The urinary system consists of paired kidneys and the urinary tract, which comprises paired ureters, a urinary bladder and a urethra (Fig. 13.1). Urine is produced by the kidneys and passes along the ureters to the urinary bladder, where it is stored until it is voided via the urethra. The route taken by urine along the urethra to the exterior is an independent closed system in females, but is shared with the reproductive system in males (Chapter 15).
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| Fig. 13.1 |
The urinary system is essential in maintaining the homeostasis of the body. It does this by regulating the water and mineral salts in, and the acid–base balance of, blood. It is particularly important in excreting toxic molecules containing nitrogen (e.g. urea and creatinine) produced by the breakdown of endogenous proteins. The urinary system also ensures that useful molecules in blood, e.g. proteins and carbohydrates, are not lost during the formation of urine. The toxic molecules and excess ions, dissolved in water, leave the kidneys as urine, which passes along the urinary tract before being voided at micturition. In addition, the kidneys produce and secrete into blood two molecules, renin and erythropoietin. The former is important in regulating blood pressure and the latter in stimulating the formation of red blood cells.
Kidneys
The gross structure of a kidney is best described as seen in a longitudinal hemi-section (Fig. 13.2). Facing towards the mid-line of the body, each kidney has an indentation forming a hilum. At the hilum blood enters and leaves each kidney in, respectively, a renal artery and a renal vein, and urine drains into a ureter.
The gross appearance of the kidney displays an outer and an inner region, the cortex and the medulla. The cortex has a granular appearance due to the presence of spherical structures about 200μm in diameter known as renal corpuscles. These corpuscles filter blood in the initial stage in the formation of urine. The cortex also contains convoluted tubules involved in forming urine. Renal corpuscles are not present in the medulla and the medulla appears smooth or may show striations. Straight and arching tubules are present in the medulla and they also are involved in forming urine. Renal corpuscles and tubules may be distinguished in histological sections even at low magnification (Fig. 13.3). In humans, the medulla projects centrally as several pyramids and the cortex extends as columns between the pyramids (Fig. 13.2). The apices of the pyramids project as renal papillae into urine-filled spaces (minor calyces). Urine drains from tubules in the medulla into the calyces. Each minor calyx drains urine into major calyces that together form the renal pelvis. The relatively large, fluid-filled space of each renal pelvis drains into a ureter (Fig. 13.2). Fat cells surround the renal pelvis, the ureter and vessels at the hilum of the kidney and pack a space known as the renal sinus. A dense layer of fat surrounds each kidney.
The blood supply to kidneys ensures that the whole volume of blood in the body passes through the kidneys every 5 minutes or so. The arrangement of blood vessels is based on supplying (and draining) each medullary pyramid and its associated cortical tissues separately, a unit described as a renal lobe. (The human kidney is multilobar, but uni-lobar kidneys occur in many species.) Each renal artery branches and forms interlobar arteries. In turn, interlobar arteries branch further and then form arcuate arteries which arch between the cortex and medulla (Fig. 13.4). Branches from arcuate arteries eventually supply each renal corpuscle with an afferent arteriole. This arrangement is essential in filtering the blood (see below) as well as supplying oxygen and nutrients.
Each afferent arteriole divides within a renal corpuscle into capillaries, known as glomerular capillaries (Fig. 13.5), and these drain from the corpuscle into an efferent arteriole (Fig. 13.6). This arrangement of capillaries draining to an arteriole is unique to the kidney and results in a relatively high blood pressure in glomerular capillaries (Fig. 13.5) which aids the filtration of blood. Efferent arterioles divide and form a second network of capillaries. Some of these capillaries are straight vessels, vasa recta, which lie between straight tubules in the kidney whereas others form an extensive meshwork surrounding convoluted tubules in the cortex (Fig. 13.6). The second set of capillaries supplies oxygen and nutrients to the cells of the kidney and is also involved in modifying the filtrate and forming urine. Veins drain capillaries (other than glomerular capillaries) in the kidney and the routes they take closely follow the arterial pattern.
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| Fig. 13.6 |
Nephrons and collecting ducts
A nephron is the functional unit of the kidney (Fig. 13.6) and there are about a million in each kidney in humans. A nephron consists of a renal corpuscle, which filters blood, and the uriniferous tubule attached to it which drains and modifies the filtrate. Eventually, the modified filtrate becomes urine and drains, via collecting ducts, into the renal pelvis.
Renal corpuscles
Each renal corpuscle consists of glomerular capillaries (a renal glomerulus) (Fig. 13.5) within a Bowman’s capsule. The capsule is shaped like a hollow, double-walled cup (Fig. 13.6) and it is lined by epithelial cells. The outer parietal layer of Bowman’s capsule is lined by squamous epithelial cells (Fig. 13.7) and it is continuous with the inner visceral layer of epithelial cells. The cells of the inner layer, known as podocytes, have numerous cytoplasmic processes which abut the capillaries of the glomerulus. The endothelial cells lining the capillaries and the podocytes (and their fused basement membranes) form the filtration barrier between the blood and the filtrate. The space between the inner and outer layers of the capsule receives the filtrate and is known as the urinary space (Fig. 13.7) even though the filtrate is not yet urine.
Each Bowman’s capsule has two poles, and it is extremely rare to see all the features of each pole in any one (histological) section of a corpuscle.
Filtration
Blood in glomerular capillaries is filtered through the endothelium lining the capillaries. It then traverses the fused basement membranes of the endothelium and of the adjacent podocytes (the visceral epithelial cells of Bowman’s capsule) (Fig. 13.9).
Filtration is affected by:
■ the pressure of blood in the afferent arteriole
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