The function of the urinary system is to maintain homeostasis of the body. It also regulates fluid and electrolytes and removes waste through the formation of urine. And, the urinary system also assists in the regulation of blood pressure. This system consists of two kidneys, two ureters, a bladder and a urethra. The structures of the urinary system are responsible for the formation and expulsion of urine from the body.
This article will look at the structures of the urinary system, follow urine from formation to excretion and look at some of the hormones that assist the urinary system in its functions. These are 8 Facts about the Urinary System Every Nursing Student Should Know.
Fact #1: The Structures of the Urinary System Include the Kidneys, Ureters, Bladder and the Urethra.
The kidneys are bean-shaped organs. They are located on the posterior wall of the abdomen between the 12th thoracic vertebra and the 3rd lumbar vertebrae. The kidneys are situated outside and behind the peritoneal cavity, or retroperitoneally. They each weigh approximately 4-5 ounces. The ribs and a layer of fat surround and protect the kidneys. Also, connective tissue suspends the kidneys from the abdominal wall and allows them to stay in place. This connective tissue along with lymphatic circulation and blood vessels is called the renal capsule.
The renal capsule covers the exterior of the kidney. The renal hilum is the indented portion of the kidney that gives it the bean shape. Blood enters the kidney through the renal hilum via the renal arteries. Also, blood leaves the kidney through the renal hilum via the renal veins.
The renal cortex, the renal medulla, and the renal pelvis make up the kidney. The renal cortex is located at the outermost edge of the kidney. It contains the nephrons and filters blood as it enters the kidney.
The renal medulla is the middle portion of the kidney. The renal medulla contains the renal pyramids. These are triangular shaped collecting ducts. Thus, they collect urine formed in the kidneys. The medulla has only one blood supply. This is the vasa recta. The vasa recta is a network of capillaries that follow the loop of Henle.
The renal pelvis is the innermost portion of the kidney. It is essentially a funnel. It funnels urine into additional collecting tubules. These collecting tubules are the major and minor calyces. The calyces are cup-shaped tubes that collect urine that drains from the renal pyramids. The renal pelvis collects and transports urine into the ureters. The renal pelvis is the proximal portion of the ureters.
As urine is formed it flows through the kidneys to the ureters. The ureters are long muscular tubes. Each ureter originates from the kidney and extends down to the pelvic area and connects to the bladder. Urothelium lines the inside of the ureters. This layer prevents the reabsorption of urine. The urine moves from the kidneys to the bladder by peristalsis. The peristalsis is the contraction of the smooth muscle of the ureter wall.
The urinary bladder is a sac located behind the pelvic bone. It is a reservoir for urine until it is expelled from the body. The urinary bladder can hold approximately 400ml but can distend to hold even more volume. The inside layer of the bladder is lined with rugae. The rugae are ridges that allow the bladder to expand when it is filled with urine. The wall of the bladder consists of muscle fibers known as the detrusor muscles. These muscles allow the bladder to contract to expel the urine. The filling and emptying are of the bladder are under the control of the sympathetic and parasympathetic nervous system. The bladder has two inlets from the ureters and one outlet for the urethra.
The urethra extends from the bladder to the penis in males and an opening anterior to the vagina females. There is an internal urethral sphincter which controls the exit of urine from the bladder. The prostate surrounds the urethra in males. Consequently, this is why males have problems with the passage of urine when they have prostate problems.
Fact #2: A Function of the Kidneys is to Filter Blood
There is a network of blood vessels throughout the kidneys. The kidneys receive blood from the aorta via the renal arteries. Renal arteries enter the kidneys at the hilum. Next, the renal arteries branch into the segmental arteries, the lobar arteries, and the interlobular arteries. The interlobular arteries branch into the arcuate arteries and supply the cortex with blood. The arcuate arteries eventually branch off into the afferent arterioles. The afferent arterioles lead into the glomerulus which is a network of capillaries.
Efferent arterioles leave the glomerulus and travel to a group of capillaries called the peritubular capillaries surrounding the collecting tubules at the renal nephron. The peritubular capillaries wrap around the nephron. When the blood leaves the nephron it travels to the arcuate veins, the interlobular veins and then the lobar veins. However, there are no segmental veins like the segmental arteries. Blood leaves the kidney via the renal veins.
