Usually, when we think of the urinary system, we think about getting rid of waste products in our urine. The urinary system, however, involves more than just waste removal. The urinary system plays many important roles in the maintenance of homeostasis. This means this system helps to regulate the internal conditions of the whole body. For instance, if the body is dehydrated, the body will function to conserve the liquid. Consequently, the body does not produce large volumes of urine. Much of this proper maintenance of homeostasis is a function of the kidneys.

The roles of the urinary system include filtering the blood to remove toxins, maintaining water levels, maintaining appropriate levels of some vitamins and minerals, maintaining acid-base and electrolyte balances, and interacting with the circulatory system to help regulate blood pressure and red blood cell count. In a three-way interaction with both the respiratory and circulatory systems, the urinary system helps stabilize blood oxygen and carbon dioxide levels.

The final outcome of the above functions of the urinary system is excretion. Excretion is the removal of wastes generated by the normal processes of cell metabolism in the body. Such metabolic wastes include urea, uric acid, creatinine, creatine, bilirubin, and ammonia. The metabolic wastes originate in the cells throughout the body and are moved into the blood. If allowed to accumulate, these wastes would be toxic to the body. All of the organs of the urinary system are involved in the removal of these metabolic wastes by contributing to the process of excretion. Other body systems that are also involved in excretion are the respiratory system, integumentary system (the skin), and digestive system. Excretion and elimination are two similar processes. Excretion specifically refers to the removal of the waste products of metabolism from the body. Elimination is the expulsion of undigested or unmetabolized waste products from the body.

The major functions of the urinary system include:

• Excretion of metabolic waste products such as urea, uric acid, creatinine, bilirubin, and ammonia,
• Maintenance of homeostasis, or the ability of the urinary system to regulate its internal conditions,
• Detoxifying harmful substances in the body,
• Maintenance of proper water levels, vitamin and mineral levels, and acid-base and electrolyte levels,
• Interaction with the respiratory and circulatory systems helps to stabilize blood oxygen and carbon dioxide levels.

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.

As noted above, much of the maintenance of proper chemical balance, or homeostasis, is the function of the kidneys. The kidneys 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.

The process begins with blood that contains metabolic waste entering each of the two kidneys through the renal artery. Urine is produced by microscopic structures called nephrons in the kidneys. Once filtered, the blood exits through the renal vein. Urine leaves each kidney through a ureter. Each ureter transports the urine via peristalsis to the urinary bladder (a hollow, muscular chamber that collects and stores urine). A single urethra transports urine from the urinary bladder to the outside of the body. 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.

This process through the smooth muscles is otherwise known as peristalsis. An internal urethral sphincter muscle and an external urethral sphincter muscle help keep the urine in the bladder until the process of urination. The internal urethral sphincter muscle surrounds the neck of the urinary bladder at the juncture of the bladder with the urethra. This smooth muscle opens reflexively when the bladder muscle contracts and builds up pressure. The external urethral sphincter muscle is located at the external opening of the urethra. This skeletal muscle is under voluntary control, and as a result of voluntary control, urination can be delayed for a time.

Urine is a fluid of variable composition that requires specialized structures to remove it from the body safely and efficiently. Blood is filtered, and the filtrate is transformed into urine at a relatively constant rate throughout the day. This processed liquid is stored until a convenient time for excretion. All structures involved in the transport and storage of urine are large enough to be visible to the naked eye. This transport and storage system not only stores the waste, but it protects the tissues from damage due to the wide range of pH and osmolarity of the urine, prevents infection by foreign organisms, and for the male, provides reproductive functions.

The process of urination is known as micturition which begins with blood carrying various wastes that enter each of the two kidneys through the renal artery and ends when urine exits out of the body through the urethra.

Learn By Doing 15.2

Match the location number on the diagram to the correct name and definition.

	Name	Definition
	Ureters	This is where the nephrons produce urine.
	Urinary bladder	Urine travels from the kidneys by circular contraction and is transported to the bladder through these.
	Kidney	Here is where urine is temporarily contained by internal and external sphincter muscles.
	Renal artery and vein	Here is where the entire process ends when urine exits out of the body through this.
	Urethra	The urinary process begins with blood carrying various wastes entering each of the two kidneys through one of these. Filtered blood exits the kidney through one of these.

