Skin

The skin is used to excrete urea, excess water, salts, ammonia. Functions of the skin. Protection of the underlying tissues from entry of microorganisms, physical damage and ultra violet rays from the sun. Regulation of body temperature/thermoregulation. Excretion. Reception of stimuli of heat, cold, pain, touch and pressure. 5. Synthesis of vitamin D. Storage of fats.

ADAPTATION TO FUNCTION OF DIFFERENT PARTS OF THE SKIN

The skin consists of two main layers: epidermis and dermis.

A. The epidermis- it consists of 3 layers: cornified layer, granular layer and malpighian layer.

1. Cornified layer- it is the outermost layer of the skin made up of dead cells that are covered with flexible substance called keratin that; a) Provides protection against mechanical/ physical damage. b) Provides protection against entry of pathogens. c) Reduces loss of water by evaporation.

• The cells of cornified layer are continuously lost through friction and are replaced by cells from granular layer.
• The thickness of cornified layer varies in the body e.g. it is thickest in areas of high friction like palms of hands and soles of feet but thinnest on lips and eyeballs.

Granular layer this is a middle layer consisting of living cells that give rise to the cornified layer.

Malpighian layer- this is the innermost layer consisting of actively dividing cells that give rise to the new epidermis.

The cells also have a pigment called melanin that gives the skin its colour and protects it against harmful effects of ultra violet (UV) rays from the sun.

B. The dermis- it consists of:

Sweat/ sudorific glands- consists of secretory cells secrete more sweat from surrounding blood vessels and tissues (consisting of excretory wastes e.g. excess water, traces of urea, lactic acid, ammonia and excess mineral salts) when it is hot.  Sweat is brought/ flows to the skin surface through sweat duct. Water in sweat evaporates/sweat evaporates carrying with it latent heat of vaporization thus cooling the body/ reducing body temperature.  And it also helps in excretion of lactic acid, traces of urea, excess water, ammonia and excess mineral salts and regulation of salt and water balance (osmoregulation).  In cold weather, little or no sweat is produced thus heat is conserved.

Blood vessels/arterioles Blood vessels contain blood that supplies nutrients and oxygen to skin tissues and remove waste products and carbon (IV) oxide.

• When it is hot/ when temperature rises above optimum the arterioles dilate (vasodilate), more blood flows near the skin surface hence more heat is lost through conduction and convection.
• When it is cold/ temperature fall below normal, the arterioles constrict (vasoconstrict), less blood flows near the skin surface hence less heat is lost/ heat conserved/ insulates the body against heat loss.

Hairs and erector pili muscle.-Hairs grow in hair follicles. Erector pili muscle attaches the hair follicle to the epidermis. Hair follicle is supplied with sensory nerves to increase sensitivity of the skin and blood vessels that supply nutrients and remove waste products. They are also used in thermoregulation. When it is cold/ when temperature drops below normal, erector pili muscles contract, causing the hairs to stand erect. Hairs trap air which is a poor conductor of heat, preventing heat loss/ conserving heat. When it is hot/ when temperature rises above normal, erector pili muscles relax, hairs lie flat on the skin surface, trapping less air hence more heat is lost.

Sebaceous glands- They are attached and open into the hair follicle. They secrete sebum which keeps the hair and epidermis flexible/ supple/soft and waterproof. Sebum also contains

Nerve endings of nerve cells- they detect stimuli of pain, pressure and temperatures of the external environment to enable organism to respond to changes in the environment. Subcutaneous layer/adipose tissue.- this is a layer of fat cells beneath the dermis. It binds the skin to the muscles and other organs deep in the body. It also stores fats and acts as an insulator against heat loss/ conserves heat. It also contains a special type of cells that manufactures vitamin D when exposed to the sun.

How is the skin adapted to protection?

The cornified layer is made of dead cells; which protects the body against physical/mechanical injury/ damage; prevents entry of pathogens and poisonous chemicals;  The sebaceous gland produces an oily secretion /sebum; that protects the skin against invasion by micro-organisms (acts as antiseptic).  Malpighian layer contains melanin; which protects the body against harmful ultra-violet rays.

ROLE OF SKIN IN EXCRETION AND HOMEOSTASIS.

Role of the skin in excretion.

Sweat glands secrete sweat containing excess water, mineral salts, ammonia, traces of urea, lactic acid. Sweat flows through the sweat duct to the skin surface and the wastes are lost.

