This section describes the kidneys and their structures.

The Kidneys

The kidneys are located in the dorsal lumbar section of the midsection and are retroperitoneal.  They, along with the adrenal glands located on top of them, are covered with--and well protected by--a layer of fat.  The kidney's main roles are to regulate volume (water content and cell volume, sodium content and ECF volume) and composition (concentrations of K, phosphate, bicarbonate, pH) of the body fluids. Through renal plasma clearance (C=UV/P) the kidneys clean the body fluids of non-volatile end products such as urea, uric acid, and creatinine. Clearance of secreted and filtered solutes can approach renal plasma flow. Other solutes such as proteins, amino acids and glucose are conserved by the normal kidney and have zero clearance. The kidney produces hormones (erythropoietin, renin-angiotensin and calcitriol). It has also metabolic functions, participating in degradation of peptides such as some hormones, in fasting gluconeogenesis and in transformations of amino acids (glutamine to NH4, synthesis of arginine and glycine.)

Renal Anatomy

The kidneys are situated between the last thoracic and third lumbar vertebrae. The right kidney often sits slightly lower than the left. These organs lie between the muscles of the dorsal body wall and the peritoneal lining.  Grossly, the structure of the kidney consists of the cortex, medulla (inner and outer zones of outer medulla and papilla or inner medulla), pyramids, renal calyxes and pelvis, and ureters.  The size and weight of the kidneys is about 300-400g.  They consist of about 0.5% of body weight in humans. 

The kidneys are protected and anchored by three concentric layers of connective tissue;  a layer of collagen fibers covers the outer surface of the entire organ.  This layer, the renal capsule , is also known as the fibrous tunic of the kidney.  A layer of adipose tissue called the adipose capsule surrounds the renal capsule. This layer can be quite thick, so that adipose capsule often obscures the outline of the kidney. Collagen fibers extend outward from the renal capsule through the adipose capsule to anchor the kidney to surrounding structures. These fibers create a dense outer layer known as the renal fascia . Posteriorly the renal fascia fuses with the deep fascia surrounding the muscles of the body wall. Anteriorly the renal fascia forms a thick fibrous layer that is covered by the peritoneum.

Superficial and Sectional Anatomy of the Kidney

Each kidney has the shape of a lima bean, with a prominent indentation, or hilus , facing medially. A typical adult kidney measures approximately 10 cm (4in.) in length, 5.5 cm (2.2 in.) in width, and 3cm (1.2 in.) in thickness. The surface of the organ is covered by a dense fibrous capsule, and in sectional view the inner portion of the capsule folds inward at the hilus to line an internal cavity, the renal sinus. Renal vessels and the ureter draining the kidney pass through the hilus and branch within the renal sinus. Renal vessels and the ureter draining the kidney pass through the hilus and branch within the renal sinus. A thickened, external portion of the capsule extends across the hilus and stabilizes the position of these structures.

Seen in section, the kidney can be divided into an outer cortex and an inner medulla . The medulla contains 6 to 18 conical renal lobes , or pyramids , whose tips, or papillae , project into the renal sinus. Each pyramid has a series of fine grooves that radiate from the papillae. Renal columns extend from the cortex inward toward the renal sinus between adjacent renal pyramids. The columns have a distinctly granular texture, similar to that of the cortex.

Where the ureter enters the renal sinus it expands to form an enlarged chamber called the renal pelvis . The pelvis branches to form two major calyces and each major calyx gives rise to four or five cup-shaped minor calyces . Each minor calyces surrounds the exposed papilla of a single renal pyramid. Ducts with in the papilla connect to the wall of the calyx and discharge the urine produced in the cortex and medulla. As production continues, the urine passes through the calyces and into the ureter.

Histological Organization

The nephron is the basic unit of renal structure and function; it has a Malpigian corpuscle, with a vascular glomerulus within a matrix formed by mesangial cells and an epithelial Bowman's capsule. The capsule joins a series of tubules starting with the proximal tubule and followed by the loop of Henle, the distal tubule, and ending in the collecting ducts.  The proximal tubule has convoluted early and intermediate segments S1 and S2 in the renal cortex and a straight segment S3 which enters the outer medulla.  The loop of Henle has medullary thin descending and thin ascending limbs and a thick ascending limb with outer medullary and cortical segments.  The cortical distal diluting segments includes the early distal tubule, that makes contact with the afferent arteriole at the macula densa cells forming the juxtaglomerular apparatus. This is followed by the cortical distal convolutions and the connecting segment.  The collecting duct has cortical, outer medullary and inner medullary segments.  There are about 1 million nephrons per kidney (±250,000).  There are three types of nephrons: (1) Superficial nephrons (30% in humans) with the glomerulus in the outer cortex and the loop of Henle that bends in the outer medulla.  (2) Midcortical nephrons with the glomerulus in the mid cortex and short loops that bend in the outer medulla (10%). Other mid cortical nephrons have loops of intermediate length that bend at various points in the inner medulla (50%).  (3) Juxtamedullary nephrons have glomeruli in the inner cortex next the medulla and long loops that reach the tip of the papilla (10%). The proportion and length of the long loops of Henle increase in proportion to the urine concentrating capacity

Each nephron empties into the collecting system. A collecting tubule connected to the distal convoluted tubule carries the filtrate toward a nearby collecting duct. The collecting duct leaves the cortex and descends into the medulla, carrying fluid toward a papillary duct that drains into the renal pelvis.

