.the kidney

>>>> Anatomic Of Kidney :
In anatomy, the kidneys are bean-shaped excretory organs in vertebrates. Part of the urinary system, the kidneys filter wastes (such as urea) from the blood and excrete them, along with water, as urine.
The medical field that studies the kidneys and diseases of the kidney is called nephrology (nephro- meaning kidney is from the Ancient Greek word nephros; the adjective renal meaning related to the kidney is from Latin).
In humans, the kidneys are located in the posterior part of the abdomen. There is one on each side of the spine; the right kidney sits just below the liver, the left below the diaphragm and adjacent to the spleen. Above each kidney is an adrenal gland (also called the suprarenal gland). The asymmetry within the abdominal cavity caused by the liver results in the right kidney being slightly lower than the left one.
The kidneys are retroperitoneal, which means they lie behind the peritoneum, the lining of the abdominal cavity. They are approximately at the vertebral level T12 to L3. The upper parts of the kidneys are partially protected by the eleventh and twelfth ribs, and each whole kidney is surrounded by two layers of fat (the perirenal and pararenal fat) which help to cushion it. Congenital absence of one or both kidneys, known as unilateral or bilateral renal agenesis occurs. In very rare cases, it is possible to have developed three or even four kidneys.
In a normal human adult, each kidney is about 10 cm long, 5.5 cm in width and about 3 cm thick, weighing 150 grams. Together, kidneys weigh about 0.5% of a person's total body weight. The kidneys are "bean-shaped" organs, and have a concave side facing inwards (medially). On this medial aspect of each kidney is an opening, called the hilum, which admits the renal artery, the renal vein, nerves, and the ureter.
The outer portion of the kidney is called the renal cortex, which sits directly beneath the kidney's loose connective tissue/fibrous capsule. Deep to the cortex lies the renal medulla, which is divided into 10-20 renal pyramids in humans. Each pyramid together with the associated overlying cortex forms a renal lobe. The tip of each pyramid (called a papilla) empties into a calyx, and the calices empty into the renal pelvis. The pelvis transmits urine to the urinary bladder via the ureter.
>>>> Blood supply :
Each kidney receives its blood supply from the renal artery, two of which branch from the abdominal aorta. Upon entering the hilum of the kidney, the renal artery divides into smaller interlobar arteries situated between the renal papillae. At the outer medulla, the interlobar arteries branch into arcuate arteries, which course along the border between the renal medulla and cortex, giving off still smaller branches, the cortical radial arteries (sometimes called interlobular arteries). Branching off these cortical arteries are the afferent arterioles supplying the glomerular capillaries, which drain into efferent arterioles. Efferent arterioles divide into peritubular capillaries that provide an extensive blood supply to the cortex. Blood from these capillaries collects in renal venules and leaves the kidney via the renal vein. Efferent arterioles of glomeruli closest to the medulla (those that belong to juxtamedullary nephrons) send branches into the medulla, forming the vasa recta. Blood supply is intimately linked to blood pressure.


>>>> Nephron :









The basic functional unit of the kidney is the nephron, of which there are more than a million within the cortex and medulla of each normal adult human kidney. Nephrons regulate water and soluble matter (especially electrolytes) in the body by first filtering the blood under pressure, and then reabsorbing some necessary fluid and molecules back into the blood while secreting other, unneeded molecules. Reabsorption and secretion are accomplished with both cotransport and countertransport mechanisms established in the nephrons and associated collecting ducts.

>>>> Structure and Function of the Kidneys: Overview
The kidneys have three basic mechanisms for separating the various components of the blood: filtration, reabsorption, and secretion. These three processes occur in the nephron (See Figure), which is the most basic functional unit of the kidney. Each kidney contains approximately one million of these functional units. The nephron contains a cluster of blood vessels known as the glomerulus, surrounded by the hollow Bowman's capsule. The glomerulus and Bowman's capsule together are known as the renal corpuscle. Bowman's capsule leads into a membrane-enclosed, U-shaped tubule that empties into a collecting duct. The collecting ducts from the various nephrons merge together, and ultimately empty into the bladder.

A. Renal Corpuscle
Blood first enters the kidney through the renal artery (See Figure), which branches into a network of tiny blood vessels called arterioles. These arterioles then carry the blood into the tiny blood vessels of the glomerulus. It is here, in the renal corpuscle, where filtration occurs. The glomerulus filters proteins and cells, which are too large to pass through the membrane channels of this specialized component, from the blood. These large particles remain in the blood vessels of the glomerulus, which join with other blood vessels so that the proteins remain circulating in the blood throughout the body. The small particles (e.g., ions, sugars, and ammonia) pass through the membranes of the glomerulus into Bowman's capsule. These smaller components then enter the membrane-enclosed tubule in essentially the same concentrations as they have in the blood. Hence, the fluid entering the tubule is identical to the blood, except that it contains no proteins or cells.
B. Tubule
The tubule functions as a dialysis unit, in which the fluid inside the tubule is the internal solution and the blood (in capillaries surrounding the tubule) acts as the external solution. Particles may pass through the membrane and return to the blood stream in the process known as reabsorption, which is analogous to the movement of particles from the internal to the external solution in the dialysis experiment you performed in lab. The reabsorption of many blood components is regulated physiologically, as discussed below. Alternatively, particles may pass through the membrane from the blood into this tubule in the process known as secretion, which is analogous to the movement of particles from the external solution into the dialysis bag in the experiment you performed in lab. The most important particles that are secreted from the blood back into the tubules are H+ and K+ ions, as well as organic ions from foreign chemicals or the natural by-products of the body's metabolism.
C. Collecting Duct
The blood components that remain in the nephron when the fluid reaches the collecting duct are excreted from the body.The collecting duct from one nephron meets up with many others to feed into the ureter. The ureters (one from each kidney) enter the bladder, which leads to the urethra, where the liquid waste is excreted from the body. Hence, the material that is filtered and secreted from the blood into the tubule, less the amount that is reabsorbed into the blood, is ultimately excreted from the body.
The localization of each of these processes within specific components of the nephron is summarized in Table 1, above.


>>>> Membrane Channels
From the overview of kidney function above, it is clear that blood components (e.g., water, ions, sugars) must be able to pass between the nephron tubules and the blood-filled capillaries surrounding them. But recall from the Introduction to this experiment (in the lab manual) that phospholipid-bilayer membranes are not permeable to polar molecules, because the interior lipid region of the membrane is nonpolar. Thus, the polar components of blood could not cross the membranes surrounding the tubules (See Figure), unless these membranes contained special channels to allow the passage of polar species. The channels required to allow the passage of polar blood components are formed by proteins embedded in the phospholipid-bilayer membrane (See Figure). Proteins that form channels in the membrane typically have membrane-spanning cylindrical shapes: there is a hydrophobic surface that can interact with the “tail” region of the phospholipid-bilayer membrane and a hollow internal core that forms the pore. These proteins form a "tunnel" from the aqueous phase on one side of the membrane to the aqueous phase on the other side of the membrane. The size of the tunnel determines the size of the particles that will be able to pass through the channel. If the internal core of the protein channel is lined with hydrophilic amino-acid residues, then the channel allows passage of polar or charged particles between the two aqueous sides of the membrane. (See Figure) shows a representative ion channel, with hydrophilic residues lining the internal core and hydrophobic residues lining the regions of the protein that contact the lipid tails in the interior of the membrane. These channels may be left open continuously, or they may be opened and closed by elaborate cellular gating mechanisms, as we will see below for three representative cases in the kidneys. In either case, passage of particles through the membrane is dictated by the size, shape, and polarity of the channel.