When enough urine has been produced in the ____1____ it leaves the through the ureters and urine is then transported to the ____2____.____3____ occurs in the smooth muscles and is the process of how stored urine moves from the kidneys, through the ureters, and into the bladder. This process using smooth muscle is otherwise known as peristalsis. The process of urination is referred to as ____4____ and urine is ultimately released through the ____5____. This body part connects the bladder to the outside of the body.

1	2	3	4	5
ureters kidneys bladder	ureters kidneys bladder	Circular contraction Circular reflexes Cyclical elongation	micturition micturition circular contraction	urethra sex organs kidneys

Osmoregulation and Osmotic Balance

Osmoregulation and osmotic balance are important bodily functions, resulting in water and salt balance. This regulation equalizes the number of solute molecules across a semi-permeable membrane by the movement of water to the side of higher solute concentration.

Recall that osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, in contrast, doesn’t dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s fluids can be divided into three categories depending on where they are located: 1) blood plasma, 2) the cytosol within cells, and 3) interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable membranes. Semi-permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by the movement of solutes and/or water across the membrane.

As seen in the figure, a cell placed in water tends to swell due to the gain of water from the hypotonic or low-solute environment. A cell placed in a solution with higher solute concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or high solute environment. Isotonic environments have an equal concentration of solutes inside and outside the cell; this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.

Need for Osmoregulation

Biological systems constantly interact and exchange water and nutrients with the environment by way of the consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

The primary function of the urinary system is to maintain homeostasis of fluid and small molecules within the body. We will start with a molecular characterization of urine followed by molecular and tissue-level descriptions of the structures that are responsible for the formation of the urine. Then we will follow the movement of material from the bloodstream, through the functional units of the kidney through the other organs of the urinary system, and out of the body.

Molecular level – The Components of Urine

Our input of water varies greatly from day to day. Despite major differences in fluid intake, the total volume of fluid within the body remains relatively constant. The body’s fluid homeostasis is achieved largely due to the kidneys’ ability to regulate how much water is excreted in urine. If a person with healthy kidneys drinks a large volume of fluids, the kidneys will produce a large volume of urine. When a person with healthy kidneys does not drink enough fluids or experiences significant fluid loss, the kidneys will produce a small amount of concentrated urine to conserve water.

Urine is composed of urea, chloride, sodium, potassium, creatinine, and other compounds (ions, inorganic and organic). Urea is an organic breakdown product of nitrogenous materials. The table below lists some of the clinical characteristics of normal urine.

The Nephron, Unit of Function in the Kidney

As the chief functional organ in the urinary system, the kidneys excrete nitrogenous (nitrogen-containing) wastes and are involved in regulating the volume, composition, and pH of the blood. The kidneys receive one-fourth of total cardiac output, a reflection of their function as blood processors. Each kidney contains about one million nephrons, the structural and functional units of the kidneys. Nephrons are made up of epithelial cells with an underlying non-cellular layer or basement membrane that separates the filter in the lumen fluid from the interstitial space.

Each nephron is made up of a high-pressure capillary bed called a glomerulus and a renal tubule. The ends of multiple nephrons join a common collecting duct. The nephrons are involved in three functions: filtration, reabsorption, and secretion. The structure within each nephron that actually filters blood plasma is the renal corpuscle.The renal corpuscle includes the glomerulus and glomerular capsule. The renal tubule is attached to the glomerular capsule and receives the filtered fluid, called glomerular filtrate. Very thin and a little over an inch long, the renal tubule has three major consecutive segments that the filtrate flows through a proximal convoluted tubule(PCT), the nephron loop (loop of Henle), and a distal convoluted tubule(DCT).

The Glomerulus

The renal corpuscle is made up of a tangled capillary network called the glomerulus and a cup-shaped structure called the glomerular capsule (Bowman’s capsule) surrounding the glomerulus. The glomerular capsule has an external layer made of simple squamous epithelium. Although this layer is not involved in the production of filtrate, it helps to maintain the shape of the capsule. An inner layer adheres to the glomerular capillaries and is composed of a special type of simple squamous epithelial cells called podocytes. These cells interlock, forming filtration slits. Filtrate from the glomerulus passes through filtration slits and enters the capsular space (Bowman’s space).

