Human Bio 10-11

Kidney Anatomy

Retroperitoneal, between T12-L5, right kidney slightly lower because of the liver

Cortex: outer region

Medulla: contains renal pyramids

Renal pelvis: funnels urine into ureter

Urine flow pathway

Renal pyramid → minor calyx → major calyx → renal pelvis → ureter

Nephrons

The functional units of the kidney, with each kidney having over 1 million nephrons each

Renal corpuscle: initial, blood-filtering component

• Glomerulus: specialized tuft of looping, fenestrated capillaries where blood is delivered + filtered

• Bowman’s capsule: cup-shaped epithelial structure surrounding the glomerulus, collecting the glomerular filtrate → directing it into renal tube

Renal tubule: process fluid filtered from the blood

• PCT (reabsorbing majority of filtered product back into blood) → Loop of Henle (concentration gradient so kidneys can produce concentrated or dilute urine) → DCT (fine tunes electrolyte balance + maintains body’s acid-base and fluid balance)

Glomerular Filtration

Step 1

Blood pressure forces fluid out of glomerulus into capsule

• Water, ions, glucose, amino acids and wastes but NOT proteins or blood cells

Tubular Reabsorption

Kidneys selectively recover water and essential solutes from filtrate and return them to the circulating bloodstream.

• Glucose, amino acids, water, Na⁺

• Main site is PCT

Tubular Secretion

Transfers waste products, toxins, and excess ions directly from the peritubular capillaries into the tubular lumen (urine)

• Removes substances not filtered by the glomerulus like drugs, creatinine, H⁺, K⁺

Crucial for balancing blood pH, managing potassium levels, eliminating wastes + foreign substances

Main site is proximal tubule

Net Filtration Pressure (NFP)

Outward pressure: glomerular hydrostatic pressure

Inwards pressures: capsular pressure and osmotic pressure

Positive NFP = filtration occurs

Glomerular Filtration Rate (GFR)

Amount of filtrate formed per minute

• normal is 125 mL/min

Increased GFR = more urine and lower blood pressure

Decreased GFR = less urine, retain fluid

Intrinsic control of GFR

Myogenic mechanism: blood vessel stretches → constricts automatically

Tubuloglomerular feedback: macula densa senses NaCI levels

• High NaCI = constricts afferent arteriole → reduce GFR

Extrinsic control of GFR

SNS: low BP/stress = constricts afferent arteriole → lowers GFR → conserves fluid

RAAS (Renin-Angiotensin-Aldosterone System)

• Low BP triggers: renin, angiotensin II, aldosterone

• Effects: increases Na+ reabsorption, water follows → BP increases

Hormones of the urinary system

ADH: antidiuretic hormone

• Inserts aquaporins into collecting ducts

• More water reabsorbed

= Less + more concentrated urine

Aldosterone: controlling the balance of sodium and potassium in the blood to regulate BP

• Na+ reabsorption, K+ secretion

• Water follows Na+

= Higher blood volume and BP

ANP: signals the kidneys to excrete excess sodium and water

• Na+ loss, water loss

= BP lower

Loop of Henle

U-shaped, hairpin-like tubule in the kidney’s nephron that recovers water and sodium chloride from urine, helping the body conserve water by creating a high-osmolarity gradient in the surrounding tissue (medulla)

1. Thin Descending Limb

• Highly permeable to water BUT NOT to salt

2. Thin Ascending Limb

• Impermeable to water BUT permeable to salt

3. Thick Ascending Limb

• Impermeable to water

Countercurrent Mechanism

The opposing flow and differing permeabilities of the descending and ascending limbs create a countercurrent multiplier system.

By continuously pumping salts out of the ascending limb and trapping water in the descending limb, the kidney establishes a deeply concentrated core.

This concentration gradient allows collecting ducts to reabsorb water + produce highly concentrated urine when the body is dehydrated

• Medullary gradient: 300 → 1200 mOsm

Concentrated vs Dilute Urine

High ADH: more water reabsorbed → concentrated urine

Low ADH: less water reabsorbed → dilute urine

Diuretics

Increase urine output

Like alcohol that inhibits ADH, caffeine and loop diuretics

Clinical kidney problems

UTIs: bacteria entering urinary tract

• Females more susceptible due to shorter urethra

Urethritis = urethra inflammation

Cystitis = bladder inflammation

Pyelonephritis = kidney infection

Kidney stones: crystallized salt

Renal failure: low GFR ( GFR <15mL/min )

• Dialysis or transplant

Chronic kidney disease: GFR <60mL/min

Urine composition

Normal urine

• 95% water

• Slightly acidic

• Yellow from urochrome

• Contains urea, uric acid, creatinine

Abnormal = blood/protein in urine

Urination

Micturition

• Detrusor contraction

• Internal sphincter opens

• External sphincter relaxes (voluntary)

Juxtaglomerular Apparatus (JGA)

(Complex, JGC)

A microscopic structure within the kidney that regulates blood pressure + filtration rate

• Macula Densa: cell in the distal tube that function as chemoreceptors for NaCI concentration in tubular fluid

• Juxtaglomerular (JG) cells: smooth muscle cells in the afferent arteriole that synthesize, store, and release renin = helping control blood pressure

When NaCI levels are low, the JGA signals JG cells to release renin, activating RAAS to increase blood pressure + fluid retention

RAAS

Renin-Angiotensin-Aldosterone System

Vital hormone pathway that regulates BP, blood volume, and fluid balance. When BP or sodium levels drop, the kidneys release the enzyme renin to trigger a cascade that retains water + constricts blood vessels

1. Renin release

2. Angiotensin I Formation

3. Angiotensin II Formation

4. Angiotensin II Effects

5. Aldosterone action

Glomerular Filtration Membrane

Specialized, three-layered sieve within the kidney’s nephrons that filters blood plasma while retaining blood cells and large proteins

• In the glomerulus

• Allows water + small solutes into Bowman’s capsule based on size and charge = fast step in urine formation

Inner layer: fenestrated endothelium

Middle layer: glomerular basement membrane

Outer layer: visceral epithelium / podocytes