An outline of the pathway of blood through the kidneys is as follows
- Renal artery
- Segmental arteries
- Lobular arteries
- Interlocular arteries
- Arcuate arteries
- Afferent arterioles (Arteries = Afferent)
- Glomerulus (capillaries)
- Efferent arterioles (Exit vein = Efferent)
- Peritubular capillaries
- Arcuate veins
- Interlobular veins
- Lobar veins
- Renal vein
Fact #3: The Nephrons are the Functional Unit of the Kidney.
There are millions of nephrons in each kidney. The nephrons are a system of funnels and tubules which consist of the renal corpuscle, and the renal tubule. The renal corpuscle is the filter. It consists of the glomerulus, which is a group of capillaries, inside the Bowman’s capsule. The renal tubule extends from the Bowman’s capsule to a convoluted segment and on to a straight segment. And, this segment begins at the proximal convoluted tubule. Eventually, the renal tubule joins to the loop of Henle, distal convoluted tubule, and collecting tubule. Finally, the renal tubule drains urine into the renal pelvis.
The glomerulus is a network of capillaries. It is located between the afferent and efferent blood vessels. The Bowman’s capsule encloses the glomerulus. The glomerular capsule is another name for the Bowman’s capsule. Endothelium surrounds the capillaries and makes up the inner layer of the glomerulus. This layer makes a great filter. This filter allows the passage of small molecules across its membrane. However, it does not allow large molecules like blood and albumin to cross the membrane. As the glomerulus allows the passage of fluids and substances across its membrane, the Bowman’s capsule will fill with this filtered solution called glomerular filtrate. So, the blood remains in the capillaries of the glomerulus. And, Bowman’s capsule contains the glomerular filtrate.
The juxtaglomerular apparatus is important to the urinary system because this is where renin is released. See the discussion on renin below. The juxtaglomerular cells are located where the afferent arterioles enter the glomerulus. The macula densa is sodium sensing cells located between the afferent and efferent arterioles. The juxtaglomerular cells and the macula densa cells make up the juxtaglomerular apparatus.
The tubule component of the nephron begins in the space of the Bowman’s capsule. The renal tubules begin with a convoluted segment and then a straight segment. These segments consist of the proximal tubule, the descending and ascending limbs of the loop of Henle, and the distal tubule, the calyces, renal pelvis, and ureters. The glomerular filtrate travels through this system of tubules.
To begin with, as the glomerular filtrate leaves the glomerulus and enters the renal tubule it first crosses the proximal tubule. The proximal convoluted tubule is another name for the proximal tubule. From the proximal tubule, the glomerular filtrate travels to the loop of Henle. The loop of Henle consists of the descending loop and the ascending loop. From the loop of Henle, the glomerular filtrate travels to the distal tubule. The distal convoluted tubule is another name for the distal tubule. Finally, as the glomerular filtrate leaves the distal tubules and it travels to the calyces. The calyces lead to the renal pelvis, then to the ureters and eventually the renal bladder.
Fact #4: Urine Formation Requires Three Processes.
The kidneys are responsible for the formation of urine. Also, they regulate water balance in the body. The kidneys do this by controlling the composition and volume of the urine. The formation of urine consists of three processes: glomerular filtration, tubular reabsorption, and tubular secretion.
Glomerular filtration is the first step in urine formation. It is the movement of fluid and small molecules across the glomerular capillary membrane. It begins as blood flows from the renal arteries to the afferent arterioles into the glomerulus. Then, from the capillaries of the glomerulus, the filtrate moves into the Bowman’s space of the glomerulus. The Bowman’s capsule filters this fluid. Filtration is dependent upon adequate blood flow through the glomerulus. Next, from the Bowman’s capsule, the filtrate enters into the renal tubules. Subsequently, tubular reabsorption or tubular secretion controls the fluid. The glomerular filtration rate is the amount of fluid filtered by the glomerulus over a period of time.
During tubular reabsorption, fluid moves from the renal tubules into the peritubular capillaries. These peritubular capillaries carry the reabsorbed fluid into venous circulation. The fluid travels into the body by way of the renal veins. Therefore, these fluid remain in the body. The reabsorption of sodium is the primary function of the proximal tubules.
During tubular secretion, fluid moves from the peritubular capillaries into the renal tubules and drains into the collecting tubules. Renal tubules carry the fluid which eventually leaves the body in the form of urine. As a result, the body uses tubular reabsorption and tubular secretion to regulate the variety of electrolytes and substances in the body.
Fact #5: Antidiuretic hormone (ADH)
The antidiuretic hormone (ADH) is also called vasopressin. The hypothalamus signals the release of the ADH. The posterior pituitary gland secretes ADH. The ADH controls the concentration of the final urine. It increases water permeability and reabsorption. When there is a decrease in blood pressure or an increased concentration of ions in the blood, you have the secretion of the ADH. The ADH acts by increasing the membrane permeability of distal tubules or collecting ducts.
