16 Chapter 16: Urinary System
The urinary system has roles you may be well aware of: cleansing the blood and ridding the body of wastes probably come to mind. However, there are additional, equally important functions played by the system. Take for example, regulation of pH, a function shared with the lungs and the buffers in the blood. Additionally, the regulation of blood pressure is a role shared with the heart and blood vessels. What about regulating the concentration of solutes in the blood? Did you know that the kidney is important in determining the concentration of red blood cells? Eighty-five percent of the erythropoietin (EPO) produced to stimulate red blood cell production is produced in the kidneys. The kidneys also perform the final synthesis step of vitamin D production.
If the kidneys fail, these functions are compromised or lost altogether, with devastating effects on homeostasis. The affected individual might experience weakness, lethargy, shortness of breath, anemia, widespread edema (swelling), metabolic acidosis, rising potassium levels, heart arrhythmias, and more. Each of these functions is vital to your well-being and survival. The urinary system, controlled by the nervous system, also stores urine until a convenient time for disposal and then provides the anatomical structures to transport this waste liquid to the outside of the body. Failure of nervous control or the anatomical structures leading to a loss of control of urination results in a condition called incontinence.
This chapter will help you to understand the anatomy of the urinary system and how it enables the physiologic functions critical to homeostasis. It is best to think of the kidney as a regulator of plasma makeup rather than simply a urine producer. As you read each section, ask yourself this question: “What happens if this does not work?” This question will help you to understand how the urinary system maintains homeostasis and affects all the other systems of the body and the quality of one’s life.
Watch this video from the Howard Hughes Medical Institute for an introduction to the urinary system.
Learning Objectives
After studying this chapter, you should be able to:
- Identify the basic structures of the urinary system, including nephrons.
- Recognize and/or describe the processes used to carry out the functions of the urinary system.
xx.1 Anatomy of the Urinary System
The kidneys, illustrated in Figure, are a pair of bean-shaped structures that are located just below and behind the liver in the abdominal cavity. The adrenal glands sit on top of each kidney and function as a component of the endocrine system. 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. Urine is carried from the kidneys to the urinary bladder via the ureters, which are approximately 30 cm long. As urine passes through the ureters, it does not passively drain into the bladder but rather is propelled by waves of peristalsis (smooth muscle contractions). The bladder collects urine from both ureters . During late pregnancy, its capacity (typically several hundred milliliters) is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical. In females, the urethra is relatively short length, about 4 cm, and is less of a barrier to fecal bacteria than the longer male urethra (approximately 20 cm). This length difference is the best explanation for the greater incidence of urinary tract infections (UTIs) in women. The urethra in males also has a reproductive function, as it transports semen (sperm and accessory fluids).
Kidney Structure
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 renal corpuscles; nephron tubules can be found throughout the renal cortex and renal pyramids, the multiple tissue masses that make up the majority of the renal medulla. There are, on average, eight renal pyramids in each kidney. Urine that is produced by the nephrons travels into the renal pelvis and then into the ureters, which carry the urine to the 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 and ends with the exiting of the renal veins to join the inferior vena cava, which transports blood back to the right atrium of the heart. The renal arteries split multiple times to form other blood vessels before branching into numerous afferent arterioles, and then enter the capillaries supplying the nephrons.
As mentioned previously, the functional unit of the kidney is the nephron, illustrated in Figure. Each kidney is made up of over one million nephrons that dot the renal cortex. A nephron consists of three parts—a renal corpuscle, a renal tubule, and the associated capillary network.
Renal Corpuscle
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.
Renal Tubule
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. The second part is called the loop of Henle, because it forms a loop (with descending and ascending limbs). The third part of the renal tubule is called the distal convoluted tubule (DCT). The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts. The urine will ultimately move into the renal pelvis and then into the ureters.
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.
Go to this website to see another section of the kidney and to explore an animation of the workings of nephrons.
xx.2 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 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 this filtrate, called urine, will be transported into the renal pelvis and then to the ureters. This entire process is illustrated in Figure.
Glomerular Filtration
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.
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.
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.
By the time the filtrate reaches the DCT, most of the water and solutes have been reabsorbed. If the body requires additional water, more 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.
Section Summary
The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the correct concentration of solutes in body fluids. They are made up internally of three distinct regions—the cortex, medulla, and pelvis.
The blood vessels that transport blood into and out of the kidneys arise from and merge with the aorta and inferior vena cava, respectively. The renal arteries branch out from the aorta and enter the kidney where they further divide.
The nephron is the functional unit of the kidney, which actively filters blood and generates urine. The nephron is made up of the renal corpuscle and renal tubule. The nephron filters and exchanges water and solutes with two sets of blood vessels and the tissue fluid in the kidneys.
There are three steps in the formation of urine: glomerular filtration, which occurs in the glomerulus; tubular reabsorption, which occurs in the renal tubules; and tubular secretion, which also occurs in the renal tubules.
xx.3 Hormonal Control of Urine Concentration
While the kidneys operate to maintain osmotic balance and blood pressure in the body, they also act in concert with hormones. Hormones are small molecules that act as messengers within the body. Hormones are typically secreted from one cell and travel in the bloodstream to affect a target cell in another portion of the body. Different regions of the nephron bear specialized cells that have receptors to respond to chemical messengers and hormones. In this section, you will learn about two hormones, aldosterone and antidiuretic hormone, that control urine concentration.
Aldosterone
Aldosterone is a hormone synthesized by the adrenal cortex that affects urine concentration by regulating sodium levels in the blood. Almost all of the sodium in the blood is reclaimed by the renal tubules under the influence of aldosterone. Because sodium is always reabsorbed by active transport and water follows sodium to maintain osmotic balance, aldosterone manages not only sodium levels but also the water levels in urine. Aldosterone favors the production of a concentrated urine by the water following the reabsorbed sodium ions. A decrease in the secretion of aldosterone means that less sodium gets reabsorbed in the renal tubules; therefore, more of it gets excreted in the urine. Patients who have Addison’s disease have a failing adrenal cortex and cannot produce aldosterone. They lose sodium in their urine constantly, and if the supply is not replenished, the consequences can be fatal.
Antidiurectic Hormone
Diuretics are drugs that can increase water loss by interfering with the recapture of solutes and water from the forming urine. They are often prescribed to lower blood pressure. Coffee, tea, and alcoholic beverages are familiar diuretics. Antidiuretic hormone or ADH, as the name suggests, helps the body conserve water when body fluid volume, especially that of blood, is low. It is formed by the hypothalamus and is stored and released from the posterior pituitary gland. It acts by inserting aquaporins, protein channels that allow water to leave, in the collecting ducts and promotes reabsorption of water. This action results in the formation of a concentrated urine. ADH also acts as a vasoconstrictor and increases blood pressure during hemorrhaging.
Section Summary
Hormonal cues help the kidneys synchronize the osmotic needs of the body. Hormones like aldosterone and anti-diuretic hormone (ADH) help regulate the needs of the body as well as the communication between the different organ systems.
Adapted from Openstax Human Biology