Functions of Renal Tubules

When we ingest something, that undergoes many different processes to get digested. Egestion is an important process in which the unwanted material of the body is thrown out. Different organisms have different excretory products. For e.g., uric acid, ammonia, urea etc. Every species has its own way of excretion and have different organs and processes for excretion. Sometimes, we often get confused about excretion and secretion. So, to clarify, the word secretion means that production of a particular compound for a certain function whereas excretion is the removal of waste products from the body.  Different organs which are used for excretion are kidneys, skins, etc. The mechanism behind the process of excretion is that whenever our body digests food, it leaves some waste product or residue which is harmful to our body, and it needs to be removed for that excretion is there. As we have told you that different organisms have different excretory products and on the basis of the excretory products organisms are classified into three categories.
First is ammonotelic animals, organisms that have ammonia as their excretory product. For e.g., Amphibians, Porifera, Platyhelminthes, Crustaceans, etc. Second is ureotelic animals, they have urea as their excretory product. For e.g., Fish, adult amphibians, mammals, etc. Third is uricotelic animals, they have uric acid as their excretory product. For e.g., Reptiles, Birds, Insects, etc. 

Urine Formation

Urine is the excretory product in liquid form. It is very important for the nitrogen cycle of the earth. It is used for fertilizing the soil and also has a role in the development and growth of plants. It has many other advantages like it is used in making gunpowder in ancient times. Urine is also used for tanning leather and dyeing of textiles.
In the formation of urine in humans, there is a proper mechanism that includes many steps that are as follows.

1. Glomerular filtration
2. Reabsorption
3. Secretion

Glomerular Filtration Rate

It is defined as the total amount of the fluid filtered by the kidney per unit time, i.e., from the kidney capillaries to Bowman’s capsule. The difference between the high blood pressure of afferent arteriole vs low blood pressure of efferent arteriole. When any solute gets filtered and is not absorbed by the kidney, then the rate of glomerular filtration is equal to the renal clearance rate. There are many different methods to calculate GFR. The most commonly used method is that GFR is equal to the ratio of urine flow multiplied by urine concentration by plasma concentration. One of the main functions of kidney is the maintenance of glomerular filtration rate. Volume of blood plasma that is cleared of creatinine per unit time is called as creatinine clearance rate. This creatinine clearance rate is useful to measure GFR. Through the blood test, the amount of GFR and creatinine clearance rate can be calculated. Many chronic kidney diseases can be identified by the amount of GFR in the body, such as albuminuria. In man the average GFR is 100-130 average 125Ml/min/1.73m2. In women, this range is 90-120ml/min/1.73m2. In children the range differs according to the amount of insulin clearance, i.e., 110ml/min/1.73m2 till two years.

Renal tubules 

Lengthy and tangled structured renal tubules that arise from the glomerulus and are parted into three different parts based on their particular functions.

• It stays in the renal cortex and is called proximal convoluted tubule (PCT) as because of its vicinity to the glomerulus.
• The loop of Henle is the second portion. It is called so because of the loop it forms that get along through the renal medulla.
• The third fragment is called as distal convoluted tubules (DCT) and this is bounded to the renal cortex. 

PCT (Proximal Convoluted Tubules)

• Mostly reabsorption occur in PCT as the blood carried by renal artery gets filtered by the glomerulus, and then it goes to PCT.
• PCT is responsible for reabsorption of amino acids, proteins, glucose, electrolytes etc.
• The reabsorption process is an active process in PCT, i.e., it needs energy for its excess.
• Simple cuboidal epithelium lines the surface area used for reabsorption.
• PCT maintains the acid-base and electrolyte balance in the body by secreting selective ions like hydrogen, potassium, ammonium in the filtrate in exchange for bicarbonate ions.

Functions

• Mostly sodium ion gets reabsorbed here only as it has to supply these ions to other downstream sites so that they can balance the rate of absorption of ions so to maintain the balance of sodium ions in steady state.
• Maintains the pH of the filtrate by exchanging hydrogen ions for bicarbonate ions.
• Secretion of organic acids like creatinine and bases is also the function of PCT.

Henle’s Loop

• Reabsorption is limited in this portion, as it only reabsorbs water and electrolytes.
• This loop is divided into two parts, ascending limb and descending limb.
• These limbs show different functions and absorbs different minerals like ascending limb is assigned for the reabsorption of electrolytes, so it cannot reabsorb water.
• The descending limb is used for the reabsorption of water, and it is not allowed to reabsorb electrolytes. 
• As the reabsorption is done, the filtrate gets diluted as it moves further.

