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.
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.
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).
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.
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.