In this section, you will explore the following questions:
- How does the structure of the kidneys relate to its function as the main osmoregulatory organs in mammalian systems?
- How is the nephron the functional unit of the kidney and how does it actively filter blood and generate urine?
- What are the roles of glomerular filtration, tubular reabsorption, and tubular secretion in the formation of urine?
Connection for AP® Courses
Much of the information in this section is outside the scope for AP®. Although the curriculum does not require you to have detailed knowledge about the structure and physiology of the kidneys, learning how we filter blood to eliminate wastes—while also retaining vital water and other substances—provides an opportunity to apply concepts we’ve explored previously. Lack of kidney function can have detrimental effects on our health.
In coordination with the circulatory and endocrine systems, the human excretory system serves several functions: excretion of metabolic wastes, maintenance of water-salt balance (osmoregulation), maintenance of pH balance, and production of hormones. The organs of the system are the kidneys, ureters, urinary bladder, and urethra. Specialized cells of the kidney called nephrons (not to be confused with neurons of the nervous system) are closely associated with capillaries. Urine formation by nephrons involves three steps: filtration, in which water, nutrients, and wastes pass from the blood into the nephron; reabsorption, in which nutrients such as glucose and most water are reabsorbed back into the blood; and secretion, in which additional wastes and H+ are added to urine to maintain homeostatic pH. The processes of diffusion, osmosis, and active transport ensure that the urine excreted from the body is hypertonic, thus preventing dehydration. For example, water is reabsorbed throughout nephron tubules back into capillaries via osmosis, and in another part of the tubule (ascending loop of Henle), Na+ and Cl– are actively transported into the interstitial fluid.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 4 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
|Big Idea 4||Biological systems interact, and these systems and their interactions possess complex properties.|
|Enduring Understanding 4.A||Interactions within biological systems lead to complex properties.|
|Essential Knowledge||4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts.|
|Science Practice||6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.|
|Learning Objective||4.9 The student is able to predict the effects of a change in the component(s) of a biological system on the functionality of an organism(s).|
|Essential Knowledge||4.A.4 Organisms exhibit complex properties due to interactions between their constituent parts.|
|Science Practice||1.3 The student can refine representations and models of natural or man-made phenomena and systems in the domain.|
|Learning Objective||4.10 The student is able to refine representations and models to illustrate biocomplexity due to interactions of the constituent parts.|
|Enduring Understanding 4.B||Competition and cooperation are important aspects of biological systems.|
|Essential Knowledge||4.B.2 Cooperative interactions within organisms promote efficiency in the use of energy and matter.|
|Science Practice||1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.|
|Learning Objective||4.18 The student is able to use representations and models to analyze how cooperation interactions within organisms promote efficiency in the use of energy and matter.|
Although the kidneys are the major osmoregulatory organ, the skin and lungs also play a role in the process. Water and electrolytes are lost through sweat glands in the skin, which helps moisturize and cool the skin surface, while the lungs expel a small amount of water in the form of mucous secretions and via evaporation of water vapor.
Kidneys: The Main Osmoregulatory Organ
The kidneys, illustrated in Figure 32.5, are a pair of bean-shaped structures that are located just below and posterior to the liver in the peritoneal cavity. The adrenal glands sit on top of each kidney and are also called the suprarenal glands. Kidneys filter blood and purify it. All the blood in the human body is filtered many times a day by the kidneys; these organs use up almost 25 percent of the oxygen absorbed through the lungs to perform this function. Oxygen allows the kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. The filtrate coming out of the kidneys is called urine.
Externally, the kidneys are surrounded by three layers, illustrated in Figure 32.6. The outermost layer is a tough connective tissue layer called the renal fascia. The second layer is called the perirenal fat capsule, which helps anchor the kidneys in place. The third and innermost layer is the renal capsule. Internally, the kidney has three regions—an outer cortex, a medulla in the middle, and the renal pelvis in the region called the hilum of the kidney. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters. The renal cortex is granular due to the presence of nephrons—the functional unit of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight renal pyramids in each kidney. The renal pyramids along with the adjoining cortical region are called the lobes of the kidney. The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
Because the kidney filters blood, its network of blood vessels is an important component of its structure and function. The arteries, veins, and nerves that supply the kidney enter and exit at the renal hilum. Renal blood supply starts with the branching of the aorta into the renal arteries (which are each named based on the region of the kidney they pass through) and ends with the exiting of the renal veins to join the inferior vena cava. The renal arteries split into several segmental arteries upon entering the kidneys. Each segmental artery splits further into several interlobar arteries and enters the renal columns, which supply the renal lobes. The interlobar arteries split at the junction of the renal cortex and medulla to form the arcuate arteries. The arcuate “bow shaped” arteries form arcs along the base of the medullary pyramids. Cortical radiate arteries, as the name suggests, radiate out from the arcuate arteries. The cortical radiate arteries branch into numerous afferent arterioles, and then enter the capillaries supplying the nephrons. Veins trace the path of the arteries and have similar names, except there are no segmental veins.
