Regulation and Control Homeostasis.

Islets of Langerhan:

• This is made up of two types of cells: α-cells and β-cells.
• α-cells produce the hormone glucagon when the concentration of glucose in the blood is too low, and β-cells produce insulin when the concentration of glucose in the blood is too high.
• Since the pancreas secretes these two hormones directly into blood capillaries, it acts like an endocrine gland in this regard. [An endocrine gland is, by definition, a gland which secretes hormones directly into the blood stream].  

The function of the two hormones in regulating the blood sugar level is summarized below:

Insulin: glucose to glycogen

Glucagon: glycogen to glucose

Note: Insulin does NOT directly convert glucose into glycogen. Instead, it opens gates in cell membranes, allowing glucose to pass into cells where enzymes may convert it into glycogen. It also inhibits the conversion of non-carbohydrate sources and glycogen to glucose. On the other hand, glucagon fits into receptor sites on cell membranes and activates the enzymes inside the cell which convert glycogen into glucose. The glucose then passes out of the cells and into the blood, raising blood sugar levels.

There are two other sources of glucose apart from that converted from glycogen by the effect of insulin (glycogenolysis). These are:

• Digestion of carbohydrates from our diet.
• Gluconeogenesis, which is the conversion of non-carbohydrate compounds (e.g. the organic residue from deamination of amino acids) to glucose.

Excretion: 

Excretion: This is the removal of potentially toxic products which are produced as a result of metabolism.

Excreted Substances:

• Urea, Ammonia, Uric acid (nitrogenous compounds)
• Bile
• Salts
• Carbon Dioxide

Importance of Excretion:

• The removal of metabolic waste substances which are by-products of major metabolic pathways is needed to prevent the unbalancing of the chemical equilibria of reactions. Many metabolic reactions are reversible and the direction of reaction is determined by the relative concentrations of reactants and products.
• Many of the waste substances are toxic and inhibit enzymes involved in metabolic pathways.
• Excretion regulates the ionic content of the body fluids. Salts behave as electrolytes and dissociate in the aqueous medium of living organisms. The balance between these ions must be maintained within narrow limits to prevent the collapsing of physiological and biochemical activities. For example, the dissociation of NaCl into Na+ and Cl- slows down action potential due to the presence of Na+ ions.
• The solute potential of body fluids depends on the relative amounts of solutes and water. The regulation of water is therefore a must, in absence of which osmotic potential changes with disastrous effects.
• The pH level of the body is regulated through excretion. This is necessary for enzymes in the body to work efficiently.

Kidney:

Functions:

• To maintain the salt and water balance of the body.
• To maintain the pH of the body.
• To maintain plasma Potassium, Calcium and Phosphate ion levels.
• To enable the body to lose unwanted metabolites such as urea, uric acid, etc.
• To excrete certain substances such as drugs.

Gross Structure:

• There are two bean shaped kidneys situated at the back of the abdominal cavity
• Each kidney is about 10 cm long, 6 cm wide and 3 cm deep.
• The left kidney lies slightly forward compared to the right kidney.
• They are well supplied with blood from the renal artery
• They are enclosed in a fibrous coat (capsule) which is surrounded with fats.

L.S of Kidney:

The kidney has three regions:

• The Cortex: This is dark red in color and is where the Bowman’s capsules and Tubules are situated.
• The Medulla: This is pale red in color and is where the loop of Henle is situated. The collecting ducts extend through this region as well.
• Pelvis: This is white in color and is the region where urine is collected.
• Ureter: This carries the urine to the bladder

Structure of a Nephron:

The nephron can be divided into 5 main parts:

1. Bowman’s Capsule:

• This is like a ball of pavement epithelial cells which has been pushed in at one side.
• The invagination contains a dense network of capillaries called the glomerulus.
• Blood flows into the glomerulus through the afferent vessel, and leaves it through the efferent vessel. The efferent vessel is considerably narrower than the afferent vessel. Blood flows from the efferent vessel into a network of capillaries, which finally ends in the renal vein.
• Leading from the Bowman’s capsule is a tubule, which divides into three sections.

2. Proximal Convoluted Tubule:

• This is the longest part of the tubule and is highly coiled.
• It is well supplied with blood capillaries.

3. Loop of Henle:

• This is U shaped, with a descending limb and an ascending limb.
• It descends from the cortex to the medulla and then back into the cortex again.

4. Distal Convoluted Tubule:

• This is coiled and leads into one of the collecting ducts.

5. Collecting Ducts:

• The collecting ducts carry urine down into the pelvis of the kidney
• This drains into the ureter which carries the urine down into the bladder.
• Leading from the bladder is a tube called the urethra which carries urine to the outside.

Functioning of the Nephron:

The functioning of the Nephron involves two main stages

Ultrafiltration:

• Ultrafiltration is filtration under pressure.
• The filtrate from the Bowman’s capsule is identical to blood plasma except for the fact that it does not contain plasma proteins.
• Filtration of the plasma occurs because the pressure inside the glomerulus is high. This forces filtration of the blood into the Bowman’s capsule.
• The high pressure is created because of the fact that the afferent vessel is much wider than the efferent vessel. Also, blood coming into the afferent vessel is at a very high pressure because it comes from the renal artery.
• The blood is forced through the walls of the Bowman’s capsule, which is made up of pavement epithelial cells.
• The walls of the glomerular capillaries are pierced by tiny holes, which are large enough to let through all constituents of  blood plasma e.g. glucose, urea, etc. but small enough to retain all cells e.g. red blood cells, lymphocytes, etc.

