Introduction:
Literally, the word excretion means elimination of any matter from the body of an organism. Different organs and systems like digestive system, respiratory system, excretory system and skin are involved in the process of excretion. However, here the term excretion refers to elimination of principal products of metabolism except carbon dioxide i.e. removal of ammonia, urea, uric acid, creatinine, various pigments and inorganic salts.
Why do we need this system?
Among the assimilated nutrients in animals, carbohydrates and lipids are metabolized to CO2 and H2O. protein and nucleoprotein metabolism produce waste nitrogen in various forms in different animals. The waste nitrogen proves toxic if it is concentrated in the cell, therefore it must be removed from the body. That’s why we need excretory system.
Organs Involved in urinary system:
Urinary system of consist of twin of bean shaped organs called kidneys. A pair of ducts called ureters which arise from each kidney, a muscular sac like organ the urinary bladder and a tube called urethra which arise from bladder and empties urine outside the body. The bladder end of urethra has a sphincter valve. Which controls release of urine from the bladder.
Kidney: The workhorses of urinary system are the kidneys. These are twin bean shaped organ, reddish brown in color, each about the size of fist or 10cm, protected by floating ribs. Right kidney is Pushed down by the liver, so it is slightly lower than the left kidney. Surrounded by three layers (Outside layer called Renal Fascia, Middle layer called Adipose capsule and last layer or inner layer called Renal capsule give kidney its distinctive shape). Kidney is enclosed by thin membranous covering called Peritoneum. On the top of each kidney adrenal glands are present which release Harmons. The kidney received blood through renal arteries, which directly arise from the dorsal aorta. Blood drained out from each kidney by a renal vein. Site from which both renal artery and renal vein enter and leaves called Hilus.
Functions of kidney:
The kidneys do the major function of the urinary system. The other parts of the system are mainly passageways and storage areas. Functions of kidney are:
1. Homeostatic function: It maintains constancy of the interval environment of body by
(a) Regulating blood ionic composition Kidneys regulate the blood levels of several ions like Na+, K+, Ca++, Cl- etc.
(b) Regulating blood pH - It maintain blood pH by excreting variables amount of H+ into urine and conserving H CO3- in the blood.
(c) Regulating blood volume - It adjust blood volume by conserving or eliminating water in urine.
(d) Regulating blood pressure - It regulate blood pressure by secreting renin.
(e) Maintain blood osmolarity - By separately regulating loss of water and loss of solutes in the urine. Kidneys maintain relatively constant blood osmolarity 300 milli osmoles/liters.
3. Regulating blood glucose level: Kidneys can also synthesize glucose from amino acid, glutamine etc.
Internal Structure of Kidney:
Kidney is divided into three main parts. Outer region is called Renal Cortex. Middle region is called Renal Medulla which is consist of many cones shaped structure called pyramids. Urine is dropped continuously from the tips of pyramids into funnel shaped space called Pelvis of the ureter.
Nephrons:
Each kidney is composed of about a million microscopic tubules or nephrons. It is regarded as structural ad functional unit of kidney, which performs osmoregulation as well as excretion.
Each nephron begins with a spherical structure called Malpighian body. It lies in the cortex of kidney. Malpighian body is composed of cup-like Bowman’s capsule. Inside this cup lies a dense network of capillaries known as Glomerulus.
The Bowman’s capsule gives out a coiled tubule known as Proximal Convoluted Tubule. The PCT narrow and descends down into the medulla, makes a U turn and comes back in the cortex. This narrow U-shaped part of tubule is known as loop of Henle. Thus, it has descending and an ascending limb. The ascending limb of the loop of Henle becomes larger in diameter in the cortex and forms a coil called Distal Convoluted Tubule. The DCT finally opens into a common Collecting duct. The latter receives urine from a number of nephrons and drains into the pelvis of the kidney.
Blood supply to the glomerulus of a nephron begins as an Afferent Arteriole, which arise from renal arteriole. From the Glomerulus, blood is carried by the Efferent Arteriole into two capillary networks. One of the networks called Peritubular Capillaries is associated with proximal and distal convoluted tubules while the other one called Vasa recta runs straight parallel besides the limbs of the loop of Henle.
