Excretion and osmoregulation

“When you replace ‘Why is this happening to me?’ with ‘What is this trying to teach me?’, everything shifts.”

GOOD DAY, fellow travellers. I hope that you had a chance to read last week’s lesson on the kidneys and to practise some of the drawings. Some of those drawings are exam material, so practise drawing and labelling them! This week, we will continue our study of excretion. Last week’s lesson concluded with the composition of the filtrate. Let us revise this. The composition is as follows: COMPOSITION OF THE FILTRATE

Glucose, amino acids, vitamins, hormones, salts and urea (useful substances as well as waste are filtered).

The body cannot afford to lose the useful substances so these are reabsorbed into the blood. Reabsorption takes place in the:

1. Proximal convoluted tubule

2. Loop of Henle

3. Distal convoluted tubule 1. PROXIMAL CONVOLUTED TUBULE

The reabsorption taking place in theproximal convoluted tubule is known as selective reabsorption.

Do you remember the term ‘selectively permeable’? Well, selective reabsorption operates in the same way, only some things are reabsorbed, i.e., the body selects the things that are to be reabsorbed.

■ Glucose and amino acids and a high quantity of the minerals are reabsorbed by active transport. Do you know why this transport has to be active? Do you remember the lessons on the movement of substances in living organisms? Reminder: It has to be active because the substances are absorbed against their concentration gradients.

■ Water is also reabsorbed (however, this reabsorption is not selective).

■ In individuals with diabetes mellitus, the amount of glucose present in the blood is at levels that are so high that the

amounts the blood that are exceeded. cannot be reabsorbed by 2. LOOP OF HENLE

Water is reabsorbed in the Loop of Henle. The longer the loop of Henle, the more the water is reabsorbed. Desert animals have much longer loops of Henle, with the kangaroo rat having the longest. Reabsorption of water prevents dehydration of the body. 3. DISTAL CONVOLUTED TUBULE

■ This facilitates the reabsorption of water and minerals. This reabsorption is controlled by hormones and the product produced after reabsorption is urine.

■ In man, the kidneys filter about 170 litres of water per day, yet only 1.5 litres of urine are excreted. COLLECTING DUCT

This collects urine from several nephrons at the same time. The urine is passed down through the ureters and is stored in the bladder. BLADDER

The bladder has muscular walls which can stretch, enabling it to hold large quantities of urine. The urethra leads from the bladder. The sphincter muscles keep the bladder tightly closed until it is full. Young children cannot control the operation of these muscles, but adult mammals can consciously do so. COMPOSITION OF BLOOD IN RENAL ARTERY AND RENAL VEIN RENAL ARTERY

■ Contains more water.

■ Contains a high concentration of urea.

■ Salt concentration is high.

■ Contains more O2 and less CO2. RENAL VEIN

■ Contains less water. Some water is lost in urine.

■ More CO2 and less O2.

■ Salt concentration is low.

■ Contains little or no urea, lost as urine. OSMOREGULATION Definition:

Osmoregulation is the regulation of the salt and water content of the blood in order to ensure that the concentration of the blood remains the same at all times.

■ Daily activities such as sweating and eating can change the concentration of the blood.

■ This control is an example of a feedback mechanism known as negative feedback and is also a part of the body’s homeostatic mechanisms. HOMEOSTASIS

All the mechanisms are used to maintain a constant internal environment.

The mechanisms are controlled by three main parts of the body. These are:

1. Sense organs

2. Responding organs

3. Control centre

The kidney is also an osmoregulatory organ. The hypothalamus (sense organ) monitors the concentration of the blood as it passes through it. If the blood becomes too concentrated from:

■ Excessive sweating.

■ Ingesting large amounts of salt.

■ Drinking too little water.

The hypothalamus sends a message to the pituitary gland (control centre). The pituitary secretes ADH – antidiuretic hormone – into the blood.

ADH stimulates the walls of the distal convoluted tubules and the collecting ducts to reabsorb more water from the

glomerular filtrate – hence, urine which is more concentrated is produced. If the blood becomes too dilute due to:

■ Too little sweating.

■ Drinking large amounts of water.

■ Low salt intake.

ADH is not released and very little water is reabsorbed – hence, urine which is more dilute is produced. FEEDBACK MECHANISM FOR OSMOREGULATION

Water conservation is not restricted to animals; plants also conserve water. ADAPTATIONS IN PLANTS TO CONSERVE WATER

Plants that live in areas where water is very short are known as xerophytes. These plants have adaptations which serve to reduce water loss. These include:

■ Reduction of leaves to spines, e.g., cacti.

■ Stomata sunken in grooves, e.g., oleander.

■ Leaves rolled into a cylinder, e.g., marram grass.

■ Waxy cuticles – cacti with cuticle on stems.

■ Some plants store water, e.g., succulents such as agave – sisal and century plant, aloe vera. This water helps the plant to survive when there is a really severe shortage. Water is stored in thickened stem or leaves.

■ Extensive root system to absorb water quickly when it rains.

■ The ability to trap CO2 at night so that the stomata can be closed during the day.

Aren’t living organisms marvellous? See you next week! Monacia Williams is an independent contributor.