Significance of Urea Excretion Discussion Essay

Tubular reabsorption: Nephrons are the kidney’s functional units, and around a million of them are present in each kidney. They are an essential part of the excretory system and are responsible for urine formation. The afferent arteriole is responsible for bringing in the blood to the head of the glomerulus, while the efferent arteriole transports the blood back into circulation. The vasa recta extend the efferent arteriole and help in the re-absorption and secretion of certain substances to and from the nephron and circulation. The four main steps for water excretion are filtration, reabsorption, secretion, and excretion.

Filtration: The most significant part of the nephron that carries out filtration is the renal corpuscle. The glomerular filtrate enters the capsule by an afferent arteriole, and the filtration happens through the ultra filtration barrier. Particles with a size of less than 1.8\text{ nm}1.8 nm (\text{N}{{\text{a}}^{+}}Na
+
and \text{Cl}^{-}Cl
−
) pass freely, whereas particles of more than 3.6\text{ nm}3.6 nm (hemoglobin) are not filtered. The bottom layer is composed of the endothelium, the middle layer is the basement membrane, and the outer layer has podocytes which are part of the Bowman’s capsule.

Reabsorption: In this process, water and solutes move from the nephron back into the blood circulation. This process is mainly carried out by the proximal convoluted tubule (PCT), distal convoluted tubule (DCT), the loop of Henle (LOH), and collecting tube (CT). The PCT absorbs \text{Na}^{+}Na
+
, \text{Cl}^{-}Cl
−
, {{\text{K}}^{+}}K
+
, glucose, amino acids, and water. The descending LOH reabsorbs water while the ascending loop reabsorbs \text{Na}^{+}Na
+
, \text{Cl}^{-}Cl
−
, {{\text{K}}^{+}}K
+
. The DCT is responsible for the reabsorption of \text{Na}^{+}Na
+
, \text{Cl}^{-}Cl
−
, {{\text{K}}^{+}}K
+
, \text{Ca}^{2+}Ca
2+
, \text{Mg}^{2+}Mg
2+
, \text{HCO}^{3-}HCO
3−
and the CT reabsorbs \text{Na}^{+}Na
+
, \text{Cl}^{-}Cl
−
, water, and urea.

Secretion: Here, the water and solute particles move from the blood circulation into the nephron. The PCT secretes creatinine, drugs, and H+ ions. The DCT is responsible for the secretion of {{\text{H}}^{+}}H
+
and {{\text{K}}^{+}}\text{ ions}K
+
ions.

Excretion: The urine formed by the above process then passes into the urinary bladder through the ureter. Here, the urine is stored temporarily and is released outside by the urethra with the help of a circular muscle called the sphincter muscle.

ADH and water absorption
During urine excretion, the nephrons absorb a small quantity of urea. Therefore, the concentration of urea increases as the water moves up the nephrons. When it reaches the collecting tube, the concentration is very high and can pose several problems. Antidiuretic hormone plays a vital role in water reabsorption. The ADH is responsible for reabsorbing excess water out of the fluid in the collecting tubule.

The ADH activates the water channels in the collecting duct, allowing water transport into the high concentration area (medulla). However, since urea is a solute, it resists moving out of the collecting tubule. It adds solute and concentrations to the collecting duct, making it more difficult to remove the water. To counter this, ADH releases aquaporin into the collecting duct for water reabsorption and activates urea channels in the collecting duct. These urea channels allow urea to diffuse into the medulla.

Significance of Urea Excretion
The kidney filters about 1.2\text{ L}1.2 L of blood every minute in the human body. The blood contains urea, a waste product of the excess protein in the body. The average amount of urea in a healthy individual is 7-20\text{ mg/dL}7−20 mg/dL. It is vital to eliminate urea since excess levels of urea can be toxic for the body. In addition, the kidney is responsible for filtering out the urea, excess water, and ions.

Each kidney comprises about a million microscopic units called nephrons. As the blood flows in the kidney, the renal artery splits into smaller arteries and eventually becomes capillaries. Each capillary then forms a glomerulus which has tiny pores in it to squeeze out the blood. The filtrate comprises water, urea, amino acids, glucose, and ions.

As the filtrate passes along, anything the body needs to keep is taken back in the blood by selective re-absorption.

However, since urea is a waste product, the blood carries it through the nephron, eventually reaching the collecting duct.

This structure transports urea to the pyramid area of the kidney, which collects all the waste from the nephrons.

The urine now flows down the ureter, and the body stores it in the urinary bladder till the person feels the urge to urinate.