Chapter 17: Excretory System
Recommended Article : 【Biology】 Table of Contents for Biology
8. Sweat
1. Regulation of Ingestion
2. Nitrogenous Wastes
⑴ Generation of Nitrogenous Wastes
① Amino Acid Breakdown : Reaction in which amino groups are removed during amino acid metabolism, producing ammonia
○ Liver combines ammonia with CO2 via the ornithine cycle to synthesize urea
○ Mitochondria and cytoplasm are involved
○ Urea formation occurs in the cytoplasm
○ Kidneys secrete ammonia into renal tubules
○ Amino Group Transfer Reaction (Glutamine, Asparagine) : Reaction transferring amino groups to form 2-oxoacids
Figure 1. Ornithine Cycle
② Nucleic Acid Breakdown : Production of uric acid
○ Humans lack enzymes for uric acid breakdown
○ 25% excretion via digestive system, 75% excretion via kidneys
○ Accumulation of uric acid can lead to gout and uric acid kidney stones (composed of uric acid, phosphates, calcium, oxalates, etc.)
○ Gout : Main causes include genetics, kidney disorders, medications, diabetes, alcohol consumption, etc.
⑵ Characteristics of Nitrogenous Wastes and Animal Physiology
Figure 2. Types of Nitrogenous Wastes
① Ammonia (NH3, ammonia)
○ Characteristics : High solubility, high toxicity (because ammonia readily accepts H+ ions)
○ (During excretion) Water requirement : 1 mL per 1 g of nitrogen
○ Relevant animals : Mostly cartilaginous fish, aquatic invertebrates
② Urea
○ Characteristics : Much weaker toxicity compared to ammonia (10,000 times), excretable in concentrated form
○ Energy required for urea synthesis
○ (During excretion) Water requirement : 1 mL per 50 g of nitrogen
○ Relevant animals : Mostly mammals, amphibians, cartilaginous fish, some bony fish
③ Uric Acid
○ Characteristics : Non-toxic, low solubility (semi-solid state), minimal water consumption during excretion (water-saving effect)
○ Requires more energy than urea synthesis
○ (During excretion) Water requirement : 1 mL per 500 g of nitrogen
○ Relevant animals : Birds, reptiles, terrestrial arthropods
⑶ Evolution and Environmental Impact on Nitrogenous Wastes
① Methods of waste elimination while preventing water loss : Urea, uric acid
② Determined by Reproductive Strategy
○ Eggs of shell-less amphibians, embryos of mammals : Preferentially excrete soluble waste (ammonia becomes more toxic upon accumulation)
○ Eggs of birds, reptiles with shells : Excrete uric acid (non-toxic even upon accumulation)
③ Influenced by Habitat
○ Example : Terrestrial turtles (uric acid), aquatic turtles (urea, ammonia)
④ Endotherms have more nitrogenous waste compared to ectotherms
3. Types of Excretory Systems
⑴ Protonephridia (Flame Cell System) : Excretory system of flatworms
① Protonephridia branch like tree branches, with flame cells (composed of cap cells and flame cells) at the terminal end
○ Flame cells have a flame-like appearance, giving them their name
② Cell-to-cell filtration occurs through a membrane connecting interstitial fluid and flame cells
⑵ Metanephridia : Excretory system of annelids responsible for osmoregulation and waste excretion
① 1st Filtrate enters metanephridium through tubules surrounded by cilia
② 2nd Tubule cells alter the composition of the filtrate as it flows through the tubules
③ 3rd Diluted urine is expelled outside through the excretory pore
⑶ Malpighian Tubules : Excretory system of terrestrial arthropods
① 1st Salts, water, and nitrogenous wastes enter Malpighian tubules
② 2nd Malpighian tubules excrete urine into the hindgut
③ 3rd Feces and urine are expelled through the anus
⑷ Kidneys : Excretory system of vertebrates
4. Structure of Kidneys
⑴ Pathway of Urine Formation : Nephron → Collecting Duct → Renal Papilla → Ureter → Bladder → Urethra
① Renal Cortex : Nephron
② Renal Medulla : Henle’s loop, collecting duct, renal papilla
③ Related Blood Vessels : Afferent arteriole → Peritubular capillaries → Efferent arteriole
⑵ Nephron
