This unit discloses on temperature and various absorption and regulation mechanism procedures in the body.

The efficiency range of thermal neutrality

This is the temperature range between a lower critical temperature and the high critical temperature, during which physical mechanisms alone are capable of regulating the body temperature.

The graph shows that above the high critical temperature, the body’s cooling mechanisms fail, the metabolic rate increases as temperature rises.

Below the low critical temperature, physical mechanisms are helped by chemical means. The metabolic rate increase as temperature falls.

Animals living in cold places have a lower critical temperature than those living in warm places.

Also the curve for the metabolic rate below critical temperature is less steep and lower lethal temperature much lower for cold dwellers than for warm dwellers as shown below.

Man is disadvantaged cause he is endothermic with a high critical temperature of ~ 27; yet with skunty hair, he would not have explored the temperature regions and the poles had he not used behavioural methods of putting on clothes.

Temperature regulation in plants

Plant tissues can generally tolerate wide fluctuations in temperature than most animals but need to thermo regulate to avoid enzymes denaturing and tissues freezing. Plants gain and loss heat by the same physical processes as animals ie radiation, conduction, convection and evaporation.

Prevention of overheating

Transportation results in cooling of the plant.

Plants with in dry condition, parenchyma cells turgidity and leaves and stems droop. This reduces the SA exposed to the sun.

Plants have shiny cutides which reflect heat and small leaves prevent excessive heat gain.

Prevention of freezing

Heat gain is less frequently a problem to plants.

  • Leaves are oriented so that they do no shade each other to take maximum advantage of light at any one time.
  • By loosing easily damaged leaves on by producing seeds or spores which are very resistant to low temperature. The usually occurs in areas that experience winter.

Plants survive long periods of very low temperatures without suffering tissue damage.

Plants of different regions are adapted to different temperature ranges, hence plant in temperature regions thrive best between 250C and 350C whereas the optimum conditions for the aratic ad alpine plants may be as low as 150C skin, kidney, liver, lungs.

The skin excretes excess H20, urea and salts. The liver secretes bile pigments. The lungs excrete CO2 and H20 vapour.

Nitrogenous excretory products

These are produced by catabolism of proteins and nucleic acids.

The immediate nitrogenous waste product of deamination of proteins is ammonia.

Ammonia may be excreted immediately on converted into the major nitrogenous compunds i.e. urea and uric acid with differ in their solubility and toxicity.

The nature of the excretory product is determined by;

  1. The metabolic capability of the organism, i.e. the enzymes present.
  2. Availability of water to the organism i.e. habitat
  • Extent to which water loss is controlled by the organism.

Classification of animals according to the nature of their excretory nitrogenous product.

There are three types:-

  1. Ammoniotellic
  2. Ureotellic

Summary of the major difference among the three types


There is a degree of correlation between excretory products, embryonic environment and at adult habitat e.g. the tadpole excretes, ammonia while the adult amphibians excretes urea.


  • Very toxic
  • The high solubility of ammonia helps in its rapid excretion as ammonia ions in most aquatic organisms before it reaches concentrations toxic to the organism.
  • Being highly toxic, it needs large quantities of water to be diluted and hence excreted by aquatic organisms.


Less toxic and less soluble than ammonia hence less water needed for its excretion because the tissues can tolerate large concerns of it.


Uric acid and its salts are ideal excretory products for terrestrial organisms and organisms producing dei…….. eggs (shelled eggs) since they combine a high nitrogen content with low toxicity and low solubility. They can be stored in cells tissues and organs without producing any toxic or adverse osmeregulatory effects, and require a minimal amount of water for the excretion.

As the concentration with uric acid in the tissue rises, it settles out as solid orystals or precipitate.

Uric acid and ammonium urate are forms in which nitrogenous excretion occurs in insects, lizards, birds snakes.

Humans and apes excrete small quantities of uric acid produced from the break down of nucleic acid but not from breakdown with proteins.

Other examples of nitrogenous excretory products.

Trimethyl amine oxide, excreted by teleast (bony fish).

Purines, adenine and guanine are excreted directly by spiders and some animals but in other organisms these are broken down into uric acid.

  1. The majority of mammals have uricase in the liver which converts uric acid to the excretory product albintoin. It is also excreted by dipreran insects e.g. housefly.
  2. Allantoic acid, some bony fish produced allantoic acid by oxidizing allantoin.
  • Creative and its derivative creative.

Why is it that even when a person stops taking proteins in their diet, he still excretes urea.

The body forms the non essential amino acids.


This is the passing back of the useful substances of the giomerular filtrate into the blood flowing in the surrounding capillaries so that they are not lost with urine.

This occurs mainly at the proximal and distal convulated tubules, although the loop of henic and collecting duct also help in reabsorption of water.

Structure of convoluted tubules in relation to function.


Adaptions of convoluted tubules to their function.  

