Functional anatomy of nephron

Question 1

Describe the nephron and list and compare the two types of nephrons

Answer 1

Nephron
- Structural and functional unit of the kidney ~ 1-2 million
- Both types are comprising two components:
- renal corpuscle (filtration of blood)
- renal tubule (reabsorption and secretion)

There are two types of nephrons:
- superficial (cortical) nephron
- the juxtamedullary nephron

Types of Nephron
1. Cortical Nephrons:
- 85% of all nephrons
- Short nephron loops to the border of the outer and inner medulla
- Short thin descending limb
- No thin ascending limb
- Efferent arterioles branch into peritubular capillaries around PCT and DCT

2. Juxtamedullary Nephrons:
   
   • 15% of all nephrons
   • Corpuscles are located near corticomedullary junctions
   • Very long nephron loops, maintain salinity gradient in the medulla and help conserve water – produces more concentrated urine ^[long in camels]
   • Thin limbs reaching the inner medulla
   • Efferent arterioles branch into vasa recta around the long nephron loop

Note: one collecting duct to one lobule
aks all nephrons in same lobule drain into same collecting duct

Question 2

Describe renal corpuscle

Answer 2

The renal corpuscle consists of glomerulus, Bowman’s capsule, and the filtration barrier.
- tubular and vascular poles
- three layers of renal corpuscle (in the filtration barrier): visceral layer, parietal layer, urinary/capsular/Bowman’s

Question 3

Describe mesangial cells

Answer 3

Structure
- located between capillaries
- adhere to endothelial cells
- enveloped by basement membrane
- cytoplasm is surrounded by mesangial matrix

(postulated) functions:
- contract to regulate glomerular blood flow
- remove molecules trapped in basement membrane
- support glomerulus

Question 4

Describe the filtration barrier

Answer 4

Filtration is a non-specific process as compared to re-absorption and secretion.
Filtration occurs based on size and charge (BM)

The filtration barrier consists of:
1. Endothelial cells of capillaries with fenestrations
- Fenestrations, 70nm in diameter
- Only stop blood cells from passing through
2. Glomerular basement membrane
- 300nm thickness, much thicker than normal capillary basement membrane
- Fusion of endothelium basal lamina (BL) and podocyte BL
- One central lamina densa (type IV collagen and laminin) - physical barrier
- Two lamina rara on each side (fibronectin) with negatively charged sites (glycosaminoglycan) - charge barrier freely
- Stop particles > 10nm and proteins > 69kDa (albumin) from crossing (so, glomerular filtrate = plasma - proteins)
3. Podocytes (visceral layer of Bowman’s capsule) with filtration slits
- Filtration slit between foot processes (pedicels), 25 nm wide
- Slit diaphragm, 6 nm thick
- Contain actin filaments to contract

Question 5

Describe the renal tubules

Answer 5

The renal tubules are responsible for urinary excretion through filtration, secretion, and reabsorption. Mostly reabsorption occurs.

Filtration:
- process by which the water and the solutes in the blood leave the vascular system through the filtration barrier and enter the Bowman’s capsule.

**Secretion:
- process of moving substances from the tubule lumen from the cytosol of epithelial cells that form the walls of the nephron

**Reabsorption:
- net movement of substance from lumen to blood vessel
- mainly occurs at the loop, especially where it is thin

**Excretion:
- exiting of material from the body

Question 6

Describe the composition fo the different tubules

Answer 6

• PCT: located in renal cortex, simple cuboidal epithelium, microvilli, abundant mitochondria near base
• DCT: in renal cortex, made of simple cuboidal epithelium, NO brush border, abundant mitochondria near base
• loop: mainly in renal medulla, with cuboidal epithelium in thick limbs (simple?), and squamous over thin limbs. Wide lumen

Question 7

Identify and interpret the microscopic features of the renal corpuscle, including the glomerulus, Bowman’s capsule, filtration barrier, and juxtaglomerular apparatus.

see notes

Question 8

Describe flow of urine

Answer 8

see anatomy renal system deck

Question 9

Describe the blood supply of kidneys and explain why it is unique

Answer 9

The renal blood supply is unique and vital for the kidney’s function.

1. Rich blood supply compared with other organs
   
   • Kidneys weigh 0.5% of total body weight, but receive 25% of the total cardiac output. Still, it is considered an altruistic organ, as it is not using blood for its own needs, but filtering for the body
2. Distribution of blood in kidney
   
   • 90% in the cortex (through cortical glomeruli)
   • 10% in the medulla (through juxtamedullary glomeruli)
3. High pressure system for filtration (for high efficiency)
   
   • High pressure afferent arterioles (70 mmHg)
   • High pressure glomerular capillaries (50 mmHg, other systemic cap. 25)
   • High pressure efferent arterioles (30 mmHg)
   • Low pressure peritubular capillaries (10 mmHg, other systemic cap. 25)

What makes it especially unique is that there are two capillary systems in sequence:
- glomerular capillaries arising from afferent arteriole to filter blood, aka no gas exchange here
- peritubular capillaries or vasa recta, arising from efferent arterioles, to reabsorb from tubules and facilitate gas exchange

Question 10

Recall arterial supply and venous drainage

Answer 10

see renal anatomy deck

Question 11

Describe the juxtaglomerular apparatus

Answer 11

Juxtaglomerular Apparatus
The juxtaglomerular apparatus is involved in local and systemic feedback mechanisms. It is a complex feedback mechanism.

• Lacis cells are contractile cells that provide support to afferent and efferent arterioles, as well secreting Epo, to enhance formation of RBCs
• Macula densa cells are responsible for local feedback. Specialised epithelial cells, of TAL. Contacts glomerulus. Appear plaque like.
• detects sodium or fluid change. Inc resutls in release of paracrine agents aka ATP, to cause contraction of afferent, reducing RBF and thus GFR
• alteration of GFR affects pressure
• JGA cells are specialised smooth muscle cells, located on walls of afferent arterioles
• involved in systemic feedbac cv macula densa
• they produce, store release renin due to signalling from baroreceptors on afferent arterioles (due to decrease in BP)
• ## drugs that targets this include diuretics, ACEIs, ARBs; tektuma

Question 12

Summarise the functions of the kidney

Answer 12

The kidney serves three major functions: maintaining homeostatic balance, removing toxins and metabolic waste, and performing endocrine functions:

1. maintenance of homeostatic balance (fluid balance, electrolyte balance)
2. removal of toxins and metabolic waste
3. Endocrine function: release of hormones (i.e. EPO, renin) for the maintenance of blood pressure, erythrogenesis etc. Specifically:
   
   • Juxtaglomerular cells:
     
     • Secretion of the enzyme, renin, which cleaves circulating angiotensinogen to release angiotensin I to regulate blood pressure!
   • Proximal tubules:
     
     • Hydroxylation of 25-hydroxyvitamin D3 to an active form, 1, 25- dihydroxyvitamin D3 to regulate calcium and phosphorous metabolism
       
       • Interstitial cells and lacis cells (juxtaglomerular cells):
     • Secretion of the glycoprotein hormone, erythropoietin, which enhances the formation of red blood cells!