Adaptations/ role of the skin in homeostasis.

a) Osmoregulation. The skin has sweat glands that secrete sweat containing excess water and mineral salts/ ions and other wastes e.g. urea hence maintaining water and ions balance. The water and mineral lost in the sweat contributes to osmotic changes of the body cells and fluids. On a hot day, the body loses a lot of water and some mineral salts, resulting in the sensation of thirst being felt due to tissue dehydration. The osmotic balance is however restored by drinking large volumes of water and intake of mineral salts in the diet.

b) Thermoregulation. It has blood arterioles/ vessels that vasodilate (vasodilation) when it is hot/ when the body temperature rises above normal, the hypothalamus is stimulated which sends impulse to the skin causing more blood flows near the skin surface hence more heat is lost. When it is cold/ when the body temperature falls below normal, the arterioles constrict/ vasoconstrict (vasoconstriction), less blood flows near the skin surface hence less heat is lost/ heat is conserved. It has erector pili muscles which contract when it is cold/ when the body temperature falls below normal. This causes hairs to stand erect trapping more air. Air which is a poor conductor of heat insulates the body against heat loss/ prevents heat loss.

When the temperature is raised above the normal/when it is hot, erector pili muscles relax, hair lies flat on the skin surface, less air is trapped and more heat is lost. It has sweat glands which secrete more sweat when the temperature is raised above the normal/when it is hot. Water in sweat evaporates/sweat evaporates, taking up heat from the body hence the body is cooled/temperature lowered. When it is cold/ when temperature drops below normal sweat glands are less stimulated/there is less sweating leading to less vaporization of water/less heat loss.

Study question

1. State one homeostatic role of the human skin.  Thermoregulation, osmoregulation, regulation of salts.

2. Name three structures of the skin essential for its homeostatic function. Sweat glands. Hair. Erector pili muscle. Blood vessels/ arterioles.. Nerve endings.

3. Explain why the amount of sweat increases with increase in temperature. This is because of increase in latent heat of vaporization hence causing evaporation/ vaporization of sweat to cool the body.

   KIDNEY.

   It is used to excrete urea, uric acid, excess water, salts. It performs the following functions:

   1. Excretion
   2. Osmoregulation (regulation of water and ionic content in blood)
   3. Regulation of pH of blood and tissue fluid. A mammal has a pair of kidneys located in the abdominal cavity. They are bean-shaped and red in colour. The adrenal glands are located above each kidney which secrete hormones.

   Parts of a kidney.

   1. Hilum- it is a depression located on the concave side of the kidney through which the renal artery enters to supply blood from the heart containing oxygen, nutrients and urea and renal vein which removes blood from kidney from the kidney to the heart containing carbon (IV) oxide.
   2. Cortex – this is the outer part of the kidney and contains glomeruli, Bowman’s capsule and the convoluted tubules of the nephrons.
   3. Medulla – this is the inner part of the kidney. It is red in colour and contains the loop of Henle and the collecting ducts.
   4. Pyramids- they are conical
   5. Pelvis- this is a swollen cavity which is white in colour.
   6. Medulla – this is the inner part of the kidney. It is red in colour and contains the loop of Henle and the collecting ducts.
   7. Pyramids- they are conical structures that extend from medulla.
   8. Pelvis- this is a swollen cavity which is white in colour.

   THE NEPHRON.

   This is the basic functional unit of the kidney. Each kidney contains about 5 million nephrons. Parts of the nephron.

   1. Afferent arteriole- brings blood to the nephron from the renal artery.

   2. Efferent arteriole - takes blood away from the nephron to the renal vein.

   3. Bowman’s capsule- cavity which encloses the glomerulus.

   4. Glomerulus (Plural- glomeruli) - a network of capillaries enclosed by Bowman’s capsule.

   5. Proximal convoluted tubule- it is coiled and extends into Loop of Henle.

   6. Loop of Henle- U- shaped part of the nephron.

   7. Distal convoluted tubule- it is coiled and leads into a collecting duct/ tubule

   8. Collecting duct- it receives urine from several nephrons and directs it to the pelvis and ureter.

   ROLE OF THE KIDNEY IN EXCRETION/ URINE FORMATION/HOW THE KIDNEY/ NEPHRON WORKS (FUNCTIONS)

   The kidney/ nephron receives blood from the renal artery through afferent arteriole into the glomerulus. Blood contains urea, blood cells, dissolved food substances, water, hormones and blood proteins. The afferent arteriole receives blood at a higher pressure from the renal artery. The afferent arteriole entering the glomerulus is wider that the efferent arteriole leaving it. The higher blood pressure, narrower lumen of glomerulus and narrowness of the efferent arteriole than the afferent arteriole causes resistance and high pressure in the glomerulus.

