Chapter 50 Excretory System

Question 1

Why is homeostasis of the ECF important?

Answer 1

1) Solute concentration of ECF determines water balance of cells in body
2) Ionic composition of ECF includes cell functions
3) Health of cells requires elimination of nitrogenous wastes

Question 2

What do excretory systems control and how?

Answer 2

• Control volume, concentration, and composition of ECF and excrete wastes
  A) Filtration: remove cells and large molecules (blood plasma is filtered via capillaries)
  B) Secretion: modifies the filtrate
• kidney (urine)
  C) Reabsorption: concentrates the waste
• Kidney: water, ions, nutrients, and necessary biomolecules

Question 3

What is urine composed of?

Answer 3

• Urine is 95% water, 5% waste products
• Wastes include nitrogenous wastes (urea) and ions

Question 4

What is osmosis? What are the three types of cells under osmosis? What is osmolarity?

Answer 4

• Osmosis causes water to enter or leave the cells. (determines size and shape of cell)
  a) Isotonic: rates of water movement in and out are equal
  b) Hypotonic: cells take up water (swell + burst)
  c) Hypertonic: cells lose water (shrivel)
• Osmolarity: number of moles of osmotically active solutes per liter of solvent

Question 5

What are the parts of the excretory system and their functions?

Answer 5

1) Kidneys (2): create urine
2) Ureters: transport urine to bladder
3) Bladder: stretches, stores, and eliminates urine
4) Urethra: transports urine out of body via 2 sphincters:
a) Smooth muscle (involuntary)
b) Skeletal muscle (voluntary)

Question 6

What are the parts of the kidney?

Answer 6

1) Cortex = outer layer
2) Medulla = internal core
3) Pelvis = basin-like proximal part of ureter

Question 7

What separates the cortex and medulla in the kidneys?

Answer 7

Renal veins and arteries

Question 8

What is a nephron and what is it made of?

Answer 8

• A nephron is the functional unit of the kidney
  1) Glomerulus: located in cortex (capillaries, blood filtration)
  2) Bowmans Capsule/Proximal Convoluted Tubule: located in cortex for absorption
  3) Loop of Henle: located in medulla (urine concentration)
  4) Distal Convoluted Tubule: located in cortex for secretion

**Collecting ducts are not part of nephron, but urine collects there

Question 9

How is blood filtered in the production of urine?

Answer 9

1) Afferent arteriole: carries blood to glomerulus
2) Glomerulus: dense bed of capillaries where blood is filtered
- highly permeable to water, ions, and small molecules (not large molecules)
- blood pressure drives water and solute movement into Bowmans capsule
3) Efferent arteriole: carries blood out of glomerulus

Question 10

How does reabsorption take place in the production of urine?

Answer 10

1) Bowman’s Capsule: cup-like sac that receives filtrate from glomerulus
- Podocytes: specialized cells that wrap around glomerular capillaries
2) Renal Tubules: modifies filtrate from glomerulus
- reabsorbs ions, nutrients, water, and returns to blood

**Glucose/aa are only produced in the proximal convoluted tubule

Question 11

How does secretion take place in the production of urine?

Answer 11

1) Filtrate is further modified in the distal convoluted tubule (NaCl/H2O)
2) Peritubular capillaries transport substances to and from the renal tubules
- surround proximal and distal convoluted tubules
- Vasa recta: the peritubular capillaries that run parallel with the loop of Henle

Question 12

What is the countercurrent multiplier mechanism and which organisms are they found in?

Why does it work so well?

Answer 12

• Enables the mammalian kidney to produce urine that is 4x more concentrated than the blood plasma
• Found in mammals
• Goal is to remove water from filtrate to prevent dehydration of organism

Works due to:
1) different degrees of permeability to ascending and descending Loop of Henle to water
2) due to active Na+ transport in ascending loop

Question 13

What is the function of the descending Loop of Henle?

Answer 13

1) Impermeable to ions
2) Permeable to water
3) Osmotic differential pulls water out
4) Composed of thin cells with no microvilli

Question 14

What is the function of the ascending Loop of Henle?

Answer 14

1) Reabsorbs Na+ and Cl-
2) Impermeable to water
3) Has thick cells with many mitochondria

Question 15

What is the purpose of the thin ascending limb and the thick ascending limb?

Answer 15

• The thin ascending limb receives concentrated fluid from descending limb which allows diffusion of NaCl into interstitial fluid
• The thick ascending limb transports NaCl and raises the concentration of the interstitial fluid

Question 16

How does the distal convoluted tubule and the collecting duct work?

Answer 16

• The distal convoluted tubule concentrates further as NaCl and H2O are reabsorbed
• The collecting duct uses osmotic loss of water to increase concentration

Question 17

What are the steps of blood flow in the kidneys?

Answer 17

1) Afferent renal arteriole
2) Glomerulus
3) Efferent renal arteriole
4) Vasa Recta

Question 18

What are the steps of urine flow?

Answer 18

1) Bowman’s Capsule
2) Proximal Convoluted Tubule
3) Loop of Henle
4) Distal Convoluted Tubule
5) Collecting Duct
6) Renal Pelvis
7) Ureters
8) Bladder
9) Urethra

Question 19

How do kidneys regulate acid-base balance?

Answer 19

• Bicarbonate ions are buffers in blood
• Lungs control CO2 levels in blood, more CO2 = more H+ (acidic)
• Kidneys remove H+ and add HCO3- (basic)

Question 20

What is H2O volume important for regulating and how does it do that?

Answer 20

• critical for blood pressure and salt concentration
  1) reabsorbing more water from nephron
  2) reabsorbing more Na+ from nephron (leads to more H2O absorption)

Question 21

How do blood pressure and blood osmolarity affect H2O volume?

Answer 21

Low H2O volume= Low blood pressure
High H2O volume = high blood pressure

Low H2O volume = concentrated salt (high osmolarity)
High H2O volume = more dilute salt (low osmolarity)

Question 22

How does the Renin-Angiotensin-Aldosterone System work?

Answer 22

• If blood volume falls (due to low water volume), the afferent arteriole detects it and produces renin (in kidneys)
• Renin: converts inactive angiotensinogen to angiotensin

Question 23

What does angiotensin do?

Answer 23

1) Constricts efferent renal arterioles to stop/slow blood leaving glomerulus
2) Stimulates release of aldosterone which leads to Na+ reabsorption from kidney nephron. (increases water volume)
3) Stimulates thirst in the brain so you take in more water

Question 24

What does the anti-diuretic hormone (ADH/vasopressin) do?

Answer 24

• Produced in hypothalamus and stored in posterior pituitary
• Stimulated by rise in blood osmolarity
• Inhibited by rise in blood pressure

1) ADH targets collecting tubules and increases water absorption via aquaporin proteins (AQP-2)
2) Stimulates thirst so you take in more water
- ADH is blocked by alcohol which leads to excessive urination and dehydration

Question 25

What does the atrial natriuretic peptide (ANP) do?

Answer 25

• Release by increased venous blood return which stretches the atria and muscle fibers
• High blood volume and pressure puts strain on the heart
• ANP decreases reabsorption of Na+ in kidney
• More loss of Na+ causes lower blood volume and pressure

Question 26

How is kidney failure caused?

Answer 26

• Retention of salts and water –> high BP
• Retention of urea –> uremic poisoning
• Decrease in pH –> acidosis