EXCRETORY SYSTEM
The following steps are involved in the excretory system.
STRUCTURE OF VERTEBRATE KIDNEY: -
The main excretory organs of vertebrates are kidney. Kidneys are mesodermal in origin. Each adult kidney is composed of a number of structural and functional units called nephrons or, nephric tubules or the uriniferous tubules. Basically, there are three types of kidneys met within the vertebrates.
(i) Pronephros: It is the embryonic kidney of all vertebrates except Myxine and Belladona, it is the head kidney that remains functional in the larvae of cyclostomes, fishes and amphibians but later degenerates
(ii) Mesonephros: It is the opisthonephric kidney of adult cyclostomes, fishes and amphibians and also serves to be the embryonic excretory organ in amniotes.
(iii) Metanephros: It is the functional kidney of the adult, reptiles, birds, mammals and man. The detailed study of metanephric kidney is discussed below.
✅In man, the two kidneys lie behind the peritoneum on either side of the vertebral column extending from 12th thoracic to 3rdlumbar vertebrae. In man and most mammals, the left kidney is usually a little higher than the right kidney, reaching the level of the eleventh rib. In healthy man, each dark red bean shaped kidney weighs between 120-170 gms and measures from 11-13 cm in length. Each kidney is covered by a layer of fibrous connective tissue, called renal capsule.
✅The physiological anatomy reveals that each kidney comprises about 1.2 million nephric tubules or nephrons, functional unit of kidney. Each nephron is 4–5 cm. long. It is divisible into an outer granular cortex and inner paler medulla. Hilus is the inner medial border of the kidney which is somewhat concave and has an indentation through which passes the renal arteries, veins, nerves and the lymphatics, the pelvis or the ureter.
✅The funnel shaped upper end of pelvis is formed by the joining together of about three or four major calyces each of which is intern made up of several short branches or minor calyces. The medulla of kidney comprises of some ten to fifteen pyramids which project into the calyces and separated by the renal columns of Bertini.
Structure of single nephron: -
The renal tubule or nephron was first described by Marcello Maiphigi (1666) and later by Bowman (1842). Approximately 2.2million nephrons are present in both kidneys i.e., (1.1 million in each kidney). In an adult person, a single nephron measures about 5-6 cm and is divisible into two parts.
1) Malpighian or renal corpuscle
2) Renal tubule.
(1) Malpighian or renal corpuscle: Each nephric tubule begins as a blind dilated cup shaped sac called Bowman’s capsule. It is double layered and situated in the cortex of kidney. The Bowman’s capsule is invaginated by a tuft of some 40-50capillary loops, each covered by special epithelial cells known as Podocytes. Podocytes have slit pores through which ultra-filtration occurs and forms the Glomerulus. The glomerular capillary tuft surrounded by its capsule is termed as Malpighian corpuscle. Blood enters the capsule through afferent renal arteriole and after passing through glomerular capillary network it is collected by the efferent renal arteriole.
(2) Renal tubule: The long portion of the nephron following the Bowman’s capsule is termed as renal tubule. It is divided into the following regions.
✅ The proximal convoluted tubule,
✅The loop of Henle (Descending limb and ascending limb)
✅ The distal convoluted tubule
✅Collecting duct
• PCT: The proximal convoluted tubule lumen of which is continuous with Bowman’s capsule is about 12-24mm in length lies in the cortex of kidney. It consists of large columnar cells with a brush border produced into numerous microvilli of 1 to 4mµ in size.
• The loop of Henle: It is a hair pin-shaped part of nephron after PCT. The loop of Henle consists of one descending limb and one ascending limb Descending limb of loop of Henle is permeable for water and is impermeable for salts while ascending limb of loop of Henle is impermeable for water. Ascending limb of loop of Henle is called diluting segment of nephron.
• DCT: The next part of nephron that is again highly convoluted is, called distal convoluted tubule (DCT). It lies in renal cortex. It is made up of cuboidal epithelial cells but have few scattered microvilli. DCT opens into collecting tubule.
✅Collecting tubule (CT): All collecting tubules receive many functional tubules from other nephrons and finally open into the renal pelvis as the papillary duct of Bertinii.
✅Types of nephrons in mammalian kidney
(a) Cortical Nephrons: -
The nephrons situated in cortical region are, called cortical nephrons. In cortical nephrons, loop of Henle is short. In a kidney 15 to 35% nephrons are cortical nephrons.
(b) Juxtamedullary nephrons: -
Nephrons situated near medulla part are, called juxtamedullary nephrons. It is formed by the DCT and glomerular afferent arteriole. It is so named because it lies next (juxta)to the glomerular. In juxtamedullary nephrons, loop of Henle is long and parallel blood vessels, called vasa recta are present. In a kidney 65 to 85% nephrons are juxtamedullary nephrons. It works only in condition of stress.
It consists of three types of cells.
