Control of the Internal Environment for AP Biology
Practice problems for these concepts can be found at: Human Physiology Review Questions for AP Biology
The next stop on our tour is the kidney (see Figure 15.3 for an overview of the human excretory system). The kidneys lie on the posterior wall of the abdomen. The renal artery and vein bring blood to and from the kidney, respectively. Kidneys are divided into two major regions: an outer region called the cortex, and an inner region called the medulla. These two regions are full of nephrons, the functional units of the kidney. The medulla is divided into structures called renal pyramids, which dump urine into the major and minor calyces. From here, the urine is sent toward the bladder via the ureter. When contracted to urinate, the bladder sends the urine through the urethra to the outside world.
We've pulled the bus right up to one of over a million nephrons in each kidney. The nephron is composed of a renal corpuscle, proximal tubule, loop of Henle, distal tubule, and collecting duct system. If you look closely, you will see that the renal corpuscle is made up of glomerular capillaries surrounded by Bowman's capsule.
Osmoregulation and Excretion
The blood that enters via the renal artery is sent to the various nephrons by the branching of the renal artery into smaller and smaller vessels that culminate in the capillaries of the glomerulus. The blood pressure is the force that leads to the movement of solutes such as water, urea, and salts into the lumen of Bowman's capsule from the glomerular capillaries. From here, the fluids pass down the proximal tubule, through the loop of Henle, and into the distal tubule, which dumps into the collecting duct. The various collecting ducts of the kidney collectively merge into the renal pelvis, which leads via the ureter to the bladder.
As I mentioned moments ago, fluid moves from the capillaries into the lumen of the nephron as a result of the force of blood pressure. During this process of filtration, the capillaries are able to let small particles through the pores of their endothelial linings, but large molecules such as proteins, platelets, and blood cells tend to remain in the vessel. As the filtrate progresses along the tubule, plasma solutes such as urea are added by the process of secretion, a selective process that helps to create a solute gradient. It is important to realize that much of what is dumped into the tubule originally is reabsorbed—nearly all the sugars, water, and organic nutrients. The combination of reabsorption and secretion help the nephron to control what gets released in the urine. The following chart outlines in detail what happens in the various parts of the nephron:
Proximal tubule Reabsorbs 75 percent of NaCl and water of filtrate. Nutrients such as glucose and amino acids are reabsorbed unless their concentration is higher than the absorptive capacity. Glucose in urine is an indicator of diabetes, for this reason. Descending loop of Henle Freely permeable to H2O but not NaCl. Assists in control of water and salt concentrations. Ascending loop of Henle Freely permeable to NaCl but not water. Assists in control of salt concentration. Distal tubule Regulates concentration of K+ and NaCl. Helps control pH by reabsorbing HCO3– and secreting H+. Collecting duct Determines how much salt is actually lost in urine. The osmotic gradient created in the earlier regions of the nephron allows the kidney control in the final concentration of the urine.
The body controls the concentration of the urine according to the needs of the system. When dehydrated, the body can excrete a small volume of hypertonic concentrated urine (little water in the urine; it is dark yellow). But in times of excessive fluid, the body will excrete a large volume of hypotonic dilute urine to conserve the necessary salts (lots of water in the urine; it is clear). This is controlled by hormones and is discussed in more detail in a later section, but briefly: ADH (antidiuretic hormone) is released by the pituitary gland; it increases permeability of the collecting duct to water, leading to more concentrated urine. Aldosterone, released from the adrenal gland, acts on the distal tubules to cause the reabsorption of more Na+ and water to increase blood volume and pressure.
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