Hello, welcome to this blog on the topic, Excretion In Mammals, the Excretory systems and the mechanism of excretion.
Table Of Contents
- The excretory systems of mammals
- Types Of excretory organs used by mammals.
- The structure of the kidney
- The structure of the urinary tubules
- Mechanism of excretion in mammals (formation of urine).
- Revision Questions
Excretion In Mammals
There are four types of excretory organs used by mammals, these are:
- Lungs
- Skin
- Liver
- Kidney
In excretion in mammals, the lungs excrete water vapour and carbon dioxide, the liver excretes bile pigments called bilirubin (derived from the decomposition of hemoglobin), 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 shape and reddish brown organ located on 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-shape masses of tissues called pyramids. The pyramids open into a funnel-shape cavity called the pelvic. The kidney has many capillaries which are branches of the renal artery and renal vein.
The Excretory Systems Of Mammals
The excretory system of mammals include:
- The kidney,
- The ureters
- The bladder
- The urethra
- and 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 away blood from the kidney. A long narrow tube called the ureter connect 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 start in the Cortex as a cup-shape 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 takes a U-shaped loop called the Henle’s loop before re-entering the cortex. In the cortex, the tubule become 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 pelvic. Together with many other tubules, it pours its content into a wider main collective duct which eventually join up and opens into pelvic at the apices of the pyramids. All along its course, the tubules is closely associated with several networks of blood capillaries, the renal artery branches in the kidney. Each branches 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 process involved in the formation of urine occur in three phases which are:
- Ultra filtration
- Selective reabsorption
- Hormonal secretion.
1. Ultra filtration: In first phase, blood is brought to the kidney by renal arteries. As its 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 glomerulus filtrate now flow down the tubule. At the proximal convulated tubule and Henle’s loop, some water, sugar, amino acid and salt 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 in finally formed. The urine gradually trickles into the renal pelvic propelled by peristalsis through the ureter into the urinary bladder. When the bladder is full, it contracts, and discharges the urine through urethra.
Revision Questions
- Outline four types of excretory organs in Mammals.
- What are the excretory systems of mammals?
- Discuss the the structure of the kidney
- State and explain the mechanism of excretion in mammals