Kidneys

The kidneys are the centre of the urinary system, and they are responsible for the formation of urine, regulation of body water, and acid-base and electrolyte balance. They have to keep blood volume and the other body fluids at the correct level – otherwise, each time we have a drink, our blood would be diluted and its volume would increase. Extra fluid might be diverted to the tissues, but these would rapidly become swollen with the extra fluid in the condition termed oedema (or dropsy, as it was once called). The kidneys also produce two important hormones which have an effect on blood pressure and mediate in the production of new red blood cells. The way they function is, of course, determined by how they are constructed and by the method of control that the body exerts on them.

Structure

The two kidneys are located high up on the back wall of the abdomen, one on each side of the spine. Each measures 10 cm long by 5 cm wide and is 3 cm thick. The normal adult kidney weighs about 100 gm. The kidney is surrounded by a very tough layer of fibrous tissue called the capsule. This forms a layer between the kidney substance and the thick mass of fat which surrounds it, called the perirenal fat, the function of which is to protect the kidney against injury. Located on top of each kidney are the adrenal (or suprarenal) glands, endocrine organs that produce powerful hormones such as adrenaline and nor-adrenaline. The kidney itself has two fairly distinct layers, each with its own contents. The outer layer is called the cortex, and contains the majority of the specialized units of the kidney, called the nephrons, packed tightly together. The area located near the centre of the kidney is called the medulla, and is arranged in pyramid-like units called the renal pyramids. The apex of each of these pyramids is called a renal papilla and it is here that collecting ducts discharge urine into funnel-shaped structures called calyces. These structures have a waterproof lining found only in the urinary tract. One or more renal papillae may project into a single calyx. All the calyces join together towards the middle of the kidney to form the large funnel-shaped structure called the renal pelvis. This emerges from the middle of the kidney where it joins with the ureter, which carries urine to the bladder. Just behind the renal pelvis, and in contact with it, are the renal artery and vein. The renal artery, a branch of the descending aorta, carries blood into the kidney to be filtered; the renal vein carries processed blood away from the kidney to the inferior vena cava and thus to the heart. Together these three structures are known as the hilum of the kidney.

Nephrons

The kidney’s job of filtering the blood and producing urine is actually performed by tiny structures called nephrons. There are about one million nephrons in each kidney. Each nephron is divided into two parts: the renal corpuscle and the renal tubule (tubule means small tube). The renal corpuscle is itself divided into two structures, called the glomerulus and its surrounding Bowman’s capsule, and is responsible for the filtration of the blood plasma, the watery part of the blood, in the initial stage of urine production. Blood reaches the glomerulus in a tiny artery or arteriole, called the afferent arteriole, branching off the kidney’s own artery, the renal artery. This arteriole changes into a tightly coiled network of small capillaries in the glomerulus. This ball-shaped coil of single cell layered capillaries bulges into a structure rather like a microscopic egg cup – the Bowman’s capsule. The Bowman’s capsule forms the start of the renal tubule.

Inside the capsule, the glomerulus is enveloped by a layer of unusual cells called podocytes. Each cell has long foot-like projections which wrap around the capillaries, and has slit pores in between each projection. These pores are important in the filtration mechanism. A membrane called the basement membrane separates the glomerulus from the capillaries. The elements of blood that are not filtered in the glomerulus leave in another arteriole called the efferent arteriole. This arteriole is smaller than the afferent arteriole and this arrangement causes an increase in the blood pressure within the glomerulus.

Renal tubule

From the Bowman’s capsule, the renal tubule – a tube 55 mm long, lined by a single layer of cells – extends to the collecting ducts. The tubule has three separate sections. First, immediately after leaving the Bowman’s capsule, the tubule is thrown into a mass of twists and turns known as the proximal convoluted tubule. Second, after emerging from these meanders, it continues to form a long hairpin loop, called the loop of Henle. At the end of the loop of Henle the tubule is again thrown into a mass of irregular coils; this second coiled section is called the distal convoluted tubule. Immediately emerging from the distal tubule is a wider tubule called the collecting tubule. Many thousands of these are arranged in parallel rows throughout the kidney. In turn these collecting tubules drain into wider tubes, called the collecting ducts, which run at right-angles to the collecting tubules. Many tubules drain into a single collecting duct. Near the apex of the pyramids, the collecting ducts drain into a wider bore tube known as the duct of Bellini. Many of these ducts, grouped together, open at the apex of the papilla and drain urine into the calyceal system.