NEET Biology Notes Excretory Products And their elimination Excretory System of Human
Excretory system of human consists of the following parts :
- Kidneys (two)
- Ureters (two)
- Urinary bladder (one)
- Urethra (one)
These are bean-shaped, chocolate brown structures lying in the abdomen, one on each side of the vertebral column just below the diaphragm. The left kidney is placed a little higher than the right kidney (but reverse in rabbit). Concavity of kidney called hilum or hilus is always inwardly directed. These form the urine and. control osmotic pressure within the organism with respect to external Environment.
The excretory system of human consists of a pair of kidneys, two ureters, a urinary bladder and a urethra. The diagrammatic view of human excretory system showing its different parts is given below:
The functions and structure of kidney, with its different parts and their organisation is given in the figure below:
- Structure of NephronIt is the structural and functional unit of kidney. These are also called as renal tubules or uriniferous tubules. There are about one million nephrons in each kidney. Each nephron is about 6 cm long.
It is differentiated into four regions :
- Bowman’s Capsule
It is double walled cup and is lined by thin flat cells called podocytes. It contains group of capillaries called glomerulus. Glomerulus in the kidney acts as a dialysing bag.
- Proximal Convoluted Tubule (PCT)
It is highly coiled (convoluted) tubular structure. It is about 12-24 mm in length.
Almost whole of vitamins, glucose, amino acids, sodium and potassium, etc., are reabsorbed by active transport.
- Henle’s Loop
It is U-shaped segment. Loop of Henle is long in mammals and birds, which secrete hypertonic urine, but short in other vertebrates like reptiles, etc.
- Distal Convoluted Tubule (DCT)
It is connected to the collecting duct. Active reabsorption of some Na+ takes place. It is impermeable to H20. The DCTs of many nephrons open into a straight tube called collecting duct, where large amounts of water could be reabsorbed.
Two narrow tubes arising from the hilum are called ureters. Each of which is about 30 cm in length. These bring the urine downwards and open into urinary bladder. ‘
- Urinary Bladder
Each ureter opens in urinary bladder, It temporarily stores the urine. It can hold about 0.5-1.0 L of urine. It is absent in birds. In both reptiles and birds, ureters and rectum open into a common sac called cloaca.
It is a muscular and tubular structure, which extends from neck of bladder to outside. In females, this tube is small and serves as a passage of urine only. In males, it is long and functions as a common passage for urine and spermatic fluids
Urine is formed by mechanism of glomerular filtration, reabsorption and secretion as described below:
- Glomerular Filtration
The glomerular capillary blood pressure causes filtration of blood through three layers, i.e. the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and a basement membrane between these two layers.
The epithelial cells of Bowman’s capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores. Blood is filtered very finely through these membranes, so also considered as ultrafiltration.
The amount of the filtrate formed by the kidneys per minute is called Glomerular Filtration Rate (GFR), i.e. 125 mL/min, i.e. 180 L/day. Juxta-Glomerular Apparatus (JGA) is a sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole.
A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular blood flow and thereby the GFR back to normal.
The net driving force, which is responsible for ultrafiltration through glomerular blood capillaries and Bowman’s capsule is provided by the difference between hydrostatic pressure at which the blood enters the glomerulus and the sum of those pressure which resist the hydrostatic pressure called Glomerular Filtration Pressure (GFP).
About 99% of the filtrate is usually reabsorbed by the renal tubules. This is called reabsorption. The tubular epithelial cells in nephron carryout this by passive or active mechanisms, e.g. glucose, amino acids, Na+, etc., are reabsorbed actively while nitrogenous wastes are absorbed by passive transport.
Tubular secretion is opposite to tubular reabsorption. Most substances that enter the tubule by tubular secretion move by active transport.
It plays a minor role in function of human kidneys but in animals like marine fishes and desert amphibians, whose nephrons do not have developed glomeruli. Their urine formed is maintained by the tubular secretion of urea, creatinine and mineral ions.
Composition of Urine
It is pale yellow coloured fluid due to the presence of urochrome pigment. It is acidic (average pH 6.0) in nature and is slightly heavier than water. It has a faint aromatic odour due to urinod. Daily urine output in normal adult is 1.5-1.8 L Water is 95-96%, urea is 2%, salt is 2% (NaCl, chloride) and some other substances like uric acid, creatinine, etc., are 0.4%.
