EXCRETION IN MAMMALS

There are four types of excretory organs used by mammals. These are: lungs, skin, liver and kidneys. The lungs excrete water vapour and carbon dioxide, the liver excretes bile pigments called bilirubin (derived from the decomposition of haemoglobin), the skin excretes water, salts and urea through sweat while the kidney excretes water, mineral salts, urea, uric acid and creatinine. In mammals, kidneys are the major excretory organs.

 

Structure of the Kidney

The mammalian kidney is a bean shaped and reddish brown organ located in the posterior end of the abdomen. The right kidney is slightly lower in the body than the left. In a longitudinal section, the kidney has two distinct regions- an outer cortex and an inner medulla. 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 kidney has many tiny capillaries which are branches of the renal artery and renal vein.

 

The Excretory Systems of Mammals

The excretory system of mammals include the kidneys, the ureters, the bladder, the urethra and the associated renal artery and renal vein. The kidney is supplied with blood vessels. The renal artery supply blood to the kidney while the renal vein takes away blood from the kidney. A long narrow tube called the 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 Structure of the Urinary Tubule

The urinary tubule or nephron 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 convoluted 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 convoluted tubule. The tubule bends once again and completes its course in the medulla.

The tubule widens as it approaches the pelvis. Together with many other tubules, it pours its contents into a wider main collecting ducts which eventually join up and open into pelvis at the apices of the pyramids. All along its course, the tubule is closely associated with several networks of blood capillaries, the renal artery branches in the kidney. Each branch divides into a mass of blood capillaries in the Bowman’s capsule. This mass of blood 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 occur in three phases which are

• ultra filtration.
• selective reabsorption.
• hormonal secretion.

1) Ultra filtration: In the first phase, blood is brought to the kidney by renal arteries. As it circulates through the capillaries or 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 convoluted 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.