[Photo credit: Jordi March i Nogué]
In mammals, there are four excretory organs:
- lungs, which excrete water vapor and carbon dioxide;
- skin, which excretes water, salts and urea through sweat;
- liver, which excretes bile pigments called bilirubin; and
- kidneys, which excrete water, mineral salts, urea, uric acid, and creaunine.
The excretory system of mammals include the kidneys, the ureters, the bladder, the urethra, the associated renal artery and renal vein. The kidney is supplied with blood vessels. The renal artery supplies blood to the kidney while the renal vein takes takes away blood from the kidney.
A long narrow tube called ureter connects the kidney to the urinary bladder where urine is stored temporarily. The urinary bladder leads to the urethra which opens to the exterior from which urine is finally passed out.
THE KIDNEY
THE KIDNEY
The mammalian kidney is a bean shaped and reddish brown organ, located at the posterior end of the abdomen. The right kidney is slightly lower in the body than the left kidney. The paired kidneys are the excretory organs of mammals. They remove unwanted nitrogenous substances like urea and other ammonium compounds from the blood. The also maintain the osmotic pressure of the blood by controlling the excretion of water and salts.
In a longitudinal section, the kidney has two distinct regions: cortex and medulla. The cortex is the outer region and the medulla is the inner region. Over one million fine narrow tubules called the urinary tubules pass through both regions. They open at the tips of triangular-shaped masses of tissues called pyramids.
The pyramids open into a funnel-shaped cavity called the pelvis. The pelvis is continuous with the ureter. The kidney has many tiny capillaries which are branches of the renal artery and renal vein.
THE URINARY TUBULE
The urinary tubule is the functional unit of the kidney. Each urinary tubule starts in the cortex as a cup-shaped structure called the Bowman's capsule. The capsule opens into a short coiled tube referred to as the proximal convulated tubule. Then it straightens out as it passes into the medulla where it makes a U-shaped loop called the Henle's loop before re-entering the cortex.
In the cortex, the tubule becomes coiled again to form the distal convulated tubule. The tubule bends once again and completes its courses in the medulla.
The tubule widens as it approaches the pelvis. Along with many other tubules, it pours its contents into a wider main collecting ducts which eventually join up and open into the pelvis at the apices of the pyramids.
The renal artery branches in the kidney. Each branch breaks into a mass of blood capillaries in the Bowman's capsule. This mass of capillaries is called the glomerulus.
The capillaries in the glomerulus rejoin to form a blood vessel leading out of the capsule. This vessel then branches into a capillary network around the urinary tubule before rejoining to form a branch of renal vein.
MECHANISM OF EXCRETION IN MAMMALS (FORMATION OF URINE)
The processes involved in the formation of urine are as follows:
- ultra filtration;
- selective reabsorption; and
- hormonal secretion.
1. Ultra filtration: In the first phase, blood is brought to the kidney by renal arteries. As it circulates through the glomerulus of each Bowman's capsule, water, urea, nitrogenous compounds, mineral salts, sugars, glucose and plasma solutes are filtered into the capsule.
This process of filtering materials from the glomerulus into the Bowman's capsule is called ultra filtration.
2. Selective reabsorption: The fluid in the capsule or glomerular filtrate now flows down the tubule. At the proximal convulated tubule and Henle's loop, some water, sugars, amino acids and salts which are useful to the body are reabsorbed into the blood capillaries against concentration gradient or by active transport.
This process of reabsorbing useful materials back into the blood is called selective reabsorption.
3. Hormonal Secretion: The fluid in the tubule becomes more concentrated as it flows through the distal tubule where more water is reabsorbed by the action of Anti-diuretic Hormone (ADH) and urine is finally formed.
The urine gradually trickles into the renal pelvis and propelled by peristalsis through the ureter into the urinary bladder. When the bladder is full, it contracts and discharges the urine through the urethra.
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