The Urinary System

Excretion: The removal of metabolic wastes from the body as a result of cellular activities

Examples of Excretion:

Skin – Excretes sweat (a solution of water, salt, and some urea) - Used to cool the body 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 – Excrete urine, which contains a combination of the end products of metabolism.

Urinary System Functions

Kidney: Produce urine

Ureters: Transport urine to the bladder

Bladder: Storage of urine

Urethra: Elimination of urine

Position of the kidney in the body:

The two kidneys lie 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 collects 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 receptor trigger urination, and the fluid is excreted.

Microscopic Anatomy of the Kidney:

The kidney is composed of over 1 million nephrons (or renal corpuscles)

Bowman’s capsule and proximal convoluted tubule are located in the cortex.

Loop of Henle in the medulla

Distal convoluted tubule in the cortex

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, no glucose)

Each nephron has its own blood supply. Including 2 capillary regions:

1. At the Glomerulus

2. Surrounding the rest of the nephron

Nephron

URINE FORMATION

1. Pressure filtration

2. Selective reabsorption

3. Tubular excretion

4. Reabsorption of water

5. Excretion

Function of the Nephron

1. Pressure filtration: Blood enters the Glomerulus which is surrounded by the Bowman’scapsule. Here, occurs. Water, nutrients, andwastes enter the Bowman’s Capsule. Large organic molecules donot enter. The liquid that collects in the Bowman’s capsuleis blood plasma minus blood proteins.Filtered Blood Components:• Water • Nitrogenous Wastes • Nutrients • Salt (ions)Non-Filtered Blood Components:• Formed elements- (blood cells and platelets )• ProteinsBlood 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 theconstriction of the Glomerulus thus the blood pressure isincreased to an adequate level.

Steps of Urine Formation

2. 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. Requires ATP.

3. Tubular Excretion:

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

5. Concentrated Urine from the collecting tubule collect and goes to the Renal pelvis

4. Reabsorption of Water:

The body can not afford to lose all this water. It must bereabsorbed into the blood. The cells of the loop of Henle activelypump Na+ out of the tubule and K+ into the nephron.The result of this is that the negative ion (Cl-) and water followthe sodium by osmosis (to even out electrical and concentrationgradients)

Water is reabsorbed along the entire length of the nephron.There fore urine is concentrated. The blood in the capillaries arehypertonic to the fluids in the fluid spaces. The loop of Henlecreates a high osmotic pressure in the medulla. The fluid going upthe distal side of the loop of Henle has a greater concentration ofwastes and little nutrient value.

5. Excretion:

Concentrated Urine from the collecting tubule collect and goes to the Renal pelvis

Antidiuretic hormone (ADH) - Negative feedback

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.

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. That is why drinking alcohol, results in a lot of urination and dehydrated.

Aldosterone - Negative feedback

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