The Proximal Convoluted Tubule
The proximal convoluted tubule (convoluted refers to the coiled shape) tubule connects the glomerular capsule to the nephron loop. The surface of the simple cuboidal epithelial cells making up the proximal convoluted tubule are covered in microvilli. The microvilli and the length of the proximal convoluted tubule dramatically increase the surface area available for reabsorbing water and solutes and for secreting substances into the filtrate.

The Nephron Loop
The renal corpuscle, the proximal convoluted tubule, and the juncture between the proximal convoluted tubule and the nephron loop are located in the outer rim of the kidney, called the renal cortex. The first part of the nephron loop, the descending limb of the nephron loop, drops into the middle of the kidney, called the renal medulla. In the renal medulla, the loop makes a sharp, almost 180-degree turn back toward the renal cortex as the ascending limb of the nephron loop. The ascending limb is continuous with the distal convoluted tubule. The ascending and descending limbs of the nephron loop have two distinct parts: a thin section of the limb and a thick section of the limb. In the thin section of the limb, the diameter of the tubule is distinctly smaller than the diameter of the rest of the nephron tubules. In the thin sections of the limbs, the epithelium is thinner simple squamous epithelium that is permeable to water. In the thick sections of the limbs, the epithelium is simple cuboidal epithelium that is highly impermeable to water. Regardless of being in the thin or the thick segments, the lumen is the same size as the lumen in the rest of the renal tubule.

Distal Convoluted Tubule
The final segment of the nephron is the distal convoluted tubule. As the ascending limb of the nephron loop reaches the renal cortex, it becomes the distal convoluted tubule. The distal convoluted tubule extends to the collecting duct. The distal convoluted tubule is composed of simple cuboidal epithelium with very few microvilli and no brush border.

Capillaries of the Nephron

The glomerulus is not the only capillary bed associated with nephrons. Peritubular capillaries are branches of the efferent arterioles that drain the glomeruli and recover most of the filtrate produced in the renal corpuscle. Glomerular capillaries differ from other capillary beds in the body because they are both supplied by and drained by arterioles. The feeder afferent arterioles are branches of the cortical radiate arteries. The draining efferent arterioles branch into the peritubular capillary network around the proximal and distal convoluted tubules or the vasa recta around the nephron loop. The diameter of the draining efferent arterioles is smaller than that of the afferent arterioles, giving the efferent arterioles higher resistance. Because of this, the glomerulus has a high blood pressure that allows it to filter high volumes of fluid and solutes out of the blood and into the glomerular capsule. The nephrons’ segments reabsorb approximately 99 percent of this filtrate. The peritubular capillaries adhere to neighboring convoluted tubules and drain into neighboring venules. These low-pressure and porous capillaries easily reabsorb the water and solutes that the tubule recovers from the filtrate. In some nephrons, rather than breaking up into peritubular capillaries, the efferent arterioles form clusters of thin-walled vasa recta. Important for the formation of concentrated urine, the vasa recta are long, straight capillaries that reach deep into the medulla alongside the longest nephron loops where they can collect reabsorbed substances from the loop segments.

Collecting Ducts
As the functional units of the kidneys, the primary role of the nephrons is to filter plasma, reabsorb what the body would like to keep and excrete the rest. Any substances not reabsorbed in the tubules of the nephrons flow into one of the thousands of collecting ducts in the kidney. Each collecting duct receives fluid from several nephrons and then transports it to an area called the renal pelvis.

Urine Formation

To produce urine, nephrons and collecting ducts carry out three basic functions: glomerular filtration, tubular reabsorption, and tubular secretion.

The first step in urine production is glomerular filtration. In this process, water and most of the solutes in blood plasma pass through the wall of glomerular capillaries, first into the glomerular capsule and then into the renal tubule. The product of glomerular filtration is referred to as glomerular filtrate.

