Location of Kidneys, Kidney Structure, and Kidney Functions
Glomerular Filtration, Filtration Pressure, Filtration Rate, and Regulation of Filtration Rate
Tubular Secretion and Urine Composition
Ureters, Urinary Bladder, Micturition, and Urethra
The organs of the urinary system are the kidneys, ureters, urinary bladder, and urethra. This system functions to remove waste products from the blood stream. The waste products are excreted from the body in the form of urine.
Kidneys are bean-shaped organs the are reddish brown in color. They are covered by tough, fibrous capsules.
The kidneys are in a retroperitoneal position. This means they lie behind the abdominal cavity. They lie on either side of the vertebral column at about the level of the lumbar vertebrae.
The medial depression in a kidney is called the renal sinus. The entrance of the sinus is called the hilum and it contains the renal artery, renal vein and ureter.
The ureter is a tube that carries urine out of the kidney. Inside the kidney the ureter expands as the renal pelvis. The renal pelvis divides into calyces inside the kidney.
The outermost layer of the kidney is termed the renal cortex while the middle portion of the kidney is termed the renal medulla. The renal medulla is divided up into triangular shaped areas called renal pyramids. The renal cortex covers the pyramids and also dips down between the pyramids. The portion of the cortex between pyramids is called a renal column.
Within the substance of the kidneys are millions of microscopic structures called nephrons.
Click here to see the structure of the kidney
The kidneys remove metabolic waste products from the blood. The metabolic wastes are combined with water and ions to forms urine. The kidneys also secrete the hormone erythropoietin which helps to regulate red blood cell production. The kidneys also secrete renin which is hormone that helps to regulate blood pressure.
The following is a summary of blood flow through the kidney:
Renal artery --> interlobar arteries --> arcuate arteries --> interlobular arteries --> afferent arterioles --> nephron (this is where waste products are removed from blood)
The renal artery branches into interlobar arteries. Interlobar arteries run between renal pyramids. The interlobar arteries branch into arcuate arteries. The arcuate arteries for arcs over the medullary pyramids. The arcuate arteries branch into interlobular arteries. The interlobular arteries are found in the renal cortex. The interlobular arteries branch into afferent arterioles. The afferent arterioles deliver the blood to the nephron where waste products are removed from it.
Blood eventually leaves the kidney through the renal vein.
Click here to see renal blood vessels
Nephrons are considered the functional units of the kidneys; in other words these microscopic structures carry out the functions of the kidney!
Nephrons are microscopic. Each kidney contains about 1 million nephrons. Each nephron is made of a renal corpuscle and a renal tubule.
A renal corpuscle is made of a group of capillaries called a glomerulus and a capsule that surrounds the glomerulus called a glomerular capsule. So a renal corpuscle is made of a glomerulus plus a glomerular capsule. The renal corpuscle is where the filtration of blood occurs.
Extending off the glomerular capsule are the renal tubules of a nephron. Directly attached to the glomerular capsule is the proximal convoluted (convoluted means twisted) tubule. The proximal convoluted tubule eventually straightens out to become the nephron loop. The nephron loop is a tubule that curves back toward the renal corpuscle. The nephron loop starts to twist again and now it becomes the distal convoluted tubule. Distal convoluted tubules from several nephrons merge together to form collecting ducts. So, the renal tubules of a nephron are (in order) proximal convoluted tubule, nephron loop, and distal convoluted tubule.
Click here to see the structure of a nephron
An afferent arteriole delivers blood to a glomerulus (remember this is a group of capillaries). The blood leaves the glomerulus through efferent arterioles. The efferent arterioles deliver blood to peritubular capillaries (they are called peritubular capillaries because they are wrapped around the tubules of the nephron). Blood leaves the peritubular capillaries through veins of the kidneys. By the time the blood leaves the peritubular capillaries, it has been cleansed of waste products. So here is a summary of the blood flow through a nephron: afferent arteriole to glomerulus to efferent arteriole to peritubular capillaries to veins of the kidney.
Click here to see the structure of a nephron with its blood supply
Two structures make up the juxtaglomerular apparatus - 1) the macula densa and 2) juxtaglomerular cells.
The macula densa is an area of the distal convoluted tubule that touches afferent and efferent arterioles. Juxtaglomerular cells are simply enlarged smooth muscle cells in the walls of either the afferent arteriole or the efferent arteriole. The macula densa and the juxtaglomerular cells associate with each other to form the juxtaglomerular apparatus.
The juxtaglomerular apparatus secretes the hormone renin which regulates blood pressure.
Click here to view the structure of a juxtaglomerular apparatus
The three processes of urine formation 1) glomerular filtration, 2) tubular reabsorption and 3) tubular secretion. They are described in this section,
Glomerular filtration takes place in the renal corpuscles of nephrons. In this process the fluid part of blood is forced from the glomerulus (the capillaries) into the glomerular capsule. The fluid in the glomerular capsule is not called glomerular filtrate.
Filtration pressure is simply the amount of pressure forcing substances out of the glomerulus into the glomerular capsule. It is largely determined by blood pressure. If your blood pressure is too low, the fluid part of blood will not be forced into the glomerular capsule.
