Introduction To The Kidneys

Question 1

What are the 4 main functions of the kidney?

Answer 1

1. Regulation of body fluid volume
2. Regulation of body fluid composition i.e. electrolytes
3. Excretion of metabolic waste and toxins e.g. drugs
4. Endocrine functions e.g. regulates BP, blood cell levels, vitamin D and calcium homeostasis
   Key processes are filtration, reabsorption & secretion to eventually form urine which is excreted

Question 2

What is the kidney made up of mainly in terms of its microstructure?

Answer 2

Numerous uriniferous tubules (nephron & collecting duct) and associated blood vessels

Question 3

What is the functional unit of the kidney?

Answer 3

Filtration, reabsorption and secretion is mainly in the NEPHRON. There is roughly 1 million per kidney. The nephron is made up of the renal corpuscle, proximal convoluted tubule, loop of henle and distal convoluted tubule. Nephrons are either cortical (~80%) or juxtamedullary (~20%) depending on position of renal corpuscle. Many nephrons drain into the same collecting duct which then drain urine out to the renal papilla.

Question 4

What is the cortex and medulla of the kidneys made up of?

Answer 4

Cortex = mostly renal corpuscles, proximal and distal convoluted tubule 
Medulla = mostly loops of henle and collecting ducts

Question 5

Provide an overview of the kidney microstructure.

Answer 5

Uriniferous tubule -> nephron + collecting duct -> renal corpuscle + proximal convoluted tubule + loop of henle + distal convoluted tubule -> glomerulus + Bowmans capsule

Question 6

Describe the blood supply to the kidneys.

Answer 6

Need good blood supply and utilise 1L per minute of cardiac output
The renal artery goes in and splits off into segmental arteries -> interlobar arteries (between renal pyramids) -> arcuate arteries (run along corticomedullary junction) -> interlobular arteries (give off afferent arterioles to the renal corpuscles) -> further branches to uriniferous tubules

Question 7

Describe the capillaries of the kidneys and explain why they are unique in this sense too.

Answer 7

Blood passes through 2 capillary beds:

• Afferent arterioles bring blood into high pressure glomerular capillaries for filtration
• Efferent arterioles form another capillary bed which is the low pressure peritubular capillaries for reabsorption/secretion (vasa recta involved here too)
• Unique as a organ because it has 2 capillary beds

Question 8

What is the renal corpuscle, what is it made up of what does it do?

Answer 8

• Made up of glomerulus capillaries + Bowmans capsule (double walled cup surrounding glomerulus)
• Filters blood to form initial filtrate (from vascular pole to urinary pole)
• Outer layer = parietal (containment -> simple squamous)
• Inner layer = visceral (filtration -> modified simple squamous i.e. podocytes)
• Mesangial cells provide support between glomerular capillary loops interspersed

Question 9

What is the filtration barrier, what is it made up of and what does it do?

Answer 9

Podocytes wrap round the glomerular capillaries and their interlinked foot processes form filtration slits/gaps. The filtration barrier is formed from:

1. Glomerular capillary endothelium (fenestrated)
2. Basement membrane (negatively charged)
3. Epithelial cells (podocytes)
   - > limits passage of substances from blood based on size, charge + shape into Bowmans capsule

Question 10

What is the proximal convoluted tubule, what is it made up of and what do it do?

Answer 10

• Unmodified filtrate leaves Bowmans space and enters the proximal convoluted tubule
• Majority of water, sodium, chloride, amino acids and glucose reabsorption takes place
• Also secretion of some drugs and waste molecules
• Most metabolically active and has lots of mitochondria and transporters to shift materials across
• Made up of simple cuboidal cells with microvilli forming a brush border to increase surface area

Question 11

What is the loop of henle, what is it made up of and what does it do?

Answer 11

3 main parts:
1. Thin descending limb: permeable to water, no active reabsorption/secretion of solutes (simple squamous epithelium)
2. Thin ascending limb: impermeable to water, essentially no active reabsorption/secretion of solutes (simple squamous epithelium)
3. Thick ascending limb: impermeable to water, active reabsorption of sodium & other solutes (simple cuboidal cells)
= hyperosmolar interstitium in medulla important in concentration of urine

Question 12

What is the distal convoluted tubule, what is it made up of and what does it do?

Answer 12

• Early + late parts
• Early part passes back to vascular pole of its own renal corpuscle to form part of JGA (loops back to afferent and efferent to provide feedback)
• Active reabsorption of Na+ & other solutes and secretion of K+ & H+
• Simple cuboidal cells
• Variable water permeability depending on precise part and presence of ADH

Question 13

What is the Juxtaglomerular apparatus (JGA),what is it made up of and what does it do?

