​ ​ ​ ​ ​ ​ The Excretory System

Large amounts of carbon dioxide and water are produced by the metabolism of carbohydrates, fats and
proteins

Nitrogenous waste are formed during the metabolism of proteins

Defecation is the passing out of faeces for the undigested food through the rectum

Passing out carbon dioxide through the lungs is a part of respiration

The process of removal of chemical wastes (mainly nitrogenous wastes) from the body is known as
'excretion'

The main excretory product is urea

Wastes that have to be eliminated

Carbon dioxide and water
-​ The lungs excrete carbon dioxide from our body through the expired air
-​ The extra water is released out of the body in the form of sweat by the skin
Nitrogenous Wastes
-​ These include urea, uric acid and ammonia.
-​ Urea is excreted out through the kidneys in the form of urine
Excess salts
-​ Such common salt (NaCl) and
-​ Even some excess water-soluble vitamins (B and C) need to be eliminated
Bile pigments (bilirubin)
-​ They are the breakdown products of the haemoglobin of the dead RBCs
-​ The liver cells extract it from the circulation and
-​ Secrete it into the bile juice
-​ Which is then poured into the duodenum through the common bile duct
-​ They are modified in the intestine to pigments which give faeces their yellowish brown colour

The Excretory Organs:

1.​ Kidneys: These are the primary excretory organs eliminating nitrogenous wastes in the form of
urine
2.​ Skin: Sweat excreted by the sweat glands of the skin primarily excrete water and sodium chloride
3.​ Lungs: They excrete carbon dioxide from our body through the expired air
4.​ Liver: Liver detoxifies ammonia by converting it to urea

Kidneys
-​ Located on either side of the backbone and protected by the last two ribs
-​ Bean shaped organs
-​ The right kidney is at a lower level then the left one because of the presence of the liver

Ureter
-​ A tube that arises from the notch (hilum) of each kidney.
-​ Connects to the urinary bladder at the back.
-​ Transports the urine from the kidney into the urinary bladder

Pelvis
-​ Front end of the ureter is slightly expanded inside the kidney
Urethra
-​ A tube that empties the urine from bladder to the outside of the body

Sphincter Muscle
-​ A circular muscle at the opening of the bladder into the urethra
-​ Relaxes during urination (micturition)
-​ Controlled by an impulse from the brain

Urinary Bladder
-​ Lower part of the abdomen
-​ Temporary storage of urine

Internal Structure of the Kidneys

-​ A longitudinal section of the kidney shows two main regions
-​ outer dark cortex and an inner lighter medulla
-​ Cortex:
1.​ Outer region
2.​ Dark-colored
-​ Medulla:
1.​ Inner region
2.​ Lighter-colored
3.​ Made of finely striped tissue
-​ Medullary Structure:
1.​ Composed of several conical pyramids
2.​ The apex (tip) of each pyramid is called the papilla
-​ Nephrons (Uriniferous Tubules):
1.​ Numerous minute tubules in the kidney
2.​ Known as nephrons or uriniferous tubules
3.​ These are the structural and functional units of the kidney
-​ Portions of the cortical tissue extend in between adjacent renal pyramids to form renal columns

Structure of a Kidney Tubule

1.​ Bowman's Capsule: ​​ ​ ​ ​ ​ ​ C
-​ Thin-walled cup structure.
-​ Single cell thick epithelium
-​ Outer concavity holds a mass of blood capillaries called the glomerulus
-​ The Bowman’s capsule and the glomerulus together are called as the malphigian capsule of
the renal capsule allon
2.​ Proximal Convoluted Tubule (PCT):​ ​ ​ ​ C
-​ Also called the first convoluted tubule.
-​ It is the starting convoluted region of the tubule.
-​ Nearer to the Bowman’s Capsule
3.​ Loop of Henle:​ ​ ​ ​ ​ ​ ​ M
-​ Middle U-shaped part of the tubule.
-​ Shaped like a hairpin.
4.​ Distal Convoluted Tubule (DCT)​ ​ ​ ​ ​ C
-​ End part of the kidney tubule
-​ Opens into a collecting duct.​ ​ ​ ​ M

Both the Bowman’s capsule and the proximal convulated part lie in the cortex giving it a dotted
appearance in the sectional view
The total length of all the tubules together is more than 60 km
-​ Because it provides a huge surface for the reabsorption of usable substances especially water as
the contents move through them

