Renal Physiology Presentation PDF

Renal Physiology by Dr.Mai Mohammed Hasan Professor of Medical physiology The urinary system or renal system excretes water, salt, urea, alcohol, drugs, hormones and many other substances in urine. Urine is made in the kidneys and contains many waste products which is toxic if n...

Renal Physiology by Dr.Mai Mohammed Hasan Professor of Medical physiology The urinary system or renal system excretes water, salt, urea, alcohol, drugs, hormones and many other substances in urine. Urine is made in the kidneys and contains many waste products which is toxic if not eliminated from the body. The urinary tract is the system of ducts and channels that conduct urine from the kidneys to the exterior and it consists of two kidneys, two ureters, a bladder and a urethra.  Kidneys The kidneys are two bean-shaped organs, located on either side of the body, one each side of the spine. They are delicate organs and are surrounded and protected by the bones of the lower ribs and spine, the bulky skeletal muscles of the back and abdominal wall. A tough membrane called capsule and a layer of fat surround each kidney. Examination of the cut section in the kidney shows different areas: 1. Renal cortex: the outer region. 2. Renal medulla: the inner region with pyramid-shaped areas. 3. Renal pelvis: the central cavity which narrows into the ureter. 4. Blood vessels. 5. Ureter: the tube which drains urine to the bladder. Structure of the kidney  Functions of the kidneys The ability of the kidneys to excrete urine is essential to their function in regulation of the composition and volume of all body fluids. 1. Filter waste products from the blood to maintain homeostasis of body fluids. 2. Excrete excess electrolytes including sodium (Na+), potassium (K+), calcium (Ca2+) and phosphate (PO4 3–). 3. Control body fluid volume which is essential for normal functions of the cardiovascular system. 4. Monitor blood pressure. 5. Activate vitamin D obtained from synthesis in the skin or from the diet. 6. Secrete erythropoietin: a hormone essential for the production of red blood cells in the bone marrow. 7. Excrete foreign substances, e.g. drugs, pesticides and toxins.  Nephrons  Nephrons (renal tubules) are the microscopic functional units of the kidneys, and each kidney contains about a million of them.  The beginning of the tubule lies in the renal cortex and is expanded into a cup- shaped capsule named Bowman’s capsule that encloses a small network of capillaries called the glomerulus.  Bowman’s capsule extends into the proximal convoluted tubule, which dips down into the renal medulla as the loop of Henle, then back to the distal convoluted tubule to the collecting duct. Capillaries surround the tubule until it joins the collecting duct which leads to the renal pelvis of the kidney. Structure of the nephron Structure of the nephron  Functions of the nephrons As blood passes through the kidneys, three key processes take place in the nephrons that result in urine formation: I. Ultrafiltration: the blood in the glomerulus that filters out urea, salt, water, glucose and other small particles from the blood. II.Reabsorption: essential substances are brought back into the blood in the capillaries surrounding the tubule, e.g. water, glucose, electrolytes. III.Secretion: of substances such as acid or drugs into the filtrate.  Functions of the nephrons I-Ultrafiltration  Ultrafiltration or glomerular filtration occurs in the glomerular capillaries, which act as a semipermeable membrane.  The efferent arteriole carries blood away from Bowman’s capsule has a smaller diameter than the afferent arteriole, so the resistance in the efferent arteriole is higher than that in the afferent arteriole.  The pressure difference forces small molecules from the blood to be filtered into Bowman’s capsule. The fluid filtered in this way is called glomerular filtrate.  Glomerular filtration rate The rate of filtration from blood depends on the sum of two forces, one forcing the substances out from the capillaries into Bowman’s capsule, the other forcing them back again. 1. Hydrostatic pressure in glomerular capillaries tends to push water and solutes into Bowman’s capsule. 2. Forces that oppose glomerular filtration are: i. Colloid osmotic pressure of blood which tends to absorb fluid back into blood. ii. Hydrostatic pressure of the filtrate already in Bowman’s capsule. Forces that determine fluid filtration by the glomerular capillaries II-Reabsorption Reabsorption allows many useful solutes filtered out from the blood in Bowman’s capsule to return to the blood in different segments of the nephron. 1-The proximal convoluted tubule is responsible for reabsorbing most of the water and solutes in the filtrate. 2-The descending limb of the loop of Henle is permeable to water but impermeable to sodium chloride. 3-The ascending limb of the loop of Henle actively transports sodium chloride back into the blood. However, this segment is impermeable to water. 4-The distal convoluted tubule and collecting duct adjust the final composition and volume of the filtrate to match the needs of the body, depending on the action of the hormones ADH (antidiuretic hormone) and aldosterone. II-Reabsorption III-Secretion into the nephrons Few substances are added to the final urine mainly hydrogen and potassium ions, nitrogenous wastes ammonia, urea, uric acid and some drugs, e.g. penicillin. 1. Secretion of hydrogen ions and ammonium Secretion of hydrogen ions (H+) and the nitrogenous waste ammonium (NH4+) are linked to reabsorption of bicarbonate ions (HCO3-). 2. Secretion of potassium ions Potassium ions (K+) are normally in low concentrations in plasma, and are secreted into final urine under the effect of the hormone aldosterone. 3. Secretion of drugs Active secretion against a concentration gradient takes place in the distal part of the nephron. Functions of different parts of the nephron  Antidiuretic hormone and water excretion When the amount of water in the blood falls below a set level due to dehydration, vomiting or excessive sweating, the hypothalamus sends a message to the pituitary gland which releases ADH. Antidiuretic hormone or vasopressin controls the amount of water reabsorbed by the kidneys to keep the concentration of water in the blood nearly constant in a process known as osmoregulation. Antidiuretic hormone and water excretion Water balance Regulation of water input and output  Micturition Micturition (urination; voiding) is the process of emptying the bladder. Bladder The bladder is a hollow muscular sac on the pelvic floor and varies in size according to the volume of urine it contains. Rugae are folds in the bladder lining that allow the bladder to expand if filled with urine. Trigone: a triangular area on the floor of the bladder ,very sensitive to stretch. Detrusor muscle contracts and expels urine. Internal urethral sphincter The bladder neck muscles The external urethral sphincter Urinary Bladder  Micturition a-Involuntary (reflex) voiding Involuntary voiding occurs in infants and young children until 2-3 years. It is a complex process and requires maturation the nervous system brain, brainstem, spinal cord and nerves of the sympathetic and parasympathetic divisions of the autonomic nervous system. Involuntary (reflex) voiding  Micturition b-Voluntary control of voiding The urge to empty the bladder is perceived as an uncomfortable sensation that is correlated with the level of filling of the bladder. The brain begins the process of voiding urine by sending motor nerve impulses from the brainstem to the spinal cord. Impulses from spinal neurons stimulate the contraction of the detrusor muscle until the pressure of urine is high enough to empty the bladder. Voluntary control of voiding