Re-absorption:

        Substance                Glomerular Filtrate                Urine

        

        Water

        Protein

        Sodium Ions

        Chloride ions

        Glucose

        Urea

• After the glomerular filtrate has been formed, it flows along tubules and eventually emerges from a collecting duct as urine. The fluid is modified as it flows along the tubule.
• Glucose and amino acids are reabsorbed in the proximal tubule and chloride and sodium ions in the distal tubule.
• Relatively little urea is reabsorbed in normal circumstances. In fact, there is evidence that it may be actively secreted into the tubules from the blood.
• Water is reabsorbed in both the proximal and distal tubules.
• Re-absorption of glucose takes place by active transport. As for the salts, sodium ions are reabsorbed actively and the chloride ions then follow passively along the electrochemical gradient produced. Some water is reabsorbed passively by osmosis following the active re-absorption of salts, which results in concentration increase, and so forth.
• The cells of the tubules have abundant microvilli and mitochondria, proving this.
• The Loop of Henle is responsible for causing a vigorous osmotic flow of water from the collecting ducts, thereby reabsorbing water and causing the urine to be concentrated. It achieves this as follows:

• As the renal fluid floes along the ascending limb of the Loop of Henle, salt is actively removed from it and deposited in the surrounding tissue.
• From there, salt diffuses into, and balances with the fluid in the descending limb. This active transfer takes place at all levels of the Loop of Henle.
• At any given level the effect of this is to raise the concentration in the descending limb above that in the ascending limb, so that as the renal fluid flows down the descending limb, it becomes more and more concentrated and as it flows up the ascending limb it becomes more and more dilute.
• The effect of this system is to produce a region of particularly high salt concentration in the deep part of the medulla.
• The collecting duct passes through this region before opening into the pelvis.
• As the renal fluid passes down the collecting duct, water passes out of it by osmosis. This raises the concentration of the urine.
• Meanwhile, the water is taken away in the bloodstream via the vasa recta.

The Kidney as a regulator:

• If the amount of glucose in the body exceeds a certain threshold value, the kidneys excrete it in the urine.
• The relative amounts of water and salts reabsorbed are strictly geared to the body’s needs.
• Osmoreceptors in the brain are capable of detecting rise and falls in the osmotic pressure of the blood. They stimulate production of hormones to counteract the situation.
• If the body is short of water, the osmoreceptors in the hypothalamus detect a rise in blood osmotic pressure, which stimulates the pituitary glands to produce and release ADH (anti diuretic hormone) into the blood stream.
• The target organ for ADH is the kidney, and its function is to increase the permeability of distal convoluted tubules and particularly the collecting ducts. This allows more water to be withdrawn from the urine, resulting in more concentrated urine.
• If the body has excess water, the osmoreceptors stop stimulating the production of ADH. The opposite results ensue.
• NOTE: this is only fine control as most of the water is absorbed earlier by the distal convoluted tubule.

[Toole and Toole Pg 487 Fig 23.11]

Transmission of Nerve Impulses:

 

        Action Potential                                         Resting Potential

• Impulses travel along neurons as electrical impulses.
• When the axon is at rest, the concentrations of ions on either side of the membrane are different. Briefly, there is an excess of K+ ions inside the axon and an excess of Na+ ions outside.
• The ionic difference is maintained by the active pumping of Na+ ions out of the axon and K+ ions into the axon through the sodium-potassium pump.
• Also, the membrane is permeable to the outward diffusion of K+ ions, but is relatively impermeable to the inward diffusion of Na+ ions. K+ ions therefore, tend to diffuse out of the axon more rapidly than Na+ ions diffuse in, therefore creating a slight surplus of positive charge outside the axon. The inside is therefore relatively negative.
• This effect may be enhanced by the presence of negative organic ions within the axon to which the nerve membrane is impermeable.
• When an impulse is generated and passes along the axon, the membrane suddenly becomes permeable to Na+ ions, which, being about ten times more concentrated outside the membrane, diffuse into the axon.
• This event takes place rapidly and reverses the resting potential: the inside of the axon becomes positive relative to the outside, resulting in action potential.
• Small local currents at the leading end of the region of depolarization excite the next part of the axon, so the action potential is propagated along. This is an example of positive feeback.
• As Na+ ions enter the axon, K+ ions begin to leave. This marks the beginning of the recovery process in which the inside of the axon regains its negative charge.
• The Na+ ions inside the axon are expelled by the sodium-potassium pump, which brings in K+ ions as a result. This restores the normal distribution of ions which exists when the axon is at rest.
• NOTE: The change in potential across the membrane, resulting initially from the local currents at the leading end of the action potential, causes specific protein channels to open up in the membrane through which the ions readily pass. This is why sodium ions can enter the axon easily during the passage of an impulse.
• Depolarization of the membrane corresponds to the rising phase of the action potential, repolarization to the falling phase.
• After an axon has transmitted an impulse it is impossible for it to transmit another for a short period. The reason is that after it has been active the axon has to recover, ionic movements have to occur and the membrane has to be repolarized before another action potential can be transmitted. This period is called the refractory period.

 

Synapses:

• Synapses link one nerve cell to another.
• When an impulse arrives at a synaptic knob it alters the permeability of the presynaptic membrane to calcium ions by opening channel proteins which allow the entrance of Ca+2 ions.
• The influx of these ions causes synaptic vesicles containing acetylcholine to move towards the presynaptic membrane.
• The synaptic vesicles discharge their contents, the transmitter fluid acetylcholine into the synaptic cleft.
• Acetylcholine diffuses across the cleft and attaches to specific sites on the post-synaptic nerve cell so that a positive charge develops on that part of the cell.
• When this positive charge builds up to a critical level, an action potential is generated in the nerve cell.
• Once acetylcholine has done its job, it is broken down by an enzyme cholinesterase.
• Synapses ensure that impulses travel in only one direction as synaptic vesicles are present only at the presynaptic membrane.