Therefore, the increase in membrane permeability allows for an increase in the reabsorption of more fluid back into circulation. With the increase circulating fluid, the blood volume will increase. When the blood volume increases, the blood pressure will increase. The increase in circulating blood volume will also dilute the concentration of ions in the blood. Also, there is less secretion of urine with the reabsorption of more fluids. Hence, this is why it is called the ANTIdiuretic hormone.
Fact #6: The Renin-Angiotensin-Aldosterone System (RAS) is a Sequence of Chemical Reactions.
The renin-angiotensin-aldosterone system (RAS) regulates blood volume through a series of chemical reactions. When there is a decreased blood flow to the kidneys, there is the activation of the RAS. The distal tubules along with the afferent arterioles create an area known as the juxtaglomerular apparatus. The juxtaglomerular apparatus is where renin is secreted and enters the bloodstream. There the renin interacts with another protein produced by the liver called angiotensinogen. The renin converts angiotensinogen to angiotensin I. Then, angiotensin I interact with an enzyme produced by the lungs called angiotensin-converting enzyme (ACE). Finally, the angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II. As a result, angiotensin II causes an increase in thirst, an increase in the secretion of the antidiuretic hormone (ADH), increased secretion of aldosterone and causes vasoconstriction.
A decrease in blood flow activates the RAS. A decreased blood flow is usually a signal of decreased blood volume. Therefore, the increased thirst causes an increase in fluid intake, increasing fluid volume. Further, the activation of ADH and aldosterone causes a decrease in urination, thereby keeping more fluid in circulation. And finally, vasoconstriction caused an increase in the blood pressure due to a decrease in the diameter of the blood vessels.
Fact #7: Aldosterone Acts on the Renal Tubules.
Aldosterone is an adrenocorticosteroid. The adrenal cortex secretes aldosterone. When the amount of sodium in the blood decreases, there is the secretion of aldosterone. Aldosterone causes the reabsorption of sodium ions back into the blood in the distal tubule and ascending limb of the loop of Henle. Also, as the sodium is reabsorbed back into the body, water follows the sodium. Therefore, as more water is reabsorbed, there is less urine expelled. So, aldosterone causes an increase in fluid volume and decreased urine output.
Also, aldosterone is secreted when there is excess potassium circulating in the blood. When the potassium in the blood increases, aldosterone causes the excretion of potassium ions in the urine at the distal tubule and the ascending limb of the loop of Henle.
Fact #8: Omoslality vs Osmolarity.
Osmosis is the ability of a substance to pull fluid toward an area of higher concentration from an area of lower concentration. An example is if you have two containers and one side has more particle than the other making it more concentrated. Osmosis moves fluid into the concentrated side making each container equal in concentration but not necessarily volume. The amount of pressure required to stop the flow of fluid by osmosis is osmotic pressure.
Osmolality is the concentration of solute in water or the number of dissolved particles in water. It is measured in osmoles (units of osmotic pressure) dissolved per kilogram of solution. The osmolality affects the movement of the fluid by osmosis. The serum osmolality reflects the concentration of electrolytes, especially sodium.
When the osmolality of the blood increases, the antidiuretic hormone is secreted. An increase in osmolality means that the blood has a higher concentration of particles (blood cells) to fluid (plasma). Therefore, the secretion of the ADH increases the membrane permeability of the distal tubules and collecting ducts. There is more reabsorption of fluid. This results in an increased in circulating fluid volume. There is a decrease in serum osmolality due to the increase in circulating fluid volume. The blood is essentially more diluted.
Osmolarity is the ratio of solute (solid) to water or the concentration of the solute per volume of water. So, it is essentially the number of particles in a liter of solution. Osmolarity is measured in milliosmoles per liter (mOsm/L). Although osmolality and osmolarity are both the measurement of the concentration of solute in the fluid, osmolality is more often used for clinical purposes.
Osmolality is the number of milliosmoles per kilogram (mOsm/kg) in water, or concentration of molecule per weight.
Osmolarity is the number of miliosmoles per liter (mOsm/L) of solution, or the concentration of molecules per volume.
The urinary system maintains homeostasis of the body, regulates fluid and electrolytes. Also, it removes waste through the formation of urine. This article 8 Facts About the Urinary System Every Nursing Student Should Know will give you a better understanding of the structures and functions of the urinary system.
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