Excretion and Waste Removal

• Excretion is the removal of metabolic waste substances from the body.
• Metabolic wastes include carbon dioxide and water from aerobic respiration in cells.
• Urea is produced by the breakdown of excess proteins (amino acids) in the liver.
• Medical drugs and dietary minerals in excess are also considered excretory substances.

The Danger of Waste Products

• Waste products can be toxic if allowed to reach high concentrations.
• Carbon dioxide dissolves in water to form an acidic solution, which can lower the pH of cells, reducing enzyme activity.
• High amounts of waste products can cause body fluids to become overly concentrated.
• Waste products take up storage space needed for more useful molecules.

Excretion in Plants

• Waste products in plants include oxygen, carbon dioxide, water vapor, and other unwanted chemical substances.
• During the day, the rate of photosynthesis is typically higher than the rate of respiration.
• More oxygen is released than used in respiration during the day.
• Less carbon dioxide is released than used in photosynthesis during the day.
• The net effect is that oxygen is in excess.
• At night, when there is no photosynthesis, only respiration occurs.
• Oxygen is used and carbon dioxide is produced.
• Carbon dioxide is in excess and becomes a waste product.
• Excess gases diffuse out of the plant via the leaf through the stomata.
• Most of the water vapor lost from a plant is not a waste product of metabolism.
• The water is instead drawn up from the roots in the transpiration stream.
• Chemical waste materials in plants can be stored in dying tissues like leaves. When the leaves fall off the plant, the substances are removed.
• In autumn, the leaves turn various colors due to the presence of these chemical waste products.

Excretion in Humans

• Humans have organs specialized for the removal of certain excretory products.
• The excretory organs include the kidneys, lungs, and skin.
• Kidneys excrete urea, water, and mineral ions.
• Lungs excrete carbon dioxide and water.
• Skin excretes excess minerals (e.g., sodium) and water.
• The liver breaks down proteins (amino acids) into urea (urea production).

The Urinary System: Structure and Function

• The two key functions are filtering waste products from the blood and expelling them from the body as urine, and controlling the water levels of the body (osmoregulation).
• The urinary system includes two kidneys joined to the bladder by two tubes called the ureters.
• The urethra is another tube which carries urine from the bladder to outside the body.
• Each kidney is connected to:
  • The renal artery, which comes from the aorta and delivers oxygenated blood to the kidney
  • The renal vein, which delivers deoxygenated blood from the kidney to the vena cava.
• The kidneys regulate water content of the blood.
• The kidneys excrete toxic waste products of metabolism.
• The kidney has three region:
  • The cortex, which is the outermost region
  • The medulla, which is the inner section
  • The renal pelvis, which is the tube linking the kidney to the ureter.
• Each kidney contains structures called nephrons, also known as kidney tubules or renal tubules.
• Nephrons start in the cortex, loop down into the medulla, and back up to the cortex.
• The contents of the nephrons drain into the renal pelvis, and the urine collects there before flowing into the ureter for storage in the bladder.

The Nephron

• The nephron is made of a kidney tubule with several sections:
  • Bowman's capsule
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting duct
• Surrounding the tubule is a network of capillaries; which sit inside the Bowman’s capsule.

Ultrafiltration & Selective Reabsorption

• In the kidney, blood is first filtered (ultrafiltration) before key substances are reabsorbed back into the blood by selective reabsorption.

Ultrafiltration

• Arterioles branch off the renal artery and lead to each nephron, where they form a ball of capillaries (the glomerulus) sitting inside the cup-shaped Bowman's capsule.
• The capillaries get narrower as they go further into the glomerulus which increases the pressure, causing smaller molecules in the blood to be forced out of the capillaries and into the Bowman's capsule where they form the filtrate. This process is known as ultrafiltration.
• The substances forced out of the capillaries are glucose, water, urea, and salts.
• Some are useful and are reabsorbed back into the blood further down the nephron.

Reabsorption of Glucose

• Glucose is reabsorbed at the proximal convoluted tubule by active transport.
• The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules

Reabsorption of Water & Salts

• As the filtrate drips through the loop of Henle, necessary salts are reabsorbed back into the blood by diffusion.
• As salts are reabsorbed back into the blood, water follows by osmosis.
• Water is also reabsorbed from the collecting duct depending on how much water the body needs at that time.