   Because of the high pressure, small sized substances (water, urea, glucose, mineral salts, and amino acids) filter through the glomerulus into Bowman’s capsule/ capsular cavity/ space in the process called ultrafiltration. The mixture is called Glomerular filtrate. Ultrafiltration is important to allow excretion/removal of metabolic/ nitrogenous waste/ urea. Large sized substances (blood cells and blood proteins) do not filter through, instead they flow to efferent arteriole. This is because of small pores of glomerulus and Bowman’s capsule.

   The glomerular filtrate then flows to the proximal convoluted tubules where selective reabsorption takes place. Amino acids, glucose, vitamins, and some water are reabsorbed through diffusion and active transport and water through osmosis. Selective reabsorption is important to allow return/ reabsorption of useful substances back into the blood stream. The filtrate then flows to the descending arm/ limb of loop of Henle where water is selectively reabsorbed through osmosis. Water re-absorption is influenced by active uptake of sodium ions at the ascending loop of Henle which increases osmotic pressure of tissue fluid and Anti-diuretic hormone (ADH) secreted by pituitary gland.

   At the ascending arm/ limb of Loop of Henle, sodium ions /salts / mineral salts/ ions are selectively re-absorbed through active transport. Re-absorption of sodium chloride ions is controlled by aldosterone secreted by adrenal glands. The filtrate then flows to distal convoluted tubule where water is reabsorbed through osmosis. The water reabsorption is influenced by active uptake of sodium ions at the loop of Henle and Antidiuretic hormone. The filtrate/ urine then flows to collecting duct where water is reabsorbed through osmosis. Urine contains urea, excess salts, water, some ammonia, some uric acid. Urine flows to the pelvis and then to the urinary bladder through the ureter. Urinary bladder stores the urine for some time before it is eliminated through the urethra (through urination).

   ADAPTATIONS OF THE NEPHRON TO ITS FUNCTION.

   1. Afferent arteriole has a wider lumen than efferent arteriole creating high-pressure that enables ultra-filtration to occur at the glomerulus.
   2. The glomeruli (capillaries) have tiny pores to facilitate ultra-filtration.
   3. The proximal and distal convoluted tubules have the following adaptations:

   i. The lumen wall has villi and microvilli which increase the surface area for reabsorption. ii. The tubule is long and highly coiled to increase surface area and slow down movement of filtrate/ creating more time for efficient reabsorption. iii. It is one cell thick/has thin epithelium to reduce the distance travelled by substances hence faster reabsorption of materials.

   iv. The tubule is well vascularized/ well supplied with blood capillaries for transportation of reabsorbed materials/ create a steep concentration gradient for efficient transport of reabsorbed materials. v. The cells lining the tubule have numerous mitochondria which provide energy for active transport/ reabsorption of substances.

   5. The Loop of Henle has the following adaptations: a) It is long to increase the surface area for re-absorption of mineral salts and water.

      b) It has a counter current flow established between the filtrate flow and the large supply of blood in capillaries. This creates a large/steep diffusion gradient for efficient re-absorption.

   Renal adaptations of a desert animal e.g. desert rat

   1. Has a small glomerulus to reduce the amount of water filtered into the glomerular filtrate which helps in water retention.
   2. Has a long loop of Henle to increase the surface area for water re-absorption conserving water/ causing less water to be lost in urine. Renal adaptations of aquatic animal e.g. hippo.
   3. Has a large glomerulus to increase the amount of water filtered into the glomerular filtrate.
   4. Has a short loop of Henle to reduce water re-absorption.

   Factors that promote re-absorption of water in the distal convoluted tubule.

   1. The active uptake of mineral salts at the loop of Henle that raises the osmotic potential of blood.
   2. Secretion of Antidiuretic hormone (ADH) from the pituitary gland which increases permeability of the tubules and blood capillaries to water.

Factors that determine the quantity and concentration of urine in animals

1. Habitat e.g. whether terrestrial or aquatic.
2. Physiological adaptations.
3. Structural adaptations e.g. possession of a long loop of Henle.
4. Environmental temperature- as temperature increases, the amount of urine produced decreases. This is because increase in temperature increases sweating which raises the osmotic pressure of blood. This causes more water to be reabsorbed into the bloodstream in the kidney tubules resulting in production of little but concentrated urine.

Role of the kidney in excretion/ purification of blood.

