Urinary system of a human body

The role of the urinary system



The waste products that accumulate in the body of the organisms must be removed from the body so that the organisms are protected from internal toxic substances. Hence, kidneys facilitate the excretion process of the body through the specialized filtration units called nephrons.   

Importance of excretion with prime focus on the importance of the removal of nitrogenous waste and carbon dioxide from the body

The cells of the body utilized nutrients and oxygen to function and additionally, produces significant metabolic wastes that include:

  • Urea, which is formed when the protein substances are broken down inside the liver
  • Carbon dioxide, which is regarded as the by-product of cellular respiration

The excretory system and its associated parts are responsible for the removal of harmful metabolic wastes from the circulatory system so that high concentrations of waste are not developed in the body (Chew and Ip, 2017).  The excretory system maintains the salt-water balance of the body; as well as helps in the removal of cellular wastes. Thus, excretion is considered to be a special entity in the regulation of homeostasis. Hence, the organism is able to maintain a constant body temperature. Nitrogenous wastes like urea are produced when the cells break proteins. Hence, kidney functions effectively to remove these harmful substances from the body.

On the other hand, carbon dioxide is extremely harmful to the organism and its high concentration is poisonous for the body. Hence, accurate removal or excretion is required from within the body. It is accomplished through three specific biological processes. One is the direct dissolved state of carbon dioxide into the bloodstream. The second one is that carbon dioxide is bound to the hemoglobin in the blood (Pavlovic, 2015). The last one is that the carbon dioxide is buffered with water depicted as carbonic acid, which is a component of the bicarbonate buffer system. The lungs are responsible for regulating the removal process of carbon dioxide so that blood pH is maintained. Carbon dioxide removal is essential as hydrogen ions are released by carbon dioxide into the blood that makes the blood extremely acidic.  

Structure and function of the kidney

The kidneys are two in number and are bean-shaped in structure. It is located at the upper backside of the abdominal cavity holding the position on either side of the spine below the ribcage. The right kidney is always at a lower position than the left due to the location of the liver. Renal hilum is a notch situated at the concave periphery of each kidney (Jourde-Chiche et al., 2019). Here, the blood supply to the kidney is facilitated by the renal artery containing nutrients and oxygen. This is also associated with the flow of waste products and the renal vein is responsible to carry the filtered blood out of the kidney. Additionally, the entry of lymph vessels, ureter and nerves are guarded by the hilum.

The ureter being muscular tube transports urine from the kidney to the bladder and the waste materials are released out of the body. A renal capsule covers the kidneys, which is a solid sheet of connective tissue. The kidneys are further covered with fat layers and are anchored to the rear abdominal wall to protect the organ from further desiccation (Li et al., 2015). The inner organ is usually segregated into three parts, namely, the cortex, medulla, and pelvis. The major functions performed by the kidneys are filtration of toxic substances from the bloodstream. Moreover, it retains, as well as removes electrolytes and water by producing urine. The kidney parenchyma tissue consists of millions of filtration units denoted as nephrons. Each nephron includes renal corpuscles and renal tubule and together they form the functional unit.  

Image result for diagram of a kidney

Figure 1: Anatomy of Kidney

(Source- Li et al., 2015)

Processes of ultrafiltration and selective reabsorption within a nephron

The first stage of urine formation is ultrafiltration that begins with the entrance of blood into the glomerulus through afferent arteriole. Bowman's capsule receives water, ions, glucose and amino acids that leave the bloodstream (Lawrence et al., 2018). The afferent arteriole is wider than the efferent arteriole that generates extensive hydrostatic pressure. The leaky capillaries enhance the movement of fluid in and out of the bloodstream.