During tubular reabsorption, cells of the tubule reabsorb almost all water and a variety of solutes from the filtrate as it flows through the renal tubule and collecting duct. These reabsorbed substances are then returned to the blood in the peritubular capillaries and vasa recta. In contrast to absorption, in which new substances enter the body (e.g., through the gastrointestinal tract), reabsorption refers to substances that have been removed from the blood being returned to the blood.

In the process of tubular secretion, the renal tubule and collecting duct actively transport substances (e.g., wastes, drugs, excess ions) from the blood in the peritubular capillaries. Secretion, in this case, refers to the removal of substances from the blood. In contrast, the secretion of hormones or enzymes, for example, refers to cells releasing substances into blood, ducts, and interstitial fluid.

The normal amount of filtrate produced as a result of glomerular filtration is so high that the amount of fluid entering the proximal convoluted tubules in 30 minutes is greater than the total blood plasma volume. Clearly, some of this fluid must be reclaimed and sent back into the bloodstream to maintain water balance and ion concentrations. This reclamation process occurs through reabsorption in the tubules. Secretion is essentially the opposite of reabsorption. It removes the additional solutes from the plasma and actively transports them to the tubule. Many students get confused initially with the concept that secretion means substances end up in the urine and reabsorption means substances end up back in the blood.

The glomerular capillaries filter about one-fifth of the plasma that flows through the kidneys into the renal tubules. Glomerular filtration works like any other filtration process. For example, a coffee filter prevents large coffee grounds from passing through it while allowing the passage of water and small solutes such as flavor molecules and caffeine. In the same way, the glomerular filtration membrane blocks the passage of blood cells and proteins while allowing water and other solutes from the blood to pass into the glomerular capsule. These substances are forced through the membrane by the higher hydrostatic pressure in the glomerular capillary than in the capsular space.

As described above, a porous filtration membrane separates the inside of the glomerular capsule from the blood. It is made of glomerular capillary endothelium, the gel-like basal lamina basement membrane, and the podocyte-formed filtration slit. Water and solutes smaller than about three nanometers in diameter (e.g., glucose, amino acids, nitrogenous wastes) pass freely across the membrane between the blood and capsule. This means there are similar concentrations of these substances in both the blood and the glomerular filtrate. Some bigger molecules may find a way through the membrane, but molecules larger than five nanometers (large proteins and blood cells) do not usually get into the tubule.

Reabsorption starts as soon as filtrate enters the proximal convoluted tubules. Before the reabsorbed substances can reenter the blood, they must pass through barriers: both membranes of cells in the tubule and the endothelium of the peritubular capillaries.

Reabsorption returns about 99 percent of filtered water and many filtered solutes to the bloodstream. Although cells in the proximal convoluted tubule perform most reabsorption, epithelial cells all along the length of the renal tubule and the collecting duct also contribute to this process. To maintain normal plasma levels, almost every organic nutrient (e.g., glucose, amino acids) is completely reabsorbed. The amount of water and the number of ions reabsorbed is determined by hormonal signals and is based on homeostatic needs. The scope of reabsorption can be appreciated by looking at water. Each day, about 180 liters (190 quarts) of water are filtered in the kidneys. Only 1–2 liters (1–2 quarts) are excreted in urine; the rest is reabsorbed in the kidneys.

Tubular secretion transfers unwanted substances from the blood and tubule cells into the tubular fluid. With the exception of potassium ions, most secretion occurs in the proximal convoluted tubule. However, some secretion also occurs in the cortical regions of the collecting ducts and in the late distal convoluted tubule.

Tubular secretion is important for a number of reasons. It removes substances that are not easily filtered, such as some drugs and metabolites that are securely attached to plasma proteins. Second, it disposes of unwanted substances or end products that have been reabsorbed passively, such as urea and uric acid. Third, it eliminates excess potassium ions. Finally, it regulates blood pH. As an example, when blood pH becomes too acidic, secretion adds more hydrogen ions (acid) into the filtrate and retains more bicarbonate ions (a base).

The figure and table below summarize filtration, reabsorption, and secretion.

Kidneys

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.