Blood entering the kidney has a lot of water, salts and nitrogenous wastes e.g. urea, uric acid, ammonia and creatinine. In the glomerulus, these substances are forced out into the glomerulus by the process of ultrafiltration. They pass through the tubules without reabsorption and finally eliminated through urine and the blood leaving the kidney is purified.

Study question

1. Explain why the nephron is long and coiled. To occupy less space in the kidney, to increase surface area for selective reabsorption and allow more time for selective reabsorption.

ROLE OF THE KIDNEY IN HOMEOSTASIS

When the osmotic pressure of the blood rises above normal (as a result of dehydration), the osmoreceptors in the hypothalamus are stimulated which stimulates the pituitary gland to secrete a hormone called anti-diuretic hormone (ADH) or vasopressin into the blood. On reaching the kidney, Anti-diuretic hormone (ADH) or vasopressin causes the kidney tubules to be more permeable to water. More water is reabsorbed into the blood stream lowering osmotic pressure of the blood producing a concentrated urine.

When the osmotic pressure of the blood falls below normal, hypothalamus is less stimulated hence the pituitary gland is less stimulated, less Anti-diuretic hormone (ADH) or vasopressin is released into the blood stream. The kidney tubules become less permeable to water and less re-absorption of water into blood stream takes place hence osmotic pressure of blood rises and dilute urine is produced. Lack of ADH/Vasopressin leads to Diabetes Inspidus. When the level of sodium ions is low in the blood, adrenal glands are stimulated to release aldosterone into the blood. Aldosterone stimulate loop of Henle of kidney to reabsorb sodium ion into the blood. If the sodium ions concentration in the blood rises above optimum level, adrenal glands produce less aldosterone into the blood and less amounts of sodium ions are absorbed.

Diabetes Inspidus.

This is a condition in which in which a person excretes large amount of dilute urine. Occurs when the pituitary gland fails to produce anti-diuretic hormone or produces it in inadequate amount. The kidney tubules are less permeable to water producing large amount of dilute urine. This is called diuresis). The patient becomes dehydrated and has to compensate for the loss by drinking a lot water.

KIDNEY DISEASES.

A. NEPHRITIS This is the inflammation of the glomerulus of the kidney caused by bacterial associated infections e.g. small pox, measles, typhoid fever and sore throat. Symptoms

1. Headache.
2. Oedema (swelling of jaw).
3. Vomiting.
4. Drowsiness.
5. Nausea.
6. Fever.
7. General weakness.
8. Passing of highly coloured urine due the presence of proteins.

Control and treatment.

1. Administration of drugs/ antibiotics.
2. Renal dialysis.
3. Controlled use of salts and proteins.
4. Bed rest.

KIDNEY STONES.

Causes

1. Improper diet and inadequate of water.
2. Chemical salts in water e.g. oxalates, phosphates, urates and uric acid. These chemicals undergo precipitation and form hard deposits or stones in the pelvis and ureter hence cause blockage of urine.

Control and treatment.

i. Sharp pain on the lower back ii. Difficulty in passing out urine. iii. Pain iv. Chills v. Fever.

Control.

1. Consult a physician.
2. Take balanced diet with plenty of water.
3. Take hot baths and massage the back with hot soft material.
4. Surgical treatment which may involve kidney transplant.
5. Dialysis or artificial washing out of the wastes.
6. Use of laser beams to disintegrate the stones.
7. Use of drugs to dissolve the stones.

LIVER

It is located below the diaphragm and used to excrete the following products: bile pigments and cholesterol. It is connected to the following blood vessels:

1. Hepatic artery- which carries oxygenated blood rich in nutrients and oxygen from the heart to the liver.
2. Hepatic vein – which carries blood rich in carbon (IV) oxide and nitrogenous wastes from the liver to the heart.
3. Hepatic portal vein – which carries blood from the intestines and stomach to the liver to regulate the amount of blood sugar before it enters the general circulation.

Roles/ functions of the liver.

1. Regulation of blood sugar level.
2. Deamination.
3. Thermoregulation.
4. Regulation of haemoglobin/ excretion of bile pigments.
5. Regulation of plasma proteins.
6. Manufacture of blood cells (during foetal stage)
7. Formation and elimination of excess cholesterol.
8. Regulation of fat metabolism.
9. Storage of blood.
10. Storage of vitamins B, C, E, K and minerals.

FUNCTIONS/ ROLE OF THE LIVER IN EXCRETION AND HOMEOSTASIS.

