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Blood, Sweat and Tears (and urine!)Ion Regulation: Excretory System

• You are a sack of soup, with the soup made of water, ions and other substances

• Volume and composition of interstitial fluids (surrounds the cells) must be kept constant

• Maintaining homeostasis is the job of the excretory system

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Blood as fast food…vampire bats produce very dilute urine when feeding

Kangaroo rats never see free water…crystals in urine

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• Water enters and leaves the body by osmosis

• Concentrations of ions need to remain stable

• Excretory organs (kidneys) control the volume of interstitial fluids

– Filter out excess water and metabolic wastes– Don’t let cells and large molecules leave– Use passive transport of water

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Metabolic wastes in mammals

• carbohydrates and fats produce carbon dioxide and water

• proteins and nucleic acids produce nitrogenous wastes (nitrogen-containing) as well as CO2 and H2O

– ex. Ammonia, NH3

ammonia build up is toxic, therefore it is converted to urea

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Functions of Excretory System• excretion of metabolic waste

eliminates urea

• maintenance of water-salt balance regulates blood pressure by regulating volume

• maintenance of acid-base balance excretes extra H+ to keep blood pH at about 7.4

• secretion of hormones assist the endocrine system (calcitrol and

erythropoietin)

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The Mammalian Kidney

• The kidneys are bean-shaped excretory organs in vertebrates

• Part of the urinary system, the kidneys filter wastes (especially urea) from the blood and excrete them, along with water, as urine

• The adjective meaning “kidney-related” is renal

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Location• posterior part of the abdomen

• one on each side of the spine; the right kidney sits just below the liver, the left below the diaphragm and adjacent to the spleen

• above each kidney is an adrenal gland (also called the suprarenal gland)

• at the vertebral level T12 to L3, and the right kidney usually lies slightly lower than the left in order to accommodate the liver

• upper parts of the kidneys are partially protected by the eleventh and twelfth ribs

• each kidney is surrounded by two layers of fat (the perirenal fat and the pararenal fat) which help to cushion it

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1. Kidneys produce urine.

2. Ureters transport urine

3. Urinary bladder stores urine

4. Urethra passes urine to outside

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Blood supply

• each kidney filters blood that comes in through the renal artery

• the renal artery branches into arterioles supplying blood to glomerular arterioles

• filtered blood is collected into renal venules and leaves the kidney via the renal vein.

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Nephron: The basic unit of a kidney

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Nephron

• There are more than a million nephrons in each normal adult human kidney.

• Nephrons:

1. Filter the blood.

2. Reabsorb what’s needed

3. Secrete what’s not needed

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Structures of the Nephron

1. The Glomerulus • main filter, located in the Bowman’s capsule

• resembles a twisted mass of tiny tubes through which the blood passes.

• semipermeable, allows water and soluble wastes to pass through using concentration gradients (passive transport)

• moves water and solutes, except proteins, from blood plasma into the Bowman’s capsule (will form urine)

• filtered blood passes out of the glomerulus into the arteriole

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Glomerulus and Bowman’s Capsule

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2. Bowman's Capsule

• part of main filter, contains the nephron, the glomerulus

– within the capsule, the blood is filtered through the glomerulus and then passes out via the arteriole

• receives filtrate from glomerulus

• filtered water and aqueous wastes are passed out of the Bowman's capsule into the proximal tubule

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Fig. 49.18(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Renal tubuleExcretion

Filtration

Reabsorption to blood

Secretion from blood

Glomerulus

Bowman'scapsule

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3. Renal tubule• in charge of reabsorption and secretion

• most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood.

– reabsorption of glucose and amino acids, is driven by active transport carriers

– secretion of waste products involves transport across capillary membranes and kidney tubules.

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a) Proximal tubule

• The glomerular filtrate passes first into the proximal convolute tubule.

• Energy dependent mechanisms reabsorb all of the following constituents

– glucose – amino acids – potassium – sodium - about 80% reabsorbed – bicarbonate

• active secretion also transports other compounds e.g. penicillin

• passive reabsorption of water and chloride.

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Transport mechanisms in Proximal Cells

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b) Loop of Henle

• U-shaped tube that consists of a descending limb and ascending limb

• primary role is to concentrate the salt in the interstitium, the tissue surrounding the loop

– actively reabsorbs sodium ions– passively reabsorbs chloride and

potassium ions

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• descending limb of loop is permeable to water but completely impermeable to salt

• As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by osmosis

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• Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water

– actively pumps sodium out of the filtrate, generating a hypertonic interstitium

– filtrate becomes hypotonic

• hypotonic filtrate is passed to the distal tubule in the renal cortex.

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Reabsorption of Salt and Water

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Osmotic Gradients In Kidney

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c) Distal tubule

• similar to the proximal tubule in structure and function

• active transport of ions is regulated by the endocrine system

– some hormones cause reabsorption of ions, others cause excretion

• after travelling the length of the distal convoluted tubule, only 3% of water remains, and the remaining salt content is negligible

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4. Collecting Duct

• a pipe-like channel, where urine is concentrated

• normally impermeable to water

– becomes permeable in the presence of antidiuretic hormone, water is re-absorbed by osmosis

– levels of ADH determine whether urine will be concentrated or dilute (can reabsorb ¾ of water)

– reabsorbed water returns to blood

• Urine leaves the collecting ducts and enters the ureters to go to the bladder

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Summary

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Regulation of KidneysAntidiuretic Hormone (ADH)

• ADH is polypeptide hormone secreted by the posterior pituitary gland .

