Introduction
The urinary system consists of following parts: a pair of kidneys, which remove substances from the blood, form urine, and help regulate various metabolic processes; a pair of tubular ureters, which transport urine away from the kidneys; a saclike urinary bladder, which serves as urine reservoir; and a tubular urethra, which conveys urine to the outside of the body.

Kidneys
A kidney is a reddish brown, bean-shaped organ with a smooth surface. It is about 12 cm long, 6 cm wide, and 3 cm thick in an adult, and it is enclosed in a tough fibrous capsule.

Location of the Kidneys
The kidney lie on either side of the vertebral column in a depression high on the posterior wall of the abdominal cavity.

The upper and lower borders of the kidneys are generally at the levels of the twelfth thoracic and third lumbar vertebrae, respectively. The left kidney is usually about 1.5 to 2.0 cm higher than the right one.

The kidneys are positioned retroperitoneally, which means they are behind the parietal peritoneum and against the deep muscles of the back. They are held in position by the connective tissue and masses of adipose tissue that surround them.

STRUCTURE OF A KIDNEY
The lateral surface of each kidney is convex, but its medial side is deeply concave. The resulting medial depression leads into a hollow chamber called the renal sinus. The entrance to this sinus is termed the hilum, and through it pass various blood vessels, nerves, lymphatic vessels, and the ureter.

The superior end of the ureter is expanded to form a funnel-shaped sac called the renal pelvis, which is located inside the renal sinus. The pelvis is subdivided into two or three tubes, called major calyces (sing., calyx), and they, in turn, are subdivided into several minor calyces. A series of small elevations project into the renal sinus from its wall. These projections are called renal papillae, and each is pierced by tiny openings that lead into a minor calyx.

The substance of the kidney includes two distinct regions - an inner medulla and an outer cortex. The renal medulla is composed of conical masses of tissue called renal pyramids. The renal cortex, which appears somewhat granular in a sectioned kidney, forms a shell around the medulla. Its tissue dips into the medulla between adjacent renal pyramids, forming renal columns. The granular appearance of the cortex is due to the random arrangement of tiny tubules associated with the nephrons, the functional units of the kidney.

The kidneys also help regulate the volume, composition, and pH of the body fluids. These function involve complex mechanisms that lead to the formation of urine.

Renal Blood Vessels
Blood is supplied to the kidneys by means of the renal arteries, which arise from the abdominal aorta. These arteries transport a relatively large volume of blood; in fact, when a person is at rest the renal arteries usually carry from 15 percent to 30 percent of the total cardiac output into the kidneys.

A renal artery enters a kidney through the hilum and gives off several branches, called the interlobar arteries, which pass between the renal pyramids. At the junction between the medulla and cortex, the interlobar arteries branch to form a series of incomplete arches, the arcuate arteries which, in turn, give rise to interlobular arteries. The lateral branches of the interlobular arteries, called afferent arterioles, lead to the nephrons, which are functional units of the kidneys. Venous blood is returned through a series of vessels that correspond generally to the arterial pathways. The renal vein then joins the inferior vena cava as it courses through the abdominal cavity.

Nephrons
Structure of a nephron. A kidney contains about one million nephrons, each consisting of a renal corpuscle and a renal tubule.

A renal corpuscle is composed of a tangle cluster of blood capillaries, called glomerulus, which is surrounded by a thin-walled, saclike structure, called a glomerular capsule. The glomerular capsule is an expansion at the closed end of a renal tubule. The renal tubule leads away from the glomerular capsule and becomes highly coiled; this coiled portion of the tubule is named the proximal convoluted tubule. The proximal convoluted tubule dips toward the renal pelvis to become the descending limb of the Loop of Henle.

The tubule then curves back toward its renal corpuscle and forms the ascending limb of the blood of Henle.

The ascending limb returns to the region of the renal corpuscle, where it becomes highly coiled again, and is called the distal convoluted tubule.

Several distal convoluted tubules merge in the renal cortex to form a collecting duct, which, in turn, passes into the renal medulla, becoming larger and larger as it is joined by other collecting ducts. The resulting tube empties into a minor calyx through an opening in a renal papilla.

Blood Supply of a Nephron
The cluster of capillaries that forms a glomerulus arises from an afferent arteriole. After passing through the capillary of the glomerulus, the blood enters an efferent arteriole (rather than a venule), whose diameter is somewhat less than that of the afferent vessel.

Because of its small diameter, the efferent arteriole produces some resistance to blood flow. This causes blood to back up into the glomerulus, producing a relatively high pressure in the glomerular capillary.

The efferent arteriole branches into a complex, freely interconnecting network of capillaries, which surrounds the various portions of the renal tubule. This network is called the peritubular capillary system, and the blood it contains is under relatively low pressure.

After flowing through the capillary network, the blood is returned to the renal cortex, where it joins the blood from other branches of the peritubular capillary system and enters the venous system of the kidney.

Close by, in the walls of the arterioles near their attachments to the glomerulus, are some enlarged smooth muscle cells. They are called juxtaglomerular cells, and together with the cells of the macula densa, they constitute the juxtaglomerular apparatus (complex).