A. ROLE OF THE LIVER IN HOMEOSTASIS.

1. Regulation of blood sugar/glucose- When there is excess sugar/rises above normal, the hormone insulin is produced by interstitial cells of (Beta (ß) Islets of Langerhans of) the pancreas. Insulin stimulates liver cells to convert the excess glucose to glycogen or fats for storage, increase oxidation of glucose, inhibit conversion of glycogen to glucose lowering blood sugar level to normal/ optimum level. When the blood sugar level drops below normal, the hormone glucagon is produced by the interstitial cells (Alpha (§) Islets of Langerhans of) the pancreas. Glucagon stimulates the liver cells to convert glycogen and fats to glucose, reduce the breakdown of glucose thus raising blood sugar level to normal/ optimum level.

   When the blood sugar level drops below normal, the hormone glucagon is produced by the interstitial cells (Alpha (§) Islets of Langerhans of) the pancreas. Glucagon stimulates the liver cells to convert glycogen and fats to glucose, reduce the breakdown of glucose thus raising blood sugar level to normal/ optimum level.

   2. Production of heat energy/ thermoregulation.  When the temperature is low, the hypothalamus sends impulse to the liver. The liver increases the rate of metabolic reactions producing heat energy which is distributed throughout the body thus increasing the body temperature.

B. ROLE OF THE LIVER IN EXCRETION.

1. Deamination/ Regulation of amino acids levels- Excess amino acids cannot be stored in the body because the body does not have mechanism for storage hence are eliminated through deamination. Deamination is the process by which excess amino acids are broken down in the liver to form urea which is excreted in urine. Process of deamination. Amino group (NH2) is removed from the amino acid. Amino group combines with hydrogen atom to form ammonia.

Ammonia is taken into ornithine cycle reacts/ combines with carbon (IV) oxide to form urea. Ornithine cycle is controlled by enzyme carbonic anhydrase/orginase. The remaining organic compound is converted into glucose. Glucose is broken down to release energy or converted into glycogen/ fat for storage.

2. Detoxification/detoxication of toxic substances. The liver converts toxic substances into harmless form and eliminates them. The toxic substances arise from metabolic activities of the liver, drugs, foods or drinks. For example, hydrogen peroxide produced by actively respiring cells is toxic. Enzyme catalase in the liver catalyzes/ speeds up the breakdown of hydrogen peroxide into water and oxygen which are harmless. Hydrogen peroxide Water + Oxygen

3. Excretion of bile pigments/ haemoglobin regulation. Bile pigments produced as a result of breakdown of red blood cells/haemoglobin are removed by the liver. Haemoglobin is broken down into haem group and globin. Globin is digested into amino acids while haen group is broken down into bilirubin and biliverdin and taken to the liver/ gall bladder. Bilirubin and biliverdin are released into the gut as bile and passed out through faeces (giving it brown colour). N/B. Haemoglobin breakdown also relaeses pigment called urochrome which is eliminated in urine (giving it yellow tinge colour)

Diabetes Mellitus (Sugar Disease).

It is due to a deficiency in insulin secretion from the pancreas. The malfunctioning of the pancreas leads to failure to release insulin or release inadequate amount. This leads to very high levels of sugar in the blood that cannot be utilized by cells hence glucose is eliminated by kidney.

Signs/ symptoms.

1. Passing large amounts of urine.
2. Excretion of glucose in urine.
3. Loss of body weight.
4. Feeling of thirst.
5. Passing urine frequently/polyuria.
6. Constant feeling of thirst/ Dehydration.
7. Excessive eating.
8. Poor resistance to diseases.
9. Excessive eating/polyphagia/hyperphagia

Control

1. Administration of regular injection of the insulin hormone to patients.
2. People suffering from diabetes should avoid food rich in sugars and excessive intake of alcohol. Note Insulin is not administered orally because it is a protein and can be digested by proteases (protein-digesting enzymes).

Study question.

Describe how to test for the presence of glucose and protein in urine (describe how to test whether a patient is suffering from diabetes mellitus and proteinuria)

1. Testing for the presence of glucose (diabetes mellitus). i. Take 2ml of urine. ii. Add equal amount of Benedict’s solution. iii. Heat to boil. iv. If the colour changes from blue-green-yellow-orange, then glucose is present/ patient is suffering from diabetes mellitus.

2. Testing for the presence of proteins in urine (proteinuria/albuminuria) i. Take a sample of urine. ii. Add 2ml sodium hydroxide. iii. Add 1 % copper sulphate dropwise. iv. If purple/violet colour is observed then protein is present.