• Its principal action is to regulate the amount of water excreted by the kidneys.

• ADH, known also as vasopressin, causes the kidneys to resorb water directly from the distal tubules, thus concentrating the salts and waste products in the liquid, which will eventually become urine.

• ADH secretion by the pituitary is regulated by neural connections from the hypothalamus, which is believed to monitor either the volume of blood passing through it or the concentration of water in the blood.

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Regulation of KidneysAntidiuretic Hormone (ADH)

• Dehydration or body stress will raise ADH secretion and water will be retained.

• Alcohol inhibits ADH secretion.

• Failure of the pituitary to produce ADH results in diabetes insipidus.

• In pharmacological doses ADH acts as a vasoconstrictor. The structure and chemical synthesis of ADH was announced (1953) by Nobel laureate Vincent Du Vigneaud and others.

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Hormones Control Homeostatic Functions

• Antidiuretic hormone– Stimulates reabsorption of water by the

kidneys.

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Aldosterone

• Aldosterone is a steroid hormone produced by the outer-section of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood.

• It is synthesized from cholesterol by aldosterone synthase

• Aldosterone acts by increasing sodium reabsorption from the distal tubule and the collecting duct.

• Aldosterone is responsible for the reabsorption of about 2% of filtered sodium in the kidneys, which is nearly equal to the entire sodium content in human blood under normal conditions.

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Aldosterone

• Blood pressure receptors in the juxtaglomerular apparatus near the glomerulus detect low blood pressure.

• Specialized cells within th structure release rennin, an enzyme that converts angiotensinogen, a plasma protein produced by the liver, into angiotensin.

• Angiotensin causes the constriction of of blood vessels and raises blood pressure.

• Secondly, angiotensin causes release of aldosterone from the adrenal glands to increase sodium reabsorption and increase blood pressure.

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Fig. 49.22(TE Art)Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Low bloodpressure

Bowman'scapsule

Distalconvolutedtubule

Glomerulus

Afferentarteriole

Efferentarteriole

Loopof Henle

Increased NaCland H2O

reabsorption

Increasedblood

volume

Negativefeedback

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Low bloodflow

Proximalconvolutedtubule

Renin

Adrenalcortex

Angiotensinogen

Angiotensin II

3Juxtaglomerularapparatus

Kidney

Aldosterone

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Dialysis and kidney transplants

• Generally, humans can live normally with just one kidney.• Only when the amount of functioning kidney tissue is

greatly diminished will renal failure develop. • If renal function is impaired, various forms of medications

are used, while others are contraindicated. Provided that treatment is begun early, it may be possible to reverse chronic kidney failure due to diabetes or high blood pressure.

• If creatinine clearance (a measure of renal function) has fallen very low ("end-stage renal failure"), or if the renal dysfunction leads to severe symptoms, dialysis is commenced.

• Dialysis is a medical procedure, performed in various different forms, where the blood is filtered outside of the body.

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Kidney Dialysis Machine

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Kidney Dialysis

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Kidney Transplant

• Kidney transplantation is the only cure for end stage renal failure; dialysis, is a supportive treatment; a form of "buying time" to bridge the inevitable wait for a suitable organ.

• The first successful kidney transplant was announced on March 4, 1954 at Peter Bent Brigham Hospital in Boston. The surgery was performed by Dr. Joseph E. Murray, who was awarded the Nobel Prize in Medicine in 1990 for this feat.

• There are two types of kidney transplants: living donor transplant and a cadaveric (dead donor) transplant.

• When a kidney from a living donor, usually a blood relative, is transplanted into the patient's body, the donor's blood group and tissue type must be judged compatible with the patient's, and extensive medical tests are done to determine the health of the donor.

• Before a cadaveric donor's organs can be transplanted, a series of medical tests have to be done to determine if the organs are healthy. Also, in some countries, the family of the donor must give its consent for the organ donation.

• In both cases, the recipient of the new organ needs to take drugs to suppress their immune system to help prevent their body from rejecting the new kidney.

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Kidney Transplant

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Kidney transplant

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Kidney Transplant

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Structure

• In a normal human adult, each kidney is about 11 cm long and about 5 cm thick, weighing 150 grams.

• The kidneys are "bean-shaped" organs, and have a concave side facing inwards (medially).

• On this medial aspect of each kidney is an opening, called the hilum, which admits the renal artery, the renal vein, nerves, and the ureter.

• The outermost portion of the kidney is called the renal cortex, which sits directly beneath the kidney's loose connective tissue capsule.

• Deep to the cortex lies the renal medulla which is divided into 10-20 renal pyramids in humans.

• Each pyramid together with the associated overlying cortex forms a renal lobe.

• The tip of each pyramid (called a papilla) empties into a calyx, and the calyces empty into the renal pelvis. The pelvis transmits urine to the urinary bladder via the ureter