Excretion, Osmoregulation and Homeostasis

January 20, 2017 General Studies

Excretion, Osmoregulation and Homeostasis
??? Excretion: nitrogenous wastes (urea, uric acid), excess salts, excess water.
??? Osmoregulation: maintaining the blood at a suitable constant concentration.
??? Homeostasis: maintaining a suitable constant internal environment to sustain efficient metabolism.
Urinary System
Textbook Diagram: urinary system.
??? The kidneys are a pair of fist-sized red-brown bean-shaped structures.
??? The kidneys are attached to the back wall of the abdominal cavity.
??? They lie on either side of the backbone just above the pelvis.
??? Each kidney receives a good supply of oxygenated blood from the renal artery, a branch of the dorsal aorta.
??? The renal vein takes the deoxygenated blood from the kidneys to the inferior vena cava.
??? The blood in the renal vein has less oxygen, salt, urea and uric acid than the renal artery.
??? Urine is carried to the bladder along the ureter by peristalsis for temporary in the bladder.
??? A sphincter muscle at the junction of the bladder and urethra regulates the retention and release of urine.
??? Urine is channelled to the exterior along the urethra.

Kidney Structure
Textbook Diagram: longitudinal section of a kidney showing its internal structure.
??? A smooth thin protective cover called the capsule surrounds each kidney.
??? Below the capsule is a thick reddish granular layer, the cortex.
??? The central part of the series of triangular structures is reddish-brown, the renal pyramids, the tips of which project into the upper expanded end of the ureter known as the pelvis.

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Textbook diagram: structure and blood supply of the nephron.
??? The nephron or renal tubule is the functional unit of the kidney.
??? The nephron has a number of functionally distinct parts.
??? Each human kidney has about one million nephrons.
??? Urine is manufactured by the nephrons.
Production of Urine: Filtration and Selective Reabsorption

Details of Urine Formation
??? The glomerulus functions as a filter.
??? The glomerular capillary walls are porous.
??? The red blood cells, white cells, platelets and plasma proteins are too big to pass through the pores.
??? Therefore the glomerulus filters the blood.
??? The filtrate passing into Bowman??™s Capsule.
??? Glomerular filtrate composition is water, glucose, amino acids, vitamins, salts, urea, and uric acid.
??? About 20% of the plasma volume passes out of the glomerulus.
??? The filtration is much higher than expected.
??? The blood pressure is unusually high in the glomerulus.
??? The blood pressure is generated by the pumping action of the heart.
The high blood pressure in the glomerulus is due to:
??? The arrangement of blood vessels: arteriole ??”> capillaries ??”> arteriole
??? This arrangement is unusual – normally low-pressure venules follow capillaries.
??? The efferent arteriole is narrower than the afferent arteriole.
The higher than normal filtration at the glomerular capillaries is known as ultrafiltration.
Selective Reabsorption
??? Much useful material was lost from the blood into Bowman??™s Capsule.
??? The ???useful??™ materials are taken back into the blood from the nephron.
??? By de-selection, urea and uric acid remain in the nephron and are excreted in the urine.
??? Urine is the unabsorbed glomerular filtrate.
Proximal Convoluted Tubule (PCT)
??? Total reabsorption of glucose and amino acids.
??? Four fifths of the salts and water are reabsorbed.
??? Glucose, amino acids and salts are reabsorbed by active transport.
??? Water is reabsorbed by osmosis.
??? The cells lining the PCT are rich in mitochondria, which supply the ATP for active transport.
The Loop of Henle
This structure allows the kidney to reabsorb extra water in times of water stress. As a result it is possible for the kidney to produce hypertonic urine, i.e., more concentrated than blood plasma. A Loop of Henle is only present in mammals and birds ??” the only animals able to produce hypertonic urine.
About 5% of the water from the glomerular filtrate is reabsorbed from the Loop of Henle by osmosis.
The main function of the Loop is to develop an increasingly concentrated medulla. It accomplishes this by acting as a ???hairpin counter current multiplier??™. This allows extra water, if needed, to be absorbed from the collecting duct under the influence of ADH hormone.
Distal Convoluted Tubule (DCT)
??? Reabsorption of water is by osmosis.
??? The amount varies depending on the need of the body.
??? Water reabsorption by the DCT is under the influence of ADH (antidiuretic hormone).
??? Reabsorption of salt is by active transport.
??? The amount of salt reabsorbed depends on the needs of the body.
??? The role of the DCT is crucial in osmoregulation.
??? Osmoregulation is a major process in homeostasis.

Blood concentration is kept in check by varying the amount of water and salt reabsorbed by the kidneys nephrons.
Blood Concentration Rising
??? Cause: salty food, water loss due to sweating, inadequate water intake.
??? Response: increases water reabsorption, decreases salt reabsorption.
Blood Concentration Falling
??? Cause: excessive water intake, cold weather (sweating less than usual), diet very low on salt.
??? Response: decreases water reabsorption, increases salt reabsorption.
Note: the greater the excess protein in the diet the greater is the urea content of the urine.
Regulation of Body Fluids by the Kidney
The kidney maintains the blood at the correct composition and concentration by excretion and osmoregulation.
As a result all the other body fluids are kept at optimum condition i.e. tissue fluid and cell cytoplasm.

Role of ADH (antidiuretic hormone)
??? ADH is secreted to increase water reabsorption by the DCT and collecting duct when blood concentration rises.
??? Osmoreceptors in the brain??™s hypothalamus detect the increase in osmotic pressure of the blood.
??? This stimulates the pituitary to increase the secretion of ADH into the blood.
??? ADH is transported everywhere throughout the body in the blood.
??? The DCT and collecting duct are the target tissues of this hormone.
??? ADH causes these parts of the nephron to become more permeable to water.
??? Extra water is now reabsorbed into the blood reducing its concentration back to normal.
??? The loss of extra water from the filtrate reduces the volume but increases the concentration of the urine.
Major Homeostatic Organs: kidneys, liver, lungs, skin and brain.

Gina Nealu – Academia.edu


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