1. The human body - an orientation 1.3- 1.4

Summarizing

1.3 What are the requirements for life? and 1.4 Homeostasis is maintained by negative feedback

Atmospheric pressure is the force that air exerts on the surface of the body. Breathing and gas exchange in the lungs depends on appropriate atmospheric pressure.

At high altitudes, where atmospheric pressure is lower and the air is thin, gas exchange may be inadequate to support cellular metabolism.

excretion - 배설, 배설물	amplification - 확대, 증폭, 확충
waterfall effect - 폭포 효과 - 상층의 소비자에 마케팅을 집중하면 그 효과가 자연스럽게 아래층으로 퍼져 나가는 효과	cascade - 작은 폭포, 폭포처럼 쏟아지는 물
exquisitely - 아름답게, 절묘하게, 정교하게, 우아하게

Feedback - positive feedback and negative feedback.

Positive and negative feedback mechanisms are named for the direction of the response relative to the initial stimulus.

Negative feedback - the effector reverses the initial stimulus.

- controls which maintain some physiological function or keep blood chemicals within narrow ranges.

Positive feedback - the effector enhances the initial stimulus.

- mechanisms usually control infrequent events that do not require continuous adjustments. Typically, they set off a series of events that may be self-perpetuating and that, once initiated, have an amplifying or waterfall effect.

- mechanisms are often referred to as cascades.

Digestion is the breaking down of ingested foodstuffs to simple molecules that can be absorbed into the blood. The nutrient-rich blood is then distributed to all body cells by the cardiovascular system. In a simple, one-celled organism such as an amoeba, the cell itself is the "digestion factory", but in a multicellular human body, the digestive system performs this function for the entire body.

Metabolism - is a broad term that includes all chemical reactions that occur within body cells. It includes breaking down substances into simpler building blocks (the process of catabolism).

Reproduction - refers to both cellular reproduction (where a cell divides producing two identical daughter cells) and organismal reproduction (making a new person).

Responsiveness - the ability to sense changes (stimuli) in the environment to respond to them is a physiological ability.

- Responsiveness = excitability

- The nervous system is most involved with responsiveness.

- all body cells are excitable to some extent.

withdrawal reflex - 도피반사 - 두팔과 두다리의 피부가 강한 자극을 받았을때 모을 향하여 오므리는 것을 말함, 방어 반사	reflex - 반사작용, 반사적인 작용
pupil reflex - 동공 반사 - 빛의 밝기 정도에 따라 동공의 크기가 작아지거나 커지는 현상	residue - 잔여물, 잔여유산
chiefly - 주로	contractility - 수축성, 신축성

Water - accounts for 50% - 60% of our body weight and is the single most abundant chemical substance in the body.

- It provides the watery environment necessary for chemical reactions and the fluid base for body secretions and excretions

- we obtain water chiefly from ingested foods or liquids.

- we lose it from the body by evaporation from the lungs and skin and in body excretions.

Movement - is defined as both moving the organism through the environment using the muscular system, as well as moving substances through the internal organs of the body.

- on the cellular level, the muscle cell's ability to move by shortening is more precisely called contractility.

In the feedback cycle, the receptor detects a stimulus and sends input along the afferent pathway to the control center.

- Information (output) then flows from the control center along the efferent pathway to the effector.

- effector carries out the control center's response to the stimulus.

Metabolism depends on the digestive and respiratory systems to make nutrients and oxygen available to the blood and on the cardiovascular system to distribute them throughout the body. 

- metabolism is regulated largely by hormones secreted by endocrine system glands.

Excretion is the process of removing wastes, or excreta, from the body. If the body is to operate as we expect it to, it must get rid of non-useful substances produced during digestion and metabolism.

Digestion system - rids the body of indigestible food residues in feces and the urinary system disposes of nitrogen-containing metabolic wastes such as urea, in urine.

Carbon dioxide - a by-product of cellular respiration, is carried in the blood to the lungs, where it leaves the body in exhaled air.

Most homeostatic control mechanisms are negative feedback mechanisms. In these systems, the output shuts off the original effect of the stimulus or reduces its intensity.

- These mechanisms cause the variable to change in a direction opposite to the of the initial change, returning it to its "ideal" value.

Positive feedback - sets off a series of events that may be self-perpetuating such as labor contractions during birth and blood clotting.

hypothalamus - 시상하부	jerking - 홱 움직이다
a frantic pace - 눈앞이 핑핑 돌 정도의 속도	frantic - 정신없이 서두는, 제정신이 아닌
hectic - 정신없이 바쁜, 빡빡한

Approximately 20% of the air we breathe is oxygen. all the nutrients in the world are useless unless oxygen is also available. 

Because the chemical reactions that release energy from foods are oxidative reactions that require oxygen, human cells can survive for only a few minutes without oxygen.

The cooperative efforts of respiratory and cardiovascular systems make oxygen available to the blood and body cells.

Carbohydrates are the major energy fuel for body cells and are found primarily in plant-derived foods.

Nutrients, taken in via the diet, contain the chemical substances used for energy and cell building.

Most plant-derived foods are rich in carbohydrates, vitamins, and minerals whereas most animal foods are richer in proteins and fats.

Carbohydrates are the major energy fuel for body cells.

Proteins and to a lesser extent fats are essential for building ell structures.

Fats also provide a reserve of energy-rich fuel. selected minerals and vitamins are required for the chemical reactions that go on in cells and for oxygen transport in the blood.

The mineral calcium helps to make bones hard and is required for blood clotting.

Chemical reactions are to continue at life-sustaining rates, normal body temperature must be maintained. 

As body temperature drops below 37˚C (98.6˚F), metabolic reactions occur at a frantic pace and body proteins lose their characteristic shape and stop functioning. At either extreme, death occurs.

The activity of the muscular system generates most body heat.

Homeostasis - is so important that most diseases can be regarded as a result of its disturbance, a condition called homeostatic imbalance with advanced aging.

Our body's control systems become less efficient, and our internal environment becomes less and less stable, these events increase our risk for illness and produce the changes we associate with aging. 

Another important source of homeostatic imbalance occurs when the usual negative feedback mechanisms are overwhelmed and destructive positive feedback mechanisms take over.