PHGY 222

• Renal system or urinary system


  Has maximum excretory capacity and plays a major role in homeostasis
• Kidneys


  • Bean-shaped, about the size of a closed fist, lie against the back of the abdominal wall, outside the peritoneal cavity, just above the waistline in the lumbar area, right kidney sits slightly higher than the left one because of the liver position, about 4 1/2 inches long and 2 1/2 inches wide
• Kidneys


  • Highly vascular (contain a lot of blood vessels), divided into three main regions: renal cortex (outer region which contains about 1.25 million renal tubules), renal medulla (middle region which acts as a collecting chamber), and renal pelvis (inner region which receives urine through the major calyces)
• Nephron


  The microscopic structural and functional unit of the kidney, composed of a renal corpuscle and renal tubule
• Renal corpuscle


  • Consists of a tuft of capillaries called a glomerulus and an encompassing Bowman's capsule
• Renal tubule


  • Extends from the capsule, composed of epithelial cells with a lumen
• Functions of the kidney

  • Urine formation
  • Osmoregulation
  • Secretion of hormones
• Urine formation


  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion
• Glomerular filtration


  The first step in renal processing, a process of bulk flow where water and low molecular weight substances move from the lumen of the capillary, across the filtration membrane, and into Bowman's space
• Filtration membrane


  • Formed by three layers: Glomerular capillary membrane, Basement membrane, and Visceral layer of Bowman capsule
• Tubular reabsorption


  The process by which water and other substances are transported from renal tubules back to the blood, large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells, the reabsorbed substances move into the interstitial fluid of renal medulla and then into the blood in peritubular capillaries
• Tubular secretion


  The transfer of materials from peritubular capillaries to the renal tubular lumen, caused mainly by active transport and passive diffusion, usually only a few substances are secreted, and are typically waste products
• Substances secreted from different portions of the tubules

  • Potassium is secreted actively by sodium potassium pump in proximal and distal convoluted tubules and collecting ducts
  • Ammonia is secreted in the proximal convoluted tubule
  • Hydrogen ions are secreted in the proximal and distal convoluted tubules, maximum hydrogen ion secretion occurs in proximal tubule
  • Urea is secreted in loop of Henle
• Osmoregulation


  The active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content
• Role of kidneys in osmoregulation


  Maintain the fluid balance and the concentration of electrolytes to keep the fluids from becoming too diluted or concentrated, regulated by hormones such as antidiuretic hormone (ADH), aldosterone, and angiotensin II
• Secretion of hormones by the kidney


  Peritubular capillaries secrete erythropoietin which is necessary for formation of red blood cells, Juxtaglomerular cells secrete renin which participates in the maintenance of blood pressure through renin-angiotensin mechanism
• Applied physiology


  • Nephrotic Syndrome
  • Glomerulonephritis
  • Kidney stones
• Endocrine Glands


  • Groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of hormones into the bloodstream
  • Ductless glands so hormones diffuse directly into the bloodstream
  • Hormones are then carried in the bloodstream to target tissues and organs where they affect their metabolism and regulate total body metabolism, growth, and reproduction
• Hormones


  • Chemical messengers or regulatory molecules secreted into the blood by endocrine glands
  • The body's chemical messenger systems interact with one another to maintain homeostasis
  • Homeostasis of the internal environment is maintained partly by the autonomic nervous system and partly by the endocrine system
• Autonomic Nervous System (ANS) vs Endocrine System


  • ANS is concerned with rapid changes, while endocrine control is mainly involved in slower and more precise adjustments
  • Some chemical messengers are secreted due to response to neural stimuli, thus a sharp distinction between the nervous and endocrine systems cannot always be drawn on the basis of the chemicals they release
• Chemical Classification of Hormones

  • Amines
  • Polypeptides and proteins
  • Glycoproteins
  • Steroids
• Polar (Water-soluble) Hormones