① Functional unit of kidneys, each kidney has 1.25 million nephrons (totaling 145 km), composed of renal corpuscle and renal tubule
② Renal Corpuscle (Malpighian Body) : Collectively refers to glomerulus and Bowman’s capsule, located in the renal cortex
○ Glomerulus : A cluster of capillaries
○ Bowman’s Capsule : Double-layered capsule surrounding the glomerulus
③ Renal Tubule
○ Macula Densa : Cells surrounding the renal tubule, prevents free movement of ions by tight junctions
○ Proximal Convoluted Tubule (PCT) : Present in the cortex. Close to the afferent arteriole
○ Henle’s Loop : Present in the medulla. Found only in mammals and birds
○ Distal Convoluted Tubule (DCT) : Present in the cortex. Close to the efferent arteriole
④ Cortical Nephron, Juxtamedullary Nephron
○ Nephrons span both the renal cortex and medulla
○ Cortical Nephron : Nephron with a bias toward the cortex
○ Juxtamedullary Nephron : Nephron with a bias toward the medulla
⑤ Intercalated Cells : Present in the DCT and collecting duct
○ Type A Intercalated Cells : Active during acidosis. Secrete H+ into the lumen. Reabsorb HCO3-. Reabsorb K+
○ Type B Intercalated Cells : Active during alkalosis. Reabsorb H+. Secrete HCO3-. Secrete K+
5. Kidney Functions
⑴ 20-25% of cardiac output (500-600 mL/minute) passes through the afferent arterioles
⑵ Filtration, Reabsorption, Secretion, Excretion
① Filtration : Small molecules (H2O, salts, etc.) are non-selectively filtered in the glomerulus
② Reabsorption : Tubule cells transport useful substances from the filtrate back to the body fluids
③ Secretion : Toxic substances and excess ions from body fluids (capillaries) are secreted into the filtrate (tubules)
④ Excretion : Filtrate leaves the kidneys and is expelled from the body
⑶ 1st. Filtration
① Glomerular Filtration Pressure : Driving force for filtration
○ Glomerular Filtration Pressure = Glomerular Capillary Pressure (Blood Pressure) - (Bowman’s Capsule Osmotic Pressure + Plasma Colloid Osmotic Pressure) = 55 - (30 + 15) = 10 (mmHg)
○ Plasma Colloid Osmotic Pressure : Caused by proteins in the blood that pull water back into the blood
○ Bowman’s Capsule Osmotic Pressure : Counteracts the hydrostatic pressure generated in Bowman’s capsule
② Difference in blood flow between afferent and efferent arterioles determines filtration rate
○ Afferent arterioles are larger in diameter compared to efferent arterioles
○ Constriction of afferent arterioles : Decreases blood flow to glomerular capillaries, reducing glomerular capillary pressure → Decreased filtration rate
○ Constriction of efferent arterioles : Limits blood outflow, increasing glomerular capillary pressure → Increased filtration rate
○ Dilation of afferent arterioles : Increases blood flow to glomerular capillaries, raising glomerular capillary pressure → Increased filtration rate
○ Dilation of efferent arterioles : Facilitates blood outflow, decreasing glomerular capillary pressure → Decreased filtration rate
○ Vasoconstriction : Increases pressure in the arterial portion, decreases in the venous portion
③ Filtration Rate
○ Renal Blood Flow (RBF) (unit: mL/min) : Blood flow through renal vessels in 1 minute
○ Renal Plasma Flow (RPF) (unit: mL/min) : Plasma flow through renal vessels in 1 minute
○ Renal Threshold : Concentration at which a substance starts to be secreted
○ Hematocrit = Blood Hematocrit = Plasma / Blood = RPF / RBF
○ PAH : Completely removed from plasma
○ Creatine : Not reabsorbed or secreted in the renal tubules
○ Produced by breakdown of creatine phosphate in muscles at a fairly constant rate
○ Cleared by kidneys, frequently used to measure GFR
○ Used as a safety marker for kidney function
○ Negative Pressure: No secretion and reabsorption through the nephron
○ Chart for Filtration Rate Calculation: Calculation of filtration rate can be understood through the principle of mass conservation.