(1) The macula dense, a part of the distal convoluted tubule of the same nephron. It acts as a chemoreceptor and are stimulated by decreased NaCl concentration and thereby cause release of rennin.
(2) Juxtaglomerular cells or granular cells which secrete rennin. These cells are smooth muscle cells of afferent arteriole which supply blood to the glomerulus. They are baroreceptors and respond to changes in the pressure gradient. They are innervated by sympathetic nerves.
(3). Extraglomerular mesangial cells (Laci’s cells) These cells are located at the junction. between afferent and efferent arterioles. They are contractile and play a role in regulation of GFR.
MECHANISM OF RENAL EXCRETION (URINE FORMATION): -
As mentioned above, kidneys are the chief organs of nitrogen excretion invertebrates. Each minute, the two kidneys filter approximately 1200 ml of blood to collect 125 ml of filtrate and form about 1 to 2 ml of urine (per minute) and thus eliminate extra water, nitrogenous wastes and inorganic salts from the body. The entire mechanism of renal excretion involves three main steps.
• Ultra filtration
• Selective reabsorption
• Tubular secretion
(A) Ultra-Filtration: -
Described first by Ludwig (1844) and later modified by Richards (1942), the ultra-filtration or glomerular filtration is the first step in urine formation. The glomerular capillaries receive the renal blood supply through afferent renal arteriole which form a high-pressure bed in glomerulus, and it is approximately 60 mm Hg (Guyton,2003). Opposing the effects of blood pressure in the glomerulus are-
(1) Colloidal osmotic pressure of plasma proteins
(2) Tubular pressure of the Bowman's capsule itself, which are approximately 32 mm Hg and 18 mm Hg, respectively. Thus, in the Bowman's capsule a blood pressure of about 60 mm Hg is being opposed by an internal pressure of approximately 50 mm Hg (colloidal osmotic pressure plus the tubular pressure) and this spares a filtration pressure of about 10 mm Hg (called the net filtration pressure) which is sufficient to cause filtration of the non-protein substances of plasma across the glomerulus (fig. 4.6). Everything except plasma proteins is filtered and the fluid collected is called ultrafiltrate. The total amount of glomerular filtrate formed each minute in all the nephrons of both kidneys is known as the glomerular filtration rate. It is approximately 125 ml/min or 180 liters/day.
Filtration Coefficient (Kf) is defined as total amount of ultra-filtrate formed by all the nephrons of both kidneys per minute per 1mm of Hg of net Filtration pressure.
Kf = 125/10 = 12.5 ml/min/mm of Hg
Another noteworthy feature of ultrafiltration is the tremendous permeability of the glomerular capillary membrane which is several hundred times higher than any capillary membrane inhuman body. Histologically it is made up of three layers:
(1) endothelium of the capillary,
(2) basement membrane, and
(3) A layer of epithelial cells that line the surface of Bowman's capsule. The fenestrae and slit pores of these layers increase permeability but prevent the filtration of all particles having an average diameter greater than 70 A. Since plasma proteins are slightly larger than 70 Aº diameter, it is possible for the glomerular membrane to prevent the filtration of all those substance with molecular weight equal to or higher than those of the plasma proteins.
✅Factor affecting Glomerular Filtration Rate (GFR)
Any change in any of these pressures affects GFR.
✔ When the blood pressure in the glomerulus raises GFR increases.
✔ When colloidal osmotic pressure (CPO) increases, GFR decreases provided other two factors are constant (COP decreases in starvation and due to under nutrition).
✔ When the tubular pressure in the Bowman's capsule increases, GFR decreases.
✔ Increase in blood volume increases GFR.
✔ Increase in cardiac output increases GFR.
✔ Decrease in the number of functional nephrons will decrease GFR (Some nephrons become non-functional due to old age).
✔ Sympathetic discharge will produce constriction in afferent arteriole and this decreases GFR.
Determination of GFR: -
GFR is taken as one of the kidney function tests. By quantifying GFR one can understand how the kidney handles various substances. Thus, substances with excretory rate less than filtration rate must undergo tubular reabsorption whereas substances with excretion rate more than filtration rate must undergo secretion.
✅Clearance Test: -
The clearance of a substance may be defined as the concentration of that substance in plasma same as in glomerular filtration and is neither reabsorb nor secreted by the tubular epithelium. It is a measure of the rate of glomerular filtration. Inulin, a polymer of fructose found in the roots of certain plants is easily filtered, it is neither reabsorbed nor secreted and has a clearance equal to GFR.
✅Transport Maximum (Tm): -
Another parameter of renal excretory function is the transport maximum which is the maximum ability of the kidney either to reabsorb or secrete a given material. For example, Tm of glucose is defined as the maximum amount of glucose that can be reabsorbed by all nephrons of both the kidneys per minute. Average TmG = 360 mg/min (320 mg/min in females and 375 mg/min in males). If the tubular load of glucose becomes 400 mg then it will be excreted in urine at the rate of 40 mg/min. When tubular load of glucose is less than TmG then no glucose is lost in urine.
