Page 190 - 84 human physiolofy part-1

Chapter 10

Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the
renal circulation. The glomerular blood pressure provides the driving force for fluid and solutes to be
filtered out of the blood and into the space made by Bowman's capsule. The remainder of the blood not
filtered into the glomerulus passes into the narrower efferent arteriole. It then moves into the vasa recta,
which are collecting capillaries intertwined with the convoluted tubules through the interstitial space,
where the reabsorbed substances will also enter. This then combines with efferent venules from other
nephrons into the renal vein, and rejoins with the main bloodstream.

Afferent/Efferent Arterioles

The afferent arteriole supplies blood to the glomerulus. A group of specialized cells known as
juxtaglomerular cells are located around the afferent arteriole where it enters the renal corpuscle. The
efferent arteriole drains the glomerulus. Between the two arterioles lies specialized cells called the
macula densa. The juxtaglomerular cells and the macula densa collectively form the juxtaglomerular
apparatus. It is in the juxtaglomerular apparatus cells that the enzyme renin is formed and stored.
Renin is released in response to decreased blood pressure in the afferent arterioles, decreased sodium
chloride in the distal convoluted tubule and sympathetic nerve stimulation of receptors (beta-adrenic)
on the juxtaglomerular cells. Renin is needed to form Angiotensin I and Angiotensin II which stimulate
the secretion of aldosterone by the adrenal cortex.

Glomerular Capsule or Bowman's Capsule

Bowman's capsule (also called the glomerular capsule) surrounds the glomerulus and is
composed of visceral (simple squamous epithelial cells) (inner) and parietal (simple squamous
epithelial cells) (outer) layers. The visceral layer lies just beneath the thickened glomerular basement
membrane and is made of podocytes which send foot processes over the length of the glomerulus. Foot
processes interdigitate with one another forming filtration slits that, in contrast to those in the
glomeruluar endothelium, are spanned by diaphragms. The size of the filtration slits restricts the
passage of large molecules (eg, albumin) and cells (eg, red blood cells and platelets). In addition, foot
processes have a negatively-charged coat (glycocalyx) that limits the filtration of negatively-charged
molecules, such as albumin. This action is called electrostatic repulsion.

The parietal layer of Bowman's capsule is lined by a single layer of squamous epithelium. Between
the visceral and parietal layers is Bowman's space, into which the filtrate enters after passing through
the podocytes' filtration slits. It is here that smooth muscle cells and macrophages lie between the
capillaries and provide support for them. Unlike the visceral layer, the parietal layer does not function
in filtration. Rather, the filtration barrier is formed by three components: the diaphragms of the
filtration slits, the thick glomerular basement membrane, and the glycocalyx secreted by podocytes.
99% of glomerular filtrate will ultimately be reabsorbed.

The process of filtration of the blood in the Bowman's capsule is ultrafiltration (or glomerular
filtration), and the normal rate of filtration is 125 ml/min, equivalent to ten times the blood volume
daily. Measuring the glomerular filtration rate (GFR) is a diagnostic test of kidney function. A
decreased GFR may be a sign of renal failure. Conditions that can effect GFR include: arterial pressure,
afferent arteriole constriction, efferent arteriole constriction, plasma protein concentration and colloid
osmotic pressure.

190 | Human Physiology