Human Excretory System: Excretory Organs

 

The process of formation of urine, its transformation, its temporary storage and its elimination from body form a system Called urinary system which is commonly Called excretory system. It Consists of kidneys, ureter, urinary bladder and urethra.

• kidneys: 1 pair in dorsal anterior abdominal Cavity each on either side of vertebral Column below diaphragm at the level of T12 to L3.

Each Kidney is bean shaped and reddish dark Coloured.

Length= (10-12) Cm, width= (5-6) cm and thickness= (2-3) Cm

weight =150gm in male and 130gm in female.

Blood supply= Renal artery. Blood collection = Renal vein.

 Each kidney is bean shaped with outer Convex surface and inner Concave surface. The inner surface has a deep indentation through which renal artery enters inside and the renal vein and Ureter Comes out.

protection: By floating Ribs.

Remains Covered by a fibrous layer Called renal Capsule outer to which a layer of adipose tissue called adipose Capsule is present which further remains Covered by a fibrous layer Called renal fascia.

The Rt. kidney is positioned slightly lower than  Lt. Kidney Probably due to the presence of highly developed liver in right Side. They are mesodermal in origin , metanephric and retroperitoneal.

Development of Kidney:

During embryonic development , mesoderm forms a plate like structure Called nephrotome which contains fine tubules Called nephros. kidney is formed from nephrotome while nephron is formed from nephros.

The anterior part of nephrotome is called pronephros, while middle part is called mesonephros and posterior part is called metanephros.

on the basis of development 3 types of kidneys are found:

1) Pronephric kidney: kidney developed from pronephros of nephrotome. The nephrons are of very simple type and not differentiated into various parts. eg. Cyclostomata , tadpole of Frog etc.

2) Mesonephric Kidney: kidney developed from mesonephros of nephrotome. The nephron in this type of Kidney is differentiated into Bowman's Capsule and simple tubule. eg: Most fishes and adult amphibians.

3) Metanephric Kidney: Kidney developed from metanephros of nephrotome. The nephron is well developed and differentiated into Bowman's Capsule, PCT, LOH and DCT.

eg. Reptiles, Aves and mammals.

LOH is less developed in reptiles , incompletely developed in Aves and well developed in mammals. LOH is mammalian character.

Retroperitoneal Organ refers to those organ Whose some part lies behind peritoneum. The peritoneum lies ventral to kidney.

The retroperitoneal organs are:

supra renal gland, Aorta & inferior venacava, Duodenum, pancreas, ureters, Colon ( Ascending & descending) ,kidneys, Esophagus and Rectum. @ SAD PUCKER

Internal structure of kidney:

It is studied with the help of it's  vertical section. kidney Consists of two parts; outer renal Cortex and inner renal medulla.

Renal Cortex is outer one -third part and seems granular. It Consists of malphigian body, PCT and DCT of nephron.

Renal medulla is inner two- third part and seems striated .It consists of loop of Henle of nephron and Collecting ducts. The medulla remains divided into a number of conical regions Called as medullary pyramids or renal pyramids. Inside pyramids parts of nephron (loop of Henle) collecting duct , duct of Bellini and blood vessels are present. The broad part of pyramid lies towards Cortex and Called base while narrower part lies towards inner side called apex or renal papillae. The Cortex extends deep into medulla in between successive renal pyramids and Called renal Column of Bertini.

About 7-18 pyramids are present.

A small duct arises from each renal papillae Called minor Calayx. Thus, 7-18 minor Calayces are present. These minor Calayces unit with each other to form major Calayces. About 2-3 major Calayces are present. These major Calayces unite with each other to form a funnel shaped space Called renal pelvis which lies behind renal medulla. This renal Pelvis comes out from hilum as ureter.

 

Nephron or Uriniferous tubule: Are structural and functional unit of kidney. Inside each kidney about 1.1 million nephrons are present. Each nephron is independent and Can produce miniscule amount of Urine. Each nephron is short tubular structure and Consists of glomerulus and tubule. The tubule Consists of Bowman's Capsule, proximal Convoluted tubule (PCT), Loop of Henle  (LOH) and Distal convoluted tubule (DCT).

The glomerulus and Bowman's Capsule are Collectively Called renal corpuscles or Malphigian body.

Bowman's Capsule is double layered cup shaped blind sac like structure. It's outer or parietal layer remains lined by single layer of squamous epithelial Cells. It's inner or visceral layer remains lined by squamous Cells  with foot like processes and Called podocytes. The foot like processes are Called pedicels.

The space between outer layer and inner layer is Called Cavity of Bowman's Capsule.