Table 1. Filtration Rate Calculation Chart
○ Higher filtration rate of positive ions due to the negative charge of the Bowman’s capsule
Figure 3. Influence of Size and Charge of Substances on Filtration
④ Regulation of GFR (Glomerular Filtration Rate)
○ Afferent Arteriole: Constriction decreases RPF (Renal Plasma Flow) and GFR, Dilation increases RPF and GFR
○ Efferent Arteriole: Constriction decreases RPF, increases GFR, Dilation increases RPF, decreases GFR
⑤ Glomerular Filtration Barrier
Figure 4. Glomerular Filtration Barrier
○ Component 1: Capillary endothelial cell: Refers to fenestrae or pores, allowing the passage of substances smaller than 70 nm.
○ Component 2: Glomerular basement membrane: High collagen density, negatively charged, allows passage of substances smaller than 6-6.5 nm.
○ Component 3: Podocyte extension: Thin membrane connecting podocytes, with a width of approximately 4-11 nm.
⑥ Changes in GFR with Age
Table 2. Changes in GFR with Age
⑦ Classification of CKD (Chronic Kidney Disease)
Table 3. Classification of CKD
(Source: CKD Work Group, 2013)
⑷ 2nd. Concentration of Filtrate
① Model of Concentration of Filtrate
○ Two-solute Model: Maintains high osmotic pressure of inner renal medulla due to NaCl and solutes.
○ Countercurrent Multiplier: Consumes energy to establish concentration gradient.
② Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Reabsorption of 75% of filtered fluid (67% NaCl, H2O)
○ Na+ Reabsorption (Active transport, Na+/K+ pump) → Cl- reabsorption (Passive transport) for charge balance
○ H2O Reabsorption (Passive transport)
○ Aquaporin-mediated water movement is facilitated diffusion, one type of passive transport, in response to Na+ reabsorption.
○ HCO3- Reabsorption: Na+-linked secondary active transport, contributes to acid-base balance in body fluids.
○ K+ Reabsorption: Passive transport, followed by conditional secretion in the distal convoluted tubule.
○ Secondary Active Transport of Hydrophilic Nutrients
○ Related to the threshold for secretion
○ Threshold (Plasma threshold): Concentration at which a substance starts being secreted
○ Not related to contraction of afferent and efferent arterioles
○ 100% Reabsorption of Glucose: Na+-linked secondary active transport. 320 mg glucose/minute, using glucose transporter GluT2.
○ 100% Reabsorption of Amino Acids: Na+-linked secondary active transport.
○ Secretion: NH3, solutes, creatinine, drugs, etc.
○ pH Regulation: H+ secretion
Figure 5. Renal Threshold
③ Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Descending Limb of Henle: Impermeable to water, permeable to NaCl. AQP-1 is expressed.
○ Water exchange between Descending Limb of Henle and renal medulla
○ Osmolarity comparison: Filtrate < Renal medulla
○ Filtrate gets concentrated as water is reabsorbed from Descending Limb of Henle to renal medulla.
○ Without countercurrent multiplication, renal medulla remains diluted.
○ Water exchange between Straight Vasa Recta and renal medulla
○ Return of osmolarity: Osmolarity increases in descending Straight Vasa Recta, then decreases as it ascends from the bottom of Henle’s loop, leading to water reabsorption.
○ High Na+ concentration in descending Straight Vasa Recta leads to high water reabsorption, increasing blood pressure.
④ Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Impermeable to water, permeable to NaCl → NaCl reabsorption occurs
○ Tight junctions prevent water reabsorption.
○ Thin Ascending Limb: Reabsorption of Na+ only, passive transport
○ Thick Ascending Limb: Reabsorption of Na+ and Cl- together, active transport [Countercurrent multiplication 1]
⑤ Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Variable substance transport due to hormonal regulation, mostly active transport
○ Na+ pump regulated by RAAS (Renin-Angiotensin-Aldosterone System)
○ 1st. Decreased water → Decreased GFR
○ 2nd. Slower filtrate generation → Increased Na+ reabsorption in thick Ascending Limb
○ 3rd. Dilute urine stimulates JGA (Juxtaglomerular Apparatus)
○ 4th. JGA stimulates granular cells
○ 5th. Granular cells secrete renin
○ 6th. Renin activates angiotensinogen to angiotensin I in the liver
○ 7th. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE)
○ 8th. Mechanisms to conserve water, including aldosterone, are activated
○ Na+-K+ Pump: Moves 3 Na+ ions out and 2 K+ ions into cells
○ Na+-K+-Cl- co-transporter in Ascending Limb follows Na+ concentration gradient, leading to Na+, K+, and Cl- reabsorption in the lumen of the tubule.
○ Na+ reabsorption (Active transport) → Cl- reabsorption (Passive transport) for charge balance
○ Conditional expression of aquaporin channels for water transport
○ H2O reabsorption (Passive transport)
○ Active transport of HCO3- for pH regulation: Reabsorption of bicarbonate ions
○ K+ secretion: Active transport to regulate filtrate osmolarity
○ Selective secretion: NH3, solutes, creatinine, drugs, etc.
○ pH Regulation: H+ secretion for pH control
⑥ Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Na+ pump regulated by RAAS
○ 1st. Decreased water → Decreased GFR
○ 2nd. Slower filtrate generation → Increased Na+ reabsorption in thick Ascending Limb
○ 3rd. Dilute urine stimulates JGA
○ 4th. JGA stimulates granular cells
○ 5th. Granular cells secrete renin
○ 6th. Renin activates angiotensinogen to angiotensin I in the liver
○ 7th. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE)
○ 8th. Mechanisms to conserve water, including aldosterone, are activated
○ Na+-K+ Pump: Moves 3 Na+ ions out and 2 K+ ions into cells
○ Na+-K+-Cl- co-transporter in Ascending Limb follows Na+ concentration gradient, leading to Na+, K+, and Cl- reabsorption in the lumen of the tubule.
○ Na+ reabsorption (Active transport) → Cl- reabsorption (Passive transport) for charge balance
○ Conditional expression of aquaporin channels for water transport
○ H2O reabsorption (Passive transport)
○ Central diabetes insipidus: Problem in hypothalamus → Normal with ADH administration
○ Nephrogenic diabetes insipidus: Problem with ADH receptors
○ Acid-base balance
○ Type A Intercalated Cells: ↑ H+ → ↑ K+ → Hyperkalemia
○ Type B Intercalated Cells: ↓ H+ → ↓ K+ → Hypokalemia
○ Changes in H+ and K+ move together
○ Reabsorption of solutes: Continuous storage of solutes in renal medulla to increase medullary osmolarity [Countercurrent multiplication 2]
○ Some of the stored solutes in renal medulla move into the filtrate of Henle’s loop
⑦ Changes in Osmolarity of Adjacent Vasa Recta (Peritubular Capillaries)
○ Blood in descending vessels has higher osmolarity
○ Blood in ascending vessels has lower osmolarity
○ Therefore, the change in osmolarity of blood passing through the kidney is nearly 0
⑸ 3rd. Excretion
① Urine excretion: 1.5 liters/day
○ Bladder can store up to 500 ml, releases 200-300 ml of urine at a time
○ Reabsorption from filtrate reduces volume
② Excretion of 10 g of salt per day (= 99.5% reabsorption of salt)
Figure 6. Function of Nephron
6. Regulation of the Kidney
⑴ Antidiuretic Hormone (ADH): Also known as vasopressin
① Produced in hypothalamus, stored in posterior pituitary, and released
② Function 1: Osmoregulation
○ Step 1: Increase in osmolarity
○ Step 2: ADH secretion triggered by osmoreceptors in hypothalamus
○ Step 3: ADH secretion
○ Step 4: Increased expression of aquaporins in Distal Convoluted Tubule and Collecting Duct
○ Step 5: Increased water reabsorption
○ Step 6: Osmoregulation (300 mOsm/L)
○ Osmoreceptors stimulate thirst, inducing drinking behavior
③ Function 2: Vasoconstriction of arterioles due to smooth muscle contraction → Increased blood pressure
④ Mechanism of increased water reabsorption by ADH
○ 1st. ADH binds to the collecting duct.
