Human waste and how it is eliminated from their body is extensively taught in this chapter. The topics covered are Urine Formation, Human Excretory System, Procedure of Concentration of the Filtrate, Functions of the Tubules, Micturition, Regulation of Kidney Function, Disorders of the Excretory System, Role that Organs play in Excretion, and more. Below is a short explanation of the topics included in this chapter.

• Explaining the Human Excretory System
• How is urine formed?
• Functions performed by the Tubules
• Procedure for Concentration of the Filtrate
• Regulations related to Kidney Function
• Micturition
• Role other organs play in Excretion
• Disorganization in the Excretory System

This chapter instructs students about the excretory system of humans, its corresponding organs, and each organ's respective functions. The chapter also highlights a vital human organ, which is the kidney. The kidney performs the function of filtering the liquid that it receives. Students would also learn how the kidneys regulate their operations, how the micturition process is carried out, and the importance the related organs hold for the excretory system.

Q1. Define Glomerular Filtration Rate (GFR)

Answer: Glomerular filtration rate- It refers to the amount of glomerular filtrate formed in all the nephrons of both kidneys per minute. In a healthy individual, it is about 125 ml/minute. The glomerular filtrate contains glucose, amino acids, sodium, potassium, urea, uric acid, ketone bodies, and large amounts of water.

Q2. Explain the autoregulatory mechanism of GFR.

Answer: The kidneys have built-in mechanisms for the regulation of the glomerular filtration rate. One such mechanism is regulated by the Juxtaglomerular apparatus. Juxtaglomerular apparatus is a microscopic structure located between the vascular pole of the renal corpuscle and the returning distal convoluted tubule of the same nephron. It plays a role in regulating the renal blood flow and glomerular filtration rate. When there is a fall in the glomerular filtration rate, it activates the juxtaglomerular cells to release renin. This stimulates the glomerular blood flow, thereby bringing the GFR back to normal. Renin brings the GFR back to normal by the activation of the renin-angiotensin mechanism.

Q3. Indicate whether the following statements are true or false:
a. Micturition is carried out by a reflex.
b. ADH helps in water elimination, making the urine hypotonic.
c. Protein-free fluid is filtered from blood plasma into  Bowman’s capsule.
d. Henle’s loop plays an important role in concentrating the urine.
e. Glucose is actively reabsorbed in the proximal convoluted tubule.

Answer:
a. Micturition is carried out by a reflex. True statement
b. ADH helps in water elimination, making the urine hypotonic. False statement
c. The protein-free fluid is filtered from blood plasma into Bowman’s capsule. True statement
d. Henle’s loop plays an important role in concentrating the urine. True statement
e. Glucose is actively reabsorbed in the proximal convoluted tubule. True statement
 

Q4. Give a brief account of the countercurrent mechanism.

Answer: The function of the countercurrent mechanism that operates inside the kidney is to conserve the water and make the urine concentrated. The countercurrent mechanism depends upon the loops of Henle, vasa recta, collecting ducts, and interstitial fluid. The blood flows in the two limbs of the tube in opposite directions giving rise to the counter-currents. The proximity between the loop of Henle and vasa recta, as well as the countercurrent in them, helps in maintaining an in increasing osmolarity towards the inner medullary interstitial fluid i.e. 300 mOsmol/L in the cortex to 1200 mOsmol/L in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of the loop of Henle which is exchanged with the descending capillary of the vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of the loop of Henle which is transported back to the medullary interstitial fluid by the collecting duct.
The countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitial fluid which helps in the easy absorption of water from the filtrate present in the collecting duct so that the concentration of the filtrate is increased. The overall function of the countercurrent mechanism is to concentrate sodium chloride in the interstitial fluid and cause water to diffuse out of the collecting ducts and concentrate the urine. This leads to the production of hypertonic urine.
 

Q5. Describe the role of the liver, lungs, and skin in excretion.

Answer: Apart from kidney, liver, lungs, and skin also play important roles in the process of excretion.