Blood Supply to the Kidney Tubules

-​ A pair of renal arteries branch off from the dorsal aorta
-​ Each branch it rebranches several times to give rise to arterioles
-​ Each arteriole enters the Bowman’s capsule
-​ Afferent Arteriole:
Enters Bowman's capsule.
Brings blood into the glomerulus.
Breaks into a network of capillaries
-​ Efferent Arteriole:
Formed by the reuniting capillaries of the glomerulus.
Carries blood away from the glomerulus
Forms the renal vein again
It leaves the kidney to pour the blood into the posterior vena cava

Ultrafiltration // Glomerular filtration ​ ​ ​ ​ ​ ​ glomerulous

-​ The process of filtration of liquid part of the blood in the glomerulous into the Bowman’s capsule
under high pressure or extraordinary force
-​ Blood flows through the glomerulus under great pressure
-​ This pressure is much higher than in capillaries elsewhere in the body
-​ The fluid entering the renal tubule is called the glomerular filtrate
-​ It consists of water, urea, salts, glucose, other plasma solutes

The afferent arteriole has its lumen wider than the efferent arteriole thus there is a pressure difference
-​ The hydrostatic pressure causes the liquid part of the blood to filter out

Reabsorption​ ​ ​ ​ ​ ​ ​ ​ ​ ​ PCT
-​ Nature of the glomerular filtrate
The filtrate entering the renal tubule is not urine.
It is an extremely dilute solution
-​ Contents of Filtrate
Has glucose, most water, salts like sodium
-​ These materials are selectively reabsorbed into the blood
-​ Blood's normal concentration is not disturbed
-​ This process is called selective absorption

Tubular Secretion​ ​ ​ ​ ​ ​ ​ ​ ​ ​ DCT

-​ Substances like potassium ions, penicillin and other drugs get secreted
-​ These are passed into the forming urine
-​ The process happens through the cells of the tubular wall.
-​ It helps in eliminating unwanted substances.

All the steps involved in urine production require energy and hence the oxygen demand of the kidneys is 6
to 7 times higher than what is required by muscles

The filtrate left after the reabsorption and tubular secretion is called urine
Final urine passes into collecting ducts to the pelvis and through the ureter into the urinary bladder by
urethral peristalsis and due to gravity

Urine is expelled directly in females and through the penis in males

-​ By relaxation of the sphincter muscles
-​ Located at the opening of the urinary bladder into urethra
-​ By impulse from brain
-​ Micturition

Physical properties of urine

1.​ Colour: Clear yellow (due to pigment urochrome)
-​ Varies with diet
2.​ Volume: 1 to 1.5 litres per day but varies
3.​ pH: 5 to 8
4.​ Odour: ammonia-like

95% of water
5% of solid wastes (can vary depending on the food taken and time after taking the food)

Abnormal Constituents in Urine

1.​ Haematuria
-​ Blood
-​ Blood passes with urine
-​ Infection in urinary tract, kidney stones or tumor
2.​ Glycosuria
-​ Excess glucose passes with urine
-​ Diabetes mellitus (sugar diabetes)
3.​ Albuminuria
-​ High blood pressure
-​ Increased permeability of Bowman’s capsule membrane
-​ Albumin proteins pass with urine
4.​ Bile pigments
-​ Anaemia, hepatitis, liver cirrhosis
5.​ Urine sweetish
-​ Urine contains sugar glucose
-​ Insufficient insulin
6.​ Urine tasteless
-​ Too much water in urine
-​ Diabetes insipidus
-​ Insufficient ADH hormone

Regulation of Urine Output

-​ Controlled by the pituitary gland
-​ Diuresis = Increased production of urine
-​ ADH (Antidiuretic Hormone) from the posterior pituitary controls water reabsorption and urine
concentration

Uric acid is relatively less solube in water and may crystallize and get deposited in the joints causing gout

Excessive uric acid and certain salts like oxalate may be the source of kidney stones
Osmoregulation
-​ Kidneys remove waste (e.g., urea) and regulate the water and salt balance in the blood.
-​ This is called osmoregulation — it maintains the osmotic pressure of blood.