Kidney Structure

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 kidney cells to efficiently manufacture chemical energy in the form of ATP through aerobic respiration. The final filtrate coming out of the kidneys is called urine.

Externally, the kidneys are surrounded by three layers. 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 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 collecting ducts empty into the renal papillae that in turn empty into the 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.

The Ureters
The ureters are a pair of thin, muscular tubes that transport urine from the kidneys to the bladder. Beginning at the level of the second lumbar vertebra, the location of the ureters is retroperitoneal. Each ureter runs inferiorly and enters the posterolateral wall of the urinary bladder. This angle of entry is important because it helps prevent urine from flowing back into the ureters when the bladder fills with urine. In addition, accumulating urine increases the internal pressure of the bladder, and this pressure compresses and seals the distal portion of the ureters.

There are three layers in the ureter wall. The innermost mucosa lining contains transitional epithelium capable of stretching but is impermeable to urine. The ability to stretch allows the ureter wall to accommodate changing volumes of urine. The middle muscularis layer is composed of two layers of smooth muscle: an inner longitudinal layer and an outer circular layer. In the lower third of the ureter, the muscularis has a third outer layer of longitudinal muscle fibers. The muscularis layer is responsible for the peristaltic contractions needed to move urine through the ureters and into the bladder. The external layer of the ureter wall, the adventitia, is made of fibrous connective tissue and helps anchor the ureter to the abdominal wall.

When urine enters and distends the ureters, stretch receptors are stimulated. Reflexive action results in the contraction of the muscularis and movement of the urine into the bladder. The power and frequency of peristalsis are directly related to the rate of urine formation. Although the ureters are innervated by both sympathetic and parasympathetic fibers, the nervous system does not appear to have major involvement in the transport of urine in these organs.

The Urinary Bladder
The urinary bladder is a hollow, collapsible muscular sac that serves as a temporary storage facility for urine. It is located in the pelvic cavity, just posterior to the pubic symphysis. In females, the bladder lies anterior to the vagina and inferior to the uterus. In males, it is immediately anterior to the rectum. Peritoneal folds hold the bladder in place.

The bladder can hold up to about a liter of urine, although this amount varies from person to person. Despite its capacity to enlarge, an overfull bladder can burst, but it is more likely that excess urine will leak out of the urethra. When empty, the bladder collapses into a pyramidal shape. When a small amount of urine accumulates, it is spherical. When a larger volume of urine accumulates, the bladder becomes pear-shaped and ascends into the abdominal cavity. There are three openings in the bladder: two for the ureters and one for the urethra. These openings frame a triangular region at the base of the bladder called the trigone.

The bladder wall is made up of a mucosa with transitional epithelium, a submucosa, a thick muscularis called the detrusor muscle, and a fibrous adventitia. The adventitia is on the inferior surface only. In contrast, the peritoneum covers the superior surface. The detrusor muscle is composed of inner and outer layers of longitudinal smooth muscle fibers and an intermediate layer of circular muscle fibers.

The Urethra
The urethra is a small muscular tube that transports urine from the bladder out of the body. The urethra is five times longer in males (8 inches, 20 cm) than in females (1.6 inches, 4 cm). In males, the urethra is also part of the reproductive system, providing a passageway for semen as well as urine. The course of the urethra also differs between the sexes. In females, fibrous connective tissue binds it to the anterior vaginal wall, and its external urethral orifice(external opening) is located anterior to the vaginal opening and posterior to the clitoris. In males, the urethra is divided into three regions. The prostatic urethra is surrounded by the prostate. The membranous urethra passes through the urogenital diaphragm. The spongy urethra runs through the penis and ends at the external urethral orifice.

At the bladder-urethra junction, the circular fibers of the bladder’s detrusor muscle form the internal urethral sphincter. When urine is not draining from the bladder, this involuntary sphincter closes off the urethra to prevent the leakage of urine when you are not voiding. At the point where the urethra passes through the urogenital diaphragm, it is surrounded by the external urethral sphincter, a skeletal muscle. The external sphincter is voluntarily controlled and is kept contracted until voiding.

Regulation of Kidney Function

Hormonal Regulation
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.

The table below summarizes the hormones that control osmoregulatory functions.

Neural Regulation
The sympathetic nerve fibers that innervate renal blood vessels provide an extrinsic regulatory mechanism for glomerular filtration. During extreme stress or blood loss, the sympathetic nervous system must meet the needs of the body as a whole, for example, by temporarily reducing kidney activity and redirecting blood to other vital organs. In such situations, neural controls override renal autoregulation. The sympathetic nerve fibers release the neurotransmitter norepinephrine. Norepinephrine activates alpha-adrenergic receptors on vascular smooth muscle and causes afferent arterioles to constrict. The resulting reduced blood flow into glomerular capillaries lowers net filtration pressure. This decreased renal blood flow helps maintain blood volume by reducing urine output and increasing perfusion to other body tissues.

The kidneys do most of the work of the urinary system. In addition to being the major excretory organs, the kidneys are important regulators of the volume and chemical composition of blood, and they maintain the correct balance between water and salts, and between acids and bases, in the body. They also regulate blood glucose levels, produce hormones, and metabolize vitamin D to its active form, calcitriol.

The urinary system is responsible for only a part of our body’s ability to exchange material with the environment. It is not the only excretory system in the body. Several other organs, tissues, and processes are also involved in excretion. These structures temporarily store wastes, transport them for removal, or excrete the wastes and excess materials from the body.

Excretory Organs

The digestive system participates in waste removal in two ways. First, the liver detoxifies some substances. One example is the conversion of ammonia into urea. The large intestine is also considered an excretory organ. It eliminates a number of substances through defecation, including undigested food, water, carbon dioxide, water, salts, cholesterol, and heat. The respiratory system may seem like an unlikely waste remover, but when we exhale, the lungs excrete carbon dioxide and also rid the body of heat and some water vapor. Finally, the sweat glands in the skin are important excretory structures. They help dispose of excess water, heat, and carbon dioxide, as well as small amounts of salts and urea.

Table of Excretory Organs

Integrations of Body Systems

The kidneys are critical organs because they regulate blood volume and blood pressure. These organs perform this regulatory role by altering the amount of water that is eliminated in urine, as well as by releasing the chemicals renin and erythropoietin. When blood pressure drops, the kidneys secrete renin, which is both a hormone and an enzyme. Renin activates a hormonal pathway that increases blood pressure. When oxygen levels in body tissues decrease, the kidneys secrete the hormone erythropoietin. Erythropoietin stimulates the bone marrow to increase the production of red blood cells.

The kidneys regulate ion concentrations in blood plasma by returning some ions to the bloodstream and by excreting excess ions in urine. They control blood pH by adjusting the number of hydrogen ions and bicarbonate ions that are reabsorbed or secreted in urine. The kidneys also reclaim valuable nutrients from the filtrate and return them to systemic circulation.

The urinary system also affects all other body systems, whether directly or indirectly. The table below shows interactions between the urinary system and other body systems

Let’s Summarize

Learn By Doing 15.2

Match the location number on the diagram to the correct name and definition.

	#	Name	Definition
	2	Ureters	This is where the nephrons produce urine.
	3	Urinary bladder	Urine travels from the kidneys by circular contraction and is transported to the bladder through these.
	1	Kidney	Here is where urine is temporarily contained by internal and external sphincter muscles.
	5	Renal artery and vein	Here is where the entire process ends when urine exits out of the body through this.
	4	Urethra	The urinary process begins with blood carrying various wastes entering each of the two kidneys through one of these. Filtered blood exits the kidney through one of these.

When enough urine has been produced in the kidneys, it leaves the through the ureters, and urine is then transported to the bladder. Circular contraction occurs in smooth muscle and is the process of how stored urine moves from the kidneys through the ureters and into the bladder. This process through the smooth muscles is otherwise known as peristalsis. The process of urination is referred to as micturition, and urine is ultimately released through the urethra. This body part connects the bladder to the outside of the body.

1	2	3	4	5
ureters kidneys bladder	ureters kidneys bladder	Circular contraction Circular reflexes Cyclical elongation	micturition micturition circular contraction	urethra sex organs kidneys