Internal structure of kidney
The removal or elimination of waste products (in the form of solid, liquid or gas) of metabolism from the body, is called excretion. The major excretory products are carbon dioxide, excess water and nitrogenous compounds like ammonia, uric acid, urea, etc.
Modes of Excretion
Depending upon the nature of excretory products, animals exhibit different procceses of nitrogenous excretion. Different animals excrete different nitrogenous compounds. Thus, there are mainly three modes of excretion.
Modes of Excretion
- Ammonotelism ::- Excrete ammonia ( NH3 ) which is highly toxic and highly soluble in water, e.g., aquatic animals like Amoeba, paramecium, sycon, Hydra, Prawn, etc
- Uricotelism ::- Excrete uric acid, occur in animals living in dry conditions to conserve water, e.g., snakes, lizard, birds, insects, snail, etc.
- Ureotelism ::- Excrete urea( NH2CONH2 ) which is less toxic than ammmonia eg,. human , whales , seals , kangaroo ,frog, toad , etc.
Human excretory system includes the following structures
- A pair of kidneys
- A pair of ureters
- Urinary bladder
In human beings, kidneys are the main organs of excretion. A part from kidneys, the skin, lungs and liver also help in excretion.
Functions of Different Excretory Organs
Liver ::- It excretes bile, which contains bile Human Urinary System pigments (haem). It also excretes cholesterol.
Skin ::- It has sweat glands, through which excretion of water, urea and salts takes place.
Lungs ::- It acts as excretory organs for removing the waste product carbon dioxide from the body.
The human kidneys are the major excretory organs. They are reddish brown, bean shaped structures of 10-12 cm length, 5-7 cm in width, 2-3 cm in thickness, with average weight of 120-170 gms. These are seen between last Thoracic and 3rd lumbar vertebrae on both the sides of vertebral column.
Each kidney is enclosed in a thin, fibrous covering called capsule. The outer surface of each kidney is convex while the inner surface is concave having a notch called hilum through which the supply of blood occurs by renal artery and renal vein which pass in and out of the kidney.
There are two zones seen in kidney
- An outer, dark, granular cortex and
- An inner, lighter medulla
Medulla forms medullary pyramids projecting into calyces (calyx) called renal pyramid which directs inside a bag shaped cavity known as renal pelvis which projects towards ureter. Thus, the ureter of both sides, originates forward through renal pelvis and opens inside the urinary bladder.
Each kidney is made up of coiled excretory tubules, known as nephrons, the excretory as well as the functional unit of kidney and collecting ducts associated with tiny blood vessels.
The nephron is the real controller of the chemical composition of the blood. About 1 million nephrons form the excretory organ of human beings called kidney. Nephron is a complex tubular structure that consists of following two parts.
1.Malpighian body ::- body Glomerulus along with Bowman’s capsule is called the Malpighian body or renal corpuscle which filters out large solutes from the blood and delivers small solutes to the renal tubule for modification.
- Glomerulus :- is a network of capillaries formed from afferent and efferent arteriole. Afferent arterioles branch from the renal artery, which supplies blood to the kidneys while, efferent arterioles carry blood away from the glomerulus.
- Bowman’s Capsule (Glomerular capsule) ::- is a double walled cup-like structure that surrounds the glomerulus.
2. Renal Tubules Attached to each Bowman’s capsule is a thin, long tubule called Renal tubules with three distinct region. These regions are
- Proximal Convoluted Tubule (PCT)
- Distal Convoluted Tubule (DCT)
- Henle’s loop It is a hair-pin like structure present in between PCT and DCT. DCT (ascending limb of Henle’s loop) finally opens into collecting duct.
- Collecting duct In collecting duct, a number of nephrons open. All collecting ducts open in renal pelvis and through it ureter originates, which further opens in urinary bladder.
- Loop of Henle is present in medulla, while Malpighian corpuscle, PCT and DCT of the nephron is present in the cortical region.
- When loop of Henle is short and dips little into medulla, such nephron are called cortical nephrons, while loop of Henle when runs long and deep into the medulla, nephrons are called juxtra medullary nephron.]
Functions of Kidney
- Removing the excess water and nitrogenous wastes (urea and uric acid) from blood as urine.
- Maintaining the constant concentration of blood plasma (osmoregulation).
- Regulating the pH of the blood.
The pelvis of each kidney is continued as a ureter and emerges out at hilus. Ureter is a long and muscular tube. Ureters of both sides extend posteriorly and open into the urinary bladder.
It is a thin-walled, pear-shaped, white transparent sac present in the pelvic cavity. It temporarily stores the urine.
Mechanism of Urine Formation
Uropoiesis begins in the liver with the formation of urea in ornithine cycle, Further process occurs in kidneys when blood containing urea enters into it. Here, the process is completed in three major stages namely glomerular filtration, selective reabsorption and tubular secretion.
Glomerular filteration is brought about by the high blood pressure in glomerular capillaries and the process is known as ultrafiltration. Selective reabsorption helps to filter the selective molecules from the filterate by diffusion, active transport and passive transport. In tubular secretion, the leftover in the blood is transported into the filterate. It helps to maintain ionic balance of the body fluids.
The fluid and dissolved waste substances excreted by the kidneys, (i.e., through nephrons or renal tubules) constitute urine.
1. It is a transparent yellowish fluid.
2. The colour of urine depends upon its concentration and is due to pigment ‘urochrome’ which is being derived from breakdown of haemoglobin from worn out RBC’s.
The volume of urine depends on
- Fluid intake
- Level of physical activity
- Type of food taken
- Environmental temperature
Chemical Composition of Urine
- Water = 95%
Organic substance (includes uric acid, creatinine, urea, hippuric acid, phenoic substances, traces of glucose and vitamin-C) and inorganic substance (ammonia, mineral salts such as chlorides, sulphates, phosphates of sodium, calcium, potassium) – 5% Other substances are drugs, pigments, etc.
Artificial Kidney (Haemodialysis)
Kidneys are vital organs for survial. Several factors like infections, injury or restricted blood flow to kidneys reduce the activities of kidneys. This leads to accumulation of poisonous wastes in the body, which can even lead to death.
Thus, kidney failure can be coped up by artificial kidney, a device to remove nitrogenous waste produced from the blood, through dialysis. Artificial kidney is different from natural kidney in the way that there is no reabsorption involved in artificial kidney. Normally in a healthy adult, the initial filtrate is about 180 L daily. However the volume actually excreted is only 1 to 1.8 L a day, because the remaining filtrate is reabsorbed in the kidney tubules.