Though excretion in human beings takes place through lungs, skin, liver, the kidneys are the main organ of the human excretory system. They are bean-shaped organs, which weighs between 150 to 170 gms and its length ranges from 4 – 5 inches long. They are located in the retroperitoneal space in the abdominal cavity, just below the rib cage and present in the opposite direction or facing each other on both the left and right side of the body. The right kidney is slightly smaller and lower than the left kidney. Compared to males, the total area, size and the weight of the kidneys are smaller in females. Therefore, both male and female should be careful during their kidney transplantation.
The major functions of the kidneys are to:
- Maintains the body’s pH
- Reabsorption of nutrients
- Regulates blood pressure
- Excretion of wastes from the body
- Removal of excess fluid from the body
- Secret hormones that help in the production of red blood cell, acid regulation, etc.
The functional unit of the kidney is the nephron. Each kidney consists of millions of nephron which plays a significant role in the filtration and purification of blood. The nephron is divided into two portions, namely, the glomerulus and the renal tubule.
Also, read about the Regulation Of Kidney Function
Let’s go through the structural and functions of the Nephron and the renal tubules in urine formation and excretion process.
The nephrons are the basic structural and functional unit of the kidney. They are the microscopic structure composed of a renal corpuscle and a renal tubule. The word nephrons are derived from the Greek word – nephros, meaning kidney. There are about millions of nephrons in each human kidney.
Structure of the Nephron
The mammalian nephron is a long tube-like structure, its length varying from 35–55 mm long. At one end, the tube is closed, folded and expanded, into a double-walled, a cuplike structure called the Bowman’s capsule or renal corpuscular capsule, which encloses a cluster of microscopic blood vessels called the glomerulus. This capsule and glomerulus together constitute the renal corpuscle.
Functions of the Nephron
The primary function of nephron is removing all waste products including the solid wastes, and other excess water from the blood, converting blood into the urine, reabsorption, secretion, and excretion of numerous substances. As mentioned earlier, glomerulus is the specialized configuration of capillaries present within the nephron.
As the blood passes through the glomerulus with high pressure, the small molecules are moved into the glomerular capsules and travel through a winding series of tubules. The cell present in each tube absorbs different molecules excluding the glucose, water, and other beneficial molecules which are called ass the ultrafiltrate. As the ultrafiltrate molecules travel down the tubules they become more and more hypertonic, which results in more amount of water to be extracted from the ultrafiltrate before it exits the nephrons. The blood surrounding the nephrons travels back into the body through the renal blood vessels, which are free of toxins and other excess substances. The obtained ultrafiltrate is urine, which travels down via the collecting duct to the bladder, where it will be stored and released through the urethra.
The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function.
- The first part is called the proximal convoluted tubule (PCT) due to its proximity to the glomerulus; it stays in the renal cortex.
- The second part is called the loop of Henle, or nephritic loop because it forms a loop (with descending and ascending limbs) that goes through the renal medulla.
- The third part of the renal tubule is called the distal convoluted tubule (DCT) and this part is also restricted to the renal cortex.
The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts that line the medullary pyramids. The collecting ducts amass contents from multiple nephrons and fuse together as they enter the papillae of the renal medulla.
The capillaries of the glomerulus are enclosed by a cup-like structure called Bowman’s capsule. This structure extends to form highly coiled tubules called PCT. PCT continues to form the loop of Henle which ascends to DCT, which in turn opens into the collecting duct. The major function of tubules is reabsorption and the process can either be through active transport or passive transport. In addition, secretions by tubules help in the urine formation without affecting the electrolyte balance of the body.
Proximal Convoluted Tubule (PCT)
The blood brought by the renal artery is filtered by the glomerulus and then passed to the PCT. Maximum reabsorption takes place in PCT of the nephron. PCT is the region of renal tubule where reabsorption of essential substances like, glucose, proteins, amino acids, a major portion of electrolytes and water takes place. The surface area for reabsorption is facilitated by the lining of the simple cuboidal epithelium in them. Reabsorption takes place at the expense of energy, i.e., the process is active. PCT selectively secretes ions such as hydrogen, ammonia, and potassium into the filtrate and absorbs HCO3–from it. Thus, PCT maintains the electrolyte and acid-base balance of the body fluids.
Henle’s loop has a descending and an ascending limb. Being parts of the same loop, both the descending and ascending limbs show different permeability. The descending limb is permeable to water but impermeable to an electrolyte, while the ascending limb is permeable to electrolytes but impermeable to water. Since the electrolytes get reabsorbed at the ascending loop of Henle, the filtrate gets diluted as it moves towards the ascending limb. But reabsorption is limited in this segment.
Distal Convoluted Tubule (DCT)
PCT, DCT also secretes ions such as hydrogen, potassium, and NH3 into the filtrate while reabsorbing the HCO3–from the filtrate. Conditional reabsorption of sodium ions and water takes place in DCT. Thus, it maintains the pH and sodium-potassium level in the blood cells.
Collecting duct is a long, straight tube where H+ and K+ ions are secreted to maintain the electrolyte balance of the blood. This is also the region where the maximum reabsorption of water takes place to produce concentrated urine.
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