  1. The inner surface of epithelial cell bears numerous microilli (brush border) which increases S.A for reabsorption.
  2. The epithelial cells and surrounding capillaries (vasorector) are very close to each other to ease reabsorption by shortening distance through which material has to pass.
  3. The S.A on capillary side of epithelial cells is increased in infolding of the cell membrane to ease the process.
  4. Between the infoldings lie numerous mitochondria that supply energy for active transport.
  5. Each cells of convulated tubule processed large nucleus to control cell activities..
  6. Possesses pinocythic vessels which remove proteins with small molecular mass that had drained through.
  7. Surrounding capillaries carry away substances reabsorbed, maintaining a diffusion gradient.



Over 80% of the glamerular filtrate is reabsorbed in the proximal convoluted tubule, including all glucose, amino acids, vitamins, hormones and 25% of sodium chloride and water.

The mechanism of reabsorption occurs as follows.

Glucose, amino acids and ions are actively transported out of the cells of the proximal convoluted tubules into the intercellular spaces and basal channels.

  1. Once in these spaces and channels, they enter the permeable capillaries by diffusion and are carried away from the nephron.


  1. The constant removal of these substances from the cells of the proximal convoluted tubule creates a diffusion gradient between the filtrate in the lumen of the tubule and cells along which more substances pass. About half of urea returns to blood by diffusion. The remainder is excreted in urine.


Reabsorption of H20 is mainly by osmosis. The blood in the capillaries surrounding the proximal convoluted tubule is derived from the efferent vessel of the glomerulus. It is therefore rich in protein which exerts an osmotic pressure yet it is under low hydrostatic pressure. This is further raised by the active uptake of sodium ions and Cl. This causes water proximal convoluted tubule.

  1. Proteins of small molecular mass which may have been forced out of blood in Bowman’s capsule are taken up at base of microvilli by pinocytosis. As a result of all the above activities, the tulular filtrate is isotonic with blood in the surrounding capillaries.

Reabsorption of water by loop of Henle.

How process occurs

It uses the principle of the hair-pin counter current multiplier.

Adaptations to achieve to act as a hair pin counter current multiplier.

The ascending limb of loop of Henle is thicker than the descending limb resulting in the former being impermeable. Na+ and Cl (sodium chloride) is actively pumped out of the ascending limb into extracellular fluid of medulla. H20 would have followed the ions out of tubule but this is prevented by the ascending limb being impermeable to H20. This results in a dilute hypotonic filtrate flowing to the 2nd convoluted tubule. The active transfers of salts take place at all levels of the loop of Henle. The ascending limb has flattened cuboidal cells with brush border and many mitochondria for the active process.

Since the interstitial region (extracellular fluid) of the medulla is more concentrated due to the high concentration of Na+ and Cl pumped into it, water is drawn out of the filtrate in the descending limb by osmosis, which id freely permeable to it, but impermeable to Na+ and Cl. The H20 is hen drawn into the vasoractor and carried away.

As fluid flows further down descending limb, H20 continues to be withdrawn so that the contents of the tubule become progressively more concentrated yet more and filtrate from proximal convoluted tubule with more dissolved salts continues to flow into the descending limb. These 3 aspects, i.e. active removal of Na+ and Cl from ascending limb to descending limb, removal of H20 by osmosis from descending limb and continual removal of salts from increase convoluted tubule cause net result of max. concentration occurring extreme tip of Hnele both ………………. inside and outside in extracellular filtrate.

Therefore at this point, tabular fluid is hypertonic to blood.


  1. Also increase in concentration of extracellular fluid also causes osmotic extraction of water from collecting ducts.
  2. The r……………. applies the hair pin counter current multiplier because to solute (Na+, Cl) move into descending limb, H20 drawn out of it in opposite directions; the effect being overall at any one level (unit effect). But overall result is multiplied by the length of the hair pin; as the fluid moves further down its length.
  • The concentration of urine produced is directly related to length of loop of Henle and hence thickness of medule relative to cortex; which both increase progressively as the habitats become driver. E.g. “The Beaver is semi aquatic, has thin medulla, short loop of Henle and produces large volume of dilute urine whereas kangaroo rat has long loops of Hnele, thick medulla and produces small volumes of highly urine to conserve water. This also explains why some amphibians lack loop of Henle.


The composition of urine is also affected by substances actively secreted directly into it, intestinal fluid surrounding tubules. This occurs in both proximal and distal convoluted tubules but mainly in the latter.

These include; K+, H+, NH+4, and nitrogenous excretion like urea, uric acid, creatinine and drugs like penicillin.

 Importance of tubular secretion

  1. Although these substances are present in glomenular filtrate, direct tubular secretion enables greater quantities of such wastes to be eliminated than by ultra filtration alone.
  2. Helps to get rid of unwanted materials such as drugs.

Active tubular secretion in distal convoluted tubule helps to control pH of blood.

If the pH starts to fall below norm, H+ are secreted from distal convoluted tubule into filtrate also the kidney cells.

Also kidney cells make and secrete ammonia ions with combine with anions brought to the kidney and excreted as ammonium salts.

If the pH rises, the tubule secretes bicarbonate ions into the filtrate A. while pH of blood; is maintained within a narrow range.

Table showing composition of plasma and urine and changes in concentration.



Reabsorption of water and active tubular secretion accounts for high tubular secretions of urea, Na+ etc than plasma.

Concentration factor   =       Concentration in urine

Concentration in plasma



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