As mentioned previously, the functional unit of the kidney is the nephron, illustrated in Figure 32.7. Each kidney is made up of over one million nephrons that dot the renal cortex, giving it a granular appearance when sectioned sagittally. There are two types of nephrons—cortical nephrons (85 percent), which are deep in the renal cortex, and juxtamedullary nephrons (15 percent), which lie in the renal cortex close to the renal medulla. A nephron consists of three parts—a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries.
The renal corpuscle, located in the renal cortex, is made up of a network of capillaries known as the glomerulus and the capsule, a cup-shaped chamber that surrounds it, called the glomerular or Bowman's capsule.
The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function. The first part is called the proximal convoluted tubule (PCT) due to its proximity to the glomerulus; it stays in the renal cortex. The second part is called the loop of Henle, or nephritic loop, because it forms a loop (with descending and ascending limbs) that goes through the renal medulla. The third part of the renal tubule is called the distal convoluted tubule (DCT) and this part is also restricted to the renal cortex. The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla.
Capillary Network within the Nephron
The capillary network that originates from the renal arteries supplies the nephron with blood that needs to be filtered. The branch that enters the glomerulus is called the afferent arteriole. The branch that exits the glomerulus is called the efferent arteriole. Within the glomerulus, the network of capillaries is called the glomerular capillary bed. Once the efferent arteriole exits the glomerulus, it forms the peritubular capillary network, which surrounds and interacts with parts of the renal tubule. In cortical nephrons, the peritubular capillary network surrounds the PCT and DCT. In juxtamedullary nephrons, the peritubular capillary network forms a network around the loop of Henle and is called the vasa recta.
Go to this website to see another coronal section of the kidney and to explore an animation of the workings of nephrons.
Kidney Function and Physiology
Kidneys filter blood in a three-step process. First, the nephrons filter blood that runs through the capillary network in the glomerulus. Almost all solutes, except for proteins, are filtered out into the glomerulus by a process called glomerular filtration. Second, the filtrate is collected in the renal tubules. Most of the solutes get reabsorbed in the PCT by a process called tubular reabsorption. In the loop of Henle, the filtrate continues to exchange solutes and water with the renal medulla and the peritubular capillary network. Water is also reabsorbed during this step. Then, additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is, in essence, the opposite process to tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. From here, the papillae deliver the filtrate, now called urine, into the minor calyces that eventually connect to the ureters through the renal pelvis. This entire process is illustrated in Figure 32.8.
Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.
To learn more about the vascular system of kidneys, click through this review and the steps of blood flow.
Tubular Reabsorption and Secretion
Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Sodium (Na+) is the most abundant ion and most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na+ is actively transported out of the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries. However, every solute has a transport maximum and the excess is not reabsorbed.
In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration and tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, as illustrated in Figure 32.9, it acts as a countercurrent multiplier. The vasa recta around it acts as the countercurrent exchanger.
By the time the filtrate reaches the DCT, most of the urine and solutes have been reabsorbed. If the body requires additional water, all of it can be reabsorbed at this point. Further reabsorption is controlled by hormones, which will be discussed in a later section. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct. Kidneys also maintain an acid-base balance by secreting excess H+ ions.
Although parts of the renal tubules are named proximal and distal, in a cross-section of the kidney, the tubules are placed close together and in contact with each other and the glomerulus. This allows for exchange of chemical messengers between the different cell types. For example, the DCT ascending limb of the loop of Henle has masses of cells called macula densa, which are in contact with cells of the afferent arterioles called juxtaglomerular cells. Together, the macula densa and juxtaglomerular cells form the juxtaglomerular complex (JGC). The JGC is an endocrine structure that secretes the enzyme renin and the hormone erythropoietin. When hormones trigger the macula densa cells in the DCT due to variations in blood volume, blood pressure, or electrolyte balance, these cells can immediately communicate the problem to the capillaries in the afferent and efferent arterioles, which can constrict or relax to change the glomerular filtration rate of the kidneys.
A nephrologist studies and deals with diseases of the kidneys—both those that cause kidney failure (such as diabetes) and the conditions that are produced by kidney disease (such as hypertension). Blood pressure, blood volume, and changes in electrolyte balance come under the purview of a nephrologist.
Nephrologists usually work with other physicians who refer patients to them or consult with them about specific diagnoses and treatment plans. Patients are usually referred to a nephrologist for symptoms such as blood or protein in the urine, very high blood pressure, kidney stones, or renal failure.
Nephrology is a subspecialty of internal medicine. To become a nephrologist, medical school is followed by additional training to become certified in internal medicine. An additional two or more years is spent specifically studying kidney disorders and their accompanying effects on the body.
What special adaptations do organs of the excretory system have for the excretion of wastes? Loop diuretics are drugs sometimes used to treat hypertension (high blood pressure).These drugs inhibit the reabsorption of Na+ and Cl– ions by the ascending limb of the loop of Henle in the nephron. A side effect is that they increase urination. Why do you think this is the case?
Question 1 is an application of AP® Learning Objective 4.18 and Science Practice 1.4 because students are describing how the organs of the excretory system work together to provide for osmoregulation and the elimination of metabolic wastes. Question 2 is an application of AP® Learning Objective 4.9 and Science Practice 6.4 because students are making a prediction about how a change in the physiology of the nephron can affect the overall physiology of the body.