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