Chapter 19 Excretory Products and their Elimination
Define Glomerular Filtration Rate (GFR).
The amount of filtrate formed by the kidneys per minute is called glomerular filtration rater (GFR). GFR in a healthy individual is approximately 125ml/minute i.e. 180 lines per day.
Explain the autoregulatory mechanism of GFR.
This mechanism in Kidney is present to regulate the glomerular filtrate rate.
- Juxtaglomerular apparatus (JGA) is a specialized cellular region located where the distal convoluted tubule and afferent arteriole, come in contact with each other.
- A fall in GFR can activate the JG cells to release renin which acts through a complex series of reactions called renin-angiotensin aldosterone mechanism, which can stimulate blood flow and thereby the GFR back to normal.
Indicate whether the following statements are true or false :
(i) Micturition is carried out by a reflex.
(ii) ADH helps in water elimination, making the urine hypotonic.
(iii) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
(iv) Henle’s loop plays an important role in concentrating the urine.
(v) Glucose is actively reabsorbed in the proximal convoluted tubule.
Give a brief account of the countercurrent mechanism.
The loop of Henle and vcisa recta are responsible for concentrating the filtrate. The mechanism is called a counter-current mechanism. The flow of filtrate in limbs of Henle’s loop and vasa recta is in opposite direction so, forms a counter-current system. The proximity, as well as counter-current in them, maintains osmolarity increasing 300 mOsmolL-1 in the cortex and 1200 mOsmolL- 1 in inner medulla.
Describe the role of the liver, lungs and skin in excretion.
Liver: This largest gland in our body, secretes bile-containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out along with digestive wastes.
Lungs: Lungs remove large amounts of CO2, about 18 liters/day, and also significant qualities of water every day.
Skin: The sweat and sebaceous glands in the skin eliminate certain substances through their secretions. Sweat is a watery fluid containing NaCI, small amounts of urea, lactic acid etc. Sebaceous glands eliminate certain substances like sterols, hydrocarbons, and waxes through sebum.
Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex.
An adult human excretes on average, 1.5 liters of urine per day. The urine formed is a light yellow colored watery fluid that is slightly acidic (pH-6.0) and has a characteristic odor. On average, 25-30 gm of urea is excreted out per day. Various conditions can affect the characteristics of urine. Analysis of urine helps in the clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidneys. For example, the presence of glucose (Glycosuria) and ketone bodies (ketonuria) in urine are indicative of diabetes mellitus.
Match the items of Column I with those of column II.
Column I Column II
(a) Ammonotelism (i) Birds
(b) Bowman’s capsule (ii) Water reabsorption
(c) Micturition (iii) Bony fish
(d) Uricotelism (iv) Urinary bladder
(e) ADH (v) Renal tubule
(a) – (iii) Bony fish
(b) – (v) Renal tubule
(c) – (iv) Urinary bladder
(d) – (i) Birds
(e) – (ii) Water reabsorption
What is meant by the term osmoregulation?
Osmoregulation is the mechanism of maintaining water, blood, body fluid, and ionic (salt) balance in the body.
Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?
Land animals have an integument that is impervious to gas exchange. Ammonia is highly toxic and it has to be eliminated as rapidly as it is formed. It requires a large volume of water for its elimination. They do not have access to such a large volume of water needed for the elimination of ammonia. So, they are either ureotelic or uricotelic.
What is the significance of the juxtaglomerular apparatus (JGA) in kidney function?
The JGA plays an important role in the regulation of kidney function. A fall in glomerular blood flow / glomerular blood pressure / GFR can activate the JG cells to release renin which converts angiotensinogen in the blood to angiotensin I and further to angiotensin II. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. It also activates the adrenal cortex to release Aldosterone. Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR. This mechanism is known as the Renin-Angiotensin mechanism.
Name the following:
- A chordate animal having flame cells as excretory structures.
- Cortical portions projecting between the medullary pyramids in the human kidney.
- A loop of capillary running parallel to Henle’s loop.
- Columns of Bertini.
- Vasa recta.
Fill in the gaps:
- Ascending limb of Henle’s loop is……………… to water whereas the descending limb is………….to it.
- Reabsorption of water from distal parts of the tubules is facilitated by hormone………………
- Dialysis fluid contains all the constituents as in plasma except…………….