○ 2nd. Receptor stimulates the cAMP second messenger pathway.
○ 3rd. Insertion of aquaporins, including aquaporin, into the membrane on the side of the urinary bladder.
○ 4th. Efficient reabsorption of water from the urinary bladder due to aquaporins.
⑤ Causes of polyuria
○ Causes : Defective ADH receptors or inadequate production/secretion of ADH ×
○ Insufficient aquaporins leading to difficulty in water reabsorption.
⑥ Caffeine, alcohol promote diuresis by inhibiting the secretion of antidiuretic hormone (ADH).
⑦ ADH contributes more to plasma sodium concentration homeostasis than the RAAS (Renin-Angiotensin-Aldosterone System).
⑵ Renin-Angiotensin-Aldosterone System (RAAS)
① Juxtaglomerular apparatus (JGA)
○ Granular cells
○ Function : Sensing of afferent arteriole pressure, secretion of renin.
Figure. 7. Juxtaglomerular Apparatus
② 1st. Granular cells in the afferent arteriole secrete renin.
○ Renin is a type of protein-degrading enzyme.
③ 2nd. Renin activates angiotensinogen secreted in the liver to angiotensin I.
④ 3rd. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme.
○ Angiotensin-converting enzyme (ACE) : Located in the lungs. Chymase.
○ ACE2 : Prolyl carboxypeptidase.
⑤ 4th. Angiotensin II has the following functions through signal transduction via angiotensin II receptors.
○ Angiotensin II receptors : AT1 (Angiotensin II type 1 receptor), etc.
○ Function 1. Stimulation of adrenal cortex → Secretion of aldosterone → Reabsorption of Na+ in the blood → Increase in blood pressure.
○ Function 2. Constriction of afferent arterioles and efferent arterioles → Increase in blood pressure.
○ Function 3. Stimulation of ADH secretion in the posterior pituitary → Promotion of water reabsorption in the kidneys → Increase in blood pressure.
○ Function 4. Promotion of Na+ reabsorption in the kidneys.
○ Function 5. Inhibition of bradykinin, a vasodilator.
⑥ 5th. Stimulated adrenal cortex secretes aldosterone (a mineralocorticoid).
⑦ 6th. Aldosterone promotes Na+ reabsorption in the distal convoluted tubule and the collecting duct.
○ Na+ reabsorption : Involves the Na+/K+ pump, leading to increased secretion of K+ (Na+ : K+ = 3 : 2).
○ Na+ reabsorption (active transport) → Reabsorption of Cl- (passive transport) and water (passive transport) for osmotic balance.
⑧ 7th. Effect of increased blood volume on blood pressure.
⑨ 8th. Renin negative feedback
○ 8th - 1st. ATP is secreted into the blood vessels through the Na+/K+ pump.
○ 8th - 2nd. Simultaneously, ATP is used in the Na+/K+ pump, generating ADP.
○ 8th - 3rd. ADP generates adenosine.
○ 8th - 4th. Adenosine is secreted into the blood vessels.