The urine arriving at the renal pelvis is very different from the filtrate produced at the glomerulus. As it travels along the nephron and collecting system, the composition and concentration of the filtrate change. The nature of the changes and the properties of the urine are determined by the activities and properties of the epithelial cells exposed to the filtrate.  A filtration barrier is formed by fenestrated (375 Angstrom pore radius) (one angstrom equals 1x10-10 meters,) vascular endothelium,  the glomerular basement membrane (GBM), and visceral epithelial podocytes separated by slits with diaphragms.  The GBM is formed by collagen, laminin, and other extracellular matrix proteins such as negatively charged heparan sulfate proteoglycans. The GBM provides support and has a sieving function.  The GBM allows free passage of neutral molecules up to a radius of 18 Angstroms.  Negatively charged pores progressively restrict passage of large (> 18A radius) and almost completely sieve out neutral molecules larger than 40A or smaller negatively charged molecules (albumin). In disease, proteinuria may be due to loss of negative charge selectivity or to increasing numbers of large size pores. 
 

Function of the Nephron

A nephron consists of two portions: a renal corpuscle where plasma is filtered, and a renal tubule into which the filtered fluid (the filtrate) passes. Nephrons perform three basic functions--glomerular filtration, tubular secretion, and tubular reabsorption. There are two types of nephrons:  cortical nephrons and juxtamedullary nephrons.  The main difference in the two types of nephrons is the length to which the loop of Henle extends into the kidney.  Cortical nephrons, which are about eighty percent of the nephrons in humans, have a loop of Henle that does not extend past the cortex of the kidney.  Juxtamedullary nephrons, on the other hand, have a loop of Henle that extends past the cortex and into the medulla of the kidney.

Cortical and Juxtamedullary Nephrons

In a nephron, the loop of Henle connects the proximal and distal convoluted tubules. The first portion of the loop of Henle dips into the renal medulla, where it is called the descending limb of the loop of Henle. It then bends in a U-shape and returns to the renal cortex as the ascending limb of the loop of Henle. About 80-85% of the nephrons have short loops of Henle that penetrate only into the superficial region of the renal medulla. These nephrons usually have glomeruli in the superficial region of the renal cortex and are termed cortical nephrons. They receive their blood supply from peritubular capillaries that arise from efferent arterioles. The remaining 15-20% is juxtamedullary nephrons. They have glomeruli deep in the renal cortex close to the renal medulla and long loops of Henle that stretch through the renal medulla, almost to the renal papilla. They receive their blood supply from peritubular capillaries and vasa recta that arise from efferent arterioles. In addition, the ascending limb of the loop of Henle of juxtamedullary nephrons consists of two portions; the first part is the thin ascending limb and the second part is the thick ascending limb. These long-loop nephrons enable the kidneys to excrete very dilute or very concentrated urine. In glomerular filtration , substances in the blood that are small enough pass across the wall of the glomerular capillaries into the renal tubule.  As the fluid moves along the renal tubule, many useful materials are returned to the blood in peritubular capillaries and vasa recta; this is reabsorption.  As the fluid passes along the tubule, it also gains some additional materials (wastes and excess substances) from tubule cells and blood capillaries; this is secretion. By filtering, reabsorbing, and secreting, nephrons maintain the homeostasis of the blood. Needed materials are returned to the bloodstream and wastes are excreted in the urine.

From the capsular space, filtered fluid passes into the renal tubule, which as three main sections. In the order that fluid passes through them, the renal tubule consists of a (1) proximal tubule, (2) loop of Henle, and (3) distal convoluted tubule. Convoluted means the tubule is coiled rather than straight. Proximal denotes the tubule portion attached to the glomerular capsule and distal, the portion that is further away. The renal corpuscle and both convoluted tubules lie in the renal cortex of the kidney, whereas the loop of Henle extends into the renal medulla, makes a hairpin turn, and then returns to the renal cortex.

The Collecting System

The distal convoluted tubules of several nephrons empty into a single collecting duct. Collecting ducts then unite and converge until eventually there are only several hundred large papillary ducts at the apices of the renal pyramids, which drain into the minor calyces. The collecting ducts and papillary ducts extend from the renal cortex through the renal medulla to the renal pelvis. On average, there are about 30 papillary ducts per renal papilla. Although a kidney has about 1 million nephrons (each consisting of a renal corpuscle, proximal convoluted tubule, loop of Henle, and distal convoluted tubule), it has a much smaller number of collecting ducts and even fewer papillary ducts.

As the filtrate moves through the renal tubule, the osmolarity of the filtrate changes.  As it moves deeper into the medulla, it increases, and when it ascends the loop of Henle, it decreases only to increase again while going down the collecting duct.  It is this hyperosmotic condition in the medulla that allows passive transport to occur. 