- A healthy adult human excretes (on an average)………….. gm of urea/day.
- impermeable, permeable
- ADH (vasopressin)
- nitrogenous wastes
- 25 to 30 gm
VERY SHORT ANSWER QUESTIONS
Name the nitrogenous waste excreted by larval and adult stage of frog respectively.
Larval stage – ammonia, Adult stage – urea.
In which organ ammonia is converted to urea?
The excretion of ammonia is called ammonotelism.
What is hemodialysis?
The process of removal of excess urea from the blood of a patient (normally suffering from uremia) using an artificial kidney is known as hemodialysis
The presence of high ketone bodies in the urine is called ketonuria.
What are the columns of Bertini?
These are the extension of the renal cortex between the medullary pyramids as renal columns.
In which part of the nephron does filtration take place?
Bowman’s Capsule/Renal Corpuscle
What difference is observed in the ascending and descending limbs of Henle’s loop with g reference to permeability to water?
Ascending limb is impermeable to water and permeable to solutes.
Descending limb is permeable to water and impermeable to solutes.
Name the body part through which ammonia is eliminated in a bony fish.
What is vasa recta ?
The U-shaped peritubular capillary that runs parallel to the Henle’s loop is called vasa recta.
What is the driving force for glomerular filtration?
The driving force for filtration is the blood pressure in the glomerular capillaries.
How are the filtration slits formed?
The podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits.
Why is glomerular filtration also called as ultrafiltration?
Blood is filtered so finely through these membranes, that almost all the constituents of the plasma except the proteins pass onto the lumen of the bowman’s capsule. Therefore, it is considered as a process of ultrafiltration.
Name the mechanism that acts as a check for the Renin-angiotensin mechanism.
Arterial Natriuretic Factor (ANF) Mechanism.
What is uremia?
Uremia is a condition of excess accumulation of urea in the blood caused by the malfunctioning ofkidneys.
What term is given to the inflammation of glomerulus in nephron?
Which limb of loop of Henle is impermeable to water?
Bowman’s capsule and glomerulus are collectively called malpighian body.
What is afferent arteriole?
Blood vessels leading to glomerulus is called afferent arteriole.
Which hormone promotes the reabsorption of water from glomerular filtrate?
Vasopressin promotes reabsorption of water from the glomerular filtrate.
SHORT ANSWER QUESTIONS
How does the proximal convoluted tubule of the nephron contribute to homeostasis?
All essential nutrients and 70-80 percent of electrolytes and water are reabsorbed by the PCT segment. So it helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions, ammonia and potassium ions into the filtrate and by absorption of HCO3-” from it.
What are the functions of nephridia? Name an animal having protonephridia
Nephridia help to eliminate nitrogenous wastes and maintain a fluid and ionic balance. Protonephridia are present in Amphioxus, Rotifers, Planaria, etc.
Kidney do not play a major role in excretion in ammonotelic animals. Justify.
Ammonia is readily soluble in water and diffuses across the body surface.
In fish, it is excreted as ammonium ions through gill surface. So kidneys do not have any significant role in the elimination of ammonia.
What are the functions of ADH?
(i) ADH facilites water absorption from the distal tubule.
(ii) It also affect kidney function by its constrictor effects also on blood vessels.
What is the ultimate method of correcting acute renal failure? Describe.
Kidney transplantation is the ultimate method in the correction of acute renal failures (kidney failure). A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimize its chances of rejection by the immune system of the host.
Mention the role of DCT in urine formation.
Distal convoluted tubule plays the following roles:
- Conditional reabsorption of Na+ & water takes place in this segment.
- It also reabsorbs HCO3-”
- It helps in the selective secretion of hydrogen and potassium ions to maintain the pH and sodium-potassium balance in the blood.
Why do persons suffering from very low blood pressure pass no urine?
The blood passes into the glomerulus under high pressure during glomerular filtration. If blood pressure is less then it results in the failure of the ultrafiltration process in the glomerulus and hence, no urine formation occurs.
Name the passage in sequence through which urine passes from kidneys to the outside in humans. How is urine prevented from flowing back into the ureters?
Kidneys → ureters → urinary bladder → urethra
Urine is prevented from flowing back into the ureters because the terminal part of each ureter passes obliquely through the bladder wall.
(a) The two human kidneys do not occur at the same level-explain.
