Functional histology of the kidney Flashcards
Main function of the urinary system?
Homeostasis of blood
What does the urinary system regulate the homeostasis of?
- Plasma composition: by regulated excretion of water, ions and organic waste products into urine.
- Blood pressure: through the enzyme renin
- RBC content: through secretion of erythropoietin.
What percentage of cardiac output does the Kidneys receive?
25%
Purpose of Dialysis machine?
Mimics kidney role
Key parts of the kidney
- cortex
- renal artery
- renal vein
- ureter
- capsule
- medulla (divided into pyramids)
What is observed within the cortex?
Medullary rays: tubes forming this is in a way part of the medulla
Difference observed between foetal and adult kidney
Foetal kidney is more bumpy that adults’
Define Nephron
The functional unit of the kidney - where blood is filtered at the molecular level (dialysis), to produce urine, while retaining cells and large proteins.
How many Nephrons in each human kidney?
About 1 million Nephrons in each human kidney.
Order of parts of the Nephron
- Bowman’s capsule
- Glomerulus
- Proximal convoluted tubule
- Loop of Henle: thin arm
- Loop of Henle: thick arm
- Distal convoluted tubule
- Collecting duct
- Papilla
What is the Papilla?
This is where the kidney drains into the ureter
Describe the glomerulus
- Site of ultrafiltration (filtration/dialysis of blood through extremely fine molecular filter)
- A knot of capillaries
Describe the capillaries in the kidney?
- 50-100 nm
- Fenestrations shows ‘windows’ on the capillary
Pressure in the afferent arteriole?
High
Pressure in the efferent arteriole?
Low
Three sites of filtration?
- Fenestration wall
- Joint basement membrane
- Podocyte layer itself
Ultrafiltration definition
Molecular filtration through molecular membrane
Difference between most capillaries in the body and capillaries in the kidney?
Most capillaries in the body aren’t fenestrated, but in the kidney they are.
What is the basement membrane?
made by endothelial cells
What is the podocyte layer made out of?
A single-celled simple epithelium
Why does fluid move from the glomerulus into the space in the bowman’s capsule?
Because of greater pressure in the arterioles, the fluid moves out into the space in the bowman’s capsule.
Describe movement of fluid during the process of ultrafiltration
SEE NOTES
What are the three layers of the glomerular filter?
1) Cytoplasm of capillary endothelial cells
2) Thick, fused basal lamina
3) Filtration slit membranes (with ~4nm pores)
What is the filter cut-off value?
~70 kDa (e.g. albumin)
Describe the three layers in 3D view
SEE NOTES
What happens to the glomeruli when a patient has nephrotic syndrome?
- Glomerular basement
membranes become damaged and more leaky – so proteins lost in urine (proteinuria). - In one type, the basement membranes become visibly thickened, thought to be through deposition of immunoglobulins (autoimmunity).
What happens to the glomeruli when there is high blood pressure?
Some glomeruli destroyed and replaced by masses of clear “hyaline” materia
What is Hyaline?
Clear material
What is the function of proximal convoluted tubule?
Reabsorption from ultra filtrate
What is pinocytosis?
The ingestion of liquid into a cell by the budding of small vesicles from the cell membrane.
By what three means does reabsorption from ultrafiltrate occur?
- By active transport across membrane into cell: small molecules like Na+, glucose, amino acids
- By pinocytosis: macromolecules, especially proteins. These broken down in lysosomes and returned to blood.
- By passive flux: water, Cl-
Describe structures of the proximal convoluted tubule
- Long microvilli
- Lytic enzymes on surface
- Many lysosomes
- Many mitochondria
How are microvilli adapted to carry out function in the PCT?
- Long microvilli for high surface area for reabsorption
- Lytic enzymes on surface to break down macromolecules in the lumen
How are lysosomes adapted to carry out function in the PCT?
- There are many lysosomes to break down macromolecules
- endocytotic vesicles can fuse with lysosomes and be further broken down into single amino acids, which can be transported into the blood
How are mitochondrion adapted to carry out function in the PCT?
There are many mitochondria to fuel active transport (especially located near the basolateral sodium pumps)
What is the basolateral membrane made out of?
- The basolateral membrane of unpolarized cell is the surface of the plasma membrane that forms its basal and lateral surfaces
- It faces outwards, towards the interstitial and away from the lumen
Functions of the loop of Henle?
- Reabsorption of water and salts from filtrate: passive flux across epithelium, by osmosis and concentration gradients
What is the ion gradient set by in the Loop of Henle?
The countercurrent multiplier system
What is countercurrent multiplication in the kidneys?
The process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine
Describe the structure of the epithelial cell in the thin loop of Henle
- Thin squamous epithelium to allow passive fluxes
- A minimum of organelles
- Not a lot of mitochondria present like other cells
Functions of the DCT?
- Homeostasis by regulated active transport & exchange of ions
- Na+/K+,H+/HCO3-
Function of thick ascending loop of Henle?
