Diagnostic Activity: the Kidney

Diagnostic Activity: The Kidney

Name: ______

In this class you will be expected to be able to read and appropriately annotate complex information whether it be text or online research. The following selection contains many terms with which you probably are unfamiliar. Annotate the information and try your best to get a basic grasp on the concept – kidney structure and function. You will be asked to complete an individual task tomorrow (using the reading) that will assess your understanding. The results will provide information we can use to determine your initial skill level.

Kidney Structure

The kidney has abean-shaped structure having aconvexand aconcaveborder. A recessed area on the concave border is therenal hilum, where therenal arteryenters the kidney and therenal veinandureterleave. The kidney is surrounded by tough fibrous tissue, therenal capsule.

The substance, orparenchyma, of the kidney is divided into two major structures: the outerrenal cortexand the innerrenal medulla. Grossly, these structures take the shape of 8 to 18 cone-shapedrenal lobes, each containing renal cortex surrounding a portion of medulla called arenal pyramid.Between the renal pyramids are projections of cortex calledrenal columns.

The tip, orpapilla, of each pyramid empties urine into aminor calyx; minor calyces empty intomajor calyces, and major calyces empty into therenal pelvis which is connected to the ureter.

Kidney Function

Thekidneysarebean-shapedorgansthat serve several essential regulatory roles invertebrates. Thenephronis the basic structural and functional unit of thekidney. In humans, a normal kidney contains 800,000 to 1.5 million nephrons. Each nephron spans the cortex (where the initial filtering portion is located) and medulla.The nephron’s chief function is to regulate the concentrationofwaterand soluble substances likesodium saltsby filtering theblood, reabsorbing what is needed and excreting the rest asurine. Nephrons also eliminate wastes from the body, regulateblood volumeandblood pressure, and regulate bloodpH.

Structure of the Nephron

Blood enters the nephron from a ball of intertwined capillaries called the glomerulus. The glomerulus is enclosed in a cup-shaped structure called the Bowman’s capsule. The Bowman’s capsule connects to a long tube called the renal tubule. The tube has 4 segments: the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule and the collecting duct which opens to a minor calyx.

Function of the Nephron

Properties of the cells that line the nephron change dramatically along its length; consequently, each segment of the nephron has highly specialized functions.

Blood enters the kidney from the afferent arteriole which ends in a capillary tuft called a glomerulus. The glomerular blood pressure provides the driving force for water and solutes to be filtered out of the blood and into the space made byBowman's capsule. The pressure is created because the diameter of the efferent arterioles is smaller than that of afferent arterioles. The glomerular filtration membrane allows only elements that are small enough to pass through. Normal filtrate consists of water, glucose, amino acids, urea, creatinine, and solutes such as sodium chloride, potassium ions and bicarbonate ions. Toxins and drugs may also be present. Proteins or red blood cells are not present in the filtrate because they are too large to pass through the glomerular filtration membrane.

The remainder of the blood (only approximately 1/5 of all plasma passing through the kidney is filtered through the glomerular wall into the Bowman's capsule) passes into the efferent arteriole. Blood from the efferent arteriole, containing everything that was not filtered out in the glomerulus, moves into theperitubular capillaries whicheventually rejoin the main bloodstream via the renal vein.

The glomerular filtrate continues to the renal tubule. As the filtrate flows through the renal tubule, its composition is modified by the exchange of materials among the renal tubule, the capillaries, and the extracellular fluid. The movement of substances from the nephron back into the blood is known as reabsorption while the movement of substances from the blood into the nephron is known as secretion. The long and winding course of both the renal tubule and the surrounding capillaries provides a large surface area for the efficient exchange of materials.

All nutrients (glucose and amino acids) and 70-80% of the ions (potassium, sodium chloride, bicarbonate) and water are reabsorbed from the filtrate in the proximal convoluted tubule. Hydrogen ions, uric acid and drugs are secreted from the peritubular capillaries directly into the proximal convoluted tubule (they are not filtered out by the glomerulus). The filtrate then passes into the Loop of Henle.

Reabsorption in this segment is minimum. However, the loop of Henle plays a significant role incontrolling the concentration of urine by adjusting the concentration of salt in the interstitium, the tissue surrounding the loop. The descending limb of the loop is permeable to water but almost impermeable to other substances. Water leaves and thereby concentrates the filtrate. The ascending limb is impermeable to water but allows the transport of other substances. Cells in the ascending wall of the loop transport chloride ions from the filtrate to the fluid between the loops and the collecting duct. Positively charged sodium ions follow the chloride ions into the fluid. This process ensures that the sodium chloride concentration of the fluid between the loops and the collecting duct remains high and thus promotes the reabsorption of water from the last part of the renal tubule - the collecting duct.

Filtrate in the loop of Henle has a high concentration of metabolic waste products such as urea, uric acid and creatinine. By the time the filtrate reaches the loop of Henle, all the nutrients and substances that the body needs would have already been reabsorbed. The filtrate then passes into the distal convoluted tubule.

The main function of the distal convoluted tubule is to regulate sodium, potassium and pH levels. Sodium chloride and water are reabsorbed from the distal convoluted tubule into the blood stream as needed. Bicarbonate ions can also be absorbed. Hydrogen and potassium ions are secreted from the blood into the distal convoluted tubule. The pH of the blood is adjusted by the amount of hydrogen ions that are secreted from the blood into the filtrate and the amount of bicarbonate ions that are reabsorbed from the filtrate into the blood. Hydrogen ions increase acidity of the blood and bicarbonate ions decrease acidity of the blood. The filtrate then travels to the final portion of the renal tubule – the collecting duct.

In the collecting duct, sodium chloride and water are reabsorbed back into the blood stream. Water is reabsorbed because of the high sodium concentration around the tube (which was created by the loop of Henle). The collecting duct is also permeable to urea, allowing some of it to leave the collecting duct. This also causes water to move out of the collecting duct and be reabsorbed by the blood.

The plasma sodium concentration directly impacts blood pressure. When sodium chloride reabsorption from the collecting duct is increased, water reabsorption increases as well. This expands the extracellular fluid compartment which raises blood pressure.

Water, sodium chloride, potassium, bicarbonate, hydrogen ions, creatinine and urea are the components of urine which leaves the collecting ductthrough therenal papillae, emptying into therenal calyces, therenal pelvis, and finally into theurinary bladdervia theureter.