Figure 2: Structure of Nephron

(Source- Lawrence et al., 2018)

The proximal convoluted tubule (PCT) then receives the glomerular filtrate that acts as the prime site of selective reabsorption. In PCT, amino acids and glucose are reabsorbed through active transport mechanism, while passive reabsorption talks place for water through osmosis (Cullen-McEwen et al., 2016).  The substances which are filtered by the renal capsule must also be needed y the body. It includes water, glucose and some inorganic ions. These substances are taken back into the bloodstream via the PCT walls. Hence, it is known as selective reabsorption. Na+ exits the exterior surface of the PCT cell into the blood by the active transport mechanism. The concentration of Na+ is lowered inside the cell and hence, it gets diffused from inside the tubule (Conlisk et al., 2017). Protein transporters present in the cell surface receptors help in the diffusion of Na+. Glucose molecules are carried by Na+ through the transporter proteins. It is referred to as co-transport. Glucose molecules get directly diffused into the blood. The water potential is decreased when glucose and Na+ move into the blood. Hus, water movement is down the water potential gradient through osmosis.

Figure 3: Ultrafiltration and selective reabsorption in kidneys

(Source- Conlisk et al., 2017)

Role of kidney, hypothalamus and pituitary gland in the control of water content of the body

A balance has to be maintained between the water gained and the water lost by the organism. The amount of water gained is dependent on the diet followed by the organism through food and drinks, as well as the water released through cellular respiration (Blair et al., 2017). Moreover, the amount of water lost by the organism is associated with sweating, feces, urine, and evaporation. This is regulated by the action of the hormone ADH (anti-diuretic hormone). The hypothalamus when detects deficiency of water in the blood sends a signal to the pituitary gland. The pituitary gland after being triggered released ADH. ADH travels through the bloodstream and enters kidneys affecting the renal tubules. This stimulates the tubules to reabsorb water into the blood. Thus, a small volume of concentrated urine is produced by the kidney. Additionally, the level of water in the bloodstream enhances until the level reaches back to its normal state.   

Figure 4: Role of kidney, hypothalamus and pituitary gland in the control of water content of the body

(Source- Blair et al., 2017)


The excretion process in the body is carried out by kidneys, which are mainly responsible to filter nitrogenous wastes. In this report, the emphasis is made on the excretion of nitrogenous wastes and carbon dioxide and the different filtration steps mediated by the nephrons.




Blair, E.T., Clemmer, J.S., Harkey, H.L., Hester, R.L. and Pruett, W.A., 2017. Physiologic mechanisms of water and electrolyte disturbances after transsphenoidal pituitary surgery. World neurosurgery107, pp.429-436.

Chew, S.F. and Ip, Y.K., 2017. Nitrogen Metabolism and Nitrogenous Waste Excretion. In Fishes Out of Water (pp. 167-194). CRC Press.

Conlisk, A.T., Datta, S., Fissell, W.H. and Roy, S., Ohio State University Research Foundation, 2017. Selective ultrafiltration membranes for renal replacement therapies. U.S. Patent 9,737,653.

Cullen-McEwen, L., Sutherland, M.R. and Black, M.J., 2016. The human kidney: Parallels in structure, spatial development, and timing of nephrogenesis. In Kidney development, disease, repair and regeneration (pp. 27-40). Academic Press.

Jourde-Chiche, N., Fakhouri, F., Dou, L., Bellien, J., Burtey, S., Frimat, M., Jarrot, P.A., Kaplanski, G., Le Quintrec, M., Pernin, V. and Rigothier, C., 2019. Endothelium structure and function in kidney health and disease. Nature Reviews Nephrology15(2), pp.87-108.

Lawrence, E.A., Doherty, D. and Dhanda, R., 2018. Function of the nephron and the formation of urine. Anaesthesia & Intensive Care Medicine19(5), pp.249-253.

Li, X., Chuang, P.Y., D’Agati, V.D., Dai, Y., Yacoub, R., Fu, J., Xu, J., Taku, O., Premsrirut, P.K., Holzman, L.B. and He, J.C., 2015. Nephrin preserves podocyte viability and glomerular structure and function in adult kidneys. Journal of the American Society of Nephrology26(10), pp.2361-2377.

Pavlovic, M., 2015. Waste Disposal from the Body. In Bioengineering (pp. 169-185). Springer, Cham.

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