Key points: kidneys, nephron, structure of nephron, process of urine formation.
Kidneys play an essential role in excretion. They are bean shaped, reddish brown organs which are covered by renal capsule. There is a con caved side of a kidney that has a depression where a renal artery enters and a renal vein exits towards uterus. The kidneys are located at the rear wall of abdominal cavity.
Internal structure of kidney
Kidney has an outer dark cortical region, an inner medullary region and a funnel shaped renal pelvis region.
- Renal cortex: This region contains glomeruli of nephrons. It appears to be dark and dotted due to the presence of malpighian bodies.
- Renal medulla: It is a region just inner to renal cortex. It contains renal tubules, collecting ducts and blood vessels present in the form of pyramids. The conical shape of pyramid projects into pelvis.
- Renal pelvis: It is large funnel shaped space behind the medulla region. Urine is collected in renal pelvis and passed down to uterus.
NOTE: Each kidney consists of large number of filtering units called as ‘nephrons’. It has approximately 1.3 million nephrons.
Each nephron originates in the cortex region and extends into medulla region.
Structure of Nephron
A nephron is also called as uriniferous tubule which is a functional unit of kidney. Each nephron filters a part of blood and produces a small amount of urine. It consists of two parts,
- Malpighian body
- Long coiled tubule
Malpighian body is also divided into two main parts: a) Glomerulus b) Arterioles c) Bowman’s capsule
It is like a capillary network that receives its blood from afferent arteriole from renal circulation. The glomerular blood pressure provides the driving force for fluid and solutes to be filtered out and into the space made by Bowman’s capsule. The remainder of the blood which is not filtered
into glomerulus passes into the narrower efferent arteriole. It then moves to vasa recta which are collecting capillaries.
b) Afferent and efferent arteriole
Afferent arteriole supplies blood to the glomerulus. A group of specialized cells known as juxtaglomerular cells are located around the afferent arteriole, where it enters the renal corpuscle. The efferent arteriole drains the glomerulus. Specialized cells lies between two arterioles and those are called macula densa. The juxtaglomerular cells and macula densa together forms a juxtaglomerular apparatus.
Bowman’s capsule surrounds the glomerulus and is composed of visceral and parietal layers. The visceral layer lies beneath the thickened glomerular basement membrane and it is made of podocytes. The parietal layer is lined by single layer of squamous epithelium. Between the visceral and parietal layer is Bowman’s space, into which the filtrate enters after passing through podocytes. Any small molecules such as water, glucose, salts and urea pass freely into Bowman’s space but cells, platelets and large proteins cannot pass through it. As a result the filtrate leaving the Bowman’s capsule is very similar to blood plasma in composition.
c) Long coiled tubule
It consists of three important parts as follows;
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
Its various parts are modified for absorption of salts and water from the blood. It opens into collecting duct. The capillaries that take blood out from glomerulus, form a network all over the long tubule and finally unite to form a renal vein.
Formation of Urine
Urine formation in nephron involves three basic steps
Blood enters the afferent arteriole and flows into glomerulus. Blood in the glomerulus has both filterable and non-filterable blood components. The blood flows under pressure in the renal artery. Also in the glomerulus the efferent arteriole leaving the glomerulus is narrower than afferent arterioles. Therefore this pressure helps the fluid to filter out through thin capillary walls of glomerulus. The filterable components include water and dissolved molecules while non filterable components are blood cells and proteins. The glomerular filtrate is not the same consistency as urine. Ultrafiltration is purely a physical process and energy for filtration is derived from the hydrostatic pressure of blood.
Sodium chloride reabsorbed into the system increases the osmolarity of blood in comparison to the glomerular filtrate. This reabsorption process allows water to pass from the glomerular filtrate back into the circulatory system. Glucose and amino acids are reabsorbed into circulatory system. If too much glucose appears in the glomerular filtrate it increases the osmolarity of filtrate, causing water to be released into the urine rather than reabsorbed by the circulatory system. Frequent urination and unexplained thirst are warning signs of diabetes, due to water not being reabsorbed. Glomerular filtrate has now been separated into two forms reabsorbed filtrate and non-reabsorbed filtrate. Non reabsorbed filtrate is now known as tubular fluid as it passes through the collecting duct to be processed into urine. Simultaneously any excretory wastes that were not filtered out at the glomerulus are put back into the tubule by the capillaries.
Finally at the end of the tubule, as a result of all the mechanisms that are ultrafiltration, selective reabsorption, tubular secretion the filtrate contains urea, other wastes, excess salts and small amount of water. This fluid is now called as urine. It is drained into the collecting duct. A collecting duct collects urine from several nephrons. Finally all collecting ducts drain urine into a space called renal pelvis in the kidney, from where it passes out into the ureter, and then to urinary bladder where it is stored till excreted out of the body.
Micturition is the process by which the urinary bladder empties when it becomes filled. This process involves two main steps: First, the bladder fills progressively until the tension on its wall rises above a threshold level. This elicits the second step, which is a nervous reflex called the micturition reflex that empties the bladder or if this fails, at least causes a conscious desire to urinate.
In ultra filtration process blood pressure forces the fluid and small molecules like glucose, salts and amino acids are filtered. Selective re-absorption there is active re-absorption of useful molecules such as water, glucose and salts. In case of tubular secretion the filtrate contains all the excretory waste and small amount of water.
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