Excretion

Removal of nirogenous waste produce during metabolism of protein and nucleic acid.

Human excretory system includes:-
Pair of kidney
Pair of ureter
Urinary bladder
Urethra

Function of kidney

Kidneys regulate the osmotic pressure of a mammal’s blood through extensive filtration and purification, in a process known as Osmoregulation.

• Kidneys filter the blood; urine is the filtrate that eliminates wastes from the body via the ureter into the urinary bladder.
• The kidneys are surrounded by three layers:
  1.  Renal fascia
  2. perirenal fat capsule
  3. Renal caps

EXTERNAL ANATOMY

A typical adult kidney (mass – 135–150 g) is:

10–12 cm- long
5–7 cm – wide
3 cm cm- thick

• The concave medial border of each kidney faces the vertebral column. 
• Near the centre of the concave border is an indentation called the Renal hilum, through which the ureter emerges from the kidney along with blood vessels, lymphatic vessels and nerves.
• Human kidney are Retroperitoneal(covered with peritoneum) 
• Present between 12th thoracic vertebrae to 3rd lumber vertebrae. 
• Left kidney is higher than the right kidney due to position of liver in right side. 

Three layers of tissue

a) .The Renal capsule(Deep layer) – Smooth, transparent sheet of dense irregular connective tissue that is continuous with the outer coat of the ureter.

•  It serves as a barrier against trauma and helps maintain the shape of the kidney. 

b) The adipose capsule (middle layer) – Mass of fatty tissue surrounding the renal capsule. 

•  Protects the kidney from trauma and holds it firmly in place within the abdominal cavity.

c) The renal fascia(superficial layer) – Thin layer of dense irregular connective tissue 

•  Anchors the kidney to the surrounding structures and to the abdominal wall 

On the anterior surface of the kidneys, the renal fascia is deep to the peritoneum

Internal anatomy

A frontal section through the kidney reveals two distinct regions: 

1.  Renal cortex (outer)
2.  Renal medulla (inner). 

The renal medulla consists of several cone-shaped renal pyramids. 

The base (wider end) of each pyramid faces the renal cortex, and its apex (narrower end), called a renal papilla, points toward the renal hilum. 

The renal cortex, smooth textured area extending from the renal capsule to the bases of the renal pyramids. 

It is divided into an outer cortical zone and an inner juxtamedullary zone.

Ureter

Muscular tubes of 25-30cm length, 3m in diameter. 

Wall of Ureter

• Innermost-Transitional epithelium
• Middle layer-Muscular(longitudinal and circular muscle) 
• Outermost layer – Tunica adventita. 

Urine is move through ureter by peristalsis.

Urethra conduct the urine from urinary bladder to outside.
Female urethra is short.
Male urethra is long.

Urinary bladder

It is hollow muscular organ  that stores urine from the kidneys before disposal by urination. 

In humans the bladder is a hollow distensible organ that sits on the pelvic floor

Nephron

Structure and functional unit of kidey.

Each kidney contain about 1 million nephron 

Each nephron has 2 part 

• Glomerulus
• Renal tubules.

• Take a simple filtrate of the blood and modify it into urine.
• Cleanse the blood and balance the constituents of the circulation.
• Many changes take place in the different parts of the nephron before urine is created for disposal. 
• The term urine will be used here after to describe the filtrate as it is modified into true urine. 
• The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine.

RENAL CORPUSCLE

It CONSISTS OF A GLOMERULUS SURROUNDED BY A BOWMAN’S CAPSULE. 

THE GLOMERULUS ARISES FROM AN AFFERENT ARTERIOLE AND EMPTIES INTO AN EFFERENT ARTERIOLE. 

THE SMALLER DIAMETER OF AN EFFERENT ARTERIOLE HELPS TO MAINTAIN HIGH BLOOD PRESSURE IN THE GLOMERULUS.