The rate at which blood will be filtered is directly related to filtration pressure. If filtration pressure increases, the rate of filtration will increase. One way to increase filtration pressure is to constrict the efferent arteriole. Remember that blood leaves the glomerulus through the efferent arteriole. If this arteriole becomes smaller, less blood can leave it and the blood backs up in the glomerulus increasing the pressure there.
One way to decrease filtration pressure is to constrict the afferent arteriole. Remember the afferent arteriole delivers blood to the glomerulus. If the afferent arteriole is smaller, less blood enters the glomerulus and therefore pressure decreases in the glomerulus. The autonomic nervous system decides if the afferent or efferent arteriole should be constricted.
The sympathetic nervous system largely controls the rate of filtration. If blood pressure or blood volume drops in the body, the sympathetic nervous system will cause the afferent arterioles in the kidneys to constrict. Remember that when afferent arterioles constrict, glomerular filtration pressure decreases and therefore less glomerular filtrate is formed. When less glomerular filtrate is formed, less urine is ultimately formed. This means the body is retaining fluids needed to raise blood pressure or blood volume!
Conversely, if blood pressure or blood volume rises, the sympathetic nervous system can cause the afferent arterioles to dilate. Would glomerular filtration increase or decrease then? It would increase! Would more or less glomerular filtrate be formed? More! Would urine production increase or decrease? Increase!
Another way to control filtration rate is through the use of the enzyme renin. Juxtaglomerular cells secrete renin whenever the afferent arterioles sense a drop in blood pressure or when the sympathetic nervous system stimulates the cells or when the macula densa senses a decreased amount of ions in blood.
When renin is in the blood stream, it causes the formation of angiotensin II. Angiotensin II is important for maintaining blood pressure, water balance in the body, and sodium balance in the blood stream. Angiotensin II also stimulate the adrenal glands to release aldosterone. Recall that aldosterone is a hormone that is important for helping the body retain sodium and water.
Click here to view glomerular filtration
Tubular reabsorption is the second process in urine formation. In this process, the glomerular filtrate flows into the proximal convoluted tubule. Recall that glomerular filtrate is really just the fluid part of blood. There are a lot of substances in the fluid part of blood that your body would like to keep, such as nutrients, water, and ions! Remember also that the tubules of the nephron are surrounded by capillaries (the peritubular capillaries). In tubular reabsorption, all the "good things" in the glomerular filtrate pass through the wall of the proximal convoluted tubule into the blood of the peritubular capillaries. Now all the nutrients, a lot of water and many ions are put back into your blood stream!
As rule of thumb, you should remember that when sodium is reabsorbed back into the blood stream, water will also be reabsorbed. The more sodium retained , the more water retained! This is one reason why sodium leads to water retention.
Click here to view tubular reabsorption
Two hormones affect water reabsorption - 1) ADH (antidiuretic hormone) and 2 ) aldosterone.
Aldosterone causes more sodium to be reabsorbed. Remember the more sodium the more water!
ADH causes the tubules of the kidney to become more permeable. This simply allows water to move back into the blood stream.
Urea is a waste product of amino acid break down. Therefore, the more protein in your diet, the more urea formed. Urea is pushed into glomerular filtrate through glomerular filtration and then about half of it is reabsorbed back into the blood stream during tubular reabsorption. So, about half will be excreted out in urine.
Uric acid is a waste product of nucleic acid metabolism. Uric acid is also forced into the glomerular filtrate and then most of it is reabsorbed back into the blood during tubular reabsorption. Some of it is excreted in urine.
In tubular secretion substances move out of the blood in the peritubular capillaries into the renal tubules. Substances that are secreted include drugs, hydrogen ions, and some other ions. All substances left in the renal tubules after tubular secretion are destined to be excreted as urine.
Click here to view tubular secretion
The final product that reaches the collecting ducts of the kidneys is urine. It is mostly water but also normally contains urea, uric acid, a trace amount of amino acids, and various ions.
After urine is formed it flows out of the nephron into collecting ducts. The collecting ducts deliver the urine to calyces. The calyces deliver the urine to the renal pelvis. The renal pelvis delivers the urine to ureters.
Ureters are long, muscular tubes that carry urine from the kidneys to the urinary bladder. The ureters propell urine towards the bladder through peristalsis.
The urinary bladder is a distensible organ located in the pelvic cavity. Its function is to store urine until the time it is eliminated from the body.
The internal floor of the bladder contains three openings - 1) opening for the left ureter, 2) opening for the right ureter, and 3) opening for the urethra. These three opening make a triangle called the trigone of the bladder.
The wall of the bladder contains smooth muscle. This smooth muscle is named the detrusor muscle. The destrusor muscle contracts to expel urine from the bladder into the urethra.
Click here to view the male urinary bladder
Click here to view the female urinary bladder
Micturition is more commonly called urination. It is the process by which urine is expelled from the body. The stretching of the bladder from being filled with urine triggers micturition.
During micturition, the following events occur:
1) Detrusor muscle contracts.
2) The internal urethral sphincter opens. This sphincter is located just above the opening of the urethra. When this sphincter opens, you feel the urgency to urinate.
3) The external urethral sphincter opens. This sphincter is located below the internal urethral sphincter. You can voluntarily keep this sphincter closed until you get to an appropriate place to urinate.
4) After the external urethral sphincter opens, urine flows out of the bladder through the urethra.
The urethra is a tube that conveys urine from the bladder to the outside world. The urethra of the female is much shorter than the urethra of the male.