Glomerulus: A branch of renal artery which supplies blood to a nephron is called afferent arteriole which Comes into the cup of Bowman's Capsule and divides & redivides to form mass of capillaries Called glomerulus. It Contains about (30-40) Capillaries. The pedicels of podocytes wrap around the capillaries of glomerulus. The Small areas on capillaries in between pedicels are Called filtration slits or slit diaphragm or slit pore. Filtration of blood takes place through these slit pores.

The process of Ultrafiltration takes place in it.

The Capillaries in glomerulus unite to form efferent arteriole. the diameter of afferent arteriole is more than that of efferent arteriole. the efferent arteriole divides and redivides to form peritubular Capillaries which supply blood to other parts of nephron.

• proximal convoluted tubule (PCT): The outer layer of Bowman's Capsule Continues in the form of a Convoluted tubule Called PCT. It remains lined by brush-bordered Cuboidal epithelium rich in mitochondria. The process of Selective reabsorption takes place in it. It is present in  Cortex region. It is the site of maximum reabsorption.

• Loop of Henle(LOH): PCT Continues into a hair pin like or

U-shaped tubular Structure Called as LOH. It has 2 parts; Descending limb of LOH (DLOH) and ascending limb of LOH (ALOH).

DLOH: is thin and moves from Cortex to medulla. It remains lined by squamous epithelial Cells and is permeable to water but impermeable to salts.

ALOH: has thin and thick segment. It moves from medulla to Cortex. It remains lined by cuboidal epithelial Cells. It is permeable to Salts but impermeable to water. Active reabsorption of salts take place here. It is the site of minimum reabsorption.

The parallel network of capillaries around LOH is called

vasa recta.

The Counter current mechanism which develops here makes urine Concentrated.

• Distal convoluted tubule(DCT): In Cortex the ALOH Continues into a convoluted tubule Called DCT. It remains lined by Cuboidal epithelial Cells. Its water and Salt permeability is regulated by Anti-diuretic hormone(ADH) and Aldosterone hormones respectively.

• Collecting tubule (CT),collecting duct (CD) and duct of Bellini: DCT leads into a short tubular structure called CT and CTs unite with each other to form CD. They remain lined by Cuboidal epithelium. The CDs move towards medulla and they further unite with each other to form larger dust called duct of Bellini which opens at renal papillae.

 

Types of nephron: on the basis of position of malphigian body , two types of nephron are present;

1) Cortical nephron: Nephrons whose malphigian body is present in outer Cortex and their LOH extends only in outer medulla are called Cortical nephron. About (80-85)% nephrons are of this type. They function during normal fluid intake.

2) Juxta medullary nephron: Nephrons whose malphigian body is present near the junction of Cortex and medulla and their LOH extends up to deep medulla are Called juxtamedullary nephron. About (15-20)% neph \ rons are of this type. The Well developed vasa recta surrounds LOH. They are functional during low water intake.

They help to concentrate urine and Conserve water.

• ureters: 1 pair, each arises from hilum of each kidney and about (20-25) cm in length.Each ureter move towards posterior side and opens into urinary bladder. Their openings are separate but closely placed. Due to the  oblique openings, urine Cannot regurgitate into ureters when bladder Contracts. It's wall is muscular(inner longitudinal and outer circular) and internally remains lived by transitional epithelium (urothelium). Its function is to carry urine from kidney up to bladder. urine moves by peristalsis.

• urinary bladder: It is a muscular sac like pear Shaped structure for temporary storage of urine and is present in pelvic region. It has 3 parts; apex, body (fundus) and neck. Its wall is made up of smooth involuntary muscle Called detrusor muscle( outer longitudinal, middle circular and inner longitudinal) which has high power of Contraction to expell out Urine.

Internally, remains lined by transitional epithelium also called urothelium which has great power of stretching. This layer also Contains stretch receptors.

Three openings; two of ureters and one of urethra are present in urinary bladder. A roughly triangular area is present in between these openings called as trigon.

It stores about 500 ml urine. It can store up to 1000ml in full stretch.

The sympathetic nerves are responsible for relaxation of detrusor muscles while parasympathetic nerves are responsible for contraction of detrusor muscles.

• urethra: The bladder opens outside by a tubular Structure called urethra. urine passes out through it. Its opening remains guarded by internal urethral sphincter and external urethral sphincter. Internal urethral sphincter is involuntary and sympathetic nerves Contracts it while Parasympathetic nerves relaxes it.

The external urethral sphincter is voluntary and is regulated by higher inhibitory centres of brain in Cerebral Cortex. Thus, the act of micturition can be prolonged to some extent.

In female , it is about 4cm in length and opens in front of vaginal Orifice.

In male , it is about (16-20) cm in length and opens at the tip of penis. It has Prostatic part, membranous part and penile part.

As urethra is short and near to Anus , urinary tract infection (UTI) is common in female. UTI is generally Caused by E.coli.