40) Functional histology of the kidney Flashcards
What are the two parts of the Loop of Henle?
- Thin arm which descends and ascends half way up
- Thick arm which only ascends
What is the collecting tubule?
- The part of the tubule that joins with the collecting duct
What is the renal corpuscle?
- It is a structure containing the Bowman’s capsule and the glomerulus
What cells cover the glomerular capillaries?
- There is a specialised layer of cells which cover the glomerular capillaries called podocytes
- They have projections that branch off them. This branching keeps going until we reach the end (called the feet)
- The feet are interdigitating with other feet (i.e. they are interlocked) which creates slits between them to allow for filtration of blood
What are the three stages of filtration?
- Fenestration: Which are gaps between endothelial cells that line the walls of the capillary in which blood can flow through. Allows for larger molecules through but filters out excessively large substances
- Basement membrane: Which is thick and where the endothelial cells and podocytes are attached to. It allows smaller molecules to pass through by filters out larger compounds
- Filtration slits: Found between the feet of the podocytes which allow molecules that are even smaller to pass through while filtering out larger compounds
(As we move down the stages of filtration the levels of fineness increases (i.e. the molecules they allow through gets smaller and smaller)
What is the function of the proximal convoluted tubule?
- Reabsorption of ultrafiltrate
What are the mechanisms by which material is reabsorbed in the proximal convoluted tubule?
- Active transport: Small molecules against a concentration gradient (e.g. glucose or Na+)
- Pinocytosis: Macromolecules (e.g. proteins which are degraded within lysosomes and amino acids are recycled in the blood)
- Passive flux: Small molecules down a concentration gradient (e.g. H20)
What is the structure of the proximal convoluted tubule epithelial cells?
- Microvilli: Long finger like projections that increase surface area but also contain lytic enzymes on the surface to break down macromolecules
- Pinocytotic vesicles: Carrying macromolecules to lysosomes
- Lysosomes: Found in great abundance to recycle macromolecules
- Mitochondria: Found also in abundance (especially at the basal surface) to fuel active transport
What is the function of the thin arm of the loop of Henle?
- They reabsorb ultrafiltrate
What are the mechanisms by which material is reabsorbed in the thin arm of the loop of Henle?
- Passive flux only (i.e. osmosis)
What is the structure of the cells of the thin arm of the loop of Henle?
- Thin squamous epithelium (in size) to allow passive fluxes to occur
- A minimum number of organelles
What is the function of the thick arm of the loop of Henle and the distal convoluted tubule?
- Blood homeostasis
What are the mechanisms by which material is reabsorbed in the thick arm of the loop of Henle and the distal convoluted tubule?
- Through active transport and ion exchange
What is the structure of the cells of the distal convoluted tubule?
- Cuboidal epithelium (in size): Thicker than squamous to reduce passive fluxes and accommodate organelles
- Mitochondria: Found in abundance to fuel active transport
- Microvilli: Tiny amounts found which are short
What is the function of the collecting duct and collecting tubule?
- Transport urine to the ureter
- Water homeostasis as it allows the passive reabsorption of water which is regulated through epithelial permeability
What is the structure of the cells within the epithelial of the collecting duct?
- Cuboidal to columnar (in shape): Prevents unregulated passive flux of water (and urea)
- Dense membrane: This occurs at the site of cell contacts as it may help prevent passive flux
What is nephrotic syndrome?
- When glomerular basement membranes become damaged and leaky causing proteins to be lost in the urine
- In some cases the basement membrane becomes thickened which may be the deposition of immunoglobulins (autoimmunity)
How does high blood pressure cause kidney damage?
- High blood pressure damages glomeruli which is replaced by a hyaline material
- A hyaline material is one that is glassy and usually indicates that the cellular matter has been replaced by proteinaceous material
How do distal and proximal convoluted tubules differ?
- Distal tubules are smaller cells with a smoother surface that appears paler
- Proximal tubules are bigger with wispy material within the lumen (which is fixed protein)
What is the juxtaglomerular apparatus?
- Where the distal convoluted tubule loops back and meets the arterioles of the same nephron
What are the different cell types in a juxtaglomerular apparatus?
- Juxtaglomerular cells: Lining the arteriole which contain renin granules
- Lacis cells: In the middle
- Macula densa: On the distal convoluted tubule
What is the function of the macula densa?
- They sense the [Na+] in the distal convoluted tubule fluid
- They send signals to juxtaglomerular cells
What is the function of the juxtaglomerular cells?
- Receive signals from the macula densa and in doing so release renin
- They release more renin when there is a lower [Na+] in the distal convoluted tubule.
- Renin indirectly increases vascular tone and increases sodium reabsorption
What is the function of the lacis cells?
- It is speculated that they have a role in signalling between the two cells
- They are also called extraglomerular mesangial cells
What are the structures within the ureter?
- Transitional epithelium: Which surrounds the lumen
- Dense connective tissue: Located around the transitional epithelium (also called lamina propria)
- Layers of smooth muscle: Found around the dense connective tissue and allows peristalsis to occur
- Adventitia: Which is the tissue located around the layers of smooth muscle and is the outermost layer
What is transitional epithelium?
- A specialised stratified epithelium which is found only in the ureters and bladder
- It is specialised to be impermeable to urine and changes appearance on stretching
- Its appearance is like stratified squamous epithelium except the biggest cells are apical (whereas in stratified squamous epithelium the biggest cells are basal)
What is found on the apical cells of the transitional epithelium?
- There are plaques of specialised (urine-resistant) plasma membrane
- When the bladder is distended they are impermeable, rigid and protect the cells from urine (which is toxic)
- When the bladder is relaxed they fold up forming interplaque regions and will also invaginate to form pits which allows the volume of the bladder (and ureter) to decrease
