Excretion in Mammals
The four types of excretory organs used by mammals are Lungs, Skin, Liver and Kidney.
The lungs excrete water vapour and carbon(iv)oxide
The Liver excrete bile pigment called Bilirubin. The bilirubin is derived from the decomposition of haemoglobin. The Skin excretes water, salt and urea through sweat while the kidney excrete water, mineral salt, urea, uric acid and creatinine. In mammals, Kidney is the major excretory organ.
STRUCTURE OF THE KIDNEY
The mammalian kidney is a bean-shaped and reddish brown organ located in the posterior end of the abdomen. The kidney has two distinct regions, an outer cortex and inner medulla. Over one million fine narrow tubules called urinary tubules or nephron pass through both regions. Each urinary tubule is linked to a funnel-shaped cavity called pelvis. The kidney has many tiny capillaries which are branches of the renal arteries and renal veins.
The excretory system of mammals includes the kidney, ureter, bladder, urethra and the associated renal artery and vein. The kidney is supplied with blood vessels. The renal artery supply blood to the kidney while the renal vein takes blood away from the kidney.
The ureter connects the kidney to the urinary bladder which leads to the urethra which also open to the exterior from which the urine is finally passed out.
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 Bowman capsule which surround capillaries called glomerulus. The bowman capsule lead into a coiled tubule called proximal convoluted tubule. This goes down to form a u- shape structure called Henle’s loop before re- entering the cortex. In the cortex the tubules become coiled again to form distal convoluted tubule. The tubule bends once again and completes its course in the medulla where it empties into the collecting duct.
Mechanisms of excretion in mammal (urine formation)
The process involved in formation of urine occurs three (3) phases which are:
- 1. Ultra filtration.
Hormonal control of re-absorption
ULTRA -FILTERATION takes place in the Bowman’s capsule and is made possible by two processes namely.
- The pressure created by the narrowness of the capillaries, the barrier formed by the ENDOTHELIAL lining of the glomerular capillaries and the basement membrane of the Bowman’s capsule which retain molecules above a certain size while allowing smaller ones to pass through.
Blood in the renal artery and the arteriole is always under pressure, when it reaches glomerulus, it slows down by the narrowness of the capillaries and by the fact that the arterioles leading from the glomerulus in narrower than the one leading into it. A high pressure is developed and the blood containing dissolved substances like urea, water, glucose and mineral salt are filtered through the wall into the bowman’s capsule from the capsule in process is called ULTRAFILTRATION. Ultra filtration is the process by which waste and useful substance such as glucose, vitamin & mineral salt are filtered from the glomerulus to the bowman capsule as a result of the pressure created by the blood inside the capsule.
SELECTIVE RE-ABSORPTION: The fluid in the capsule of the glomerular filtrate flows down the tubule certain substances are passed back into the blood while others are retained, such as water, sugar, amino acid and salt like Sodium, Potassium, Calcium which are useful to the body are reabsorbed into the blood capillaries against concentration gradient or active transport. This process of reabsorbing useful material back into the blood is called SELECTIVE REABSORPTION.
HORMONAL CONTROL OF REABSORPTION OF WATER : The reabsorption of water can be linked partly to osmosis and to a hormone known as ANTI- DURETIC HORMONE (ADH). After reabsorption, the resulting fluid known as urine then trickles down the ureter and collect in the bladder which stretches to accommodate it, when the bladder is full, it contracts and discharge the urine through the urethra.