Human Body Systems: Lymphatic, Excretory, and Nervous Functions
The Lymphatic System: Functions & Components
The lymphatic system is involved in the exchange of substances between the circulatory system and the cells.
It collects the excess interstitial fluid that remains between the cells, returning it to the bloodstream. It also transports fats absorbed in the intestine and plays a vital role in the body’s defense system.
Key Functions of the Lymphatic System
- Return excess interstitial fluid to the blood
- Transport absorbed fat from the small intestine to the blood
- Participate in the body’s defense system
Components of the Lymphatic System
Lymph: The Circulating Fluid
Lymph is the fluid that circulates inside the lymphatic vessels; it originates from the interstitial fluid and also contains lymphocytes.
Lymphatic Capillaries: Reabsorption Vessels
Lymphatic capillaries are small vessels found in all tissues. Their function is to reabsorb interstitial fluid.
Lymphatic Vessels & Valves
Lymphatic vessels are larger vessels equipped with valves that prevent lymph from flowing backward. Key examples include the great lymphatic vein and the thoracic duct.
Lymph Nodes: Immune System Hubs
Lymph nodes are small, bean-shaped organs connected by lymphatic vessels. They are located along major arteries and veins, with high concentrations around the ears, neck, and armpits. They contain lymphocytes. During an infection, these nodes often become enlarged.
The Excretory System: Waste Elimination
Cells produce waste products as a result of cellular metabolism. If these waste products accumulate, they can be harmful. Waste is released into the blood, and the blood carries them to the organs responsible for elimination.
Excretion: Definition & Key Substances
Excretion is the biological process by which metabolic waste products are eliminated from the blood. The main substances excreted include:
Urea
A substance synthesized by liver cells from ammonia produced during protein metabolism.
Uric Acid
A substance produced from the breakdown of purines, molecules that make up nucleic acids. It is also produced by the liver.
Dissolved Mineral Salts
These are only excreted when present in excess in the blood; the primary example is sodium chloride.
Foreign Substances
These substances, broken down by cells, accumulate in the blood. Examples include certain drugs and food additives.
Organs Involved in Excretion
Kidneys
The kidneys eliminate toxic substances resulting from cell metabolism through urine.
Liver
The liver eliminates substances resulting from the decomposition of red blood cells, which are stored in bile and expelled through feces.
Lungs
The lungs expel carbon dioxide from cellular respiration.
Sweat Glands
Sweat glands are distributed throughout the skin. They form sweat using waste products from the blood, which is then excreted through skin pores.
Anatomy of the Excretory System
Kidney Anatomy
The kidneys are two organs located in the lumbar region. Each kidney consists of:
- An outer region, the cortex.
- A medulla, divided into sections.
- The renal pelvis, a cavity that collects urine (containing urea, uric acid, and mineral salts).
Each kidney is composed of millions of tiny filtering units called nephrons. Each nephron consists of a glomerulus, which is surrounded by Bowman’s capsule, and a renal tubule. Several nephrons drain into a collecting duct.
The Urinary Tract
The urinary tract carries urine from the kidneys to outside the body. It is composed of:
- Ureters: These tubes carry urine from the kidneys to the bladder.
- Urinary bladder: A muscular, elastic sac where urine is collected before being expelled. The sphincter regulates its opening. The bladder is connected to the outside through the urethra.
- Urethra: This tube carries urine from the bladder to outside the body during urination. In men, the urethra carries both urine and semen. In women, the urethra is solely for urination.
Functions of the Excretory System
The excretory system purifies the blood by forming urine and also regulates water balance in the body. Urine formation involves two main stages: filtration and reabsorption.
Filtration
Due to blood pressure and the permeability of the glomerulus capillaries, an initial filtration occurs. Water, nutrients, and waste substances are filtered. The filtered fluid then enters Bowman’s capsule, forming primary urine.
Reabsorption
As primary urine moves through the renal tubule, most water, nutrients, and mineral salts are reabsorbed back into the blood. The remaining fluid is the definitive urine.
Nervous System: Coordination & Communication
The nervous system is made up of nerve tissue. The fundamental unit of the nervous system is the neuron. The transmission of information between neurons through electrical signals is called nerve impulses.
Neurons: Structure & Classification
Neurons are the most specialized cells in the body and generally do not reproduce. They are composed of a cell body, dendrites, and an axon. Key parts include:
Cell Body
The cell body contains the nucleus and other organelles, and is filled with cytoplasm.
Dendrites
Dendrites are short projections of the cell body that receive nerve impulses from other neurons.
Axon
The axon is a longer projection, made up of cytoplasm, that carries nerve impulses from the cell body to another neuron. The axon ends in a series of branches called axon terminals.
Axons can be protected by a white substance called myelin. The axons of various neurons sometimes group together to form nerve fibers, and these can further group to form nerves.
Depending on their function, neurons are classified as:
Sensory Neurons
Sensory neurons transmit information from receptors to the central nervous system (nerve centers).
