Chapter 17: Excretory System
Recommended Article: 【Biology】 Table of Contents for Biology
8. Sweat
1. Osmotic Regulation
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
○ Via mitochondria and cytoplasm
○ Urea formation occurs in the cytoplasm
○ Kidneys secrete ammonia into renal tubules
○ Amino Group Transfer Reaction (Glutamine, Asparagine): The reaction that transfers an amino group to 2-oxalic acid.
Figure 1. Ornithine Cycle
② Nucleic acid degradation: Purines produce uric acid
○ Humans lack the enzyme for uric acid degradation
○ 25% is excreted through the digestive system, 75% through the kidneys
○ If uric acid cannot be excreted and accumulates, it leads to gout and uric acid stones (composed of uric acid, phosphate, calcium, oxalate, etc.)
○ Gout: Major causes include genetics, kidney disease, medications, diabetes, and alcohol consumption
⑵ Characteristics of Nitrogenous Wastes and Animal Physiology
Figure 2. Types of Nitrogenous Wastes
① Ammonia (NH3)
○ Characteristics: High solubility, high toxicity (because ammonia readily accepts H+ ions)
○ Water requirement during excretion: 1 mL per 1 g of nitrogen
○ Relevant animals: Mostly bony fish, aquatic invertebrates
② Urea (Harnstoff)
○ Characteristics: Much weaker toxicity compared to ammonia (10,000 times), excretable in concentrated form
○ Energy required for urea synthesis
○ Water requirement during excretion: 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
○ Water requirement during excretion: 1 mL per 500 g of nitrogen
○ Relevant animals: Birds, reptiles, terrestrial arthropods
⑶ Effects of evolution and environment on nitrogenous waste
① Methods of excreting waste while preventing water loss: urea, uric acid
② Determination by reproductive method
○ Amphibian eggs without shells, mammalian embryos: choose water-soluble excretion (urea is highly toxic when accumulated)
○ Bird and reptile eggs with shells: uric acid (non-toxic even when accumulated)
③ Determination by habitat
○ Example: terrestrial turtles (uric acid), aquatic turtles (urea, ammonia)
④ Endotherms produce more nitrogenous waste than ectotherms
3. Types of Excretory Systems
⑴ Protonephridia (flame-cell system): Excretory system of flatworms
① The protonephridial network branches like a tree; at each terminal is a flame bulb (composed of a terminal/cap cell and a tubule cell).
○ The name comes from its flame-like appearance.
② Interstitial fluid is filtered across the membrane at the junction between the terminal (cap) cell and the tubule cell.
⑵ Metanephridia: The annelid excretory system responsible for osmoregulation and waste excretion
① 1st. Coelomic fluid enters the metanephridium through a ciliated nephrostome.
② 2nd. As it flows along, tubule cells modify the composition of the filtrate.
③ 3rd. The resulting dilute urine is expelled to the outside through a nephridiopore.
⑶ Malpighian tubules: Excretory system of terrestrial arthropods
① 1st. Salts, water, and nitrogenous wastes enter the Malpighian tubules.
② 2nd. Urine is passed into the hindgut with the feces.
③ 3rd. Feces and urine are eliminated through the anus.
⑷ Kidneys: Excretory system of vertebrates
4. Structure of Kidneys
⑴ Pathway of urine formation: nephron → collecting duct → renal pelvis → ureter → urinary bladder → urethra
① Renal cortex: nephron
② Renal medulla: loop of Henle (Henle loop) of the nephron, collecting ducts, renal pelvis
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 Malpighian body and renal tubule
② 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: Nonselective filtration of low-molecular-weight substances (H2O, salts, etc.) at the glomerulus.
② Reabsorption: Transport epithelial cells return useful components from the filtrate back into the body fluids.
③ Secretion: Toxic substances or excess ions are secreted from the body fluids (capillaries) into the filtrate (tubules).
④ Excretion: The filtrate leaves the kidney 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 Hydrostatic 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 Hydrostatic Pressure: The back pressure generated within Bowman’s capsule acts as a resistance.
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
○ Blood Hematocrit = Plasma / Blood = RPF / RBF
○ Glomerular filtration rate (GFR) (unit: mL/min): The volume of glomerular filtrate produced per minute
○ Typically 125 mL/min = 180 L/day
○ The 100 mL/min shown in the calculation below is an adjusted value for ease of calculation
○ Clearance (CL) (unit: mL/min): The amount of urine produced per minute
○ PAH: Completely removed from the plasma.
○ Creatine: Undergoes neither tubular secretion nor reabsorption.
○ Produced by the breakdown of creatine phosphate in muscle; generated at a fairly constant rate in the body.
○ Cleared by the kidneys and often used to measure GFR.
○ Used as a safety marker for renal function.
○ Inulin: Undergoes neither tubular secretion nor reabsorption
○ Filtration-rate calculation chart: Calculations of filtration rate, etc., can be understood by the law of conservation of mass.
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 urea.
