THE URINARY SYSTEM

Question 1

The unitary system is composed of

Answer 1

• Two kidneys – filters blood and removes waste.
• Two ureters – carries urine away from the kidneys.
• One urinary bladder – stores urine.
• One urethra – carries urine out of the body.

Question 2

Structure

Answer 2

The kidneys are bean shaped organs located in the posterior abdominal. The right kidney is lower than the
left one due to the liver’s location.
The kidney is divided into cortex (outer layer) and medulla (inner layer).

Question 3

Inside the medulla

Answer 3

1. Pyramids – cone-shaped tissues of the kidney. The pyramids appear striped because they are formed by straight parallel segments of nephrons and collecting ducts.
2. Papilla – peak of the pyramid that face inward. Transports urine produced in the cortical section
   of the kidney to the cup-like cavities (calyces) where the urine gets accumulated before passing into the bladder through the ureter.
3. Calyces – chambers that surround the papilla and collect urine.
4. Renal pelvis – act as a tunnel for urine flowing to the ureter.

Question 4

Nephron – The Functional Unit of the Kidney:

Answer 4

There are 2 types of nephrons:
* Cortical nephrons – have small glomeruli, and are mostly located within the cortex.
* Juxtamedullary nephrons – have large glomeruli and a long loop of Henle that is found it the
pyramid.

Question 5

Nephron composition:

Answer 5

1. Glomerulus - a ball of capillaries located in Bowman’s capsule.
2. Bowman’s capsule – where lost fluid is collected.
3. Proximal convoluted tubule (𝑃𝐶𝑇) – reabsorbs all of the glucose, amino acid and other important materials of the body. It also secretes ammonia, which functions as a buffer for secreted 𝐻+.
4. Loop of Henle – does more absorptions, and also in charge of the urine concertation.
5. Distal convoluted tubule (𝐷𝐶𝑇) – it actively reabsorbs sodium and chloride. It also secretes
   ammonium and hydrogen ions.
6. Collection duct – receives processed filtrate and transports it to the papilla of the pyramid.

Question 6

Blood Supply

Answer 6

The kidneys have a portal system – the blood goes into two capillary beds one after the other (glomerulus
and peritubular capillaries).

Blood enters the kidney in the renal artery -> afferent arteriole -> glomerulus -> efferent arteriole -> peritubular capillaries -> renal vein.

Question 7

Functions

Answer 7

1. Disposal of waste – like urea (disposal of nitrogens) and creatinine.
2. Volume regulation – production of concentrated or diluted urine accordingly.
3. Endocrine – production of hormones.
4. 𝑝𝐻 balance – controls the acidity of the blood.

Question 8

Urine Production (disposal of waste)

Answer 8

1. filtration – high pressure blood enters the glomerulus and is filtered through the capillary
   membrane. Blood cells and large molecules do not get filtered. This process is only mechanical and
   takes place only in the glomerulus.
2. Reabsorption – takes place in the tubules (peritubular capillaries). Glucose is one of the most
   important molecules that gets reabsorbed.
3. Secretion – also takes place in tubules. Potassium entered drags is secreted there.

Question 9

Volume Regulation

Answer 9

Achieved thanks to two important hormones:
1. Ant diuretic hormone (𝐴𝐷𝐻) or vasopressin – produced in the posterior pituitary glen and is
important for creating concentrated urine (also create higher blood pressure and volume) by
making cells in the wall of the tubule create more aquaporin (reabsorptions of water). The trigger
for the release of this hormone is elevated blood osmolality. In case of too much water in the blood, the production of 𝐴𝐷𝐻 will be lowered in order for the kidneys to create diluted urine.

2. Aldosterone – secreted from adrenal glens (located above the kidneys) in response to low blood
   pressure. This hormone causes the kidney to reabsorb more sodium helping it reabsorb more water too. It also creates higher blood pressure and volume.

Question 10

Urine concentration:

Answer 10

• 96% water.
• 2.5% nitrogen waste.
• 1.5% salts.

Question 11

Endocrine function

Answer 11

1. Renin – released from the kidney when blood pressure drops (like in dehydration or bleeding). It
   is a blood vessel constrictor and it causes the release of angiotensin (another constrictor) and aldosterone which is involved in volume regulation. Together these hormones are called 𝑅𝐴𝐴𝑆. In short, production of renin results in higher blood pressure, due to release of aldosterone, vasoconstriction (constrictor of blood vessels), increased release of 𝐴𝐷𝐻 and thirst.
2. Erythropoietin – secreted in response to low levels of oxygen in renal artery. It stimulates 𝑅𝐵𝐶
   production in the bone marrow.
3. Vitamin 𝐷 – the final state of vitamin 𝐷 creation is in the kidney. This vitamin is important for
   calcium absorption from food and urine.

Question 12

𝑝𝐻 Balance

Answer 12

The lungs and the kidneys work together to maintain a 𝑝𝐻 of 7.35 ~ 7.45. The lungs secrete 𝐶𝑂2 to reduce
acidity; the kidneys on the other hand:
1. Actively secrete protons (𝐻+).
2. Reabsorption of bicarbonate(𝐶𝑂3 −).
3. Secretion of ammonia (𝑁𝐻3) that binds to a proton (𝐻+) to form ammonium (𝑁𝐻4) that is exerted in the urine.

**The kidney does so according to the acidity or the basic of the blood. If the blood is too acidic, it will secrete protons, but if it is too basic it will reabsorb them by reabsorption of bicarbonate.

Question 13

Values to know:

Answer 13

• Urinary bladder capacity – around 400−800 𝑚𝑙.
• Renal blood flow (𝑅𝐵𝐹) – 2.560 𝑙𝑖𝑡𝑒𝑟/𝑑𝑎𝑦 (the amount of blood flows out of the kidney).
• Glomerular Filtration Rate (𝐺𝐹𝑅)– 100 𝑡𝑜 120 𝑚𝑙/𝑚𝑖𝑛𝑢𝑡𝑒𝑠. ~180−𝑙𝑖𝑡𝑒𝑟 filtered in a day (approximately 1−2 𝑙𝑖𝑡𝑒𝑟𝑠 exerted out of the body, the rest is absorbed back).
• Blood sodium – 145 𝑚𝑖𝑙𝑙𝑖𝑚𝑜𝑙𝑎𝑟/𝑙𝑖𝑡𝑒𝑟.
• Blood glucose – ~80 𝑚𝑖𝑙𝑙𝑖𝑔𝑟𝑎𝑚/𝑑𝑒𝑐𝑖𝑙𝑖𝑡𝑒𝑟.
• Around 178 liters of water is reabsorbed and only 2 liters are excreted out of the body (99% of the
  filtrate is reabsorbed).

Question 14

Pathology

Answer 14

• Urinary tract infection (𝑈𝑇𝐼) – common in females, due to short urethra and its proximity to the anus. Common pathogen is 𝐸 −𝑐𝑜𝑙𝑖.
• Urethritis – inflammation of the urethra.
• Glomerulonephritis – inflammation of the nephron.
• Pyelonephritis – inflammation of the pelvis.
• Cystitis – inflammation of the urinary bladder.
• Acute kidney failure (𝐴𝐾𝐹) – rapid loss of renal function (up to 46 hours), less than 400𝑚𝑙 of
  urine/day, may be caused due to dehydration, blood loss, toxins and drugs.
• Chronic kidney failure (𝐶𝐾𝐹) – slow loss of renal function, decrease 𝐺𝐹𝑅, increase in toxins level
  in blood, may be caused by a genetic problem, diabetes and drugs.