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Urinary system (organa urinaria) 267

parietal layer. The parietal layer consists of flattened, sim- The relatively large pores (70–100 nm) of the capillary
VetBooks.ir ple squamous epithelium with an underlying basal lamina endothelium prevent the movement of cells and large
and loose connective tissue. The urinary space (capsular molecules out of the capillary lumen.
The continuous basement membrane limits the pas-
lumen) between the two layers of the capsule serves to
collect the ultra-filtrate (= primary urine), which leaves the sage of substances with a molecular weight of greater
renal corpuscle at the urinary pole (polus tubularis) and than 400,000 Da and/or a diameter of more than 10 nm.
passes into the proximal tubule (Figure 12.9). The main barrier is the lamina densa, which functions
primarily as a physical filter. In contrast, the laminae
Mesangium rarae act electrostatically, repelling negatively charged
The term mesangium refers to the intraglomerular molecules.
portion of the renal corpuscle that extends from the vas- The filtration slit diaphragm between adjacent foot
cular pole into the glomerulus, binding the capillary loops processes of podocytes permits the transit of substances
together (Figures 12.9 and 12.10). It is derived from mesen- with a molecular weight below 70,000 Da or a diameter
chymal connective tissue that migrates into the corpuscle of less than 7.5 nm.
together with the capillaries. The mesangium consists Consequently, the glomerular filtrate (primary urine)
of mesangial cells (mesangiocytes) and the mesangial is an ultra-filtrate of plasma. It differs from plasma in that
matrix (lamella hyalina). Long processes extending from it is largely free of proteins. Molecules such as inulin,
mesangial cells may in some instances penetrate the endo- free haemoglobin and serum albumin may be detect-
thelium to reach the capillary lumen. able in trace amounts. Primary urine is isotonic and
Mesangial cells typically have a bizarre, spindle-like iso-osmotic with blood. In addition to water and electro-
appearance with indented heterochromatic nuclei. The lytes, it contains amino acids, glucose and smaller protein
cytoplasm contains a sparse complement of fibrils that per- molecules.
mits a limited degree of cellular contraction. Numerous The volume of glomerular filtrate is determined by
matrix-filled invaginations are present on the cell surface. the filtration surface, the permeability of the filtration unit
The mesangial matrix is in contact with the lamina rara to water and the effective glomerular filtration pressure.
of the glomerular basement membrane, and the mesan- The last of these is influenced by blood pressure, col-
gium is continuous with the extraglomerular mesangial loid osmotic pressure and pressure exerted by interstitial
cells (Goormaghtigh cells, lacis cells). Functions of the tissues.
mesangium include:
Tubules of the nephron
· structural support for the glomerular capillaries, The tubules of the nephron (Figure 12.8) are comprised
· phagocytosis of substances originating from the of the:
capillaries,
· uptake of filtered substances from the glomerular · proximal convoluted tubule,
basement membrane (removal of debris), · proximal straight tubule (thick descending limb of
· production of substances that contribute to immune loop of Henle),
defence and repair of the glomerulus, · thin limb of loop of Henle (descending and ascend-
· production of glomerular basement membrane ing portions),
components and · distal straight tubule (thick ascending limb of loop
· regulation of capillary resistance/distension. of Henle) and
· distal convoluted tubule.
Binding of angiotensin II with mesangial receptors regu-
lates contraction of blood vessels, thus influencing renal The main function of the proximal tubule (tubulus
blood flow. Cytokines released in local inflammatory proximalis) is to reabsorb electrolytes and water, amino
responses result in constriction of the capillary lumen. acids, peptides, proteins and glucose for transport into
the interstitium and uptake by the peri-tubular capillary
Glomerular filtration apparatus network. The proximal tubule also secretes various sub-
(blood–urine barrier) stances, such as NH ions, water and salts, as well as heavy
+
4
The glomerular filtration apparatus has three main metals (e.g. mercury, lead), toxins and foreign matter into
components: the tubular lumen. The proximal tubule is divided into
segments with different structural characteristics.
· capillary endothelium, The initial portion of the proximal tubule (tubulus con-
· glomerular basement membrane and tortus proximalis) is tightly convoluted and has a relatively
· podocytes and filtration slits (Figures 12.9 and 12.14). narrow lumen. The loops of the proximal convoluted

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