Let’s Describe The Excretory System Of Man In A Definitive Manner.

The excretory system of man is also known as the urinary system.

And it consists of:

1. Kidney.
2. Ureter.
3. Urinary bladder.
4. Tubular urethra.

Ureter is a long tube that extends from the kidney to the urinary bladder, and its function is to transport urine from the kidney to the urinary bladder.

The urinary bladder is a hollow muscular organ that serves as a urine reservoir and is located within the pelvic cavity.

Urethra is an organ that carries urine from the urinary bladder to the outside of the body.

Kidney- Structure And Function

A kidney is a reddish-brown, bean-shaped organ with a small surface.

A fibrous capsule is a fibrous connective tissue layer that encloses each kidney.

The lateral side of the kidney is convex whereas the medial side of the kidney is deeply concave.

This medial side leads to a hollow chamber called the renal sinus and the entry to this sinus is called the hilum, where the renal artery and nerves enter and the renal vein and ureter exit.

The inner part of the kidney consists of renal pyramids. 

Urine is collected in the renal pelvis and exits the kidney through the ureter.

The kidney serves as the osmoregulatory and excretory organ.

In the case of osmoregulatory organs, the kidney controls the formation of diluted urine during the state of flooding.

And produces concentrated urine during the case of dehydration.

In the case of an excretory organ, it performs the filtration of nitrogenous wastes from the blood and its removal in the form of urine.

Nephron- Functional Unit Of Kidney 

A nephron is the smallest unit of the kidney.

Nephron consists of a renal corpuscle and renal tubule.

The renal corpuscle is composed of a network of capillaries called glomerulus and the glomerulus is surrounded by a thin, double-walled structure called a bowman’s capsule.

The renal tubule leads away from the bowman’s capsule and forms a highly coiled structure called the proximal convoluted tubule.

The proximal convoluted tubule then dips towards the renal pelvis, into the medulla forming a sharp loop called a loop of Henle.

This loop consists of an ascending limb and a descending limb, the ascending limb returns to the region of the renal corpuscle, forming a highly coiled structure called a distal convoluted tubule.

The distal convoluted tubule is connected to the collecting duct.

Many collecting ducts combine to form larger collecting ducts that empty into the renal pelvis.

Mechanism Of Urine Formation

There are three steps of urine formation.

1. Glomerular Filtration: This step takes place in the renal capsule under pressure. The pressure comes from the blood pressure known as hydrostatic pressure. Glomerular capillaries have exceptionally high blood pressure and due to such high blood pressure water and small solute molecules are filtered out of the glomerular capillaries and are connected into the Bowman’s capsule. The filtered fluid in the capsule is called glomerular filtrate.

2. Selective reabsorption: Certain substances like glucose, amino acids, vitamins, inorganic salts, and a low amount of water is reabsorbed.

3. Tubular secretion: Substances like ammonium, and hydrogen ions are secreted. The main purpose of this step is to maintain the pH of the urine which ranges from 4.8 to 7.5.

Mechanism Of Urine Concentration- Countercurrent Multiplier Mechanism

Let’s Describe The Excretory System Of Man In A Definitive Manner. The formation of hypertonic urine depends on the reabsorption of water from the loop of Henle and the collecting duct.

That is achieved by the countercurrent multiplier mechanism.

Due to the countercurrent, filtrate moving in the loop of Henle and the blood moving in the capillaries of vasa recta, water is greatly reabsorbed. 

As the fluid travels up the ascending limb, sodium chloride is transported out into the surrounding area, which is controlled by aldosterone.

This causes an increase in the concentration of water in the filtrate and a decrease in the concentration of water in the kidney space.

This results in water passing out of the descending limb by osmosis.

This movement of water is also promoted by aldosterone, which is secreted from the posterior lobe of the pituitary gland.