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Urinary System

Urinary System:

The excretory system is that system which is related with the formation of urine and its excretion out of the body. It is also called as urinary system.

The urinary system contains

  1. Two kidneys
  2. Two ureter
  3. Urinary bladder
  4. Urethra

Excretory system | Grade 9 Science and Technology


There is present two kidneys one on right side other on the left side of abdomen nears the lumbar vertebra. The Shape of kidney is bean shaped reddish brown in color. The outer surface of the kidney is convex and the inner surface is concave. The kidney is cover by the capsule, the length of the kidney is about 10cm, width is about 5cm and thickness is about 3cms & the weight of kidney is around 130-170gm.


Diagram of kidney

The covering of kidney

  1. Fibrous capsule
  2. Perinephric fat
  3. Renal fascia
  4. Para nephric fat (It is the cushion for the kidney)

Presenting parts:

  1. Upper end – more rounded and related to supra renal gland
  2. Lower end- It lies about 2.5cms above the iliac crest
  3. Medial boarder
  4. Lateral boarder
  5. Anterior surface
  6. Posterior surface

The kidneys are present in the abdomen on two sides of lumbar vertebra. The kidney contains three parts. The outer layer and tissue is called as cortex. The next layer is called as the medulla and the inner layer is called as pelvis. The cortex is the outer part of kidney and it is surrounded by the capsule. The renal pelvis is a flats funnel shaped like structure.


Each kidney contains very small structures called as nephron there are present about 1.2 million nephrons in each kidney.

The nephron manufactures the urine in the kidney. The nephrons are present partly in the renal cortex and partly in the renal medulla. The opening of all the nephrons is present in the deep part of the kidney called hilum, from this part the urine is collected by the ureters.

The blood supply in the kidney is by the renal artery and renal veins. The nerve supply is by sympathetic nerve fibers T10 to L1 and the Para sympathetic nerve are supplied from VAGUS nerve.

The functions of kidney:

The kidneys are the

  1. Main excretory organs that eliminate the metabolic waste produces like ammonia, urea, uric acid etc. by the urine formation.
  2. Regulation of extra cellular fluid volume by controlling the water and electrolyte balance.
  3. The kidneys are responsible for acid base balance in the body.
  4. The kidney have an endocrines function and secrete the following hormones
  5. Erythropoietin: It simulates the hemiparesis.
  6. Calcetnol: It regulates the calcium metabolism.
  7. Renin: By IG cells and regulates the extra cellular fluid volume and blood pressure.


There are present two ureter one on the right and other on the left side of abdomen. The ureters are the tube like structure with a length of 25cm. The diameter of the ureter is 3mm.

The ureter has three parts.

  1. Pelvis Ureter: This is funnel shaped and joins with the abdominal part of ureters and the level of lower end of the kidney.
  2. Abdominal: This is present in relation to the tip of transverse process of the lumber vertebrae.
  3. Pelvic part: It is divided in three parts
  4. Vertical part
  5. Oblique part
  6. Intravescile

Blood supply in kidney: The blood supply is by the renal lumber, peritoneal, common iliac artery, uterine and vaginal arteries.

Venous drainage (collection of blood): The blood collection is by the inferior vena cava.

Nerve supply: The nerve supply is by the sympathetic nerves, T10, T11, T12 and L1 (T-Thoracic) (L-Lumbar).

Parasympathetic: Nerve by the branches of VAGUS nerve.

Urinary bladder:

The urinary bladder is a muscular organ for the storage of the urine. The urinary bladder is present in the anterior part of the pelvic cavity the empty urinary bladder is tetrahedral in shape. The filled urinary bladder is rounded. The empty urinary bladder has the following parts.

  1. Apex
  2. Base
  3. Neck
  4. Three surfaces- superior, infralateral right and left
  5. Four borders- anterior, posterior and two lateral

The outer wall contains the serous layer and inner layer contain the mucus layer

Trigons of urinary bladder: The trigons of urinary bladder on naked eye examination: the mucosa present the irregular folds in an empty bladder, as the bladder is filled with urines the folds flatten out. The trigon is seen as a triangle in the lower part of the base of bladder where the mucosa is attached to underline muscular coat therefore it is smooth the base of trigon is formed by inter-ureteric ridge which is present between the two opening of ureters while the apexes is directed downwards and leads to the ureteric opening.

urinary bladder


Trigons of urinary bladder Image

The physiological capacity of the urinary bladder is 450ml. the anatomical capacity is 1000ml.