○ 8th - 5th. Both ATP and adenosine increase Ca2+ in vascular smooth muscle cells.
○ 8th - 6th. Increased Ca2+ moves to renin-secreting cells, inhibiting renin secretion.
Figure. 8. Renin Negative Feedback
⑨ Types of angiotensin
○ Angiotensinogen : N-terminal - Asp - Arg - Val - Tyr - Ile - His - Pro - Phe - His - Leu - Leu - Val - Tyr - Ser - R
○ Angiotensin I : Asp - Arg - Val - Tyr - Ile - His - Pro - Phe - His - Leu
○ Angiotensin II (1-8) : Asp - Arg - Val - Tyr - Ile - His - Pro - Phe
○ Angiotensin III (2-8) : Arg - Val - Tyr - Ile - His - Pro - Phe
○ Angiotensin IV (3-8) : Val - Tyr - Ile - His - Pro - Phe
○ Angiotensin V ( (1-7) : Asp - Arg - Val - Tyr - Ile - His - Pro
⑩ Plasma sodium concentration homeostasis is primarily influenced by ADH rather than the RAAS.
⑪ ACE inhibition
○ Research to find substances inhibiting ACE is actively underway to prevent hypertension.
○ Measurement method 1. Measuring the conversion of HHL (hippuryl-histidyl-leucine) to HA (hippuric acid) by ACE.
○ Measuring HA using UV-vis spectroscopy or HPLC.
○ Disadvantage : Organic solvents like ethyl acetate are used in the HA extraction process, reducing accuracy.
○ Measurement method 2. ACE kit-WST : Measuring the conversion of 3HB-GGG (3-hydroxybutyryl-gly-gly-gly) to 3HB (3-hydroxybutyric acid) by ACE.
Figure. 9. ACE kit-WST
⑶ Atrial Natriuretic Peptide (ANP)
① Secreted from atrial pressure receptors in the left atrium : Secretion increases when atrial receptors are stretched more than normal.
② Inhibits RAAS, promotes ADH antagonism.
③ Promotes excretion of salt (sodium, etc.) and water in the kidneys → Decreases blood pressure → Reduces body weight.
④ Prevents strain on the heart due to excessive fluid.
Figure. 10. Regulation of Blood Filtration
⑷ Autonomic Nervous System
① Sympathetic Nervous System : Relaxation of bladder smooth muscles (sensation of needing to urinate), contraction of urethral smooth muscles (prevents urine release).
② Parasympathetic Nervous System : Contraction of bladder smooth muscles, relaxation of urethral smooth muscles.
③ Somatic neurons (motor neurons) → Contraction of detrusor muscle in the bladder.
7. Renal Disorders
⑴ Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
⑵ Diabetes Insipidus
① Central Diabetes Insipidus: Issues with the hypothalamus or pituitary gland → Normal response to ADH administration
② Nephrogenic Diabetes Insipidus: Issues with ADH receptors
⑶ Acute Kidney Injury (AKI)
① COVID-19 infection can also cause acute kidney injury
② Treatment: AMF (amifostine)
⑷ Chronic Kidney Disease (CKD)
Table. 4. Classification of CKD
⑸ Hypercalcemia
① Acidosis, Alkalosis
② Hyperaldosteronism: Metabolic alkalosis
③ Hypoaldosteronism: Metabolic acidosis
④ Hypokalemia → Decreased H+
⑹ Renal Transplantation
① In renal transplantation, the kidney must be removed, and a new kidney must be transplanted within 48 hours.
8. Sweat
⑴ Functions of Sweat
① Temperature regulation: Utilizes the evaporation of sweat to lower elevated body temperature
② Edema prevention: Intense exercise → Increased systolic blood pressure → Increased capillary permeability → Edema → Prevention of edema through loss of moisture via sweat
⑵ Sweat glands secrete Ach under the influence of postganglionic nerves.
Input: 2015.07.24 11:25
Modified: 2019.12.19 00:07