Martini (Page 864)

Tortora (Page 859)

Tortora (Page 858)

Blood and Nerve Supply

Because the kidneys remove wastes from the blood and regulate its fluid and electrolyte content, it is not surprising that they are abundantly supplied with blood vessels. Although the kidneys make up just 1% of the total body mass, they receive 20-25% of the resting cardiac output via the right and left renal arteries. This amounts to about 1200 ml of blood per minute. Within the kidney, the renal artery divides into several segmental arteries, each of which supplies one renal segment (region). Each segmental artery gives off several branches that enter the parenchyma and pass as the interlobar arteries in the renal columns between the renal pyramids. At the bases of the renal pyramids, the interlobar arteries arch between the renal medulla and cortex; here they are known as the arcuate arteries. Divisions of the arcuate arteries produce a series of interlobular arteries, which enter the renal cortex and give off branches called afferent arterioles.

Each nephron receives one afferent arteriole, which divides into a tangled, ball-shaped capillary network called the glomerulus. The glomerular capillaries then reunite to form an efferent arteriole that drains blood out of the glomerulus. The afferent-efferent arteriole situation is unique because blood usually flows out of capillaries into venules and not into other arterioles. Since they are capillary networks, the glomeruli are part of the cardiovascular system as well as the urinary system.

The efferent arterioles divide to form a network of capillaries, called the peritubular capillaries, which surround tubular portions of the nephron in the renal cortex. Extending from some efferent arterioles are long loop-shaped capillaries called vasa recta that supply tubular portions of the nephron in the renal medulla.

The peritubular capillaries eventually reunite to form peritubular venules and then interlobular veins. The interlobar veins also receive blood from the vasa recta. Then the blood drains through the arcuate veins to the interlobar veins running between the renal pyramids, and on to the segmental veins. Blood leaves the kidney through a single renal vein that exits at the renal hilus.

The nerve supply to the kidneys is derived from the renal plexus of the sympathetic division of the autonomic nervous system. Nerves from this plexus accompany the renal arteries and their branches and are distributed to the blood vessels. Because these are vasomotor nerves, they regulate the flow of blood through the kidney by regulating the diameters of the arterioles.

Hormones of Kidney Regulation

Though the kidneys regulate the blood's composition, there are certain hormones that aid in homeostasis in a capacity relative to the kidneys.  One such hormone is antidiuretic hormone (ADH).  ADH is released from hypothalamus when osmoreceptor cells in the hypothalamus detect a rise in blood osmolarity (which is normally 300 mosm/L), normally caused by an excessive loss of water.  ADH reaches the kidneys by way of blood vessels.  Once there, the hormone acts upon the distal convoluted tubules and collecting ducts by making them more permeable to water so that more is reabsorbed.  In this way, water is conserved in the blood.  Also, ADH makes an individual thirsty, so that the lack of water problem is alleviated.

Another problem that is solved through manipulation of the kidneys is low blood pressure or low blood volume.  To aid in this there is a specialized tissue called the juxtaglomerular apparatus (JGA).  When blood pressure drops or if there is a lack of sodium in the blood, the JGA releases the enzyme renin into the bloodstream.  Renin acts on the plasma protein angiotensin, and turns it into its active form, angiotensin II.  Angiotensin II then constricts arterioles which raises blood pressure.  Raising blood pressure in the afferent arterioles increases filtration. 

Angiotensin also acts on the adrenal glands, promoting the production of aldosterone.  Aldosterone stimulates the reabsorption of sodium ions in the distal convoluted tubule.  Water then naturally follows the sodium into the blood stream because of osmosis, and raises the blood pressure.

The body also has a mechanism to lower blood pressure.  Atrial natriuretic protein (ANP) can be released by the wall of the heart's atrium.  ANP inhibits the release of renin and aldosterone.  This thereby decreases the reabsorption of sodium and lowers blood pressure and volume.

Renal Hormone Secretion

The kidneys secrete two hormones, Erythropoietin (EPO,) and Calcitriol ((1,25[OH]₂ Vitamin D₃). 

Calcitriol acts on the cells of the intestine to promote the absorption of calcium from food and on the bone to mobilize calcium from the bone to the blood.  Calcitriol diffuses into cells and, if they contain receptors for it (intestine cells do), it binds to them.  Calcitriol is 1,25[OH]2 Vitamin D3, the active form of vitamin D. It is derived from calciferol (vitamin D3) which is synthesized in skin exposed to the ultraviolet rays of the sun and precursors ("vitamin D") ingested in the diet.  Calciferol in the blood is converted into the active vitamin in two steps:  calciferol is converted in the liver into 25[OH] vitamin D3.  This is carried to the kidneys where it is converted into calcitriol. This final step is promoted by parathyroid hormone (PTH).

Basic Study Questions

  1.  What is the anatomic pathway of fluid from Glomerular Filtrate to toilet?

  2.  What are the two types of nephrons, their ratio and function?

  3.  What are the main roles of the kidney?

  4.  Where are the kidneys located?

  5.  Describe the major anatomical areas of the kidney.

Additional Study Questions