Osmoregulation

Unit 2 excretion and osmoregulation from john khoza

Osmoregulation -Is a process of maintaining salt and water balance (osmotic balance) across membranes within the body. The fluids inside and surrounding cells are composed of water, electrolytes and non electrolytes. An electrolyte is a compound that dissociates into ions when dissolves in water. A non electrolyte in contrast does not dissociates into ions in water. The body's fluids includes blood plasma, fluid that exists within cells and the interstitial fluid that exists in the spaces between cells and tissue of the body. The membranes of the body( both the membranes around cells and the membranes made of cells lining the body cavities) are semi permeable membranes and permeable to certain types of solute and to water but typically cell membranes are impermeable to solutes.

The body does not exist in isolation. There is a constant input of water and electrolyte into the system. Excess water, electrolytes and wastes are transported to the kidneys and excreted helping to maintain osmotic balance. Insuficient intake of fluid results in fluid conservation by the kidneys. Biological systems constantly interact and exchange water and nutrients with the environment by a way of consumption of food and water and through excretion in the form of sweat, urine and faces. Without the mechanism to regulate osmotic pressure or when a disease damages this mechanism, there is a tendency to accumulate toxic wastes and water which can have dire consequences 

Mammalian system have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major compartment: blood plasma, interstitial fluid and intra cellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of fluids compartments can also change temporarily. Since blood plasma is one of the fluid components, osmotic pressure have a direct bearing on blood pressure. 

The human excretory system function to remove wastes from the body through the skin as sweat, in the lungs in the form of exhaled carbon dioxide and through the urinary system in the form of urine. All of these three participate in osmoregulation and wastes removal. Here let us focus on the urinary system which is comprised of paired kidneys, ureter and the urinary bladder and urethra(figure 1). The kidneys are a pair of bean-shaped structures that are located just below the liver in the body cavity. Each of the kidneys contain more than a million tiny units called the nephron that filter blood containing the metabolic wastes from cells. All the blood in the human body is filtered about 60 times a day by the kidneys. The nephron removes wastes, concentrates them and form urine that is collected in the bladder.

We have learnt that internally the kidney has three regions an outer cortex, a medulla in the middle and the renal pelvis which is expanded end of the ureter. The renal cortex contains the "nephrons" the functional unit of the kidney. The renal pelvis collects urine and lead to ureter on the outside of the kidney. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder