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LT Notes PBC Part 2
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Notes from all the lectures given during the course Physiology Basic Concepts Part 2 about the Kidneys & Homeostasis. Includes usefull illustrations.
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LT Anatomy urinary systemEssentials functions of the kidneys;
− Conserve body fluid and electrolyte balance and remove metabolic waste
− Regulate water and electrolyte balance, tissue osmolarity, blood pressure and acid-
base balance
− Function also as an endocrine organ as it synthesizes EPO, renin and active vitamin
D for the calcium metabolism
Right kidneys lies a little bit lower than the left
kidney. They are both situated high in the upper
abdomen. Aorta is always a bit on the left and the
inferior caval vein on the right side. The kidneys
have a retroperitoneal positional. They are
surrounded by layers of fat: Perinephric fat, the
renal fascia and the paranephric fat. The renal
fascia is an enclosing membranous layer that is
open inferiorly. It also encloses the suprarenal
gland. Paranephric fat is the retroperitoneal adipose
tissue to keep the kidneys in place as they are
retroperitoneal.
Left renal vein receives the left suprarenal vein and
gonadal vein. On the right side these drain directly
into the inferior caval vein. Renal vein entrapment
syndrome is a compression of the left renal vein between the superior mesenteric artery
anteriorly and the abdominal aorta posteriorly.
The kidney is bean shaped and multilobar. However, because of growth of the interstitium
between them this lobar structure cannot really be seen from the outside in an adult kidney.
Have 12-18 lobes. The suprarenal (adrenal) gland is on top of the kidney, but is a separate
organ. The kidney is concave medially where you can also find the hilum. This is an entrance
for the renal vein (anterior), renal artery (middle) and renal pelvis (posterior). Renal pelvis
connect to the ureter.
The kidney also has a fibrous capsule. It has two layers, one with fibroblasts and one with
myofibroblasts. Myofibroblasts aid in resisting volume and pressure variations.
It has peripheral parenchyma and a central cavity called the renal sinus. The parenchyma
can be further divided into the cortex and the medulla. The cortex has the cortical labyrinths
and the renal columns. The medulla has the medullary pyramids with the renal papilla and
the medullary rays that project into the cortex. Renal corpuscles can be found in the renal
columns in the cortex. The renal sinus consists of minor and major calyces, the renal pelvis
and the perinephric fat, blood vessels etc.
The medullary ray with its surrounding cortical tissue/labyrinth form a renal lobule. Lobes are
larger units and consist of a medullary pyramid with the cortical tissue.
Renal arteries → Segmental arteries → Interlobar arteries → Arcuate arteries → Interlobular
arteries
,Interlobular veins → Arcuate veins → Interlobar veins → Segmental veins → Renal vein
The whole kidney receives 20-25% of the cardiac output. 90-95% of this goes to the cortex
which is important as the nephrons are also situated in the cortex. Vascular supply of the
segmental arteries is also related to five segments of the kidney.
The nephron is the smallest structural and functional unit of the kidney. Renal corpuscle (of
Malpighi) has the glomerulus and the Bowman's capsule. The proximal thick segment
consists of the proximal convoluted tubule (TCI) and the proximal straight tubule (TRI). The
thin segment consists of the thin part of the loop of Henle (always medulla). The distal thick
segment consists of the distal straight tubule (TRII) and the distal convoluted tubule (TCII).
The collecting tubules are outside the nephron. Follow the middle of a medullary ray. Has a
ductus colligens and ductus papillaris.
Renal corpuscles are always in the cortex. They can be subcapsular, intermediate or
juxtamedullary. Juxtamedullary nephrons have long loops of Henle. The loop of Henle has a
proximal straight tubule (TRI), a thin segment and the distal straight tubule (TRII). Within the
renal corpuscle there is ultrafiltration. TCI has selective reabsorption. TCII also has selective
reabsorption and secretion. In the Loop of Henle there is the countercurrent multiplier. The
thin descending limb has passive reabsorption of H2O. Thin ascending limb passive
reabsorption of Cl, Na and K. Within the collecting duct there is ADH controlled H2O
reabsorption.