Regulatory Functions of Kidney:
There are four regulatory functions of Kidney. They are:
Ultra-Filtration
Tubular Reabsorption
Tubular Secretion
Counter-Current Exchange
Ultra-Filtration:
It is the movement of small molecules across the glomerulus into the glomerular capsule or bowman’s capsule as a result of blood pressure. When blood enters the glomerulus, blood pressure is sufficient to cause small molecules, such as water, nutrients, salts, and wastes, to move from the glomerulus to the inside of the glomerular capsule. The glomerular walls are many times more permeable than the walls of most capillaries elsewhere in the body. The molecules that leave the blood and enter the glomerular capsule are called filtrate. Plasma proteins and blood cells are too large to be part of this filtrate, so they remain in the blood as it flows into the efferent arteriole. Glomerular filtrate is essentially protein free, but otherwise it has the same composition as blood plasma. If this composition were not altered in other parts of the nephron, death from loss of nutrients (starvation) and loss of water (dehydration) would quickly follow. Most of the filtered water is obviously quickly returned to the blood, or a person would actually die from urination. Tubular reabsorption prevents this from happening.
Tubular Reabsorption
It is the movement of substances from walls of the tubules into the associated
peritubular capillary network. The osmolarity of the blood is essentially the same as that of the filtrate within the glomerular capsule, and therefore osmosis of water from the filtrate into the blood cannot yet occur. However, sodium ions (Na_) are actively pumped into the peritubular capillary, and then chloride ions (Cl_) follow passively. Now the osmolarity of the blood is such that water moves passively from the tubule into the blood. About 60–70% of salt and water are reabsorbed at the proximal convoluted tubule. Nutrients, such as glucose and amino acids, also return to the blood at the proximal convoluted tubule. This is a selective process, because only molecules recognized by carrier proteins in plasma membranes are actively reabsorbed.
The cells of the proximal convoluted tubule have numerous microvilli, which increase the surface area, and numerous mitochondria, which supply the energy needed
for active transport. Glucose is an example of a molecule that ordinarily is reabsorbed completely because there is a plentiful supply of carrier molecules for it.
Tubular Reabsorption Chart
Tubular Secretion
Tubular secretion is the another way by which substances are removed from blood and added to tubular fluid Substances. At PCT and DCT some poisonous substances & nitrogenous bases are added into tubule from the blood stream These substances are uric acid, hydrogen ions, ammonia, creatinine, histamine, and penicillin etc. The process of tubular secretion may be viewed as helping to rid the body of potentially harmful compounds that were not filtered into the glomerulus.
Counter-Current Exchange:
There are two types of counter-current exchange taking place in kidney:
Counter-current multiplier
2nd is composed of Vasa Recta
Counter-current multiplier
The structure of the loop of Henle and associated vasa recta create a countercurrent multiplier system. The countercurrent term comes from the fact that the descending and ascending loops are next to each other and their fluid flows in opposite directions (countercurrent). The multiplier term is due to the action of solute pumps that increase (multiply) the concentrations of urea and Na+ deep in the medulla.
As discussed above, the ascending loop has many Na+ pumps that actively pump Na+ out of the forming urine into the interstitial spaces. In addition, collecting ducts have urea pumps that actively pump urea into the interstitial spaces. This results in the recovery of Na+ to the circulation via the vasa recta and creates a high osmolar environment in the depths of the medulla.
Ammonia (NH3) is a toxic byproduct of protein metabolism. It is formed as amino acids are deaminated by liver hepatocytes. That means that the amine group, NH2, is removed from amino acids as they are broken down. Most of the resulting ammonia is converted into urea by liver hepatocytes. Urea is not only less toxic but is utilized to aid in the recovery of water by the loop of Henle and collecting ducts. At the same time that water is freely diffusing out of the descending loop through aquaporin channels into the interstitial spaces of the medulla, urea freely diffuses into the lumen of the descending loop as it descends deeper into the medulla, much of it to be reabsorbed from the forming urine when it reaches the collecting duct. Thus, the movement of Na+ and urea into the interstitial spaces by these mechanisms creates the hyperosmotic environment of the medulla. The net result of this countercurrent multiplier system is to recover both water and Na+ in the circulation.
Counter-Current at Vasa Recta:
The other counter current is composed of vasa Recta, the blood vessels that run parallel to the loop of Henle. Only about 1% of the blood that flows through the kidney passes through the vasa recta at a very slower speed. The blood supply to vasa recta provides sufficient nourishment and oxygen to the cells of medulla and also takes away water absorbed from the filtrate, without affecting the concentration gradient of medulla.
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