Nephron and Urine Formation Process
Recap: In the previous blog, we learnt about the structure and function of the kidneys.
The present blog explores in detail about the key structural component of the kidneys and the urine formation process. This key component is specifically responsible for filtering out the waste material from our body. It is the structural and functional unit of the kidneys, termed as nephron (Figure 1), that filters the blood and removes the toxins. It reabsorbs the necessary constituents into the blood and carries the waste to urethra in the form of urine. It plays a vital role in maintaining the of blood volume and blood pressure during the renal circulation. A kidney consists of almost 1 to 1.5 million nephrons, compacted in the medulla and cortex region. A nephron is a tubule like structure of approximately 35-55 mm in length.
Figure 1: Position and structure of the Nephron
Nephron is made up of two parts, renal corpuscle, and renal tubule. Renal corpuscle consists of glomerulus and glomerular capsule (Bowman’s capsule), wherein glomerulus comprises of the cluster of blood capillaries that receives the blood to be filtered off in the kidneys. Here, fluid and solutes dissolved in the blood are filtered into the space called Bowman’s capsule that surrounds the glomerulus. Red blood cells and large proteins, such as serum albumin, cannot pass through the glomerulus in normal conditions. However, during major kidney problems or injuries, such proteins fail to be retained in the blood and are therefore detected in the urine. The second part of the nephron, renal tubule consists of proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT) and connecting tubule. The PCT is the first site where water and salt reabsorption take place. Further, the loop of Henle is a U-shaped tube that transfers the fluid from the proximal to the distal tubule. This part is permeable to water but completely impermeable to ions and minerals, thus, causing a large amount of water to be reabsorbed. The distal convoluted tubule and collecting duct are the final sites of reabsorption in the nephron. Finally, the tubular cells secrete substances like hydrogen ion, potassium ion, etc into the filtrate (viz. urine). The excess water and waste from individual nephrons are collected in a collecting duct and are passed to ureter as a urine. Urine contains approximately 95% water and 5% nitrogenous wastes (urea, ammonia, creatinine). Apart from these, the potassium, sodium, and calcium ions are also excreted through the urine.
Meaning of scientific terms
1. Blood capillaries: These are smallest and thinnest blood vessels that are the site for transfer of oxygen and the necessary nutrients into the bloodstream.
2. Permeable: A material/lining that allows passage of solutes/molecules from one side to another
3. Impermeable: A material/lining that does not allow the passage of certain solutes/molecules from one side to another
4. Filtrate: The liquid that is made to pass through the filter, here urine
5. Ureter: A tube that connects the collecting tube of nephron to the urinary bladder
3. Brown, D. L., Walling, B. E., & Mattix, M. E. (2016). Urinary system. In Atlas of histology of the juvenile rat (pp. 395-421). Academic Press.
About the author:
Devashree is currently doing PhD in Bioprocess Technology at Institute of Chemical Technology, Mumbai. She holds a postgraduate degree in Biotechnology from Dr D Y Patil Biotechnology and Bioinformatics Institute, Pune. She is presently working towards the development of bioartificial organs for preclinical and biomedical applications. Apart from indulging in scientific research, she loves animals and is a passionate bike rider. She also likes photography, dancing, composing poems and writing inspirational quotes.