THE BOWMAN’S CAPSULE IS DIVIDED INTO THREE LAYERS:

• OUTER PARIETAL LAYER- MADE UP OF EPITHELIAL CELLS WITH MINUTE PORES OF DIAMETER 12NM.
• MIDDLE BASEMENT MEMBRANE-IT IS SELECTIVELY PERMEABLE.
• INNER VISCERAL LAYER-IT CONSISTS OF LARGE NUCLEATED CELLS CALLED PODOCYTE(BEAR FINGER-LIKE PROJECTIONS CALLED PODOCEL) 

Renal tubule

It IS A LONG AND CONVOLUTED STRUCTURE THAT EMERGES FROM THE GLOMERULUS 

IT CAN BE DIVIDED INTO THREE PARTS BASED ON FUNCTION:-

1.  PROXIMAL CONVOLUTED TUBULE (PCT) –  IN THE RENAL CORTEX.
2.  THE LOOP OF HENLE, OR NEPHRITIC LOOP – IT FORMS A LOOP (WITH DESCENDING AND ASCENDING LIMBS) THAT GOES THROUGH THE RENAL  MEDULLA.
3. DISTAL CONVOLUTED TUBULE (DCT)- IN THE RENAL CORTEX.

Loop of Henle

Thick segment-Simple cuboidal epithelium
Thin segment-Simple squamous epithelium.

DCT

Distal  convoluted tubules

Cuboidal epithelium with fewer mitochondria and microvilli

• Conditional reabsorption of water under the effect of ADH.
• Na+ – Aldesteron
• Ca²+ – parathyroid hormone
• Reabsorption ofHCO3¯and secretion of H+, K+and NH3 to maintain pH. 

Collecting duct

Cuboidal epithelium

Conditional reabsorption of water, Na+, Ca²+. 

Permeability for urea 

PCT

⅔rd of water reabsorption and 60% of glomerular filtrate is reabsorbed. 

Water, Na+, Cl-, HCO3-, Glucose, vit. C, amino acid, K+and little amount of urea and uric acid.

Descending Limb-permeable to water only. 

Ascending Limb– permeable to ions only. 

Na+, Cl-, k+, Mg²+, Ca²+

Reabsorption is minimum. 

Urine formation

1) Glomerular filtration
2) Tubular reabsorption
3) Tubular secretion

Glomerular filtration 

• WATER AND SOLUTES ARE FORCED THROUGH THE CAPILLARY WALLS OF THE Glomerulus INTO THE BOWMAN’S CAPSULE (GLOMERULARCAPSULE)
• FILTRATE –THE FLUID THAT IS FILTERED OUT INTO BOWMAN’S CAPSULE. 
• Glomerulus filtrate-same as plasma but protein are absent. 

Contains-Water, ions, Glucose, amino acid, water soluble vitamin, urea, uric acid etc. 

Tubular reabsorption

OCCURS BOTH PASSIVE AND Actively. 

GLUCOSE, AMINO ACIDS, AND OTHER NEEDED IONS (NA, K, CL, CA, HCO3) ARE TRANSPORTED OUT OF THE FILTRATE INTO THE PERITUBULAR CAPILLARIES ( REABSORBED BACK INTO THE BLOOD)

ABOUT 65% OF THE FILTRATE IS REABSORBED IN THE PCT. 

AS THESE SUBSTANCES ARE Reabsorbed, THE BLOOD BECOMES HYPERTONICSO WATER EASILY FOLLOWS BY OSMOSIS. 

REABSORPTIONIN THE DCT IS UNDER Hormonal CONTROL ALDOSTERONE CAUSES MORE SALT TO BE ABSORBED

 ADH CAUSES MORE WATER TO BE ABSORBED

TUBULAR SECRETION

WASTE PRODUCTS SUCH AS UREA AND URIC ACID, DRUGS AND HYDROGEN AND BICARBONATE IONS ARE MOVE OUT OF THE PERITUBULARCAPILLARIES INTO THE FILTRATE; THIS REMOVES UNWANTED WASTES AND HELPS REGULATE PH

Urine

It is pale yellow in colour due to Urochrome pigment that is byproduct of red blood corpuscles(RBC) breakdown. 

Around 1-1.5 litre of urine is formed per day. 

PH =6 (vary 4.2 – 8.2) 

It can be four times as concentrated as the blood i. e-1200mosmol/L.

Heavier than water

• contain
• 95% =water
•  5%= urea, uric acid, K+, H+, NH4+, sulphate, hippuric acid, oxalate