Introduction To The Renal System
The kidneys help to
maintain the composition of extracellular body fluids, and regulate ions (e.g. Na+, K+, Ca2+,
Mg2+), acid–base status and body water. They also have an endocrine
function. Plasma is fil- tered by capillaries in the glomerulus (Chapter
32), and the composition of the filtrate is modified by reabsorption and
secretion in the nephrons. The average urine output is ∼1.5 L
per day, although this can fall to <1 L per day and increase to nearly 20 L
per day.
Gross structure
The kidneys are located on each
side of the vertebral column, behind the peritoneum. The renal artery and vein,
lymphatics and nerve enter the kidney
via the hilus, from which the renal pelvis, which becomes the ureter,
emerges (Fig. 31a). The kidney is surrounded by a fibrous renal capsule.
Internally, the kidney has a dark outer cortex surrounding a lighter medulla,
which contains triangular lobes or pyramids. The cortex contains the glomerulus
and proximal and distal tubules of the nephrons,
whilst the loop of Henle and collecting ducts descend into the
medulla (Fig. 31b). Each kidney contains ∼800 000 nephrons. The collecting ducts converge
in the papilla at the apex
of each pyramid, and empty into the calyx (plural: calyces) and
thence renal pelvis. Urine is propelled
through the ureter into the bladder
by peristalsis.
The nephron
Each nephron begins with a
capsule (Bowman’s capsule) surrounding the glomerular capillaries,
which collects filtrate (Fig. 32a), fol- lowed by the proximal tubule, loop
of Henle, distal tubule and early collecting duct (Fig. 31b).
There are two types of nephron – those with glomeruli in the outer 70% of the
cortex and short loops of Henle (cortical
nephrons: ∼85%), and those with glomeruli close to the cortex–medulla boundary and long loops of Henle
(juxtamedullary nephrons: ∼15%). The glomerulus produces ultrafiltrate from plasma (Chapter
32).
The proximal tubule is
convoluted when it leaves the Bowman’s capsule, but straightens before becoming
the descending limb of the loop of Henle in the medulla. Its walls are formed
from columnar epithelial cells with a brush-border of microvilli
on the luminal surface that increases the surface area ∼40-fold
(Fig. 31c). Tight junctions close to the luminal side limit diffusion
through gaps between cells. The basal or peritubular side of the cells shows
considerable interdigitation, which increases the surface area. The term
lateral intercellular space is often used to describe the space between
the interdigitations and basement membrane, and between the bases of adjacent
cells. The main function of the proximal tubule is reabsorption (Chapter
33).
The thin part of the loop of
Henle (∼20 μ m across) is formed from thin, flat (squamous) cells (Fig.
31d), with no microvilli. The thick ascending loop of Henle has
columnar epithelial cells similar to the proximal tubule, but with few
microvilli (Fig. 31e). At the point at which the loop associates with the juxtaglomerular
apparatus (Chapter 35), after re-entering the cortex, the wall is formed
from modified macula densa cells (Fig. 31b). The loop of Henle is
important for the production of concentrated urine.
The distal tubule is
functionally similar to the cortical collecting duct. Both contain cells
similar to those in the thick ascending loop of Henle (Fig. 31e). In the collecting duct, these principal cells are
interspersed with intercalated cells of
different morphology and function; these play a role in acid–base balance
(Chapter 36). The collecting duct plays an important role in water homeostasis
(Chapter 35).
Renal circulation
The kidneys receive ∼20% of
cardiac output. The renal artery enters via the hilus and divides into interlobar arteries running
between the pyramids to the
cortex–medulla boundary, where they split into arcuate arteries. Interlobular
arteries ascend into the cortex, and feed the afferent arterioles of
the glomerulus (Fig. 31a,b). The capillaries of the glomerulus are the site of filtration,
and drain into the efferent arteriole (not vein). Afferent and
efferent arterioles provide the major resistance to renal blood flow. Efferent
arterioles branch into a network of capillaries in the cortex around the
proximal and distal tubules (peritubular capillaries). Capillaries close
to the cortex– medulla boundary loop into the medulla to form the vasa recta
surrounding the loop of Henle; this provides the only blood supply to the
medulla. All capillaries drain into the renal veins. Ninety per cent of the
blood entering the kidney supplies the cortex, giving a high blood flow (∼500 mL/min/100 g) and
a low arteriovenous
O2 difference (∼2%). Medullary blood flow is less (20–100
mL/min/100 g).
Regulation of renal blood flow. Differential constriction of afferent and efferent arterioles strongly affects
filtration (see above; Chapter 32). The kidneys exhibit a high degree of autoregulation
(Fig. 32e), both by the myogenic response (Chapter 24) and via the
macula densa, which detects high filtration rates and releases adenosine, which
constricts afferent arterioles, so reducing filtration. Noradrenaline
(norepinephrine) from renal sympathetic nerves constricts both afferent and
efferent arterioles, and increases renin and thus the production of angiotensin
II (a potent vasoconstrictor) (Chapter 35). Many peripheral vasoconstrictors
(e.g. endothelin, angiotensin II) cause the release of vasodilating
prostaglandins in the kidney, so protecting renal blood flow.
Hormones and the kidney
Renal function is affected by a
variety of hormones that modulate the regulation of ions and water (e.g. antidiuretic
hormone, aldosterone). Renin is produced by the
juxtaglomerular apparatus and promotes the formation of
angiotensin (Chapter 35). Erythropoietin
is synthesized by interstitial cells in the cortex, and stimulates red cell
production (Chapter 8). Vitamin D is metabolized in the kidney to its
active form (1,25-dihydroxycholecalciferol), which is involved in Ca2+
and phosphate regulation (Chapters 34 and 48). Various prostaglandins are
also produced in the kidney, and affect renal blood flow.
Micturition
The constriction of smooth muscle
in the bladder wall (detrusor muscle) expels urine through the urethra
(micturition, urination). Micturition is initiated by a spinal
reflex when urine pressure reaches a critical level, but is strongly controlled
by higher (voluntary) centres. The neck of the bladder forms the internal
urethral sphincter; the external sphincter is formed from voluntary
skeletal muscle around more distal
regions of the urethra.
