2.9 HOMEOSTASIS

Definition of homeostasis:

(i)   the maintenance of a relatively stable physiological environment in an organism, 

or

(ii)  maintaining a stable internal body environment in an organism, 

or

(iii) regulation of chemical composition of body fluids in an organism.

Examples of the body conditions are:

(i)    blood pressure 

(ii)   blood glucose level 

(iii)  body temperature

3 components for feedback mechanism:

(i)   a receptor

(ii)  a control centre

(iii) an effector

The positive feedback mechanism

Positive feedback mechanism: 

- The homeostasis mechanism enhances the original stimulus.

- Example: the excretion of oxytocin for the contraction of the uterine wall during giving birth.

Negative feedback mechanism: 

-  The homeostasis mechanism stops or reduces the original stimulus.

-  Example: The excretion of glycogen to reduce blood glucose level.

* alpha cells of the islets of Langerhans of pancreas

* beta cells of the islets of Langerhans of pancreas

* Glucagon causes  glycogenolysis which is breakdown of glycogen to glucose and gluconeogenesis which is breakdown of other metabolic such as amino acid and fatty acid to glucose.

* Glucose is released from the liver cells into the bloodstream. 

* Glycogenesis is the formation of glycogen from glucose.

Human Kidney

Structure of a nephrone in the kidney

Urine Formation

(i)   Ultrafiltration

First step in the formation of urine involves glomerulus and Bowman's capsule occurs as hydrostatic pressure forces fluid from the blood in the glomerulus to diffuse into the lumen of Bowman's capsule. The high hydrostatic pressure is due to the diameter of the afferent arteriole is larger than the diameter of the efferent arteriole. 

The highly coiled glomerular capillaries provide large surface area for ultrafiltration. The capillaries of glomerulus also perforates with pores and podocytes are permeable to water and small solutes. The small solutes are salts, glucose, amino acids, vitamins, hormones and urea but lack of large molecules such as blood cells, platlets and plasma proteins which are left behind in the blood.

(ii)  Reabsorption

Useful substances such as glucose, amino acids and vitamins are transported  from the filtrate back into the interstitial fluid. Whereas wastes, excess salts and other materials remains in the filtrate. Most of the reabsorption occurs in the proximal convoluted tubule. Proximal convoluted tubule has numerous microvilli to increase the surface area for reabsorption and lots of mitochondria to provide energy in active transports of substances. Reabsorption continues as the filtrate passes through the loop of Henle and distal convoluted tubule. 

Glucose, amino acids, sodium ions (Na⁺) and chloride ions (Cl^-) are transported out to the interstitial fluid by active transport at proximal convoluted tubule and distal convoluted tubule. Water is reabsorbed by osmosis at proximal convoluted tubule, descending limb of loop of Henle and at the distal convoluted tubule. Potassium ions (K⁺) and bicarbonate ions (HCO₃^-) are reabsorbed by passive transport at the proximal convoluted tubule.

(iii) Secretion

      At the proximal convoluted tubule, H⁺ is actively secreted into the tubule to control pH level in body fluid. Ammonia is passively secreted into the tubule to prevent the filtrate becomes too acidic due to the presence of  H⁺ 

           At the distal convoluted tubule, K⁺ and H⁺ from the body fluid is actively secreted into the tubule.