K1; Structure and Functions of the Kidney

Question 1

What are is the primary role of the kidney and its corresponding categories?

Answer 1

• Excretion
  > Metabolic waste (urea; from AAs, creatinine; from muscle creatine, uric acid; from nucleic acids, end products of Hb breakdown; e.g. bilirubin, hormone metabolites)
  > Foreign substances; orally/pulmonary/inject (drugs, pesticides, food additives)

Question 2

What are the other functions of the kidney?

Answer 2

• Control of water and electrolytes (K+, Na+, Cl-)
• Control of arterial blood pressure; controlling the water and electrolytes in the body, RAAS activation via renin from kidney
• Control of acid-base balance (H+, HCO3-); lungs and body fluid buffers make up triad of acid-base balance
• Endocrine; erythropoietin (EPO); production of RBCs, renin, 1,25-dihydroxyvitamin D3 (calcitriol); kidney produces enzyme to activate Vitamin D, which is in turn involved in calcium homeostasis
• Glucose synthesis (gluconeogenesis from intermediates, usually liver); during prolonged fasting

Question 3

How does kidney failure affect its homeostatic functions? State the consequences corresponding to function.

Answer 3

Severe abnormalities in body fluid volumes and composition

• Excretion; accumulation of metabolites and toxins
• Water and electrolytes; water and salt retention, increases BP
• Acid-base balance; acidosis
• Endocrine; decreased levels of various hormones (deficiency)

Question 4

What are the types of kidney disease?

Answer 4

• Acute kidney injury (AKI); rapid loss of kidney function e.g. drug-induced, trauma. Patients often recover.
• Chronic kidney disease (CKD); develops slowly, associated with conditions such as diabetes, hypertension and chronic heart failure.

Question 5

What is the result of kidney disease?

Answer 5

Deterioration of kidney function; proteinuria, high blood pressure (due to accumulation of water and Na+), oedema (peripheral/pulmonary)

Question 6

What is the structure of the kidney?

Answer 6

• Renal capsule (protective, fibrous) and visceral fat (cushioning) on the outside
• Hilum (dent/bean shape bit) has the renal artery/vein, nerves and pelvis entering/exiting the kidney
• Cortex; outer bit
• Medulla; inner bit
• Medulla divided into multiple pyramids
• Nephrons (functional units) sit within pyramids

Question 7

How many nephrons do we have and what is the rate of loss?

Answer 7

• Each kidney composed of 1 million nephrons

- At Age > 40, natural loss of 10% per 10 years (e.g. Age 70, 30% fewer functioning nephrons)

Question 8

What does a nephron conist of?

Answer 8

• Corpuscle: C-shaped bit of tubule containing glomerulus/bowman’s capsule etc

Question 9

How much blood does the kidney receive?

Answer 9

20-25% of total cardiac output

Question 10

Where are peritubular capilaries situated and what are its functions?

Answer 10

• Between efferent arterioles and the renal vein

- Supplies blood for the rest of the tubule (the unfiltered blood from the glomerulus; waste/O2 exchange)

Question 11

What function does the PCT serve?

Answer 11

• Key site of reabsorption back into peritubular capilaries

- Single layer of cuboidal cells with microvilli that increase membrane surface area

Question 12

What is the structure of the Loop of Henle?

Answer 12

• U-shape that dips into the medulla
• Thin descending limb plunging from cortex into medulla
• Thick ascending limb
• Macula densa at end of thick ascending limb, forming part of the juxtaglomerular apparatus, teogether with renin-producing granular cells (afferent/efferent arterioles)

Question 13

Where is the DCT located and does it differ to the PCT?

Answer 13

• Begins just after the macula densa
• Extends into cortex
• Shorter and less convoluted than PCT

Question 14

Where does the collecting duct go and what is it lined with?

Answer 14

• DCT empties into collecting duct, whihc goes to the pelvis and then the uretha
• Lined with cuboidal cells
  > Principal cells (P)
  > Intercalated cells (I)
• Drains up to 8 separate nephrons per CD

Question 15

How do cortical nephrons differ from juxtamedullary nephrons?

Answer 15

Renal corpuscle:

• Cortical; lies in the outer part of the cortex
• Juxtamedullary; lies in the inner third of cortex

Loop of Henle:

• Cortical; short
• Juxtamedullary; long, extending into medulla, has thin ascending limb as well as thick

Distribution in humans:

• Cortical: 70-80% nephrons
• Juxtamedullary; 20-30% nephrons

Vascularisation:

• Cortical: entire tubular system surrounded by peritubular capillaries
• Juxtamedullary; have long efferent arterioles extending from glomerulo to medullar which subdivide into specialised peritubular capillaries known as vasa recta

Question 16

How is urine formed?

Answer 16

• Filtration (high hydrostatic pressure in glomerulus forces content out): movement of fluid from blood to lumen of nephron in renal corpuscle
• Reabsorption (volume/composition of filtrate modified): movement of filtered material from lumen of nephron back into blood (peritubular capillaries)
• Secretion: removal of selected molecules from blood & their addition to fluid in lumen of nephron
• Excretion: removal of substance from body

Question 17

How is the kidney innervated?

Answer 17

Sympathetic nervous system (no parasympathetic innervation):

• α1-adrenoceptors; mediate vasoconstriction of afferent/efferent arterioles (vascular smooth muscle)
• β-adrenoceptors; on granular cells of juxtaglomerular apparatus; stimulate renin release (cleaving Ang I to Ang II)

Question 18

What does vasoconstriction of the afferent arteriole result in?

Answer 18

• Less blood flow to glomerulus
• Lower hydrostatic pressure
• Decreased GFR

Question 19

What hormones act on the kidney and what do they do?

Answer 19

• Vasopressin from posterior pituitary (ADH); concentration of urine
• Aldosterone from adrenal cortex; water and electrolyte balance
• Parathyroid hormone from parathyroid glands; calcium/phosphate homeostasis
• Natriuretic peptides; excretion of Na+ and water

Question 20

What hormones are produced by the kidney and what do they do?

Answer 20

• Erythropoietin (EPO); production of RBCs
• Renin; RAAS activation
• Activated Vitamin D3 (calcitriol); calcium/phosphate homeostasis
• Prostaglandins; ensure good blood flow to kidney