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Kidney

May 4, 2024

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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.

  1. 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.