The urethra is the opening of the urinary bladder. It excretes the urine.

Male urethra: It is about 18-20cms from the neck of the urinary bladder up to the tip of the penis.

  1. Prostatic urethra
  2. Membranous part of urethra
  3. Panile (spongy) urethra

Blood supply: The blood supply in the urethra is by the blood vessel of prostate gland and the penis.

Female’s urethra: It is about 4cm in length. It starts from the urinary bladder to the external urethral opening. It is embedded in the arteries wall of the vagina internally mucosa of urethra is folded and contains maximum mucus glands. It is easily dilatable and present just above the female genital orifice (Central part).

Briefly describe the structure and functions of

Structure of nephron image

Composition of urine: H2O + urea + uric acid + Sodium chloride(salt) + Ca++

Urine Formation

Urine formation by nephron:

The function of the nephron includes the filtration tubular reabsorption and tubular secretions. By these functions the kidney eliminates the nitrogenous waste materials and regular the volume composition and the pH value of the blood.

Step1. Glomerular filtration:

In the glomerular filtration the maximum blood is filtered because its filtration membrane has maximum surface area and the glomerular blood pressure is much higher than the other capillary beds (Approximately 55mm of mercury is compared to 18mm of mercury). As a result of this the kidney filters about 180 liters of filtrate per day (125mm/min). The molecules smaller than 3mm in diameter such as: water + glucose + Amino acid + nitrogenous waste products pass freely from the blood into the renal tubule layer, molecule like RBC, WBC, Platelets doesn’t enter in tubule.

Step2.Tubular reabsorption:

During this process the important needed substance are returned back in the blood by the tubular reabsorption. The sodium enters in the blood by the diffusion the other substances like urea are absorbed passively (Slowly). The actively (Quickly) reabsorbed substances are glucose, Urine acid, sodium, potassium & water. The loop of Henle absorbs the potassium and chlorine and it also absorb maximum water.

Step3. Tubular secretion:

The distal convoluted tubule reabsorbs the sodium, potassium and some chlorides. The tubule secretes some toxic substances in the urine for the excretion. The kidney receives 1.2 – 1.3 liters of blood/min and produces the urine 1ml/min under the normal condition.

Therefore the urine is formed in nephron by a combination of glomerular filtration, tubular reabsorption, and tubular secretion. Finally the urine is collected by the collecting tubules.

Urine composition: Normally an adult secretes about 1-1.8 liters of urine in one day. The volume of the urine formation depends on the fluid intake, physical activity, temperature. The urine is a transparent, slightly yellowish aqueous fluid. The urine is acidic in reaction and contains the high osmotic pressure than the plasma. The specific quantity is between 1.003 – 1.040.

When the urine is allowed to stand it gives the strong ammonia smell. The urine mainly contains the water, urea, uric acid, Sodium chloride, small quantities of chlorides, sulphates and phosphate and K+ , Ca++, Mg++

The blood pressure in the glomerulus is higher than the other blood capillaries because the diameter of afferent arteriole is greater than the efferent arteriole.


Bowman’s capsule Image

Hormones affecting renal tubular function:

ADH: (Anti diuretic hormone) it increases the permeability of distal convoluted tubule and collecting tubule to water.

Aldosterone: It increases the action of sodium, potassium ATPase pump resulting in the reabsorption of sodium and increase the secretion of potassium.

ANP(Atrial natriuretic peptide): it decreases the reabsorption of sodium and water from proximal convoluted tubule and collecting tubule and of ADH and aldosterone.

Parathormone: It increases the reabsorption of calcium ion and increase phosphate secretion.

