Excretion is a process that removes the nitrogenous metabolic waste materials from our body, for e. g. urea, uric acid, creatinine. 

There are two kidneys situated on the dorsal body wall. If we take a cross-section of any one of them, there will be a cortex and a medulla. From the medulla side, there is a dent present. This dent has a renal vein and renal artery. 

The ureter is a tube that comes out of the kidney carrying nitrogenous metabolic waste. The two ureter emerge out to form a muscular bag, known as urinary bladder. Once the urinary bladder is full, signals are sent to the brain leading to the opening of the urethra. 

Working of kidneys 

In the kidney, there are millions of cells known as nephrons. There is a site of excretion. Bowman's capsule is the top part of the nephron. It holds a network of capillaries, known as the glomerulus. The glomerulus is attached to the aorta and the renal artery which changes into afferent arterioles. 

The first process in the urine formation takes place in the glomerulus. When blood reaches the glomerulus, it gets filtered there. The small molecules, for e. g. urea, amino acids, mineral ions, uric acid and water, will pass from the glomerulus into the renal capsule due to the buildup of pressure. This leads to the filtration of the blood. All the cells will remain in the capillary, and the nitrogenous waste will be filtered from capillaries to the renal capsule. 

The renal tubule is responsible for selective reabsorption. The substances required for the body for e. g. glucose, amino acids, essential salts, and water get reabsorbed into the blood. Urea, uric acid, creatinine, and some water is left behind. The collecting duct in the ureter opens and nitrogenous waste, excess ions, and excess water are secreted out. 

There are two main solutions to kidney failure:

● Kidney transplant 

● Kidney dialysis

If the kidneys stop working, the best match is found for the patient. There are chances of rejection by the immune system. Thus, immuno-suppressive drugs are required to suppress lymphocytes. 

In a box, there is a busking tube present which acts as the renal tubule. The box is filled with a dialysing bath that acts as the tissue fluid. One end of the visking tube is connected to the renal artery. The blood will go to the renal artery and the concentration of the nitrogenous waste will be higher inside the tube. Thus, these waste materials diffuse out, and water goes from a high water potential to a lower water potential through the process of Osmosis. There are essential salts and glucose present in the dialysis bath to provide immediate energy to the patient. 

The kidney transplant is a one time hastle, while the dialysis is a long, tiring process. It is also more expensive compared to getting a transplant. One thing about the kidney transplant is that it is very hard to find a perfect match/donor. Many people are not willing to donate their kidney, and there are chances of the immune system rejecting the kidney. Meanwhile in the dialysis, there is no requirement of finding a donor or using immuno suppressive drugs.