the Loop of HenleCONCENTRATION: KANGAROO RATS
AND THE LOOP OF HENLEKangaroo Rats live in extremely arid desert environmentsThey need no water besides what is in the seeds they eatThey secrete a powerfully concentrated nearly solid urineTheir secret is an extremely long Loop of Henle whose countercurrent exchange produces a concentration of sodium almost four times higher than humans in the interior of their kidneySee graphic in next slide
NEPHRON OF A KANGAROO RAT