Glucose lost in urine = tubular load of glucose - TmG
= 400-360
= 40 mg/min
(B). Selective reabsorption: -
The filtered fluid in the Bowman's capsule and in the tubule is called the glomerular filtrate. It has almost the same composition as the fluid that filters from the arterial end of capillaries into the interstitial fluids. The glomerular filtrate has no red blood cells but consists of about 0.03 percent proteins, a great amount of glucose, salts, nitrogenous end-products and water. About 99 percent of the water and almost all other useful substances present in the glomerular filtrate are to be reabsorbed back into the blood as the filtrate passes through various parts of the convoluted tubule.
✅ Reabsorption in the proximal convoluted tubule
About 65 to 80 percent of the glomerular filtrate is reabsorbed in the proximal convoluted tubule and this fraction is referred to as the obligatory reabsorption. The epithelial cells of the proximal tubule have a brush border composed of thousands of very minute microvilli which facilitate a rapid and active diffusion of sodium from the peritubular capillary blood. Each time a positively charged sodium ion is pumped out of the tubule, a negatively charged ion follows it and it is usually a chloride ion which thus leaves the tubule, since it is the most abundant negatively charged ion in the tubular fluid.
The peritubular capillary bed has a mean blood pressure of about 13 mm Hg which is slightly higher than the filtration pressure. The peritubular fluid now has a higher osmotic pressure because of an increased concentration. Both these factors influence the movement of water from the filtrate which also carries away, passively, substance like glucose, amino acids, and potassium, calcium and phosphate ions from the filtrate.
✅ Reabsorption in the loop of Henle
At the end of proximal tubule, the fluid is almost isotonic with blood. In the rest of its course, it is converted into urine of the appropriate concentration through a complicated sequence of events termed as the counter-current mechanism. The mechanism is basically associated with the anatomy of the loop of Henle which is a hair-pin loop between the proximal and distal convoluted tubules of nephrons. Entire mechanism of the counter-current system is as follows:
(a) The ascending limb of loop of Henle with thicker walls is impermeable to water but actively transports sodium and chloride ions passively from the tubular wall into the renal medulla. This causes an increase in the osmolarity of the interstitial fluid in the medulla.
(b) The increase in the osmolarity due to the sodium pump causes water to leave the thin-walled descending limb. This makes the filtrate increasingly hypertonic as it proceeds from the renal cortex towards the papilla.
(c) The Na+ secreted by the cells of the ascending limb of the loop of Henle into the interstitial fluid escape passively into the adjacent blood vessels or the descending limb of the loop following a concentration gradient.
(d) Thus, by this re transport of the sodium (and chloride) again and again the counter-current multiplier mechanism increases the concentration of sodium chloride in the medulla, and consequently determines the concentration of urine.
(3) Reabsorption in the distal convoluted tubule: -
The fluid reaching the distal tubule of the nephron is hypotonic and it is the antidiuretic hormone (ADH) from the hypothalamic pituitary which now controls the concentration of the urine. This ADH controlled water reabsorption is the distal and collection tubule is termed as facultative reabsorption, and its mechanism is simple. In water diuresis very little ADH is present in the blood, so the distal and collecting tubules become relatively impermeable to water and a dilute urine is excreted but when the body has little water (or say when the blood is more concentrated) a larger amount of ADH is present in the blood and the kidneys conserve more water because of tubular reabsorption Hence the urine becomes concentrated. Mechanism of urine formation in mammal.
✅Tubular Secretion: -
Tubular secretion is the final step in the urine formation. The epithelial lining of the tubule is able to collect electrolytes and water from the fluid but at the same time many foreign substances are readily secreted by the tubule. It is believed that some creatinine, potassium, phenol red, H ion and penicillin are the main substances secreted by the tubular epithelium in man.
✅Composition and characteristics of urine: -
The concentrated fluid entering the collecting tubule is called Urine. Through the way of pelvis and ureters, it is usually stored in the urinary bladder and is discharged from the body from time to time. The normal volume and composition of urine varies widely from day to day and is being governed by, among other things, the type of food and fluid taken and the amount of fluid lossby other agencies, a factor which itself depends upon environmental temperature, humidity, exercise and sweating etc.
Micturition: -
Micturition is the mechanism by which the urinary bladder empties itself when it becomes filled with the urine. The urine is basically collected in the pelvis of the kidney where all the nephrons of a kidney open through their collecting tubules. As urine collects in the pelvis, its pressure increases and initiates peristaltic waves occurring every 10 sec or so and travelling through the ureters at a velocity of about 3.0 cm/sec and pushing a little spurt of urine into the bladder, the stretch receptors located in the bladder wall and proximal urethra become stimulated creating a micturition reflex. The detrusor muscles, which make-up the body of urinary bladder, contract during the micturition reflex; the internal sphincter of the urethral opening relaxes and the urine is evacuated.
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