- Similar structure to DCT
- Finished with regulation of macromolecules earlier on in the nephron
- this part of the nephron focuses mainly on the ions
- regulating pH of the plasma
Describe the structures of the DCT, and how they are adapted to carry out their functions?
- Cuboidal epithelium: thicker than squamous to reduce passive fluxes and accommodate organelles
- Few, short microvilli (unlike PCT)
- Many mitochondria to form active transport, mainly at the base of the cell
How are the mitochondria observed in an epithelial cell on the DCT?
These are mainly basal and can show as a pale or striped basal area in H&E-stained sections.
Describe the functions of the Collecting duct
- Transport of urine to ureter
- Water homeostasis: using ionic gradients passive reabsorption of water
- This is regulated through epithelial permeability (the channels allowing water to move across)
Describe the structure of a collecting duct epithelial cell
- Cuboidal to columnar epithelium, to prevent passive flux of water (and urea etc)
- Specialized dense membranes at cell contacts (probably to help prevent massive passive flux)
What is distinguishable about the membranes of the collecting duct epithelial cell?
You can see the cell membranes between the cells – you can’t see them in cell membranes of other types of epithelium:
- These membrane probably have more proteins
- They have junctions that are very impermeable and stop water moving between the cells: so the water has to cross the cell and be regulated
What happens to the cells as you move down the collecting duct?
The cells get taller and more impermeable to the water crossing it
What are some differences observed between the lumen of the PCT and DCT cells (under light microscope)?
- Distal has a visible lumen
- Proximal has a lumen filled with proteins by the time it’s fixed and stained: so the protein that hasn’t been absorbed yet: lumen hard to see + microvilli make the border hard to see
What are some differences observed between DCT cells and PCT cells?
DCT: cells paler, tubule & cells smaller, lumen smoother
- clear lumen
PCT: bigger tubules and cells, wispy material in the lumen (fixed protein)
- fizzy lumen
What is the juxtaglomerular apparatus?
A specialized structure formed by the distal convoluted tubule where it loops back to meet the glomerular afferent arteriole of the same nephron
Where is the juxtaglomerular apparatus located?
Located near the vascular pole of the glomerulus
What is the main function of the juxtaglomerular apparatus?
Main function is to regulate blood pressure and the filtration rate of the glomerulus.
What is different about the macula densa cells?
- In the DCT
- crowded patch of cells, that are taller and thinner than the other cells (in cross-section).
Role of juxtaglomerular cells?
Smooth muscle cells that are modified and located on the outside of the arteriole
What is Renin?
A hormone that helps to regulate blood pressure
What are Lacis cells?
The function for these is unclear, but they are thought to play a role in communication between the macula densa and the juxtaglomerular cells
How do the macula densa and juxtaglomerular cells work together?
- The macula densa senses [Na+] in the DCT fluid
- it appears to signal to the JCs
- The JCs release renin in response to lower [Na+] in the DCT
- Renin indirectly increases vascular tone and sodium reabsorption
What does Extraglomerular mean?
Outside the glomerulus
What is mesangial tissue?
A sort of connective tissue
What are mesangial cells?
Contractile cells that constitute the central stalk of the glomerulus
What is needed in order for the renin granules to be observed under microscope?
A special stain
What is observed from a transverse section of the Urea (under low magnification)?
- lining of the ureter
- transitional epithelium
- dense connective tissue (lamina propria)
- layers of the smooth muscle
- adventitia
What can the ureter change according to volume of urine?
- The ureter is able to expand when a lot of urine is coming through
- Collapse when a lot of urine isn’t coming through.
- Ureter is actually contractile
Peristalsis in the ureter
You get waves of contractions pass along the ureter to the bladder
What is transitional epithelium?
A special stratified epithelium, found only in ureters and bladder
What are adaptations of the structure of transitional epithelium to its function?
- Specialized to be impermeable to urine.
- Changes appearance on stretching
- Somewhat like stratified squamous epithelium (SSE) when distended,
Difference between apical and basal cells?
- Apical cells are biggest and have much apical cytoplasm
- Basal cells are the smallest
Function of plaques when bladder distended (bladder full)?
These impermeable, rigid membrane patches (plaques) protect apical cells from toxic urine.
Function of plaques when bladder contracted (bladder empty)?
The rigid plaques are invaginated forming pits and vesicles in the cell, allowing cell surface area to decrease.
Where are plaques located?
Plaques of specialized (urine-resistant) plasma membrane in apical cells of transitional epithelium
Why are urinary infections quite common?
Transitional epithelium highly impermeable: leukocytes of immune system cannot readily penetrate.
- The bacteria can therefore multiply without any immune reaction
Why are UTIs more commoner in females?
Female urethra shorter. More risk of contamination, e.g. from anal region.
Advice on prevention of UTIs?
- drink plenty of fluid, to wash the infection away
- the more you drink the more fluid that flows through the bladder
- otherwise antibiotics can be used