  • Soluble in water, hydrophilic
  • Do not enter the target cells, but instead bind to receptors on the plasma membrane
• Non-Polar (Lipid-Soluble) Hormones


  • Insoluble in water but soluble in lipids, aka lipophilic hormones or hydrophobic
  • Pass through the target cell's plasma membrane, bind to intracellular receptor proteins, and act directly within the target cell
• Hormone Receptors


  • Hormone receptors are the large proteins present in the target cells
  • Each receptor is specific for one single hormone, i.e. each receptor can combine with only one hormone
  • Receptors are situated in the cell membrane, cytoplasm, and nucleus
• Hormone Interactions


  • Synergistic Effect
  • Antagonistic Effect
  • Permissive Effect
• Mechanisms of action of hormones

  1. Hormones That Bind to Nuclear Receptor Proteins

  2. Hormones That Use Second Messengers
• Characteristics of a Hormone

  • The duration of action of each hormone differs and specific to its functions
  • The concentrations of hormones required to control most metabolic and endocrine functions are incredibly small
  • The rates of secretion of the various hormones are extremely small
  • Hormone concentration in the blood primarily reflects the rate of secretion by the endocrine glands
  • Hormones are rapidly removed by target organs and by the liver
  • The half-life of a hormone ranges from minutes to hours for most hormones
  • Hormones removed from the blood by the liver are converted by enzymatic reactions into less active products
  • The effects of hormones are very dependent on concentration
• Priming effects or upregulation

  1. Variations in hormone concentration within the normal, physiological range can affect the responsiveness of target cells

  2. Small amounts of (GnRH), increase the sensitivity of anterior pituitary cells to further GnRH stimulation
• Desensitization


  • Prolonged exposure to high concentrations has been found to desensitize the target cells
  • Subsequent exposure to the same concentration of the same hormone thus produces less of a target tissue response
• Downregulation


  • High concentrations of hormones cause a decrease in the number of receptor proteins in their target cells
  • The importance of pulsatile secretion e.g. The pulsatile secretion of GnRH and LH is needed to prevent desensitization
• Feedback Control of Hormone Secretion

  1. Negative feedback: A hormone is released in response to a specific stimulus and usually its action reverses or negates the stimulus

  2. Positive feedback: amplification of the stimulus and increasing release of the hormone until a particular process is complete and the stimulus ceases
• Reading Materials: Fox: Human Physiology, K Sembulingam Essentials of Medical Physiology, Gerard J. Tortora and Bryan Derrickson. Principles of Anatomy and Physiology
• Endocrine Glands


  • Pituitary Gland
  • Thyroid gland
  • Parathyroid glands
  • Pancreas
  • Adrenal glands
  • Sex glands
• Hormones secreted by the Hypothalamus

  • Growth Hormone Releasing Hormone (GHRH)
  • Thyroid-Releasing hormone (TRH)
  • Corticotropin Releasing Hormone (CRH)
  • Gonadotropin Releasing hormone (GnRH)
  • Prolactin Releasing hormone (PRH)
• Hormones secreted by the Anterior Pituitary


  • Growth Hormone (GH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Prolactin (PRL)
  • Melanocyte Stimulating Hormone (MSH)
• Hormones secreted by the Posterior Pituitary

  • Antidiuretic hormone (ADH)
  • Oxytocin (OT)
• Hormones secreted by the Thyroid gland

  • Thyroxine (T4)
  • Triiodothyronine (T3)
  • Calcitonin
• Parathyroid gland


  Secretes Parathormone
• Hormones secreted by the Pancreas - Islet of Langerhans

  • Insulin
  • Glucagon
  • Somatostatin
  • Pancreatic Polypeptide
• Hormones secreted by the Adrenal Cortex

  • Mineralocorticoids
  • Aldosterone
  • 11-deoxycorticosterone
  • Glucocorticoids
  • Cortisol
  • Corticosterone
  • Sex Hormones
  • Androgens
  • Estrogen
  • Progesterone