Motor Neurons
Motor neurons transmit signals from the nerve centers to effector organs.
Interneurons
Interneurons connect sensory neurons and motor neurons. They are primarily located in the brain and spinal cord.
Nerve Impulse Transmission
Neurons generate and transmit nerve impulses. When a neuron is stimulated, electrical changes occur, transmitting signals from the dendrites towards the axon.
A nerve impulse is an electrical signal transmitted from the cell’s membrane to other neurons.
Neurons work together to establish connections called synapses, which allow the nerve impulse to cross from one neuron to another. A synapse is typically found between the end of one axon and a dendrite of another neuron.
Neurons act as both transmitter and receptor neurons.
Transmitter and receptor neurons are not in direct physical contact; instead, the message is transmitted across the synaptic cleft by chemical substances called neurotransmitters, released from the axon terminal.
Functions of the Nervous System
- Receive information transmitted by sensory organs.
- Interpret information and create appropriate responses.
- Transfer signals to effector organs.
- Coordinate organs for proper functioning.
- Carry out intellectual and mental functions.
- Responsible for emotions and feelings.
The Central Nervous System (CNS)
The CNS is composed of nerve centers that receive information, coordinate it, and prepare a response. It consists of the encephalon (brain) and the spinal cord.
The CNS is composed of two types of tissue: grey matter, which consists of neuron cell bodies and dendrites, forming control centers for specific functions; and white matter, which contains neuron extensions (generally axons) covered in myelin.
The Encephalon (Brain)
The encephalon is protected by the cranium and three membranes called meninges. It is also cushioned by cerebrospinal fluid.
The Cerebrum
The cerebrum is divided into two hemispheres, and its surface, called the cerebral cortex, is full of folds and furrows known as convolutions. Cerebrum functions include:
- Receiving information from sensory organs and producing sensations.
- Transmitting responses to effector organs; coordinating nervous system functions; and controlling higher functions like memory, reasoning, and intelligence.
The Cerebellum
The cerebellum controls balance and coordinates voluntary movements, such as walking.
The Diencephalon
The diencephalon includes the thalamus and the hypothalamus.
- The thalamus relays sensory signals to the cerebral cortex and plays a role in consciousness (e.g., sleep).
- The hypothalamus connects to and controls the pituitary gland, which secretes regulatory hormones to other organs.
The Brainstem
The brainstem links the encephalon and the spinal cord, controlling involuntary actions such as breathing and heart rate. The medulla oblongata, part of the brainstem, is responsible for the contralateral control, where the left side of the brain controls the right side of the body, and vice versa.
The Spinal Cord
In the spinal cord, grey matter is found in the center, while white matter is in the outer part. A central channel within the grey matter (the ependyma) runs from top to bottom and contains cerebrospinal fluid. The spinal cord extends to all parts of the body except the head.
The spinal cord has two primary functions: it produces reflex actions independently of the brain, and it carries sensory nerve impulses to the brain and motor impulses from the brain to effector organs.
The Peripheral Nervous System (PNS)
The PNS connects all receptors and effectors of the body to the CNS. It is composed of nerves: 31 pairs originate from the spinal cord (spinal nerves), and 12 pairs originate from the encephalon (cranial nerves). Nerves can be classified as sensory, motor, or mixed. The PNS is divided into two main parts: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous System
The somatic system connects sensory receptors from sensory organs to the central nervous system, and from there to skeletal muscles.
- It receives information about external stimuli and body position.
- It transmits signals for voluntary muscle contractions.
The Autonomic Nervous System
The autonomic nervous system regulates involuntary actions and has components located in the grey matter of the spinal cord and brainstem.
Sympathetic Nervous System
The sympathetic nervous system stimulates the body to react to fear, stress, or emergencies (fight-or-flight response).
Parasympathetic Nervous System
The parasympathetic nervous system promotes relaxation and ‘rest-and-digest’ functions.
These two systems are antagonistic, meaning they act in opposite ways to maintain homeostasis.
How the Nervous System Works
Stimuli perceived by internal or external receptors reach the central nervous system, where a response is generated.
Reflex Actions
Reflex actions are fast, involuntary responses controlled by the spinal cord, often occurring in emergency situations. The elements involved in a reflex arc are:
- Receptors: Perceive the stimulus.
- Sensory nerve: Transmits the impulse to the spinal cord.
- Interneurons: Generate an immediate response order within the spinal cord.
- Motor nerves: Carry the order to the effector organ.
- Effector organs: Execute the response.
Voluntary Actions
Voluntary actions are conscious responses deliberated by the encephalon (brain). The elements involved are:
- Receptors: Perceive the stimulus.
- Sensory nerves: Transmit the impulse to the cerebral cortex.
- Cerebral cortex: Interprets the information into a conscious sensation and formulates a response.
- Motor nerves: Carry the order to the effector organ.
- Effector organs: Execute the response.