Functions

• It concentrates the urine by absorbing most of the water.
• Major function is the absorption of water and electrolytes.
• Absorbing water is the function of descending limb, whereas descending limb is designated to reabsorb minerals.  

DCT (Distal Convoluted Tubule)

• Last part of the nephron, combines and vacant its components into a collecting duct.
• DCT maintains the sodium-potassium level and pH in the blood by secreting potassium, hydrogen ions in exchange for bicarbonate ions, just like PCT.
• In DCT, limited reabsorption will take place of water and sodium ions.

Functions

• It regulates the volume of extracellular fluid and also maintains electrolyte homeostasis.
• Through electrolyte pathways, they reabsorb sodium and chloride ions.
• DCT also responds to stimulation of sympathetic-nerve.

Collecting Duct

The electrolyte balance of blood is maintained by the secretion of hydrogen and potassium ions by a long, straight tube called a collecting duct.

The highest amount of water is reabsorbed here so that concentrated urine can be released.

Functions

• The main function is the purification of blood and the formation of urine.
• Another major function of the collecting duct is to transport urine to the pelvis.
• Majorly reabsorbs water and secretes NaCl.

Reabsorption

When the important nutrients get reabsorbed through neurons and are returned to the blood. This is called reabsorption.  It is termed so because these nutrients are once absorbed by the body, and it is needed to absorb again, which are on their way to becoming urine. In the epithelial cells, there is a basolateral membrane where there is a sodium-potassium ATPase channel which is used in the reabsorption process. After passing this channel, the urine gets concentrated as the nutrients get reabsorbed. The main unit of the kidney is nephrons, which are divided into different segments responsible for reabsorbing different minerals which flowed in urine formation. The fluid leaving the convoluted tubules will have the same osmotic potential as in the glomerular filtrate. In, cotransport channels through sodium gradient and secondary active transport other solutes like glucose, amino acids, etc. are reabsorbed. For reabsorption, some tubules play an important role in urine formation.

Conceptual Questions

Question 1: List down the sites in the glomerular where limited reabsorption occurs, and what are the minerals that are reabsorbed there?

Answer: 

 PCT (Proximal Convoluted Tubules). It is the first portion in the nephron where most of the reabsorption occurs. It absorbs most of the minerals, ions, electrolytes, etc. For example glucose (100%), amino acids (100%), urea (50%), sodium (70%), potassium (70%), phosphate (70%), calcium (70%), magnesium (30%) and water (70%).
DCT (Distal Convoluted Tubules). It is the second last portion where limited reabsorption occurs, as it is permeable to a few minerals and ions only. For example, sodium (5%), calcium (8%), magnesium (5%), urea, and water are variable which are reabsorbed up to some extent.

Question 2: Give the composition and importance of urine.

Answer:

 Urine is composed of 95% water, creatinine 0.1%, urea 2%, uric acid 0.03%, sodium, sulfate, potassium, and other ions and molecules, and proteins are found in traces.
Importance of urine- As we eat anything the waste products are secreted by the body which is needed to move out of the body. The liquid waste is secreted in the form of urine. The formation of urine also regulates blood osmolarity, plasma composition, and fluid volume. If it is not secreted out of the body then it will cause many different problems to the heart, kidneys, and lungs in the body. 

Question 3: What is the name of excretory glands in prawns and also give their functions.

Answer:

 Green glands are the excretory glands in the prawns which are opaque-white in color and pea-sized and are located in the coax of each antenna, and they secrete ammonia.

Question 4: Give the importance of sebaceous glands.

Answer:

 Expect from the palms, soles, and dorsum of the feet, these glands are present on the surface of the entire body. Mostly concentrated in the face, where it is the site of acne. Through sebum, these glands remove substances such as wax, triglycerides, cholesterol, etc. Skin is protected against friction by sebum, and it makes skin more impervious to moisture. 

Question 5: Name the excretory framework in amoeba.

Answer:

 Contractile vacuoles are the excretory organ in amoeba. The main excretory products in amoeba are ammonia and carbon dioxide. The excretion occurs by simple diffusion.

Question 6: Explain micturition?

Answer: 

The removal of urine outside the body through the urethra is known as micturition. It is also called as urination, voiding, enuresis, emiction, peeing, seeing, and pissing. It is a voluntary process that is under the control of every individual. It is normal for a healthy individual to pee 7-8 times per day. The pontine micturition center, cerebral cortex, and periaqueductal gray are the brain center with are involved in urination.