The Kidneys

From MyMCAT

Introduction

While the Liver controls the majority of what enters the blood, it is the Kidneys that filter the blood and regulate the concentration of salt and water.

Structure

The kidneys are bean-shaped organs, located in the posterior part of the abdomen. The outer portion of the kidney is called the renal cortex while the inside is called the renal medulla. The medulla is organized in to renal pyramids with the tip of each pyramid connecting to the pelvis, an opening at the center of the kidney which drains urine into the ureters and on to the bladder.

The basic functional unit of the kidney is a nephron, of which there are more than a million laying between the cortex and the medulla. Each nephron acts individually to filter a small portion of blood, reabsorbing vital nutrients and secreting waste into collecting ducts.

The Nephron

Each nephron is composed of an initial filtering component follwed by a tubule specialized for reabsorption and secretion.

Blood enters the glomerulus, a tightly packed capillary network, where the high pressure of the incoming blood forces water and solutes out of the blood and into Bowman's capsule. In healthy nephrons, red blood cells and proteins remain in the plasma as they are too large to be forced into the Bowman's capsule. The solute that is forced out, once collected in the Bowman's capsule continues down the convoluted tubles where reabsoption and secretion take place.

The Proximal Convoluted Tubule

Initially the solute that travels through the convoluted tubules is rich in water, salt, glucose, amino acids, and other organic solutes. Active transport is used immediately to reabsorb the vital organic solutes, specifically glucose and the amino acids as wasting any would be costly.

The Loop of Henle

Extending from the proximal tubule, this tubule extends downwards into the medulla, through a descending limb, and back up to the cortex, through an ascending limb. The descending limb is permeable only to water, and as the solute travels down this tubule towards the hypertonic medulla, water flows freely out by osmosis. At the deepest end, the tubule changes direction and solute begins to pass back up towards the cortex. The ascending tubule is no longer permeable to water and instead actively pumps out Na+ and other ions. This active transport, forces the solute to become hypotonic, but allows the surrounding medulla to maintain its high ionic concentration. (It is this process of removing water, followed by pumping of ions, which produces the concentration gradient of the medulla and makes the absorption and secretion processes possible.)

The Distal Convoluted Tubule

Unlike the proximal convoluted tubule, the distal convoluted tubule is packed with mitochondria to generate the ATP needed for active transport. The distal convoluted tubule is the last place for secretion and reabsorption before the remaining solute, which is now rich in waste material like H+ and urea, is expelled into the collecting duct.

The Collecting Duct

While for the most part, the collecting ducts are impermeable to water, the hormone ADH can allow water to be reabsorbed causing urine to become more concentrated and allowing the kidneys to retain more water. The collecting ducts of the millions of nephrons in the kidneys combined at the renal pelvis and exit the kidneys through the ureters.

Excretion

Once urine has exited the kidneys through the two ureters, it is stored in the bladder and released out of the urethra. As with the anal sphincters, the outer urethral sphincter is under voluntary control, whereas the inner urethral sphincter is under involuntary control.

Blood Flow

Blood enters the kidneys through the renal artery, where it then branches to the millions of nephrons. Blood enters the glomerulus of each nephron through the afferent arteriole and out through the efferent arteriole. The efferent arteriole then branches into the peritubular capillaries that provide an extensive blood supply to the cortex and surround the tubule of the nephron, making exchange possible. The capillaries then combine and leave the kidneys through the renal vein.