The Human Excretory System

Biology 2201 Unit 3 Section 4

Excretory System

The

kidneys

regulate the amount of water, salts and other substances in the blood. The kidneys are fist-sized, bean shaped structures that remove nitrogenous wastes (urine) and excess salts from the blood. The

ureters

are tubes that carry urine from the pelvis of the kidneys to the urinary bladder. The

urinary bladder

temporarily stores urine until it is released from the body. The

urethra

is the tube that carries urine from the urinary bladder to the outside of the body. The outer end of the urethra is controlled by a circular muscle called a sphincter.

Excretory System

The Kidney

Each kidney is composed of three sections: – the outer (renal) cortex, the (renal) medulla (middle part) and the hollow inner (renal) pelvis. The

cortex

is where the blood is filtered. The

medulla

contains the collecting ducts which carry filtrate (filtered substances) to the pelvis. The

pelvis

is a hollow cavity where urine accumulates and drains into the ureter.

The Parts of the Kidney

How blood is Filtered

The filtering units of the kidneys are the

nephrons

. There are approximately one million nephrons in each kidney. The nephrons are located within the cortex and medulla of each kidney. The tubes of the nephron are surrounded by cells and a network of blood vessels spreads throughout the tissue. Therefore, material that leaves the nephron enters the surrounding cells and returns to the bloodstream by a network of vessels.

Parts of the Nephron

Each nephron consists of the following parts: – 1) glomerulus ; – 2) Bowman’s capsule ; – 3) proximal tubule ; – 4) loop of Henle ; – 5) distal tubule ; – 6) collecting duct.

The Glomerulus

The

glomerulus

is a mass of thin-walled capillaries. The

Bowman’s capsule

is a double-walled, cup-shaped structure. The

proximal tubule

leads from the Bowman’s capsule to the Loop of Henle. The

loop of Henle

is a long loop which extends into the medulla. The

distal tubule

connects the loop of Henle to the collecting duct.

~ See Figure 11.19; p. 375.

How the kidney does its job!

Kidney Action

Blood enters Bowman’s capsule through a tiny artery — (

the renal artery

). The artery branches to form a

glomerulus

. Blood pressure forces some blood plasma and small particles into the surrounding capsule — (this is called the

nephric filtrate

). Large particles such as blood cells and proteins remain in capillaries. The nephric filtrate is pushed out of the capsule and into the

proximal tubule

. This is where reabsorption begins.

Kidney action (cont) Only materials needed by the body are returned to bloodstream — for example, 99 % of water, all glucose and amino acids and many salts are reabsorbed. Osmosis , diffusion, and active transport draw water , glucose , amino acids and ions from filtrate into surrounding cells. Small villi like projections help in the active transport of glucose out of the filtrate and speed up the reabsorption process. From here, these components return to the bloodstream.

Kidney action (cont) The filtrate reaches the end of the proximal tubule and the fluid is isotonic with the surrounding cells. Glucose and amino acids have been removed from the filtrate. The filtrate then moves to the

loop of Henle

whose primary function is to remove water from the filtrate by osmosis.

Kidney action (cont) There is in increase in sodium concentration in the cells of the loop of Henle as we move from the area of the cortex to the inner pelvis of the kidney.

This causes water to be drawn from the filtrate. These high levels of sodium are a result of active transport and results in increased concentration of filtrate (due to water loss).

Why doesn’t water on outside go back in?

The water that left the descending loop cannot enter the ascending loop because it is impermeable to water. As the water concentration in the filtrate decreases, the chlorine concentration increases and causes it to diffuse out of the tubule. This process is helped by the electrical attraction of chlorine to sodium. .

Water (cont) The filtrate moves to the

distal tubule

where tubular secretion occurs. Active transport is used to pull hydrogen ions, creatinine, drugs such as penicillin out of the blood and into the filtrate. Fluid from a number of nephrons moves from the distal tubules to a common

collecting duct

which carries what can now be called

urine

to the

renal pelvis

.

NOTE Since the kidneys control what leaves and what remains in the nephrons, they maintain the levels of water, ions and other materials nearly constant and within the limits to maintain homeostasis

.

Function of the Kidney

The principal function of the kidney is to filter blood in order to remove cellular waste products from the body. At any given time, 20 % of blood is in the kidneys. Humans can function with one kidney. If one ceases to work, the other increases in size to handle the workload.

The kidney has other functions but it is usually associated with the excretion of cellular waste such as : 1) urea (a nitrogenous waste produced in the liver from the breakdown of protein. It is the main component of urine) ; 2) uric acid (usually produced from breakdown of DNA or RNA) and 3) creatinine (waste product of muscle action).

All of these compounds have nitrogen as a major component. The kidneys are more than excretory organs. They are one of the major homeostatic organs of the body. They control water pH, secrete erythropoietin (a hormone that stimulates red blood cell production) and activate vitamin D production in the skin.

That is why a doctor can tell so much from a urine sample

.

Disorders of the Excretory System

Our excretory system is very important in maintaining homeostasis. There are certain conditions which can affect the excretory system. We will discuss some of these common disorders.

UTI (Urinary Tract Infection)

Is a very common disorder. If the bladder has become infected, it is known as

cystitis

. If the urethra is infected., it is called

urethritis

. UTI is more common in women than men because of differences in anatomy. Symptoms include painful urination burning sensation), frequent urination (even if no urine present) and bloody or brown urine. This can lead to chills, fever, nausea, vomiting and upper abdomen tenderness.

Kidney Infections

Result when an infection reaches the kidneys and becomes known as

pyelonephritis

. Common causes can be infection from elsewhere in the body or obstruction of the prostate gland (usually in older men). For children, infection can be caused by the tube that drains urine from the kidneys and the bladder.

NOTE :

If left untreated, all UTI’s can lead to permanent kidney damage and possible kidney failure. The general treatment is by antibiotics. A person needs to maintain good personal hygiene when eliminating wastes (liquid and solid forms) from the body. Also, a person should drink lots of water.

Kidney Stones

Are also a common kidney disorder. They form when chemicals in the urine precipitate out and form crystals. The most common crystals are from calcium oxalate, while others could be from uric acid and cystine. Kidney stones are more common in men than women and can reoccur at any time. Factors such as recurrent urinary bladder infections, insufficient water intake and consumption, low levels of physical activity, and too much Vitamin C and D intake can lead to kidney stones. One of the best ways to decrease the occurrence of stones is to drink lots of water and to change your dietary habits.

Kidney Stones (cont) Symptoms include severe back or abdomen pain, blood in the urine, nausea and vomiting. Diagnosis involves a complete medical examination, including X-rays. Treatment may vary from letting the stones pass through the urinary tract to ultrasound shock (or lithotripsy) to disintegrate the stones to a small size that can be passed through the urinary tract. Real large stones require surgery for removal.