We urinate fewer in the summer than in the winter and the urine pass is generally thicker b
-​ Because in the summer we lose a considerable part of water through sweating and
The kidneys have to reabsorb more water from the glomerular filtrate making it more concentrated

In Cholera patients suffer from vomiting and watery bowels

-​ There intestines are unable to absorb water into the blood there result is that their Kidneys
reabsorbed almost all of the water from the glomerral filtrate in the renal tubules and
-​ With it even by urea eventually they may die due to poisoning by the accumulation of high
quantities of urea in the body (uremia)
-​ The immediate treatment is to replenish water in the body by glucose saline drift or by giving oral
rehydration solution through the mouth

Loss of water is 2500 mL

Gain of water is 2500 mL

Metabolic water is the product of cellular respiration

Artificial Kidney
-​ If one kidney is damaged or removed, the other can function alone for excretion
-​ Artificial kidney = Dialysis machine
-​ Patience blood from the radial artery in his or her arm through the machine where the urea is
removed and purified blood is returned to a vein in the same
-​ Helps filter blood when both kidneys fail.
The part of kidney tubules where the term urine is first used for the fluid in it
-​ Distal convoluted tubule (DCT)

Afferent arteriole, glomerulus, efferent arteriole, secondary capillary network, renal vein.

Renal artery, kidney, ureter, urinary bladder, urethra.

A pair of renal arteries branch off from the aorta and enter the kidney.

Collecting duct — The collecting duct system is the final component of the kidney to influence the body's
electrolyte and fluid balance. It receives the contents of many kidney tubules and pours it as urine in the
pelvis of the kidney.

Vasa recta — The vasa recta is a secondary capillary network that branches out of the efferent arteriole.
It consists of long, hairpin-shaped blood vessels that run parallel to the loops of Henle. The hairpin turns
slow the rate of blood flow, which helps maintain the osmotic gradient required for water reabsorption.

(a) Bowman's Capsule​ ​ ​ (iv) Glomerulus

(b) Contains more CO2 and less urea​ (iii) Renal Vein
(c) Anti-diuretic hormone​ ​ ​ (ii) Regulates amount of water excreted
(d) Contains more urea​ ​ ​ (i) Renal artery

In a nephron, the blood flows through the glomerulus under great pressure. The reason for this great
pressure is that the efferent (outgoing) arteriole is narrower than the afferent arteriole (incoming). This
high pressure causes the liquid part of the blood to filter out from the glomerulus into the renal capsule.

Three nitrogenous wastes of our body:

1.​ Urea
2.​ Uric acid
3.​ Ammonia

Three organic wastes of our body:

1.​ Urea
2.​ Uric acid
3.​ Creatinine
Three inorganic wastes of our body:
1.​ Common salt (NaCl)
2.​ Iron
3.​ Calcium

Three types of fluids in different parts of a nephron:

1.​ Glomerular filtrate
2.​ Glomerular filtrate with glucose and sodium
3.​ Urine

(a) PCT — Proximal Convoluted Tubule

(b) DCT — Distal Convoluted Tubule

(c) ADH — Antidiuretic hormone

(d) ORS — Oral Rehydration Solution

(a) Kidney — It is located on either side of the backbone and protected by the last two ribs.

(b) Uriniferous tubule — Uriniferous tubule begins in the cortex; the tubule dips down to the medulla, then
return to the cortex before draining into the collecting duct.

(c) Malpighian capsule — It comprises of Bowman's capsule and glomerulus and is located in the kidney
tubule.

(d) Loop of Henle — It runs in the medulla to turn back and to re-enter the cortex to continue into the next
convoluted region of the tubule.

Micturition — Urine is expelled from the urinary bladder through the urethra by relaxation of the sphincter
muscles located at the opening of the urinary bladder into the urethra under impulse from the nervous
system. This process is called micturition.

Osmoregulation — The regulation of osmotic pressure of the blood by regulating its composition (water
and electrolyte) is called osmoregulation

Properties​ Malpighian capsule​ ​ Loop of Henle

Location​ Beginning of nephron​ Between first and second convoluted tubule.

Henle's loops and collecting ducts lie in the medulla of kidneys giving it a 'striped' appearance.

The Bowman's capsule and the proximal convoluted part of all the nephrons lie in the cortex of kidneys
giving it a 'dotted' appearance