The kidneys play a dominant role in regulating both the volume and composition of the extracellular fluid. They function to ensure that the body maintains a homeostatic internal environment. They achieve this by excreting the appropriate amounts of several types of solutes. The substances that are ex greeted are both substances that are in excess, and therefore waste, alongside foreign compounds.
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When kidneys fail, several problems result. The first-line treatment of kidney impairment, dysfunction, or failure, or dialysis and kidney transplantation, though the latter is common only for advanced (end-stage) kidney disease.
Functional Kidney Anatomy
The average weight of each of the two kidneys in the human body is approximately 150g. There are two regions seen in a cross-section of a kidney. These are the outer region, called the cortex, and an inner region called the medulla. The core taxes are typically reddish-brown and appear granulated. This is due to the presence of nephrons, which is the functional unit of the kidney.
The medulla is comprised of pyramidal tissue units, called the renal pyramids. In between the pyramids, there are spaces called renal columns. Blood vessels pass through the renal columns. Renal papillae are the structures that are at the tips of the pyramids, and they point towards the renal pelvis. On average, there are approximately 8 renal pyramids in each kidney. Both the renal pyramids, combined with the adjoining cortical region comprise the kidney lobes.
The renal pelvis, which is the region next to another called the hilum, is the concave section of the kidney where blood vessels and nerves, as well as the ureter, exit. The renal pelvis itself leads to the ureter, on the outside of the kidney. On the outside of the kidney, branches of the renal pelvis into two or three extensions comprise the major calyces, which further branch out into the minor calyces.
The Structure and Function of the Nephron
Each kidney contains approximately 1,000,000 nephrons, which are comprised of a section called the renal corpuscle and the renal tubule. The renal corpuscle is comprised of a network of capillaries called the glomerulus and the capsule; a crescent-shaped chamber that surrounds it is called the bowman's capsule. The renal tubule is a long and convoluted structure that is divided into three parts.
The first, called the proximal convoluted tubule (PCT) is in the renal cortex. The second section is called the loop of Henle as it forms a loop that goes through the renal medulla. The third and final section is called the distal convoluted tubule (DCT) and is also located in the renal cortex.
The DCT, as the name suggests, is the most distant and final part of the nephron, and it connects and empties its contents into ducts that line the pyramids. All of these collecting ducts collect contents from several nephrons and are fused as they reach the papillae.
An Overview of Kidney Function
Three processes are involved in forming urine. These are glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration involves the ultrafiltration of plasma into the glomerulus. The filtrate collects in the urinary space of the Bowman's capsule and flows downstream through the tubule lumen.
In the tubular lumen, activity alters its composition and volume. Tubular reabsorption is comprised of the transport of substances out of the tubular urine. These substances are then put back into the capillary blood which surrounds the tubules of the kidney. Reabsorbed species include important ions, metabolites such as glucose and amino acids, water, and small quantities of waste products such as uric acid or urea.
In tubular secretion, several substances are transported into the tubular urine. The terms reabsorption and secretion refer to the movement of substances out of, and subsequently into, urine, respectively. Both forms of tubular transport can be active or passive, depending on the substance being moved and other conditions. Excretion is the net sum of filtered – (reabsorbed and secreted).
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What Are the Core Functions of the Kidney and How Do These Relates to Health?
The kidney is a complex organ that works in conjunction with several other organs and systems in the body. As such, they perform a variety of important functions which include:
- Regulation of the body’s osmotic pressure (also called osmolarity) by excreting dilute or concentrated urine
- Regulation of the concentrations of several ions in the blood plasma. These include Na+, Ca2+, Mg2+, K+, HCO32- and Cl+.
- Ensuring the balance of acid /base by excreting excess H+ In the case of excess acid and secreting excess HCO32- when conditions are too basic Regulation of the volume of the exercise in a fluid by controlling Na+ and water secretion
- Regulation of arterial blood pressure through adjustments of sodium excretion and the production of substances such as renin which affect blood pressure.
- Elimination of waste products of metabolism. This includes:
- Urea which is the most prevalent nitrogen-containing product of protein metabolism
- Uric acid, which is the metabolic product from the metabolism of purines
- Creatinine, which is the end-product of muscle metabolism
- Removal of drugs and foreign and toxic compounds
- Production of a select number of hormones, which include erythropoietin and 1,25-dihydroxy Vitamin D3.
- Degradation of peptide hormones which include glucagon, insulin, and parathyroid hormone
- Synthesis of ammonia, which plays a role in acid-base metabolism in the body
- The synthesis of substances that influence renal blood flow and sodium secretion. These substances include derivatives of arachidonic acids, such as prostaglandin and thromboxane, as well as a proteolytic enzyme that produces kinins, kallikrein. Prostaglandins are lipids that are produced at sites of tissue damage or infection that are implicated in injury and illness. They control inflammation, blood flow, blood clot formation, as well as several other processes which are dependent on the site of production. Thromboxane is also a prostaglandin and stimulates blood clot formation as well as stimulating vasoconstriction to limit the diameter of a blood vessel to prevent blood loss. Kinins are proteins in the blood that cause inflammation and affect blood pressure
Overall, the kidneys perform several important functions related to the homeostatic control of the extracellular fluid. They are the critical organ for waste removal and maintenance of appropriate levels of water for osmotic pressure. In the absence of homeostatic balance, muscles, nerves, tissues, and whole systems in the body fail; in individuals experiencing kidney failure, a multitude of problems arise which include high blood pressure, heart disease, anemia, mineral and bone disorders, as well as malnutrition.
- Li PK, Garcia-Garcia G, Lui SF, et al. Kidney health for everyone everywhere – from prevention to detection and equitable access to care. Clin Nephrol. 2020;93(3):111-122. doi: 10.5414/CNWKDEditorial. PMID: 32017699.
- Ogobuiro I, Tuma F. Physiology, Renal. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538339/
- Soriano RM, Penfold D, Leslie SW. Anatomy, Abdomen and Pelvis, Kidneys. [Updated 2021 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482385/
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Last Updated: Mar 8, 2022
Hidaya is a science communications enthusiast who has recently graduated and is embarking on a career in the science and medical copywriting. She has a B.Sc. in Biochemistry from The University of Manchester. She is passionate about writing and is particularly interested in microbiology, immunology, and biochemistry.
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