Answer 13

3 components:
1. Macula densa: specialised cells in early DT that pass next to vascular pole of own renal corpuscle
2. Extraglomerular mesangial cells (Lacis cells)
3. Granular or juxtaglomerular cells in afferent arteriole
= involved in tubuloglomerular feedback and control of BP

Question 14

What is the medullary collecting duct, what is it made up of and what does it do?

Answer 14

• Final site for urine processing
• Water permeability under hormonal control by ADH
• Surrounded by medullary interstitium with a high concentration of solutes
• Key role in controlling and regulating degree of urine concentration
• Made up of simple columnar cells
• Lies parallel close to loop of henle

Question 15

What is body fluid homeostasis?

Answer 15

• Maintaining a constant volume and stable composition of body fluids essential for homeostasis
• Precise matching of intake and output
• Insensible water losses e.g. sweat (poorly regulated)
• Kidneys play major role in regulation of body fluid volume and composition which allows us in turn to freely drink and take in electrolytes without worry

Question 16

What are the main body fluid compartments?

Answer 16

Intracellular fluid (ICF) = 28L
Extracellular fluid (ECF) = 14L 
= in 70kg male, total body water is 42L (~60% bodyweight) but varies with age, gender and body fat 
- Composition of body fluid varies between compartments which are separated by semi-permeable membranes

Question 17

What makes up Extracellular Fluid (ECF)?

Answer 17

• Interstitial fluid (~11L) surrounding cells; also includes trancellular fluid (CSF + joint fluid etc.)
• Plasma (~3L) which is non-cellular component of blood

Question 18

Explain body fluid composition in different compartments.

Answer 18

• Capillary membrane: highly permeable to water and electrolytes but not plasma proteins
• Interstitial fluid is separated from plasma via the capillary membrane - they have similar compositions apart from proteins which remain in plasma
• Cell membrane separates interstitial fluid and ICF and this is permeable to water but not to most electrolytes

Question 19

What are the main cations and anions of ECF and ICF?

Answer 19

• ECF: main cation = sodium + main anion = chloride

- ICF: main cation = potassium + main anion = phosphate

Question 20

What osmolarity of body fluids should be maintained?

Answer 20

~280-300 mOsm/L

Question 21

What are the main osmotically active electrolytes of ECF and ICF?

Answer 21

ECF: Sodium
ICF: Potassium

Question 22

What is osmotic equilibrium and how is it maintained?

Answer 22

Water can move easily between all compartments
Rapid equilibration between compartments when changes occur in one compartment i.e. if osmotic equilibrium of different compartments is disturbed
Kidneys control volume and composition of plasma which in turn can influence interstitial fluid and ICF

Question 23

Where is erythropoietin (EPO) produced, what does it do and what happens if this goes wrong (E.G. of kidney function failure causing clinical disease)?

Answer 23

• Produced by interstitial cells in cortex and outer medulla and it is released in hypoxia
• GF that stimulates production of RBC precursors in bone marrow
• Kidneys are the main source so kidney disease can result in normochromic normocytic anaemia

Question 24

Explain vitamin D activation and what can happen if it goes wrong (E.G. of kidney function failure causing clinical disease).

Answer 24

• Calcium poorly absorbed from GI tract
• Absorption mediated by active form of vitamin D (calcitriol)
• Kidney produces enzyme 1α-hydroxylase which converts inactive precursor into active form of vitamin D
• Patients with chronic kidney disease develop renal bone disease due to failure to produce enzyme and inability to absorb adequate calcium from diet

Question 25

What are the 3 main processes performed by the nephron in urine formation?

Answer 25

1. Filtration
2. Reabsorption
3. Secretion
   = Urinary excretion of any substance reflects sum of these processes - urinary excretion rate = filtration rate + secretion rate - reabsorption rate

Question 26

How much filtrate and urine is produced by the body a day?

Answer 26

~180L of filtrate/day

~1.5L of urine/day

Question 27

What can the kidney do with different substances in the process of urine formation? Give examples.

Answer 27

Different substances are handled differently depending on whether kidney wants to keep them or not and processes can be altered according to needs of body to lose or retain a substance. E.G:

• Want to get rid of creatine so it is mainly filtered and excreted in urine
• Filter almost all Na+ but want to reabsorb a large amount, only some is excreted
• Glucose is filtered and then completed reabsorbed, so none is excreted
• Drugs are filtered and secreted so are mostly excreted