(b) Why are Kidneys called retro-peritoneal?
(a) Left kidney is slightly longer, narrower, median, and lies at a level 1.25 cm higher than the right kidney. The right kidney is lower in position due to the presence of the right lobe of the liver.
(b) Retroperitoneal. It is space that lies between the peritoneum and vertebral column, Kidneys occur in this space so that they are lined by peritoneum only on the ventral side.
Differentiate between Cortical Nephron and Juxtamedullary Nephron.
What is the chemical composition of human urine?
Human urine is transparent, yellowish in colour and variable in chemical composition. It
consists primarily of water (95 %), with organic solutes including urea (2.6%), creatinine, uric acid, and trace amounts of enzymes, carbohydrates, hormones, fatty acids, pigments, and mucins, and inorganic ions such as Na+, K+, Cl– Mg2+, Ca2+, and phosphates.
What is Erythropoietin? What is its function?
Erythropoietin is a glycoprotein hormone that controls erythropoiesis, or the formation of red blood cells. It acts as a cytokine (proteinsignaling molecule) for RBC precursor in bone marrow. It is produced by enterstitial fibroblasts in the kidney in close association with peritubular capillary and tubular epithelial tubule.
Long ANSWER QUESTIONS
Describe the structure of the kidney.
- Kidneys are reddish-brown, bean-shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
- Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g.
- Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter.
- Inner to the hilum is a broad funnel-shaped space called th& renal pelvis with projections called calyces.
- The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner medulla.
- The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing.: calyx). The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
Describe the role of organs other than the kidney in the process of excretion in human beings.
- The organs other than kidney involved in the process of excretion are (i) Lungs (ii) Skin (iii) Liver (iv) Intestine (v) Salivary glands.
- Our lungs remove large amounts of C02 (18 liters/day) and also significant quantities of water every day. The liver, the
- largest gland in our body, secretes bile-containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs. Most of these substances ultimately pass out alongwith digestive wastes.
- The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions.
- The sweat produced by the sweat glands is a watery fluid containing NaCl, small amounts of urea, lactic acid, etc.
- Though the primary function of sweat is to facilitate a cooling effect of the body surface, it also helps in the removal of some of the wastes mentioned above.
- Sebaceous glands eliminate certain substances like sterols, hydrocarbons, and waxes through sebum. This secretion provides a protective oily covering for the skin.
Describe the structure of nephron.
Structure of nephron
A nephron has two parts-the glomerulus and the renal tubule
(i) Glomerulus: It is a tuft of capillaries formed by the afferent arteriole, which is a fine branch of the renal artery.
(ii) Renal tubule has three-part
(a) proximal convoluted tubule
(b) loop of Henle
(c) distal convoluted tubule
(a) Proximal convoluted tubule –
- The renal tubule is closed at the proximal, end; it is expanded and curved inwardly. form a double-walled cup-shaped structure called Bowman’s capsule.
- The glomerulus is located in the hollow of the Bowman’s capsule and together they constitute the renal corpuscle.
- The lumen of the capsule is continuous with the narrow lumen of the entire tubule.
- The tubule continues to form a highly convoluted proximal convoluted tubule (PCT).
(b) Loop of Henle – It arises from the end of the proximal convoluted tubule and ends at the starting of the distal tubule.
It is hairpin-like, with a descending limb (that extends into the medulla) and an ascending limb, that crosses back to the cortex.
(c) Distal convoluted tubule –
- The ascending limb, on entering the cortex becomes the distal convoluted tubule.
- It then continues as a short straight collecting tubule, that joins the collecting duct.
- Each collecting duct receives the collecting tubule of a number of nephrons.
- Many collects converge, ran through renal pyramids, and open into the renal pelvis through the openings called renal papillae, at the tip of pyramids.
Describe the process of hemodialysis.
- Blood from the artery of a uremia patient is taken, cooled to 0°C, and mixed with an anti-coagulant like heparin.
- The unit contains a coiled cellophane tube surrounded by a fluid (dialyzing fluid) having the same composition as that of plasma except for the nitrogenous wastes.
- The porous cellophane membrane of the tube allows the passage of molecules based on the concentration gradient.
- As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood.
- The cleared blood is pumped back to the body through a vein after adding anti-heparin to it. This method is a boon for thousands of uremic patients all over the world.