W9.1_Renal Physiology

Question 1

What are the four main components of the urinary system? What are the functions of it (6)? Describe the properties of the kidneys.

Answer 1

• Four main component parts of urinary system: kidneys, ureters, urinary bladder, urethra
• Function: filter blood and remove waste by making urine, blood acid-base and osmolarity homeostasis, control red blood cell production, control blood pressure, respond to hormones (ADH, PTH, aldosterone), make hormones (renin, vitamin D, erythropoietin)
• Properties: right kidney lower than left, receives disproportionately high volume of blood, adrenal glands attached

Question 2

Regarding the kidney anatomy, explain the properties or functions of renal capsule, renal cortex, renal pelvis, renal artery, and renal vein. What is a nephron? Categorise the parts of a nephron into cortex or medulla.

Answer 2

• Renal capsule: fibrous, with fat layer on top, covers whole kidney to protect and lower risk of trauma to kidney
• Renal cortex: outer layer vs Renal medulla: inner layer
• Renal pelvis: drains urine to the ureter
• Renal artery: carries oxygenated/unfiltered blood
• Renal vein: carries deoxygenated/filtered blood
• Nephron: functional unit of kidney
• Cortex: glomerulus, convoluted tubules, cortical collecting duct
• Medulla: loop of Henle, medullary collecting duct

Question 3

Explain the properties and functions of glomerulus and proximal convoluted tubule in a nephron.

Answer 3

Glomerulus
- Made of tubular epithelium, contains a hollow Bowman’s capsule
- Receives blood from afferent arteriole then leaves at efferent arteriole
- Conducts ultrafiltration: high hydrostatic pressure forces water, ions, and small molecules to be squeezed into the Bowman’s capsule (filters 10% of renal blood plasma), leaving large blood cells/proteins behind -> forms glomerular filtrate in Bowman’s capsule
Proximal convoluted tubule (PCT)
- Columnar epithelium lining with microvilli provides a high surface area + lots of mitochondria for active secretion/reabsorption
- Site of most reabsorption actively/passively (60% of water, most of Na+, K+, Ca2+, phosphate, glucose, amino acids, drugs)
- Site of tubular secretion (H+, NH+, waste products such as creatinine, drugs through ABC/SLC transporters)
- Transport mechanisms: passive diffusion, transporters, ion channels, endocytosis via receptors)

Question 4

Explain the properties and functions of loop of Henle, distal convoluted tubule, and collecting duct.

Answer 4

Loop of Henle
- Counter-current mechanism of glomerular filtrate and blood creates an osmotic gradient
- Descending loop of Henle: permeable to water, impermeable to ions, reabsorbs high volume of water and secretes urea -> create osmotic gradient by increasing solute concentration
- Ascending loop of Henle: permeable to ions, impermeable to water, Na+ and Cl- effluxed out through ion pumps (active) to maintain salt-water balance in medulla
Distal convoluted tubule (DCT)
- Heavily controlled by hormones (PTH for calcium reabsorption, aldosterone for Na+ reabsorption and K+ secretion)
Collecting duct
- Concentrates urine by reabsorbing water, reabsorption/secretion of ions continue
- Antidiuretic hormone (ADH, vasopressin, AVP) promotes water reabsorption

Question 5

Describe the hormone production process and functions of renin, erythropoietin, and vitamin D in kidneys.

Answer 5

Renin
- Produced by juxtaglomerular apparatus (between DCT and glomerulus)
- Released in response to low sodium -> formation of angiotensin II -> acts on nephrons by promoting sodium retention in blood + acts as potent vasoconstrictor to regulate blood pressure
Erythropoietin (EPO)
- Made by fibroblasts (interstitial space outside of nephron)
- Released in response to hypoxia -> promotes red blood cell formation in bone marrow
Vitamin D
- Metabolised in kidneys into 1,25-dihydroxycholecalciferol (calcitriol) -> promotes Ca2+ and K+ absorption from gut

Question 6

Describe how drugs are excreted renally. Explain how lipophilicity affects renal excretion.

Answer 6

• Similar process as waste/toxins (filtration, reabsorption, secretion)
• Lipophilic drugs are more passively reabsorbed (concentration gradient created by ultrafiltration of drugs + diffuse better across phospholipid bilayer of membrane) -> smaller renal excretion

Question 7

Explain the properties and functions of the remaining parts of the urinary system.

Answer 7

• Ureters: transports urine from renal pelvis to urinary bladder, peristalsis conducted in smooth muscular tubes (lined with epithelium to allow stretch)
• Urinary bladder: receives urine from ureters, hollow, transitional epithelium allows reflex stretching to expand the size (in >100mL), release urine to urethra by contraction of smooth muscle
• Urethra: controlled by urinary sphincters (involuntary internal vs voluntary external)