Biology: Excretory System

Excretion Overview

• Removes metabolic waste from blood as urine

• Main organ: Kidney

• Functional unit of kidney: nephron loop

• Smaller: Afferent → glomerulus → bowman's capsule

Kidney Functions

• Filters blood and forms urine

• Regulates:

  • Blood pressure

  • Blood pH

  • Red blood cell production (via erythropoietin)

• Anatomy:

  • Cortex (outer)

  • Medulla (inner)

Nephron

Functional Unit of Kidney

Glomerulus: cortex

• Located in renal corpuscle (Bowman’s capsule + glomerulus)

• Site of blood filtration

• Afferent arteriole

• Efferent arteriole

• Podocytes

Afferent arteriole

Brings blood in

Efferent arteriole

Carries filtered blood out

• Forms peritubular capillaries (around PCT/DCT)

• Forms vasa recta (around Loop of Henle)

Podocytes

Increase filtration selectivity

Proximal Convoluted Tubule (PCT)

• Major site of reabsorption

  • Reabsorbs: Na⁺, Cl⁻, glucose, amino acids (100%)

  • Secretes: H⁺

Loop of Henle: Descending limb

• Permeable to water

• Water exits → urine concentration increases

Loop of Henle: Ascending limb

• Impermeable to water

• Ions (Na⁺, Cl⁻) reabsorbed → urine concentration decreases

Distal Convoluted Tubule (DCT)

• Reabsorbs: Na⁺, Cl⁻

• Secretes: K⁺, H⁺

• Water follows salts by osmosis

Collecting Duct

• Passive reabsorption of water

• Secretes K⁺

• Final concentration of urine

Urine Flow After Nephron

Collecting duct → Renal pelvis → Ureter → Bladder → Urethra

Renin-Angiotensin-Aldosterone System (RAAS): increase blood pressure

• Triggered by low blood pressure

• Juxtaglomerular cells detect low BP → release renin

• Renin converts angiotensinogen (liver) → angiotensin I

• ACE (lungs) converts angiotensin I → angiotensin II

Angiotensin II effects

• Stimulates aldosterone release

• Increases Na⁺ reabsorption

• Increases thirst

• Vasoconstriction: shrink blood vessels increase BP

Aldosterone

• Function: Keeps salt (Na⁺) and water in the body

• Also makes you lose potassium (K⁺)

• Goal: Raise blood pressure and blood volume

ADH (Antidiuretic Hormone / Vasopressin)

• Function: Keeps water in the body by adding aquaporins

• Goal: Raise blood volume and concentrate urine

• Blocked by alcohol → more pee

ANP (Atrial Natriuretic Peptide)

• Function: Gets rid of salt and water

• Increases urine production

• Goal: Lower blood pressure and volume

Osmoregulation: Marine (Saltwater) Fish (HYPO)

• Environment: hypertonic

• Lose water passively

• Adaptations:

  • Constant drinking

  • Rare urination

  • Salt secreted through gills

Osmoregulation: Freshwater Fish (HYPER)

• Environment: hypotonic

• Gain water passively

• Adaptations:

  • Constant urination

  • Do not drink

  • Salt absorbed through gills

Proximal convoluted tubule (PCT)

Most reabsorption (Na⁺, Cl⁻, glucose, amino acids)

Loop of Henle

• Descending = water out

• Ascending = salt out

Distal Convoluted Tubule (DCT)

Salt reabsorption, K⁺ and H⁺ secretion

Collecting Duct

Water reabsorption, K⁺ secretion

Renin-Angiotensin-Aldosterone System (RAAS)

Raises blood pressure

ADH (Antidiuretic Hormone / Vasopressin)

Retains water (aquaporins)

ANP (Atrial Natriuretic Peptide)

Lowers blood pressure

Aldosterone

• Increase Na⁺ and K⁺

• If stopped working, more water than ions