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We eat many kinds of food daily to keep our bodies nourished. These foods provide energy and important macromolecules that keep our bodies running. While the body absorbs what it needs from our food, some material is not absorbed and passed out of the body. At the same time, many metabolic processes in our cells produce waste products that must be removed.
Some main metabolic waste products in humans are carbon dioxide, water, mineral salts and urea. As we have learned from the article on respiration, the respiratory system removes carbon dioxide from the body when we exhale. A small amount of water and mineral salts can be lost through sweating, but this is not enough to efficiently remove all the waste the body needs to get rid of.
This is where the renal system comes in to carry out most of the body’s excretion of liquid waste. In this article, we will see how the renal system not only removes waste substances from the blood, but also helps keep the levels of ions such as sodium and potassium, and the amount of water in the body, within a healthy range.
The human renal system consists of several key blood vessels, the kidneys, the ureters, urinary bladder and urethra.
The blood vessels consist of the renal arteries and renal veins, which carry blood to and away from the kidneys respectively. Blood entering the kidneys through the renal arteries contain a higher concentration of waste products that need to be removed. After filtration in the kidneys, the blood leaves through the renal veins with most of these waste substances removed.
The kidneys are bean-shaped organs, each about the size of a fist. One kidney lies on each side of the body, with the left kidney usually positioned slightly higher than the right. Each kidney carries around 800,000 to 1,000,000 tiny filtering units called nephrons, which help to clean the blood by removing waste products.
Urine is produced as a by-product of cleaning the blood in the kidneys. The urine formed in the kidneys is carried to the urinary bladder through the ureters, where it is stored. During urination, urine leaves the body through the urethra, removing the waste that has been filtered from the blood by the kidneys.
The kidney is protected by a tough fibrous layer called the renal capsule, which is surrounded by a layer of fat known as the adipose capsule. Some textbooks even treat the adipose capsule as part of the renal capsule! Together, these layers help cushion the kidney and protect it from trauma and damage.
The kidney itself can be divided into two main regions, an outer cortex and an inner medulla. Within these regions are many tiny filtering units called nephrons, which filter the blood to remove waste materials and help form urine.
The nephron is the functional unit of the kidney. Here, blood is filtered to remove waste, while useful substances are returned to the blood to maintain homeostasis. Each nephron consists of Bowman’s capsule, the proximal convoluted tubule (PCT), Loop of Henle, distal convoluted tubule (DCT) and collecting duct.
The Bowman’s capsule surrounds the glomerulus (a tangled network of capillaries, like a ball of yarn). Here, blood undergoes ultrafiltration.
Blood enters the glomerulus through the afferent arteriole, passes through the capillary network, and leaves via the narrower efferent arteriole. The afferent arteriole has a wider diameter than the efferent arteriole, creating the high pressure in the glomerulus that drives movement of substances into the Bowman’s capsule.
The one-cell-thick walls of the glomerulus allow small soluble substances like water, glucose, ions and urea to pass into the Bowman’s capsule, while large components like blood cells and proteins remain in the blood.
The tangled nature of the glomerulus provides a large surface area for filtration, increasing the rate of ultrafiltration and allowing more soluble substances to enter the Bowman’s capsule.
The Bowman’s capsule then leads into the proximal convoluted tubule (PCT), a long, winding tube. From there, it flows into the straighter loop of Henle.
In the PCT, useful substances are absorbed back into the bloodstream through selective reabsorption. Water returns by osmosis, while salts are actively transported through ion channels. Glucose and amino acids are also actively transported back using specific carrier proteins, which ensure only needed substances are reabsorbed.
The loop of Henle dips down into the salty medulla of the kidney before looping back up towards the cortex. Here, it helps the kidney conserve water and produce concentrated urine by fine-tuning the reabsorption of water and salts.
The deeper we go into the medulla, the saltier (higher osmolality) the surrounding kidney tissue becomes. This creates a concentration gradient that drives the movement of water and salt in the loop.
The descending limb goes down towards the medulla, and is permeable to water but not salts. Water diffuses out into the surrounding salty tissues through osmosis, making the fluid within the loop highly concentrated by the bottom of the loop.
The ascending limb goes up towards the cortex, and its walls actively pump out salts like sodium chloride, while remaining non-permeable to water. This dilutes the fluid inside the loop while replenishing salt in the surrounding medulla, maintaining the concentration gradient.
The distal convoluted tubule (DCT) follows the loop of Henle. Here, some more water and salts are reabsorbed through osmosis and active transport respectively.
The collecting duct is the final segment of the nephron, where produced urine will be channeled to the ureters and then stored in the urinary bladder. Some reabsorption of water through osmosis still occurs at the collecting duct.
Now that we have learned how urine is produced, we can see that water reabsorption is a fine-tuned process. Reabsorption of water in the nephrons is key to maintaining the correct amount of water in our bodies.
The water potential of our bodily fluids must be kept relatively constant, as drastic changes could cause serious problems. For example, if the water potential is too high, our cells could swell and even burst! The kidneys therefore carefully control water potential through osmoregulation.
ADH (anti-diuretic hormone, aka vasopressin) is the key molecule controlling osmoregulation. When water content in the blood is low after dehydration, the pituitary gland in the brain produces more ADH, which induce the kidneys to reabsorb more water, resulting in low urine volume.
On the other hand, if the body is over-hydrated and the water content in the blood is too high, less ADH is produced and the kidneys reabsorb less water, resulting in a high volume of urine to remove excess water within the blood.
We have learned how kidneys not only facilitate excretion of waste, they also act as osmoregulators to maintain water potential within our bodies. There is no doubt the kidneys are very important organs!
However, the kidneys can be severely damaged to the point that they are unable to carry out their functions, a condition known as kidney failure. Some common causes of kidney failure include high blood pressure, diabetes, alcohol abuse, severe trauma and complications from major surgeries.
If one kidney fails, the other can still function such that we can have a normal life. However, if both kidneys have failed, kidney function can only be restored through a kidney transplant. Waiting for a transplant could take up to several years, so how do patients manage in the meantime?
Dialysis is a process where excess water, solutes and toxins from the blood are removed through a selectively permeable membrane. Two types of dialysis can be used to treat kidney failure, peritoneal dialysis and hemodialysis.
Peritoneal dialysis uses the peritoneal membrane in the torso as a natural membrane through which fluids and dissolved substances are exchanged.
A catheter is surgically placed in the abdomen. The dialysate, containing a specially formulated mix of sugars and minerals, is flowed into the peritoneal cavity. Waste products and extra water from the blood diffuse across the peritoneal membrane into the dialysate through diffusion and osmosis. After a few hours, the dirty fluid is drained out and replaced with fresh dialysate. This process is repeated several times a day.
Peritoneal dialysis can be done at home without any complicated equipment. However, there is a risk of infection at the catheter site, and may not be suitable for some patients with compromised abdomens.
Hemodialysis on the other hand, draws out blood and circulates it through a dialysis machine. This machine clears waste products out of the blood through a membrane, and returns the clean blood to the patient. Patients would visit a facility a few times a week, where each session may last a few hours.
Compared to peritoneal dialysis, hemolysis treatments are much faster and highly advantageous at removing waste quickly. However, regular hospital visits are required, and there is a risk of infection at the site where blood is drawn.
In this article, we have discussed the renal system and how it removes waste products from our bodies. Besides waste removal, the kidneys also help to regulate the amount of water in our bodies. When kidney failure occurs, these organs can no longer carry out their functions properly. In such cases, dialysis is needed to take over the waste removal role of the kidneys until a suitable kidney transplant can be done.
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Prepared by: Michelle
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