Osmoregulation and Regulation of Kidney Functions
- In humans, the kidneys play an important role in osmoregulation.
- Osmotic pressure of the blood influences many cellular activities, therefore, the amount of H20 in the body fluids must be regulated. In case, the volume of body fluids falls below the normal, glomerular filtration slows down due to decrease in blood pressure and filtration pressure in the glomerular capillaries. This stimulates the posterior pituitary lobe to release ADH (Antidiuretic Hormone).
- The ADH increases the reabsorption of water in the Distal Convoluted Tubule (DCT) and the collecting duct. Less filtration and more reabsorption of water lead to the discharge of small amount of hypertonic urine. This raises the volume of body fluids to the normal.
- Reabsorption of sodium is controlled by a hormone aldosterone produced by the cortex of adrenal glands. A reduction in the level of Na+ stimulates JGA apparatus to form a protein called renin. Renin acts as an enzyme, which converts plasma protein angiotensinogen into angiotensin.
- Reabsorption of Na+ brings about the uptake of an osmotically equivalent amount of water. Absorption of sodium and water increases the blood volume and pressure.
- Thus, the Renin Angiotensin-Aldosterone System (RAAS) maintains homeostasis. Thus, kidneys regulate body fluid osmoregulation by controlling amount of water with the help of the hormone ADH and sodium with the help of aldosterone and proteins by renin and angiotensin.
Atrial Natriuretic Factor (ANF)
An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decreases the blood pressure. ANF mechanism keeps check on the renin-angiotensin mechanism.
Role of other Organs in Excretion
- Lungs, liver and skin also help in excretion in addition to kidneys.
- Lungs remove about 18 L/hour of C02 and major quantities of water everyday.
- Liver secretes bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which passes out along with digestive water.
- Skin eliminates certain wastes with sweat, such as NaCl, some urea, lactic acid, etc. Sebaceous glands in skin eliminate substances like sterols, hydrocarbons and waxes through sebum.
Artificial Kidney and Dialysis
Artificial kidney called haemodialyzer is a machine that is used to filter the blood of a damaged kidney. This process is called haemodialysis.
Haemodialysis is the separation of small molecules from large molecules in a solution by interposing a semipermeable membrane between the solution and water. Blood of the patient is pumped from one of the arteries into the cellophane tube after cooling it to 0°C and mixing with an anticoagulant (heparin). Pores of the cellophane tube allow urea, uric acid, creatinine, excess salts, etc., to diffuse from the blood into the surrounding solution. The blood is thus, purified and then pumped into a vein of the patient. Plasma proteins remain in the blood as the pores of the cellophane are too small to permit the passage of large molecules.
Disorders of Excretory System
Some disorders of excretory system are as follows :
- Diabetes Insipidus (DI)
It is a condition characterised by excessive thirst and excretion of large amounts of severely diluted urine, with reduction of fluid intake having no effect on the concentration of urine. The common DI is caused by the deficiency of Arginine Vasopressin (AVP) also known as Antidiuretic Hormone (ADH).
It is the presence of an excessive amount of urea in the blood. It results from the decreased excretion of urea in the kidney tubules due to bacterial infection or some mechanical obstruction. Urea poisons the cells at high concentration. The high concentration of non-protein nitrogen like urea, uric acid, creatinine exists in blood due to kidney failure.
- Renal Failure (RF)
The partial or total inability of kidneys to carry-out excretory function is called renal failure or kidney failure. It is caused by tubular injury, bacterial toxins, inflammation, etc.
- Renal Calculi (Kidney Stones)
Excessive hormonal imbalance, uric acid formation, milk intake, dehydration, metabolic disturbances, etc., lead to formation of renal stones or renal calculi.
It is inflammation of renal pelvis, calyces and interstitial tissues due to local bacterial infection. Inflammation affects the counter-current mechanism and the victim fails to concentrate urine.
Symptoms are back pain and frequent and painful urination.
- Dysuria painful urination
- Polyuria unusually large amounts of urine.
- Phenylketoneuria the presence of increased level of phenylalanine in blood. It is a genetic disorder.