Homeostasis

• The regulation of the internal condition of a cell or organism.
• Some examples are:
  • Water content
  • Blood pressure
  • Temperature
  • Blood glucose concentration
  • pH
• Homeostasis maintains optimal conditions for enzyme action and cell functions to ensure that reactions in body cells can take place.
• Water is reabsorbed in the collecting ducts.
• If the blood is determined to have a high water content in the hypothalamus then less water is reabsorbed.
• If the blood has a low water content in the hypothalamus then more water is reabsorbed.
• Water content can be detected in the hypothalamus.
• The pituitary gland releases a hormone called ADH (antidiuretic hormone).
• ADH affects the permeability of the collecting ducts to water.
• The quantity of ADH released depends on how much water the kidneys need to reabsorb from the filtrate.
• Hypothalamus detects when the water content of the blood falls below a certain level, and the blood is too concentrated. It then stimulates the pituitary gland to release more ADH.
• When more ADH is released then the collecting ducts of the nephrons become more permeable to water.
• High levels of ADH leads to more water being reabsorbed from the collecting ducts.
• Low volumes of more concentrated urine are produced when ADH levels are high.
• Hypothalamus detects when the water content of the blood is above a certain level, and the blood is too dilute. It then stimulates the pituitary gland to release less ADH.
• The collecting ducts of the nephrons become less permeable to water when ADH levels are low. Low levels of ADH lead to less water being reabsorbed from the collecting ducts.
• Larger volumes of a less concentrated urine are produced when ADH levels are low.

Control of Body Temperature in Humans

• The optimum body temperature of humans is 37°C to give optimal enzyme function
• The hypothalamus contains receptors that are sensitive to the temperature of the blood.
• The skin also contains temperature receptors and sends nervous impulses to the hypothalamus.

Cooling mechanisms:

• Vasodilation is relaxing the muscles of arterioles near the skin to dilate and allow more blood to flow through capillaries causing increased heat loss.
• Sweating is the secretion of sweat by the sweat glands. Sweat evaporates from the surface of the skin, leaving a cooling effect.
• Flattening of hairs is the relaxation of the hair erector muscle causing hairs to lie flat to allow air to circulate and radiation.

Warming Mechanisms:

• Vasoconstriction is when the muscles in arteriole walls constrict and allow for less blood flow through capillaries prevent heat loss.
• Shivering is a reflex action where muscles contract very rapidly in a regular manner, this generates heat.
• Erection of hairs is when hair erector muscles contract causing hair to stand on end to trap an insulating layer preventing heatloss.

Coordination Systems in Humans:

• Nervous and hormonal systems are the two control systems in humans that help in responding to stimuli.
• The nervous and hormonal systems coordinate a suite of responses to stimuli and enable the organisms to respond to changes and coordinate and regulate body functions.
• Coordinated responses require the following:
  • Stimulus (e.g. pain)
  • Receptor (e.g cells that detect stimuli)
  • Coordination centre (which receives and processes information from receptors)
  • An effector (a muscle or gland) bringing about responses

###The Nervous and Endocrine Systems

• The nervous system relies on electrical impulses that pass along nerve cells known as neurons traveling at high speeds allowing responses to stimuli to be rapid. The nervous system coordinates activity of sensory receptors, the central nervous system, and effectors.
• The endocrine system sends information via chemical substances known as hormones produced by endocrine glands and carried by the blood. Hormones are specific and alter activity of more specific target organs involved in slower-acting control functions.
• A gland is a group of cells that produces and releases one or more substances.
• Key features of the nervous and endocrine system:
  • Nervous system uses brain, nerves/neurons and electrical impulses for fast, short lasting effects.
  • Endocrine system uses glands, chemical messengers, and bloodstream leading to a slower but longer effect until hormone is broken down.
• Functions that need instant responses are controlled by the nervous system, and functions that need slow responses are controlled by the endocrine system.

The Human Nervous System: Structure

• Consists of:
  • Central nervous system (CNS): the brain and spinal cord
  • Peripheral nervous system: all of the nerves in the body
• A bundle of neurons is know as a nerve
• Nerves spread out from the central nervous system to all other regions of the body and importantly to the sense organs
• The CNS acts as a central coordinating center for the impulses that come in from any part of the body
• There are three types of neurones:
  • Sensory neutrons carry impulses from sense organs to the CNS (brain or spinal cord
  • Relay neutrons are short and have a small cell body with many dendrites branching from it
  • Motor neurones are long and have a large cell body at one end with long dendrites branching off it
• Axon is the main long fibre – There are three main parts to the cell body axons and dendrites to speed up electrical impulses transferring.
• Axon is insulated by small uninsulated sections along its length by a fatty myelin sheath.
• Many dendrites extend out to transfer from many from others
• Neurones receives cell structures from them

Human Nervous System Functions

• Stimulus is the start, then receptors, followed by sensory nerone and finally reaching to the brain.
• From the brain the message goes to a motor neurone via a relay neurone. The effector has the response.
• Stimulus is received by sensory neurone, receptors are specialised to detect the stimulus produced by electrical impulses, travel along the CNS, and passes the to the effector.

Synapses

• Neurons pass from end to the other but not actually fully on touch.
• The ending where they almost touch it is called a synapse.
• From the small gap message is sent from neuron (presynaptic) to the post synaptic membrane and then sent off to another cell. This can happen by neurotransmitters and receptors.
• Neurotransmitters cross from one end of synapse, neurotransmitters cross from one side of synaptic cleft while the body remains. This stimulates neuron generates electrical impulse, then the transmissions are the second stimulate.
• The second neuron stimulates the second neuron to prevent being completely still.
• The electrical impulse travels there is released chemical messages this is a synapse
• The transmitters release to spread them towards where the molecules that are to be delivered are.