○ Countercurrent Multiplier: Consumes energy to establish such 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)
○ To maintain osmotic balance corresponding to Na+ reabsorption
○ Water movement via aquaporins is facilitated diffusion (a form of passive transport)
○ 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 renal threshold for secretion
○ Renal threshold (renal plasma threshold): The concentration at which a substance begins to be excreted
○ Not related to the constriction of afferent or 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, urea, 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: Water can pass through, but NaCl cannot; 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 the descending limb of Henle to renal medulla.
○ Without countercurrent multiplication, renal medulla remains diluted.
○ Water exchange between the vasa recta and the renal medulla
○ Restoration of osmolarity: The osmotic concentration within the vasa recta increases toward the bottom of the loop of Henle, then decreases as it ascends by taking up water.
○ When Na+ concentration in the vasa recta is high, the vasa recta absorbs more water, leading to an increase in blood pressure.
④ Glomerulus → Proximal Convoluted Tubule → Descending Limb of Henle → Ascending Limb of Henle → Distal Convoluted Tubule → Collecting Duct
○ Water cannot pass through, but NaCl can, resulting in NaCl reabsorption.
○ 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, and mostly active transport
○ Conditional expression of 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 macula densa.
○ 4th. The signal of macula densa stimulates granular cells
○ 5th. Granular cells secrete renin
○ 6th. Renin converts angiotensinogen, which is secreted by the liver, into angiotensin I.
○ 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: The macula densa moves 3 Na+ ions out and 2 K+ ions into cells
○ Along the Na+ concentration gradient, the Na+-K+-Cl- cotransporter of the macula densa cells reabsorbs Na+, K+, and Cl- from the tubular lumen.
○ Na+ reabsorption (Active transport) → Cl- reabsorption (Passive transport) for charge balance
○ Conditional expression of H2O channels via aquaporins
○ H2O reabsorption (Passive transport)
○ HCO3- reabsorption (active transport): Bicarbonate ions are reabsorbed by active transport for pH regulation.
○ K+ secretion: Potassium ions are secreted by active transport to regulate osmotic concentration.
○ Selective secretion: NH3, urea, 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
○ Conditional expression of 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 macula densa
○ 4th. The signal of macula densa stimulates granular cells
○ 5th. Granular cells secrete renin
○ 6th. Renin converts angiotensinogen, which is secreted by the liver, into angiotensin I.
○ 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 aquaporins (H2O channels)
○ H2O reabsorption (passive transport)
○ Central diabetes insipidus: pituitary defect → returns to normal with ADH administration
○ Nephrogenic diabetes insipidus: defect in 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 in the same direction.
○ Urea reabsorption: urea is continuously stored in the renal medulla (∴ reabsorption of urea), thereby increasing the osmolarity of the renal medulla [Countercurrent multiplication 2]
○ Part of the urea stored in the renal medulla moves back into the filtrate fluid of the loop of Henle.
⑦ Changes in osmolarity of the vasa recta adjacent to the loop of Henle
○ As blood descends in the vasa recta, its osmolarity increases.
○ As blood ascends in the vasa recta, its osmolarity decreases.
○ Therefore, the net change in osmolarity of the blood passing through the kidney is nearly zero.
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
○ Reabsorbed from the filtrate, resulting in a reduced amount.
② Excretion of 10 g of salt per day (= 99.5% reabsorption of salt)
Figure 6. Function of Nephron
6. Kidney Regulation
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 in the hypothalamus induce thirst, leading to 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 membrane receptor.
○ 2nd. The receptor activates the cAMP-mediated second messenger signaling pathway.
○ 3rd. Vesicles containing aquaporins are inserted into the luminal membrane.
○ 4th. Water reabsorption from the tubular lumen occurs efficiently via aquaporins.
⑤ Causes of diabetes insipidus
○ Cause: deficiency of ADH receptors, or impaired production/secretion of ADH
○ A reduced amount of aquaporins makes water reabsorption difficult.
⑥ Caffeine and alcohol inhibit the secretion of antidiuretic hormone (ADH), thereby promoting diuresis.
⑦ In maintaining plasma sodium concentration homeostasis, ADH contributes more significantly than the RAAS.
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. Angiotensin II: 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) for charge balance and water (passive transport) for osmotic balance.
⑧ 7th. Increased blood volume leads to increased 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 motor neurons induce contraction of the urethral sphincter.
7. Kidney Disorders
⑴ Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
⑵ Diabetes Insipidus
① Central diabetes insipidus: pituitary defect → normal with ADH administration
② Nephrogenic diabetes insipidus: defect in 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 Blood-related disease
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 Prevention of edema: vigorous exercise → increased arteriolar blood pressure → increased permeability of capillaries → edema → prevented by water loss through sweating.
⑵ In sweat glands, ACh is secreted from postganglionic neurons.
⑶ Diseases related to sweat glands and Eccrine duct
① Psoriasis
② Vitiligo
Input: 2015.07.24 11:25
Modified: 2019.12.19 00:07