Functional Histology of the Kidney Flashcards
What is the function of the urinary system?
To maintain homeostasis of:
- plasma composition
- BP
- RBC content
How is plasma concentration maintained stable?
by regulated excretion of water, ions and organic waste products into urine
How is blood pressure maintained?
through the enzyme renin via RAAS
How is the RBC content kept stable by the kidneys?
through secretion of erythropoietin
How much of the cardiac output do the kidneys receive?
Kidneys receive ~25% of cardiac output
Huge renal vessels
How do humans mimic the function of the kidneys for patients with renal failure?
Dialysis Machine
Best human engineering can provide, less effective than a kidney
What is the outer protective layer of the kidney called?
Capsule
What is the outermost inside layer of the kidney?
Cortex
What part of the kidneys are the pyramids structures a part of?
The medulla, the tip of each pyramid is the papilla
What is the calyx of the kidney?
The inner space where all the urine produced is collected
What are the vessels supplying and draining the kidneys?
Renal artery and renal veins
What is the area all the tubes connect to the kidney called?
The hilum
What is a nephron?
The functional unit of the kidney
What occurs at the nephrons?
blood is filtered at the molecular level (dialysis), to produce urine, while retaining cells and large proteins
How many nephrons are present in one human kidney?
About 1 million nephrons in each human kidney
What is a nephron composed of?
Bowman’s capsule Glomerulus (blood capillaries) Proximal convoluted tubule Loop of Henle - thin arm Loop of Henle - thick arm Distal convoluted tubule Collecting tubule (straighter, epithelium) Collecting duct Papilla
What does the renal corpuscle refer to?
Bowman’s capsule and glomerulus
What is the renal tubule?
Proximal convoluted tubule Loop of Henle - thin arm Loop of Henle - thick arm Distal convoluted tubule Collecting tubule (straighter, epithelium)
What is the function of the glomerulus?
Site of ultrafiltration (filtration/dialysis of blood through extremely fine molecular filter)
What is the glomerulus?
A knot of fenestrated capillaries covered in epithelial cells (podocytes) to allow movement of fluid during ultrafiltration
What are the 3 layers of the glomerular filter?
- Cytoplasm of capillary endothelial cell
- Thick, fused basement membrane
- Filtration slit membranes (podocytes)
What is the filter cut off size?
> 70kDa molecules are retained in the blood. Many proteins, nutrients (e..g sugars) and ions are filtered through
Explain the affect of nephrotic syndrome on the structures within a kidney
Glomerular basement membranes become damaged and more leaky – so proteins lost in urine (proteinuria).
Why may some basement membranes become thickened during nephrotic syndrome?
thought to be through deposition of immunoglobulins (autoimmunity)
What affect does high blood pressure have on the kidney structure?
Some glomeruli destroyed and replaced by masses of clear “hyaline” material
What is the function of the renal tubule?
Adjusts composition of ultrafiltrate, to recover nutrients, water etc,
and to regulate plasma composition
How does the renal tubule reabsorb from the ultrafiltrate?
By active transport across membrane into cell:
small molecules: Na+, glucose, amino acids
By pinocytosis: macromolecules, especially proteins.
Broken down in lysosomes & returned to blood.
By passive flux: water, Cl-
Describe the structure of the epithelial cells in the proximal convoluted tubule
- Long microvilli; high SA for reabsorption
- Lytic enzymes on surface; break down macromolecules
- Pinocytotic vesicles carry macromolecules to lysosomes
- Many lysosomes; break down + recycle macromolecules
- Many mitochondria; fuel active transport
(seen especially near the basolateral sodium pumps)
What is the function of the thin loop of henle?
Reabsorption of water and salts from filtrate - passive flux across epithelium, by osmosis & concentration gradients
Describe the structure of the epithelial cells present in the proximal convoluted tubule
Thin squamous epithelium to allow passive fluxes
Have minimum organelles present
What are the functions of the distal convoluted tubule?
Homeostasis by regulated active transport & exchange of ions
(Na+/K+, H+/HCO3-)
Describe the structure of the epithelial cells in the DCT
Cuboidal epithelium:
thicker than squamous, to reduce passive fluxes + accommodate organelles
- Few, short microvilli (unlike PCT)
- Many mitochondria; fuel active transport. These are mainly basal + can show as a pale or striped basal area in H&E-stained sections
What are the functions of the collecting duct?
Transport of urine to ureter.
Water homeostasis: passive reabsorption of water, regulated through epithelial permeability
Describe the epithelial cells structure in the collecting duct
Cuboidal / columnar epithelium; prevent passive flux of water (and urea etc)
Specialized dense membranes at cell contacts
Function unclear – probably helping prevent passive flux
Describe how you’d be able to identify DC cells in light microscopy
Cells are paler, smaller and have a smoother lumen
Describe the PCT cells seen using a light microscope
Bigger tubules and cells with wispy material in lumen (fixed protein)
What is the juxtaglomerular apparatus?
Where DCT loops back to meet arterioles of same nephron
What are the 3 components of the juxtaglomerular apparatus?
Macula densa
Juxtaglomerular cells
Lacis cells
What is the role of the macula densa?
Senses [Na+] in the DCT fluid. Appears to signal to JG cells
What is the role of the juxtaglomerular cells?
release renin – more so in response to lower [Na+] in DCT. Renin indirectly increases vascular tone and sodium resorption
What is the function of the Lacis cells?
Function unknown but as it lies between JXG and MD cells; most probably signals between them
Describe the structure of the Ureter
- Transitional epithelium in the centre (found only ureters)
- Connective tissue - middle area; made of collagen
fibroblasts - supports the ureter - Smooth muscle on the outside
- Adventitia - connective tissue carrying blood vessels and
nerves
What is the transitional epithelium?
Special stratified epithelium, only in ureters + bladder
Impermeable to urine
Transitions between shapes; changes appearance on stretching
Explain how the transitional epithelium change shape when stretching?
On contraction (empty) - apical cells are biggest; lots of apical cytoplasm
On distension (full) - Basal cells biggest, apical cells are flat
Explain the significance of a large apical surface?
There are plaques of specialised (urine-resistant) plasma membrane in apical cells of transitional epithelium - specialised to resist toxic substances in urine e.g. acidic v. hypertonic
Explain how the apical cells are protected during distension when the bladder is full
Impermeable, rigid plaques protect apical cells from toxic urine
How are apical cells protected during contraction when the bladder is empty?
Rigid plaques invaginated forming pits + vesicles in cells, allowing cells SA to decrease
What are the disadvantages of the transitional epithelium?
- Urinary infections are common (esp. in women)
Why are urinary infections so common?
Transitional epithelium highly impermeable - leukocytes of immune system cannot readily penetrate
Why are women at a higher risk of developing urinary infections?
Female urethra shorter. More risk of contamination, e.g. from anal region
How can we prevent getting a urinary infection?
Drinking plenty of water