Filtration barriers: There are three barriers

  1. Endothelial layer: The Endothelial layer with pore arrangements on the inner surface of glomerular capillaries.
  2. Basement membrane: It is composed of three layers. The middle layer is the proper filtering membrane it contain thin collagenous fibers arranged in network. These are present in matrix having the pores of 30-60mm in diameter.
  3. Epithelial cells: The layer of epithelial cells is present on the outer wall of the glomerular capillaries. At the surface of the epithelial cells (podocytes), these are a layer of glycocalyx which fills the gaps between the podocytes. It affects the permeability the permeability of the capsule is also affected by the electric charge. In this way the kidney filtration takes place.

Forces governing glomerular filtration: It is governed by the equation


GFR: Glomerular filtration rate

KF: Glomerular filtration coefficient (permeability)

PBC: Pressure in bowmen’s capsule

PGC: Glomerular capillary pressure

PPO: Plasma oncotic pressure

The last three factors are the forces that govern the hydrostatic forces through the glomerular membrane.

Juxtaglomerular apparatus: This is the term given to the collection of the special cells of renal tubule and the associated afferent arterials of the nephron. They are present in the anterior part just before they enter the glomerulus these cells are rich in endoplasmic reticulum, mitochondria and ribosomes. There cells synthesis and store the enzyme RENIN therefore these cells are as the baroreceptors.

Diseases related to the urinary systems:

  1. Diabetic nephropathy: This is also known as the (Wilson syndrome) and the inter capillary glomerulonephritis. This is the kidney disease which is caused by the angiopathy of capillaries in the kidney glomeruli.
  2. Kidney stones: This is also called as the renal calculi, in this case the solid crystals of dissolved minerals in the urine are found inside the kidney or ureters. They are different in size for a small grain of sand to as large as golf’s ball. The kidney stones typically leave the body by the urine. If they are larger in size and do not pass into ureter than it can cause the severe pain in the abdomen than operation is the treatment.
  3. Pyelonephritis: When the infection of the renal pelvis spreads to the complete kidney it is called as the pyelonephritis. It is mainly due to the spread of the fecal bacterium i.e. Escherichia coli, from the anal region from the urinary track. In the severe cases the kidney swells and there is the puss formation, if it is untreated the infected kidney may be severely damage but by the treatment of antibiotics normally there is total cure.
  4. Glomerulonephritis: This is the inflammation of the glomerular part of the kidney. It can be due to the drugs or toxins vascular disorder and the systemic diseases. There are two changes in the urine.

1. Hematuria

2. Proteinuria

There is also a decrease in urine formation as there is decrease in GFR.

  1. Renal failure: The uremia is a syndrome of renal failure and it includes the blood urea. It can be due to the severe hypotension or due to the glomerular disease.
  2. Hematuria: It is the presence of blood cells in the urine, it may be discoloration of urine throughout the urination. It may be due to pre renal or post renal as following.

1. Pre-renal causes:

a) Trauma of kidney

b) Septicemia

2. Post-renal causes:

a) Urethritis

b) Cystitis

C) Due to kidney damage

d) Due to urethral damage at the beginning

e) Due to vesicle damage at the end of urination

3. Renal causes:

a) Acute glomerulonephritis

b) Poly nephritis

c) Tubular damage

d) Renal infection


The osmoregulation is the maintaining the concentration of osmotic presence of the blood when the water intake is high the urine is excreted maximum, when there is very less water in the body very little urine is formed by kidney. In this way the osmotic consultation of the blood is maintain. The kidneys are extremely flexible in this operation. Whenever there is the maximum water in the cells and tissue of the body the hormone ADH (anti diuretic hormones) isn’t secreted by posterior pituitary gland.

As a result of this is no absorption of water by the walls of the tubules PCT, DCT and collecting tubule remains impermeable but the active absorption of the sodium continues. The urine becomes more and more dilute and large volume of the urine is excreted out of the body.

When there is little water in the body and this is to be reserved. In this condition the ADH is secreted by the posterior pituitary gland, as a result of this there is active reabsorption of water, sodium , potassium from the tubule (PCT, DCT, and collecting tubule) but passive reabsorption of the urea, uric acid, sodium chloride.

Therefore the concentrated urine with maximum toxic materials is excreted out of the body. In this way the nephrons of the body in this way the nephrons of the kidney maintain the balance of the osmotic pressure in the blood by secreting more or less urine this called as the osmoregulation.