The vasa recta are the blood vessels
that provide the thin loops of Henle
with the osmotic gradient in the
medulla. The vasa recta consists of
the arteriolae rectae and the venulae
rectae. It stems from the efferent
arterioles of the deep glomeruli.
Endothelium is fenestrated so that
ions and water can pass.
The proximal convoluted tubule (TCI)
has an influx of 180L/day but an efflux
of 60L/day so there is a selective
reabsorption of 65% which consists of
electrolytes and H2O (80%) and
amino acids (100%). These epithelial
cells have a compact brushes border
with microvilli to really enlarge the
surface for reabsorption. They can also distend because of the basolateral interdigitations.
Tight junctions keep the cells close so there is no leakage. Since there is a lot of selective
reabsorption there are a lot of mitochondria. Basal striations because of the mitochondria.
Junctional complex is the tight junctions and the adhering junctions.
Distal convoluted tubules have selective reabsorption of Na and H2O and selective secretion
of K and H. It does not have a real brushed border. Because of this it can be distinguished
from the proximal convoluted tubules (not as fuzzy). Moreover, the cell boundaries are more
clear as there are no real clear basolateral interdigitations.
, The collecting ducts are very important in the reabsorption of water. It selective permeability
is regulated by ADH-regulated water channels. They use the osmotic gradient of the
medullary interstitium (so not active).
The renal corpuscle is the filtration apparatus of the kidney where the primary urine is
produced. It consists of the glomerulus, the Bowman's capsule and mesangium. Capsule of
Bowman has two layers (parietal and visceral). Mesangium is important for the structural
support and maintenance of the filtration apparatus. At the vascular pole the afferent arteriole
enters and the efferent arteriole leaves. At the urinary pole the proximal convoluted tubule
begins.
Glomerulus is special as it is drained by
an efferent arteriole instead of an
efferent venule.
Visceral layer of the Bowman's capsule
consist of podocytes. The parietal layer
consist of squamous epithelial cells and
go into cuboidal epithelial cells at the
site of the proximal tubule.
The endothelial cells, the basement
membrane and the Bowman's capsule
podocytes all play a role in the filtration.
Endothelium is fenestrated (large
fenestrations without diaphragm).
Podocytes have pedicles and make up
filtration slits. The glomerular basement membrane is very thick and it the product of the
glomerular endothelium cells and the podocytes. Acts as a physical barrier and ion-selective
filter. Has three layers:
− Lamina rare interna (adjacent to capillary endothelium)
− Lamina densa (overlapping portion)
− Lamina rara externa (adjacent to the podocyte processes)
Lamina densa is a physical filter. Restricts movement of molecules larger than 70kDa
because of the type IV and type XVIII collagen. The lamina rarae act as ion-selective filters
as they are rich in polyanions (negatively charged). They restrict the movement of negatively
charged molecules also smaller than 70kDa.
Podocytes form slit pores of around 40nm and are covered by a molecular diaphragm that
act as size-selective filters. Nephrin is a zipper-like protein that forms the slit diaphragm.
Connects to actin filaments. Because of this, changes in the slit pore are also 'felt' in the actin
filaments and vice versa it can be regulated. Dynamic size selective filter.
Mesangial cells are in the renal corpuscles. Perform phagocytosis and endocytosis of
residues from the GBM and the filtration slit diaphragm. Provide structural support for the
endothelial cells of the glomerulus. Also production of inflammatory substances and
modulation of the glomerular distension.
Juxtaglomerular apparatus has three types of cells; macula densa cells, juxtaglomerular cells
and extraglomerular mesangial cells (lacis cells). Lacis cells are modified SMCs. Macula
densa cells are specialized TRIIs at the vascular pole. Is a chemo sensor for the [Na] of the
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