Unit O – Urinary System

Definition: Excretion- The removal of metabolic wastes from the body (the useless and potentially dangerous by-products of cellular reactions)  

Roles of Organs in Excretion Skin – Excretes perspiration (a solution of water, salt, and some urea) - Used to cool the body rather than secretion of wastes. - Increase in wastes during times of renal failure (kidney)

Liver – Breaks down old red blood cells - Yellow pigment in urine is also formed from the breakdown of heme by the liver. It is then deposited in the blood and later removed from the kidneys.

Lungs – Expiration removes carbon dioxide but it also results in the loss of water.

Intestine – Certain salts, such as those of iron and calcium are excreted directly into the cavity of the intestine by the epithelial cells lining it and become part of feces.

Kidneys - Remove wastes from the blood, then return “clean blood” to the circulatory system. - Excrete urine, which contains a combination of the end products of metabolism.

Entrance and exit of water Enters: 1. Drink water 2.   Absorbed from food eaten Exits: 1. Exhalation 2.     Urination 3.     Perspiration 4.     Tears (small amounts) 5.     Expectorating (phlegm) 6.     Defecation

Urinary System Functions Kidney – Produce urine   Ureters – Transport urine to the bladder Bladder – Storage of urine Urethra - Elimination of urine

Kidney Structure  

When a kidney is sliced longitudinally (sagittal section), it shows 3 regions. 1.     Outer granulated region called the cortex 2.     Striated or lined layer called the medulla 3.     Inner cavity called the pelvis where urine collects

The kidneys are in the lower, dorsal part of the abdomen. They receive blood from the renal arteries to be cleaned.

The wastes to be excreted, called urine, collect in the pelvic region of each kidney from where they are conducted by peristalsis down to the urinary bladder. When the urinary bladder becomes full, stretch receptors trigger urination, and the fluid is excreted

Microscopically, the kidney is composed of over 1 million nephrons (or renal tubules) Bowman’s capsule (which contains the glomerulus) and proximal convoluted tubule are located in the cortex.  Distal convoluted tubule is in the cortex Loop of Henle is in the medulla  Collecting ducts are located in the medulla and cortex

Renal Artery – Bring blood to the kidneys to be “cleaned” - high urea content   Renal Veins – Returns blood that is cleaned - Low urea content

Each nephron has its own blood supply. Including 2 capillary regions: 1. At the Glomerulus 2.  Surrounding the rest of the nephron

URINE FORMATION A.    pressure (glomerular) filtration B.    selective tubular reabsorption C.    reabsorption of water D.    tubular secretion E.    excretion

Function of the Nephron A. Pressure Filtration. Blood enters the Glomerulus which is surrounded by the Bowman’s capsule. Here, pressure filtration occurs. Water, nutrients, and wastes enter the Bowman’s Capsule.

Large organic molecules do not enter Bowman’s capsule (stay in blood) Large organic molecules do not enter Bowman’s capsule (stay in blood). The liquid that collects in the Bowman’s capsule is blood plasma minus blood proteins.

Water - Formed elements (blood cells and platelets)   Filtered Blood Components Non-Filtered Blood Components (taken out of blood and move into (stays in blood, does not enter Bowman’s Bowman’s capsule) capsule) Water - Formed elements (blood cells and platelets) Nutrients Nitrogenous wastes - plasma proteins Salt (ions) (see animation)

Blood pressure is very important Blood pressure is very important. If blood pressure is too low, filtration will not occur. A hormone called rennin is released from a specialized tissue in the Glomerulus that causes the constriction of the Glomerulus thus the blood pressure is increased to an adequate level.

B. Selective Tubular Reabsorption occurs mainly at the proximal convoluted tubule. Required nutrients (glucose, amino acids, vitamins, minerals, salt molecules and some water) are reabsorbed into the peritubular capillary network (blood). Requires ATP  

Reabsorption of Water. The body cannot afford to lose all this water Reabsorption of Water.The body cannot afford to lose all this water. It must be reabsorbed into the blood. The cells of the loop of Henle actively pump Na+ out of the tubule to make the medulla hypertonic.

The hormone aldosterone (secreted from the adrenal cortex above the kidney) promotes the reabsorption of Na+

The result of this is that the negative ion Cl- and water follows the sodium by osmosis (to even out electrical and concentration gradients)

Water is reabsorbed along the length of the nephron Water is reabsorbed along the length of the nephron. Therefore urine is concentrated. (see video)

The loop of Henle creates a high osmotic pressure in the medulla The loop of Henle creates a high osmotic pressure in the medulla. The fluid going up the distal side of the loop of Henle has a greater concentration of wastes and little nutrient value.

D. Tubular Secretion occurs at the distal convoluted tubule D. Tubular Secretion occurs at the distal convoluted tubule. Excess molecules in the blood, such as H+ ions, histamine, penicillin, and ammonia are secreted into the tubule from the blood, thereby helping to maintain homeostasis. Helps maintain blood pH

Excretion. The collecting duct is another site of water reabsorption, leaving concentrated urine. The urine from the collecting tubule collects and goes to the renal pelvis to be excreted via the ureter, bladder and urethra. (see video)

Antidiuretic hormone (ADH) Good example of Negative feedback loop ADH – controls reabsorption of water.   ADH increases the permeability of the distal convoluted tubule and collecting duct so that more water can be reabsorbed.

Therefore a decreased amount of urine results, but an increase in urine concentration. Water and Urea=Urine. Less water, more urea concentration. Urea is a waste product in the metabolism of proteins. With more water being reabsorbed, blood volume tends to be increased.

If too little water is taken in orally the hypothalamus releases a hormone into the nerve tract to the posterior pituitary gland.    

The posterior pituitary gland releases ADH and more water is absorbed by the kidneys to maintain blood volume. pH of blood is maintained by tubular excretion. If pH is too low, hydrogen ions are secreted into the nephron.

Alcohol has a negative effect on ADH. It sends a false message to the hypothalamus that there is too much water in the blood. ADH is not released in fact it is inhibited.

Therefore water is not reabsorbed and is lost in the urine Therefore water is not reabsorbed and is lost in the urine. That is why when you drink alcohol, you urinate a lot and become dehydrated.

Aldosterone Hormones which regulate the level of sodium and potassium in the blood. If sodium is too low, aldosterone causes more sodium to be reabsorbed. If sodium concentration is high release of aldosterone is inhibited.  

Adrenal Cortex controls this hormone Adrenal Cortex controls this hormone. The effect of this is the same as that of ADH – more water is reabsorbed due to osmosis in the loop of Henle