1. Liver- The liver is the largest gland in vertebrates which helps in the excretion of compounds like cholesterol, steroid hormones, vitamins, drugs, and other waste materials via the production of bile. Urea is formed in the liver by the ornithine cycle whereas ammonia which is a toxic substance is quickly changed into urea in the liver and hence gets eliminated from the body. The liver is also responsible for changing the decomposed hemoglobin pigment into bile pigments in the form of bilirubin and biliverdin.

2. Lungs: The lungs help in the removal of waste materials such as carbon dioxide from the body.

3. Skin: Skin possesses many glands that help in excreting waste products through pores in the skin. The two types of glands present in the skin are sweat and sebaceous glands. The sweat glands are highly vascular and tubular glands that separate the waste products from the blood and excrete them in the form of sweat. Sweat also removes excess salt and water from the body. Sebaceous glands are branched glands that secrete an oily secretion called sebum.

Q6. Explain micturition.

Answer: The process by which the urine from the urinary bladder is excreted out is called micturition. As the urine accumulates, the muscular wall of the bladder expands and causes the stimulation of the sensory nerves in the bladder, setting up a reflex action. This reflex stimulates the urge to pass out urine. To discharge urine, the urethral sphincter relaxes, and the smooth muscles of the bladder contract. This forces the urine out from the bladder. An adult human excretes about 1 to 1.5 liters of urine per day.

Q7. Match the items of Column I with those of column II :

Answer: Terrestrial animals are either ureotelic or uricotelic, and not ammonotelic because of the following two reasons:

1. Since ammonia is highly toxic, it needs to be converted into a less toxic form such as urea or uric acid.
2. Terrestrial animals need to conserve water. Ammonia being soluble in water, cannot be eliminated continuously. Hence, it needs to be converted into urea or uric acid which are insoluble in water. 
 

Q10. What is the significance of the juxtaglomerular apparatus (JGA) in kidney function?

Answer: Juxtaglomerular apparatus is an apparatus consisting of a few cells of the glomerulus, distal tubule, and afferent and efferent arterioles It is involved in maintaining the glomerular filtration rate in the kidney. JGA is found to be located in a specialized region of a nephron, where the afferent arteriole and the distal convoluted tubule are in direct contact with each other. The juxtaglomerular apparatus consists of certain specialized cells of the afferent arteriole. These cells are known as juxtaglomerular cells and they contain the enzyme renin that can sense blood pressure. When there is a decrease in the glomerular filtration rate, the juxtaglomerular cells get activated and release renin which functions to convert the angiotensinogen in the blood into angiotensin I and further into angiotensin II. Angiotensin II is a powerful vasoconstrictor that can increase the glomerular filtration rate or glomerular blood pressure. Angiotensin II further stimulates the adrenal cortex of the adrenal gland to produce aldosterone which increases the rate of absorption of sodium ions and water from the distal convoluted tubule and the collecting duct. Later on, it leads to an increase in blood pressure and glomerular filtration rate. This mechanism is known as the renin-angiotensin mechanism, ultimately leading to increased blood pressure. Juxtaglomerular apparatus plays an important role in the renin-angiotensin mechanism.
 

Q11. Name the following:
a. A chordate animal having flame cells as excretory structures.
b. Cortical portions projecting between the medullary pyramids in the human kidney.
c. A loop of capillary running parallel to Henle’s loop.

Answers:
a. A chordate animal having flame cells as excretory structures is Amphioxus.
b. Cortical portions projecting between the medullary pyramids in the human kidney are called columns of Bertini.
c. A loop of capillary running parallel to Henle’s loop is the vasa rectum.
 

Q12. Fill in the gaps :

a. The ascending limb of Henle’s loop is _____ to water whereas the descending limb is to it.
b. Reabsorption of water from distal parts of the tubules is facilitated by the hormone _______.
c. Dialysis fluid contains all the constituents as in plasma except_______.
d. A healthy adult human excretes (on an average)_______gm of urea/day.

Answer:
a. Ascending limb of Henle’s loop is impermeable to water whereas the descending limb is permeable to it.
b. Reabsorption of water from distal parts of the tubules is facilitated by the hormone Vasopressin or ADH (Antidiuretic Hormone).
c. Dialysis fluid contains all the constituents as in plasma except nitrogenous wastes.
d. A healthy adult human excretes (on average) 25-30 gm of urea/day.